phototropism experiment

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Experiment: How do plants “see” light?

Can plants really “see” the light.

Scientists call a plant’s ability to bend toward light phototropism . Even as far back as ancient Greece it’s been a big puzzle about how plants are able to do it. People experimented with how plants accomplish this amazing feat, but no one really figured out how it worked—until Charles Darwin came along, that is.

Although Darwin is most well-known for his studies on evolution, he was also a prolific scientist in general. The questions about phototropism piqued his curiosity, and he thought of an ingenious experiment to test how plants are able to see light. In this experiment, we’ll recreate what he did, and at the end we’ll dive further into the science.

3 small cups full of soil Tape, a marker, and 3 sticky notes Medium-sized box (such as a shoebox or a storage cube) 12 corn seeds Aluminum foil Small cookie sheet that fits inside the box (or another sheet of aluminum foil) 1 Straw Water

• Plant four corn seeds in each of the soil cups. Make sure they’re evenly spaced, and plant them just a half inch under the dirt.
• Water the cups, and dump out any excess water (be careful not to tip the soil and seeds out). Place the cups on the cookie sheet or aluminum foil. This will prevent moisture and dirt from soaking through the box.
• Place the cups/cookie sheet setup inside of the box. Make sure it’s open on one side so that light is coming in from an angle. Place in a windowsill, with the open side facing the sun. (You might need to stack some books underneath it to support it, if your windowsill isn’t very wide.)
• Shoot cap: Cut a small 2″ x 3″ square of aluminum foil. Wrap it around the tip of a straw to create a small, closed-ended metal cap, and slide it off. This will be placed over the tip of the growing shoot to cover any light coming in to the tip.
• Base sleeve: Cut a small 1/2″ x 3″ square of aluminum foil. Wrap it around the middle of a straw so it creates a small open-ended 1/2″ tall tube, and slide it off. This will be placed around the growing shoot so that it can grow through it.
• Check the cups each day. Once they send up a shoot about half an inch high, place either a shoot cap (on Tip seedlings) or a base sleeve (on Base seedlings) around them, depending on which cup they’re in. The control cup will get neither of the light exclusion devices. The seedlings might grow at different rates, so be sure to check each day to put the caps/sleeves on as needed. They grow fast once they germinate!
• Continue to water the seedlings as needed.
• Check the seedlings after a week. What has happened? Compare the seedlings with the caps and the sleeves to the control seedlings. Are any of them growing in certain directions?

How did the seedlings “see” the light?

If the experiment worked correctly, you should have noticed that the seedlings that were covered with caps at the tip grew straight up, while the control seedlings and the seedlings with the bases covered bent towards the light. This is phototropism in action.

Darwin correctly concluded that plants are able to “see” light using the tips of the plant shoots, rather than through the stalks. It wasn’t until a bit later that scientists figured out exactly why that was, though.

It turns out that plants are able to grow by using hormones such as auxins and gibberellins . Auxin in particular tells individual cells to reach out and grow longer, like Stretch Armstrong. It’s one of the ways that plants grow taller. Normally, plants growing with an unshaded light source will grow straight up towards the sun because auxin is evenly distributed all around the shoot.

But when the light is heavily shaded and comes in from an angle, something interesting happens. Auxin starts to concentrate on the shaded side of the plant instead, and as a result, the cells on the sunny side stay the same size but the cells on the shaded side grow longer. This causes the plant to tip and grow towards the light.

Auxin is primarily produced in the tips of the plants. This is why the plant grew straight up when you covered the tip with a cap—it couldn’t “see” the light anymore! The tips of the control seedlings and the seedlings with the bases covered could still sense the light, so they grew towards the sunlight.

Thanks to Charles Darwin and modern science, the mystery of how plants grow towards light was finally solved.

Learn more about phototropism:

To understand plant tropisms, you first have to understand plant hormones. We created an excellent page about Plant Growth Hormones  here and here .

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Lindsay graduated with a master’s degree in wildlife biology and conservation from the University of Alaska Fairbanks. She also spent her time in Alaska racing sled dogs, and studying caribou and how well they are able to digest nutrients from their foods. Now, she enjoys sampling fine craft beers in Fort Collins, Colorado, knitting, and helping to inspire people to learn more about wildlife, nature, and science in general.

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Phototropism

What is phototropism .

‘Photo’ means light, and ‘tropism’ means turning. Phototropism is the movement of plants or other organisms, such as fungi, in the direction of light or away from it. 

Types of Phototropic Movement

Based on the orientation of a plant or other organisms   in response to light, phototropism is of two types:

• Positive phototropism :   Growing in the direction of a light source, as found in the plant shoot.
• Negative phototropism (Aphotropism) : Growing away from light, as observed in plant roots .

What Causes Phototropism 

The presence of light is the most important factor in phototropism. Light energy stimulates the rate of photosynthesis in plants causing food production and generation of energy in the form of high-energy phosphate molecule , ATP . Phototropism is thus a survival strategy adopted by plants to maximize their light-harvesting capacity.

Early Experiments to Explain Phototropism

The Experiment of Charles Darwin

In 1880, Darwin and his son Frances in their experiment described the bending of grass seedlings in the presence of light. They used newly grown plants with their shoot tips covered by a sheath called the coleoptile. Through this experiment, they found that coleoptile-covered plants did not respond to the light stimulus. In separate research involving plants where either the tip or the lower part of the coleoptile is covered, they concluded that the upper part is only capable of sensing the light response.

Peter Boysen-Jensen Experiment 

In 1913, Danish physiologist Peter Boysen-Jensen, in a follow-up study to Darwin, showed that a chemical signal produced at the tip of the shoot in plants is responsible for the bending response of the coleoptiles   in response to light. 

In his experiment, he removed the tip of the coleoptile and covered the cut portion with gelatin and then reinserted the tip. To his observation, he found the coleoptile to bend towards the light. On repeating the experiment using mica in place of gelatin, he discovered that the coleoptile lost its bending response. This result proved that gelatin having pore allowed a specific chemical signal to communicate between the tip and the base of the plant while mica being impermeable prevented any such communication. 

In another experiment, Boysen-Jensen further showed that the chemical signal travels only through the coleoptile-covered plant’s shaded side. This proved that the signal is a growth stimulator.

Kenneth Thimann Experiment

Later in 1933, Kenneth Thimann isolated and identified the chemical signal responsible for plant growth and movement as indole-3-acetic acid or auxin . 

Physical Basis of the Phototropic Response

Plants sense light using specialized regions called photoreceptors. Photoreceptors are made of pigment molecules called chromophores that are responsible for absorbing light energy. On absorbing light, the chromophore initiates a signaling pathway that ultimately causes the plant to bend towards or away from the light source. 

How Does Auxin Promote Phototropism

Auxin is the growth hormone produced at the tip of plants. The mechanism showing the role of auxin in phototropic response is described in the following steps:

• The specific blue-light receptor proteins called phototropins receive the light of wavelength around 450 nanometers and get stimulated to activate several hormones, including the growth hormone, auxin.
• The phototropin molecules on the bright side absorb lots of light, while molecules on the shady side absorb much less. This causes more auxin to be transported down the shady side, and less to be transported down the bright side. 
• Auxin stimulates a decrease in cell-pH, which helps to activate a group of enzymes responsible for cell growth and elongation in the shaded regions, causing the stem to bend towards the light source. 

Examples 

Positive Phototropism

• Sunflower plants  ( Helianthus annus ): Highly phototrophic. They grow towards the sun, but they can also be seen to follow the sun’s movement from east to west. Sunflower plants need more light for growth and survival, as well as for fruiting and flowering, compared to other flowering plants.
• Seedlings : Grow vertically upwards when the amount of light is the same on all sides. However, when the light is not evenly distributed, the seedling tends to bend in the direction of light.
• Fungi  (Genus  Pilobolus ): Feed and survive on dead and decaying matter.  Pilobolus crystallinus  (commonly known as the ‘hat-thrower fungus’) uses phototropism to disperse their spores in the vegetation where herbivores consume them. 

Negative Phototropism

• The plant roots growing downwards and away from the light source shows negative phototropism
• Promotes food production in plants by photosynthesis
• Helps in growth of the plant
• Helps fungi such as  Pilobolus crystallinus  to complete their lifecycle
• Phototropism Explained – Thoughtco.com
• Phototropism & Photoperiodism – Khanacademy.org
• Phototropism – Plantcell.org
• Phototropism Experiment – Untamedscience.com

Article was last reviewed on Thursday, February 2, 2023

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Article Contents

• ‘The power of movements in plants’: Darwin's lasting legacy to the field of phototropism research

Darwin's ‘influence’: auxin and its role in phototropism

Darwin's vision: phototropin blue light receptors, getting from darwin's vision to his ‘influence’: early phototropin signalling components, power of movement meets origin: phototropism in the field, from darwin to the future: final thoughts.

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Understanding phototropism: from Darwin to today

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Jennifer J. Holland, Diana Roberts, Emmanuel Liscum, Understanding phototropism: from Darwin to today, Journal of Experimental Botany , Volume 60, Issue 7, May 2009, Pages 1969–1978, https://doi.org/10.1093/jxb/erp113

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Few individuals have had the lasting impact on such a breadth of science as Charles Darwin. While his writings about time aboard the HMS Beagle , his study of the Galapagos islands (geology, fauna, and flora), and his theories on evolution are well known, less appreciated are his studies on plant growth responses to a variety of environmental stimuli. In fact, Darwin, together with the help of his botanist son Francis, left us an entire book, ‘ The power of movements in plants ’, describing his many, varied, and insightful observations on this topic. Darwin's findings have provided an impetus for an entire field of study, the study of plant tropic responses, or differential growth (curvature) of plant organs in response to directional stimuli. One tropic response that has received a great deal of attention is the phototropic response, or curvature response to directional light. This review summarizes many of the most significant advancements that have been made in our understanding of this response and place these recent findings in the context of Darwin's initial observations.

‘ The power of movements in plants ’: Darwin's lasting legacy to the field of phototropism research

Plants are sessile by nature, and thus to maximize energy production they must rely on their capacity to move directionally, or exhibit tropic responses, in response to directional environmental cues. The way plants respond to stimuli has fascinated humans since the time of Ancient Greece ( Whippo and Hangarter, 2006 ). Although Charles (and son Francis) Darwin's ‘ The power of movements in plants ’ dealt in large part with Darwin's proposal that circumnutation could provide a unifying model to explain directional growth responses in plants ( Darwin, 1880 ), an hypothesis that has been shown to be generally incorrect, this seminal book has provided the foundation for an entire field of study focused on the tropic responses of plants.

‘ The power of movements in plants ’ proposed several key elements that shape the current research on tropic responses. Darwin, although not the first to do so (for an excellent historical literature review on tropic response research, see Whippo and Hangarter, 2006 ), proposed that plants could grow differentially (thus directionally) in response to external stimuli such as light or gravity. Second, he demonstrated that the part of the plant that perceives the stimulus is separate and distinct from the part that responds to that stimulus. In the case of phototropism, directional light is perceived in the apical portion of a young seedling and ‘transduced’ to more basally localized portions of the shoot as a differential signal that informs the plant which side is the closest to and which is the furthest from the light source such that a bending response occurs ( Fig. 1 ). Finally, Darwin proposed that an ‘influence’ (though he was unable to identify it) moves from the site of stimulus perception to the area of response where bending occurs ( Fig. 1 ).

Stylized representation of phototropism. The diagrams depicted here are meant to represent the coleoptile of a dark-grown grass seedling, such as oat, which is a classic model for phototropism. On the left is a seedling soon after exposure to unidirectional blue light (half sun to the right of the seedling). The ‘eye’ is used to represent Darwin's proposal that light sensing occurred within a specific region of the seedling (in this case the tip of the coleoptile), not that Darwin was ascribing any anthropomorphic properties to the plant per se . The downward pointing black arrows represent the downward flow of the ‘influence’ (we now know as auxin) that Darwin proposed moved from the site of light perception to the site of differential growth that results in curvature. The horizontal black arrows reflect the lateral movement of auxin occurring from the lit to the shaded side of the seedling that has been demonstrated to occur in a variety of plants. On the right is a seedling that has developed a curvature response after a refractory period ( t x ) during which the differentially accumulated auxin (green shading) promotes localized growth (plus signs).

Just after the turn of the century Boysen-Jensen (1911) was able to gain further insight into Darwin's ‘influence’ in an experiment that used pieces of mica to disrupt the proposed influence's flow, and the results of those experiments confirmed that the ‘influence’ does indeed participate in a plant's response to directional stimuli, such as light. In particular, Boysen-Jensen's experiments suggested that Darwin's ‘influence’ flows from the tip of the plant toward the base in the unlit side of the plant, and that this directional and differential movement of the ‘influence’ is critical for the plant's bending response.

Although textbooks generally credit the Dutch plant physiologist Fritz Went with the identification on Darwin's ‘influence’ as the now well-understood plant hormone auxin, the actual history of auxin's chemical identification is a bit more complicated. First, it is important to give shared credit for the physiological identification of auxin to the Russian plant physiologist Nicolai Cholodny, who, while Went was working with grass coleoptiles ( Went, 1926 ), was generating similar results with grass roots ( Cholodny, 1927 ). It is also critical to note that, in actuality, it is Kogl and colleagues ( Kogl and Haagen-Smits, 1931 ) at Utrecht University (where Went did his graduate work) that deserve credit for the first chemical identification of an ‘auxin’ from human urine. Cholodny (1928) and Went (1928) each independently proposed a similar mechanism by which auxin could mediate tropic responsiveness, which was later simply renamed the Cholodny–Went theory ( Went and Thimann, 1937 ). In brief, the Choldony–Went theory combines Darwin's hypotheses with those of the auxin pioneers to propose that an asymmetric accumulation of auxin occurs in response to a tropic stimulus, and that this asymmetric gradient of auxin stimulates the differential growth response that results in tropic curvature. While other models have been proposed, the Cholodny–Went theory is still the prominent one used to explain a plant's response to tropic stimuli.

Following the initial proposal of the Cholodny–Went theory, a number of hypotheses have evolved regarding how the unequal distribution of auxin occurs, particularly in response to phototropic (directional light) stimulation. For example, Went and Thimann (1937) hypothesized that the unequal auxin accumulation occurs due to either light inactivation of auxin on the stimulated side, light-induced inhibition of the production of auxin, or light-induced transport of the auxin from the lit side to the shaded side. A study by Briggs et al. (1957) showed that introducing a physical barrier between the lit and the shaded side of corn coleoptiles disrupts the formation of an auxin gradient, providing evidence against the arguments that light induces the destruction, or the inactivation, of auxin. Subsequently, Briggs (1963) published additional data that provided support for a hypothesis that the unequal distribution of auxin was due to a lateral movement or transport of auxin. Specifically, these data showed that, in maize coleoptiles, an increase in the amount of curvature in response to light was correlated to an increase in the amount of auxin present on the ‘shaded’ side of the coleoptile ( Fig. 1 ).

Pickard and Thimann (1964) applied radio-labelled auxin, indole-3-acetic acid (IAA) in particular, to maize coleoptiles to trace the path of auxin during phototropism. It was found that IAA moves laterally across the coleoptile from the lit to the shaded side under both the pulse (first positive) and extended (second positive) irradiation conditions ( Fig. 1 ). Based on similar radio-tracer labelling studies, Shen-Miller and Gordon (1966) proposed that light promotes a lateral accumulation of auxin by inhibiting polar auxin transport. Gardner et al. (1974) obtained additional support for the notion that light stimulates the lateral movement of auxin, although their data did not support a role of light-mediated inhibition of polar auxin transport.

In recent years, genetic studies in the model plant Arabidopsis thaliana have identified proteins that appear to function as auxin transport facilitators ( Leyser, 2006 ). At least five auxin transport proteins have been associated with stem/shoot phototropism: AUX1 (AUXIN-RESISTANT 1; Stone et al. , 2008 ), PIN1 (PIN-FORMED 1; Blakeslee et al. , 2004 ), PIN3 ( Friml et al. , 2002 ), MDR1 (MULTIDRUG-RESISTANT 1), and PGP1 (P-GLYCOPROTEIN 1; Noh et al. , 2003 ). Studies in Arabidopsis have also led to important findings about how a gradient of auxin established by such transport facilitators leads to differential growth. For example, semi-dominant loss-of-function mutations in the NPH4 (NON-PHOTOTROPIC HYPOCOTYL 4) / ARF7 (AUXIN RESPONSE FACTOR 7) locus, and dominant gain-of-function mutations in MSG2 (MASSUGU 2)/IAA19 and AXR5 (AUXIN-RESISTANT 5) / IAA1 result in severely impaired phototropic and gravitropic responses ( Liscum and Briggs, 1996 ; Watahiki and Yamamoto, 1997 ; Stowe-Evans et al. , 1998 ; Harper et al. , 2000 ; Park et al. , 2002 ; Tatematsu et al. , 2004 ; Yang et al. , 2004 ). NPH4/ARF7 is a transcriptional activator whose activity is repressed in the presence of the MSG2/IAA19 and AXR5/IAA1 ( Liscum, 2002 ). In the presence of elevated levels of free auxin, MSG2/IAA19 and AXR5/IAA1 are rapidly degraded by a 26S proteasome that requires the SCF TIR1 complex containing the auxin receptor TIR1 to target these proteins for degradation ( Tan et al. , 2007 ). This, in turn, allows homodimerization of the NPH4/ARF7 protein and transcription of ‘auxin responsive genes’ ( Tatematsu et al. , 2004 ; Celaya et al. , 2009 ). A recent transcript profiling study in Brassica oleracea has identified a number of genes that appear to represent targets of NPH4/ARF7 regulation in response to tropic stimulation; these include genes encoding proteins involved in the regulation of free auxin levels, additional transcriptional regulators, and proteins involved in the regulation of cell wall extensibility ( Esmon et al. , 2006 ).

In addition to proposing the existence of a mobile ‘influence’ that was necessary for tropic responses (we now know this ‘influence’ to be auxin; see above), Darwin made observations, again presented in ‘ The power of movements in plants ’, that indicated that tropic curvatures in response to light were not general light responses but specific with respect to light quality. In particular, Darwin was able to demonstrate that the blue region of the electromagnetic spectrum is the most effective portion of the spectrum with respect to the induction of phototropism. These findings have, like those of Darwin's tropic ‘influence’, provided the impetus for a large number of studies over the past 100 or so years. Yet only within the past decade or so have the molecular details of how plants ‘see’ blue light cues ( Fig. 1 ) to induce phototropism, ‘Darwin's vision’ if you will, have become known.

As was the case with the elucidation of the molecular mechanisms underpinning the role of auxin in phototropism, Arabidopsis genetics was also a major factor in the identification of the photoreceptor molecules mediating phototropism in higher plants. The first of these photoreceptors identified at the molecular level is phototropin 1 (phot1) ( Huala et al. , 1997 ), originally designated NPH1 (for its n on- p hototropic h ypocotyl mutant phenotype; Liscum and Briggs, 1995 ). The PHOT2 gene was subsequently identified based on its high degree of sequence homology to PHOT1 ( Jarillo et al. , 2001 ; Sakai et al. , 2001 ). Phototropins regulate not just phototropism, but a number of additional blue light responses, including stomatal opening, chloroplast movements, leaf movements and expansion, and rapid inhibition of stem growth ( Christie, 2007 ).

The functions of the phototropins in these responses are both overlapping and distinct. For example, in the case of phototropism, phot1 (see Briggs et al. , 2001 , for a description of nomenclature) is the dominant receptor, mediating response across a wide range of fluence rates (e.g. 0.01–100 μmol m -2 s -1 ), whereas phot2 appears to operate only at higher fluence rates (>10 μmol m -2 s -1 ) ( Sakai et al. , 2001 ). By contrast, with respect to stomatal regulation, both phot1 and phot2 contribute over the entire range of effective fluence rates ( Kinoshita et al. , 2001 ; Kinoshita and Shimazaki, 2002 ). The interplay between the phototropins is even more complex when one considers blue light-induced chloroplast movements. In high-light conditions, chloroplasts move away from the upper surface of the leaf to avoid photobleaching ( Wada et al. , 2003 ), a response that is mediated solely by phot2 ( Jarillo et al. , 2001 ; Kagawa et al. , 2001 ). However, in low light, both phot1 and phot2 appear to contribute equally to the accumulation of chloroplasts along the upper surface of the leaf to maximize photosynthetic light capture ( Wada et al. , 2003 ).

PHOT1 and PHOT2 , being duplicate genes, encode proteins that are strikingly similar in their overall sequence and structure ( Christie, 2007 ). Structurally, the phototropins consist of two major parts: (i) an amino-terminal photosensory domain, and (ii) a carboxyl-terminal Ser/Thr protein kinase signalling domain ( Fig. 2 ). Both portions of the protein are necessary for phototropic signalling and much has been learned in recent years about how each portion functions and is regulated, as discussed below.

Domain organization of the phototropin blue light receptors. Both phototropins (phot1 and phot2) share the same basic organization with two amino-terminal LOV ( l ight, o xygen, and v oltage) domains and a carboxyl-terminal protein kinase domain. Although not shown here, a single molecule of FMN (flavin mononucleotide) is associated with each LOV domain as a light-harvesting cofactor.

LOVing blue light: photosensory mechanism of phototropins

The photosensory domain of a phot contains two ∼110 amino acid islands with homology to each other that are critical for photoreceptor activity ( Christie, 2007 ): LOV1 (light, oxygen, voltage) and LOV2 ( Fig. 2 ). The LOV domains are members of the larger PAS (Per, Arnt, Sim) domain superfamily ( Huala et al. , 1997 ; Crosson et al. , 2003 ). Each of the LOV domains binds a single molecule of blue light-absorbing flavin mononucleotide (FMN) ( Christie et al. , 1998 ), imparting photoreceptor function to the phototropins ( Christie et al. , 1999 ; Salomon et al. , 2000 ).

As shown in Fig. 3 , photosensitive LOV domains undergo a unique photocycle in response to absorption of blue light ( Celaya and Liscum, 2005 ; Christie, 2007 ; Matsuoka et al. , 2007 ). In darkness, the FMN chromophore is bound non-covalently to the LOV domain as a singlet ground state molecule. This state, which is capable of absorbing blue light, is referred to as LOV D 447 ( Salomon et al. , 2000 ; Crosson and Moffat, 2001 ; Swartz et al. , 2001 ) and absorption of a single photon of blue light results in the generation of an excited singlet FMN, which is rapidly converted into a red-shifted triplet state (LOV L 660 ) ( Swartz et al. , 2001 ; Corchnoy et al. , 2003 ; Kennis et al. , 2003 ; Kottke et al. , 2003 ). The triplet state flavin rapidly decays to form a covalent adduct between the C(4a) atom of the FMN and the cysteine within a highly conserved motif (GXNR C FLQ) in the LOV domain; a state with a near UV-shifted absorption maximum designated LOV S 390 ( Salomon et al. , 2000 ; Crosson and Moffat, 2001 , 2002 ; Swartz et al. , 2001 ; Kasahara et al. , 2002 ; Fedorov et al. , 2003 ; Kennis et al. , 2003 ; Kottke et al. , 2003 ). This FMN-cysteinyl adduct is completely reversible in darkness ( Salomon et al. , 2000 ; Swartz et al. , 2001 ; Kasahara et al. , 2002 ; Kennis et al. , 2003 ) or after absorption of a second near UV photon ( Kennis et al. , 2004 ). Thus, LOV domains cycle between two major states (with a transient intermediate): the dark state (LOV D 447 ) and the lit state (LOV S 390 ), depending upon the light condition.

Proposed photocycle of the phototropin LOV domains. The photocycle begins with the absorption of a blue photon of light by the dark-state (LOV D 447 ) and subsequent conversion to an excited singlet state (asterisk). The excited singlet state is then converted to the red light-absorbing (LOV L 660 ) excited triplet state (T), which is hence converted into the near-UV-absorbing covalent adduct (LOV S 390 ) that represents the active state. Both the singlet and active LOV S 390 states can be converted to the initial dark-state by incubation in darkness. Details of this photocycle are described in the text. Approximate half-times of reactions are given.

It is generally accepted that the LOV S 390 FMN-cysteinyl adduct represents the active signalling state of a phototropin ( Crosson et al. , 2003 ; Celaya and Liscum, 2005 ; Christie, 2007 ; Matsuoka et al. , 2007 ). Consistent with this model, replacement of the critical cysteine with either serine or alanine eliminates the formation of LOV S 390 ( Salomon et al. , 2000 ; Swartz et al. , 2002 ; Kottke et al. , 2003 ), and expression of a PHOT1 transgene containing the cysteine to alanine mutation in both LOV1 and LOV2, or LOV2 alone, fails to complement the aphototropic phenotype of a phot1 null mutant ( Christie et al. , 2002 ). It is interesting to note that a LOV1 cysteine to alanine single mutant transgene does compliment the aphototropic phot1 mutant phenotype, indicating that the two LOV domains are not equal with respect to physiological function ( Christie et al. , 2002 ; Sullivan et al. , 2008 ). Similarly, LOV1 is dispensable, whereas LOV2 is sufficient on its own to mediate function of phot2 in the chloroplast avoidance response ( Kagawa et al. , 2004 ).

The aforementioned findings raise an obvious question: what is the functional role of the LOV1 domain? Several independent studies ( Salomon et al. , 2004 ; Nakasako et al. , 2004 ; Katsura et al. , 2008 ), suggest that LOV1 may function as a dimerization motif; a finding certainly not at odds with the fact that LOV domains are a sub-class within the larger PAS domain superfamily ( Crosson et al. , 2003 ). It is also interesting to note that the quantum efficiency for conversion of LOV D 447 to LOV S 390 is about 10-fold higher in LOV2 than LOV1 in phot1 ( Salomon et al. , 2000 ; Kasahara et al. , 2002 ; Iwata et al. , 2005 ), although once photoconverted the LOV1 domain is longer-lived than LOV2 ( Kasahara et al. , 2002 ; Iwata et al. , 2005 ). These observations suggest that at least in the case of phot1, the predominant receptor mediating phototropism, the LOV2 domain is considerably more ‘photodynamic’ than LOV1, and that selective pressures in nature are stronger on LOV2 versus LOV1. It remains to be determined why, if it is not functioning to regulate phototropin activity, LOV1 remains photosensitive at all.

Sharing the LOV: protein kinase domain activation

As already mentioned the phototropins contain a Ser/Thr protein kinase domain in their carboxyl-terminal regions ( Christie, 2007 ). While no native substrates for the phot protein kinase domain, other than the phototropins themselves, are currently known ( Christie, 2007 ; Matsuoka et al. , 2007 ), mutational studies have demonstrated that the catalytic activity of this domain is apparently necessary for phototropic signal-output ( Christie et al. , 2002 ; Cho et al. , 2007 ). Because of this latter fact much effort has been focused in recent years on understanding how the blue light-dependent formation of LOV2 S 370 leads to activation of the protein kinase domain.

While initial X-ray crystallography studies suggested that only minimal changes occur in the tertiary structure of a LOV domain during photocycling ( Crosson and Moffat, 2001 , 2002 ; Fedorov et al. , 2003 ), solution spectroscopy provided clear evidence that, in fact, the structural rearrangements associated with the formation of LOV S 370 are fairly pronounced, especially if the polypeptides being examined also encompassed regions carboxyl-terminal to LOV2 ( Swartz et al. , 2002 ; Corchnoy et al. , 2003 ; Eitoku et al. , 2005 ; Iwata et al. , 2005 ). Nuclear magnetic resonance (NMR) studies by Harper and colleagues ( Harper et al. , 2003, 2004 ) identified an alpha-helical region (designated the Jα-helix) that resides between LOV2 and the protein kinase domain, which, in darkness, associates with the solvent-exposed surface of the β-sheet portion of the LOV2 core region facing away from the FMN chromophore. Upon blue light-induced FMN-cystenyl adduct formation the Jα-helix becomes disordered and dissociates from LOV2 ( Harper et al. , 2004 ). A number of mutations were identified that could mimic the aforementioned ‘dissociated state’ in the absence of light exposure, and, when introduced into a full-length phot1, these same mutations resulted in light-independent autophosphorylation of the phot1 protein, suggesting that the LOV2-Jα-helix interaction acts to repress the protein kinase activity of phot1 ( Harper et al. , 2004 ). In support of such a LOV2 domain ‘repression model’ ( Fig. 4 ), in vitro studies have shown that an isolated protein kinase domain from phot2 is catalytically active against casein, a common in vitro substrate for protein kinase assays ( Matsuoka and Tokutomi, 2005 ). These results further suggest that phototropins may target proteins other than themselves for phosphorylation in planta , although again no such substrates are currently known.

‘Repression domain’ model for photoactivation of the phototropin protein kinase domain. (A) In darkness the LOV2 domain and cis -associated Jα helix adopt a compressed closed configuration that appears to repress the activity of the carboxyl-terminal protein kinase domain. (B) Upon absorption of a photon of blue light by the non-covalently associated FMN molecule [conversion of LOV2 D 447 to LOV2 L 660 is represented by the transition from oval FMN in (A) to starred FMN in (B)], the active cysteinyl-LOV2 domain adduct is formed to induce progressive structural changes in the LOV2 [represented by the larger blue LOV domains in (B) as compared to smaller grey LOV domains in (A)] that result in unfolding of the Jα helix and de-repression of the protein kinase domain. The active protein kinase domain can then catalyse the autophosphorylation of phototropin (blue balls with a P in each) and currently unknown substrates. The positions of the autophosphorylation sites are generally representative of those determined experimentally but are not meant to depict precisely and exclusively those sites. (This figure is available in colour at JXB online.)

Moving distances with LOV: intracellular localization of phototropins is dynamic and light regulated

Movements of the phototropins are not limited to the angstrom-level intramolecular movements upon formation of LOV S 390 , rather the entire phototropin protein appears to move from one part of the cell to another in response to blue light. For example, while phot1 is normally tightly associated with the plasma membrane in dark-grown seedlings, probably through its carboxyl-terminal protein kinase domain ( Kong et al. , 2006 ), blue light induces the relatively rapid (within minutes) movement of some proportion of phot1 to intracellular locations ( Sakamoto and Briggs, 2002 ; Wan et al. , 2008 ). Similar relocalization properties have also been observed for phot2; although the intracellular compartment to which phot2 moves appears to be the Golgi ( Kong et al. , 2006 ). At present it is unknown exactly how phototropin movement is linked to a particular physiological response, however, a recent study by Han and colleagues ( Han et al. , 2008 ) suggests that this dynamic response may be coupled with receptor adaptation/desensitization or signal attenuation. Specifically the authors found that blue light-induced relocalization of phot1 can be largely, if not completely, prevented by prior exposure to red light ( Han et al. , 2008 ); light conditions that also lead to phytochrome A-mediated enhancement of phot1-dependent phototropism ( Stowe-Evans et al. , 2001 ; Han et al. , 2008 ). Thus it would appear that plasma membrane-localized phot1 is more ‘active’ in terms of phototropic signalling than internalized phot1. Certainly the dynamic nature of phototropin localization represents fertile ground for future studies.

One of the biggest questions currently facing the community of researchers who study phototropism at the molecular level is: how does phototropin activation lead to auxin-regulated differential growth (curvature)? While the details of this process still remain largely unknown, several components of this ‘black box’ have been identified, most notably three phot1-interacting proteins: NPH3 (NON-PHOTOTROPIC HYPOCOTYL 3; Motchoulski and Liscum, 1999 ), RPT2 (ROOT PHOTOTROPISM 2; Inada et al. , 2004 ), and PKS1 (PYTOCHROME KINASE SUBSTRATE 1; Lariquet et al. , 2006 ).

NPH3 and RPT2 are paralogous proteins that represent the founding members of the moderately sized NRL ( N PH3/ R PT2- L ike) protein family (33 members in total) in Arabidopsis ( Celaya and Liscum, 2005 ; Celaya et al. , 2009 ). Members of the NRL family, including NPH3 and RPT2, share five regions of primary sequence conservation ( Fig. 5 ): DIa ( D omain Ia ) and DIb, together comprising an amino-terminal BTB ( B road-Complex/ T ramtrack/ B ric-à-Brac) domain ( Aravind and Koonin, 1999 ; Stogios et al. , 2005 ); DII, resembling no known structural or functional motif; and the remaining two regions, DIII and DIV, together representing the Pfam ‘NPH3 domain (PF03000)’ of unknown function ( Finn et al. , 2008 ). The NRL family also exhibits several regions of conserved predicted secondary structure that have diverged in sequence ( Fig. 3 ); most notably a carboxyl-terminal coiled-coil ( Lupas and Gruber, 2005 ) that is present in approximately half of the family members ( Celaya and Liscum, 2005 ; Pedmale and Liscum, 2007 ). Although the functional roles of each of the aforementioned ‘domains’ are currently not fully understood, the coiled-coil has been shown to represent the phot1-interaction domain of NPH3 ( Motchoulski and Liscum, 1999 ; Pedmale and Liscum, 2007 ), while the BTB domain can mediate interaction between NPH3 and RPT2, at least in yeast ( Inada et al. , 2004 ). Recent studies have shown that the BTB domain of NPH3 can also mediate interaction with CULLIN 3 (CUL3) ( Pedmale and Liscum, 2007 ). These latter results suggest that NPH3 may represent the substrate adapter component of a CUL3-based E3 ubiquitin ligase, a recently recognized role for many BTB-containing proteins ( Krek, 2003 ; Pintard et al. , 2004 ; van den Heuvel, 2004 ; Willems et al. , 2004 ; Stogios et al. , 2005 ; Perez-Torrado et al. , 2006 ).

Domain organization of the NRL ( N PH3/ R PT2- L ike) family of proteins. Members of the NRL family, including the founding members and phototropic signal transduction components NPH3 and RPT2, share five domains of conserved sequence homology designated DIa to DIV. They also contain two regions of conserved predicted secondary structure; an amino-terminal BTB domain (encompassing most of DIa and DIb) and a carboxyl-terminal coiled-coil (C-C).

While no target for ubiquitination by an NPH3-CUL3 complex has yet been reported, one can imagine that such a target might function in the regulation of auxin transport. Findings that phototropic stimulation fails to induce an asymmetric distribution of auxin across the coleoptile in the rice mutant cpt1 ( c oleoptile p hoto t ropism 1 ) ( Haga et al. , 2005 ) is consistent with this hypothesis. CPT1 encodes the rice orthologue of Arabidopsis NPH3 ( Haga et al. , 2005 ), thus placing NPH3/CPT1 downstream of phot1 and upstream of the regulation of auxin redistribution. Recent studies suggest that regulation of auxin transport may represent a common function of NRL family members. For example, mutations in the NPY/ENP/MAB4 ( N AKED P INS IN Y UC MUTANTS/ E NHA N CER OF P INOID/ MA CCHI- B OU 4 ) subfamily of the NRL superfamily appear to influence auxin-mediated organogenesis through alterations in auxin movement ( Cheng et al. , 2007 ; Furutani et al. , 2007 ), probably through genetic interactions between the NPY proteins and the AGC kinases PID (PINOID), PID2, WAG1 (denotes the ‘wagging’ root growth it mediates), and WAG2 ( Cheng et al. , 2008 ). This latter observation is particularly intriguing as phot1 is also an AGC kinase ( Bögre et al. , 2003 ; Galván-Ampudia and Offringa, 2007 ).

The PKS1 protein was originally identified as a negative regulator of phytochrome signalling and to serve as a substrate for phytochrome's protein kinase activity ( Fankhauser et al. , 1999 ), but has since been shown to function as a positive regulator of phototropism as well and physically to interact with both phot1 and NPH3 ( Lariquet et al. , 2006 ). At present, it is not understood how PKS1 (or PKS2 and PKS4; Lariquet et al. , 2006 ) influences phototropism at a molecular level, but it is tempting to speculate that the PKS proteins may bridge the enhancing influences of phytochrome on phot1-dependent phototropism ( Liscum and Briggs, 1996 ; Parks et al. , 1996 ; Janoudi et al. , 1997 ; Stowe-Evans et al. , 2001 ; Liscum, 2002 ), possibly through influences on phot1 localization ( Han et al. , 2008 ). It is also worth noting that NPH3, like PKS1 ( Fankhauser et al. , 1999 ), is a phosphoprotein whose phosphorylation state and functional activity is light-dependent; whereas red light stimulates the phosphorylation of PKS1 in a phytochrome-dependent fashion ( Fankhauser et al. , 1999 ), blue light results in the desphosphorylation of NPH3 that is dependent upon the presence of phot1 ( Pedmale and Liscum, 2007 ). Thus it would appear that the signalling capacity of both NPH3 and PKS1 are regulated by similar post-translational mechanisms linked to the photoreceptors through which the former molecules signal.

Though Darwin (1880) hypothesized that phototropic responses are adaptive to a plant, and this proposal has been reiterated many times over the past 100 plus years in one form or another ( Iino, 1990 ; Liscum and Stowe-Evans, 2000 ; Christie, 2007 ), it has only been within the last few years that this hypothesis has actually been experimentally tested. Galen et al. (2004) have shown that the fitness of field-grown Arabidopsis plants carrying loss-of-function mutations in PHOT1 are significantly lower than that of wild-type plants grown in the same plots. Somewhat surprisingly, in contrast to previous proposals that stem phototropism would represent the adaptive response in nature ( Iino, 1990 ), this study found that root phototropism was the trait coupled to fitness, and only under high light conditions ( Galen et al. , 2004 ). A subsequent study demonstrated that negative root phototropism (bending away from directional blue light) enhances the ability of the plant to access water, which under high light conditions is more abundant deeper in the soil because of increased evaporation near the surface ( Galen et al. , 2007 a ). Three life history traits in particular were shown to be influenced dramatically by the ability of a root to access water in arid conditions: (i) seedling establishment, (ii) accumulation of biomass in established plants, and (iii) fecundity of plants reaching adulthood ( Galen et al. , 2007 a ). These studies provide an exciting potential avenue to develop plants capable of growing in more arid environment that maintain, or even increase, their production value through genetic engineering of phot1 signalling ( Galen et al. , 2007 b ).

Darwin's ‘ The power of movements in plants ’ undoubtedly stimulated an entire field of study on plant responses to the environment. Since publication of this seminal work our understanding of phototropism in higher plants has expanded tremendously. Several significant findings have been made: identification and characterization of the photoreceptors controlling phototropism; identification of auxin as the major growth regulator involved in the development of phototropic responses, and elucidation of its mechanistic basis of action; identification of several signalling components functioning between photoperception and auxin responsiveness; and characterization of an adaptive significance for phototropism under natural growth conditions. However, the goal remains fully to elucidate all of the molecular components, from the reception of light to movement, that contribute to the phototropic response, as well as the ecological variables that have provided the selective pressures for the evolution of this response in nature. In all these regards, there is much work left to do. The next century, like the last, is likely to bring many answers to such questions, leading to an even greater appreciation of just how important ‘ The power of movements in plants ’ really is!

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Phototropism

Reviewed by: BD Editors

Phototropism Definition

Phototropism is the ability of a plant, or other photosynthesizing organism, to grow directionally in response to a light source.

Plants and other autotrophs need to manufacture their own food; they usually do this through photosynthesis . Through photosynthesis, organisms convert water, carbon dioxide (CO 2 ) and light into sugars, which are used for energy and growth.

Plants are sessile , meaning they cannot move around to acquire what they need, so in order to maximize the amount of light that they receive through the leaves, they use phototropism.

Positive phototropism is the response of a plant toward a light source, while negative phototropism (also called “aphototropism ”) causes growth in the opposite direction. Plant roots usually use negative phototropism although additionally they use “ gravitropism ”, which is the response to gravitational pull.

How Does Phototropism Work?

The plants first sense the light using photoreceptors . Photoreceptors are special molecules consisting of a protein and a pigment that absorbs light called a chromophore . When light is absorbed by the chromophore, the protein changes shape, initiating a signaling pathway . Plants use signaling pathways to initiate processes such as gene expression, hormone production and growth. The specific photoreceptors which are responsible for detecting light during phototropism are called phototropins .

In 1880, Charles Darwin and his son Frances, discovered that seedlings with the very tip of a sheath called the coleoptile covered did not respond to light, whereas those with the lower part of the coleoptile covered did. From this they theorized that the light-sensing activity took place within the tips of the plant.

Plants contain a hormone called auxin , which coordinates many growth and behavioral processes throughout their life cycles. It is this auxin, which is responsible for the curvature of the stems, allowing plants to grow in a certain direction.

When the phototropins are activated by a light source, auxins are redistributed up the coleoptile and toward the side of the stem where the phototropins are less active – the shaded side.

The auxin activates proton pumps , which lowers the pH of the cells, making them more acidic. This acidification activates enzymes called expansins , which cause the cell walls to become more flexible by breaking the hydrogen bonds . When cell walls are less rigid, the cell walls are able to grow larger and faster than usual.

The larger size of the cells on the shaded size causes an asymmetry of cell size within the stem, and thus, the stem bends toward the light.

Examples of Phototropism

An example of a plant that is highly phototrophic is the sunflower ( Helianthus annus ).

Not only do sunflowers grow toward the sun, they can be visibly seen to track the sun’s movements from East to West throughout the day. At night, the heads move back from West to East in anticipation of the next day’s sunrise.

Scientists have discovered that sunflowers need more light for growth and survival, as well as fruiting, and flowering, than most other flowering plants. This finding is a likely reason for the daily solar tracking activity.

Pilobolus Fungi

Fungi in the genus Pilobolus are saprobic feeders, meaning they feed from non-living or decaying organic material.

In the case of the species Pilobolus crystallinus (commonly known as the “hat-thrower fungus”), they survive by eating the feces of grazing herbivores.

In order to gain access to feces, P.crystallinus uses an explosive propulsion technique, whereby the spores are shot from the sporangiophore in to the air and attach to vegetation. When grazing animals eat the vegetation, the spores pass through the animal’s digestive system and end up in their feces.

P.crystallinus uses phototropism so that their spores will be directed toward a light space where there is likely to be a gap in the grass, so they have a better chance of dispersal. Animals do not usually eat near dung, so they need to disperse their spores away from the dung on which their mycelium grew to increase their chances of being consumed.

Related Biology Terms

• Photosynthesis – The process by which photosynthesizing organisms convert light energy, carbon dioxide and water into oxygen and organic compounds.
• Skototropism – The growth of organisms away from a light source, often utilized by root climbers in order to find structures to attach to.
• Photoperiodism – The mechanism by which organisms respond to seasonal changes in day length.

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• v.18(5); 2006 May

Phototropism: Bending towards Enlightenment

Research on phototropism has had far-reaching consequences in the field of plant biology, from helping to refute the ancient misconception of plant insensitivity to the environment to the discovery of the plant hormone auxin and the identification of the phototropin photoreceptors. In this essay, we trace the major trends and ideas that shaped past shoot phototropism research and briefly summarize the current state of the field.

ANCIENT AND MEDIEVAL PERCEPTIONS OF PHOTOTROPISM

For centuries, poets, philosophers, artists, and scientists have noted and studied the phototropic movement of plants. In one of the earliest depictions of plant phototropism, Venus, the ancient goddess of love, transforms Clytie, a water nymph, into a plant because of her infatuation with Apollo, the sun god. Associated with her metamorphosis into a green plant, Clytie turns and follows the movement of Apollo ( Ovid et al., 1998 ). This tale of unrequited love is based on the assumption, developed by the early classical philosophers, that plants exhibit completely passive responses to the environment.

The earliest Greek philosophers, Anaxagoras (500–428 BCE) and Empedocles (495–435 BCE), believed that plants, like animals, are sensitive and capable of motion ( Drossaart Lulofs and Poortman, 1989 ). Although Plato (427–347 BCE) also believed in plant sensitivity, he rejected the idea of plant movement ( Shemp, 1947 ; Plato, 2000 ). Aristotle (384–322 BCE) argued that plants are totally passive and insensitive, and plant insensitivity served as a key criterion for distinguishing between plants and animals ( McKeon, 1947 ; Drossaart Lulofs and Poortman, 1989 ). Following Aristotle's reasoning, Theophrastus (371–287 BCE) also considered plants as passive organisms. In his botanical writings, Theophrastus recorded the phototropic (and solar-tracking) tendencies of plants, but rather than implicating any activators in the plant, he attributed the phenomenon to the sun's activity in removing fluid from the illuminated side of the plant ( Theophrastus, 1976 ). Because Aristotelian scientific philosophy placed greater value on logic alone and downplayed the need for experimental testing, Theophrastus' simple explanation of phototropism persisted until the 17th century when experimental botanists began to recognize plant sensitivity ( Webster, 1966 ).

During the middle ages, herbalists were more interested in the medicinal properties of plants than understanding plant biology. According to the doctrine of signatures, which associated the shape of a plant with its medicinal usage, phototropic plants may have been prescribed for the treatment of snake and serpent bites due to the serpentine shapes they display ( von Erhardt-Siebold, 1937 ). From today's scientific standpoint, the most significant advancement of the medieval herbals was the establishment of a nomenclature that separated plants whose flower-opening is dependent on the sun (composite) from plants that display shoot and leaf phototropism (solago) ( von Erhardt-Siebold, 1937 ).

DISCOVERING THE INDUCTIVE NATURE OF PHOTOTROPISM

During the renaissance, some early scientists began studying “natural magic,” which was reliant on the elements and occult properties of material things. In contrast with the Aristotelian disdain of experimentation, these early scientists used experimental observation in addition to classical texts to guide their thinking. Giambattista della Porta (1535–1615), probably one of the most well-known practitioners of natural magic, experimented with movement responses of cucumber seedlings. Drawing on Theophrastus' description of phototropism and anthropomorphic treatment of the response by medieval sources, della Porta described plant phototropism as a “rejoicing” response to the sun ( della Porta, 1569 ). Furthermore, in an attempt to explain seemingly similar natural phenomenon, he proposed that the same fundamental law of nature, which he called “sympathy,” governed the attraction of iron toward magnets, hens toward eggs, and the phototropic movement of plants toward the sun ( della Porta, 1569 ). Although it is unclear if della Porta actually believed the concept of plant sensitivity, his explanation of phototropism as a rejoicing and sympathetic response helped open the debate on plant sensitivity.

Francis Bacon (1561–1626), who helped shape the modern scientific method, was familiar with della Porta's writing about plant movement. Bacon recorded the tropistic movements of many different plants but held to the classical belief in plant insensitivity. As such, Bacon discarded della Porta's explanation of plant phototropism as a sympathetic or rejoicing response to the sun, and, like Theophrastus, he viewed phototropism as a simple mechanical consequence of wilting. He wrote, “the cause (of phototropism) is somewhat obscure…the part beateth by the sun waxeth more faint and flaccid in the stalk, and less able to support the flower” ( Bacon et al., 1627 ).

Intrigued by Bacon's discussion of plants, Thomas Browne (1605–1685) began studying plant physiology. As an alchemist, Browne was seeking a mystical unification of the universe and was more open to the idea of plant sensitivity than Bacon ( Webster, 1966 ). At the time, plants were thought to emit “bad air,” so Browne believed that plant movements allowed plants to avoid the bad air produced by neighboring plants. In what is probably the first crude scientific experiment on plant phototropism, Brown observed that mustard seedlings grown in front of a basement window would eventually reorient themselves toward the window after he rotated the pot ( Browne, 1658 ). When Robert Sharrock (1630–1684) repeated Browne's experiment, he concluded that the response was stimulated by fresh air and caused by growth rather than a mechanical consequence of wilting ( Sharrock, 1672 ). These early phototropism experiments are significant because they provided some of the first scientific examples of plant sensitivity.

As the leading botanical taxonomist, John Ray (1628–1705) would have recognized the taxonomic implications that plant sensitivity would have on the classical distinction between plants and animals. To escape the problem this created, Ray considered phototropic movement of plants to be a mechanical effect of temperature on growth. Believing that the rapid etiolated growth of dark-grown seedlings was caused by warmer temperatures, Ray argued that phototropism is caused by a temperature gradient across the seedling with the side closest to the window being colder and slower-growing ( Ray et al., 1686 ; Sachs et al., 1890 ). Although Ray's temperature hypothesis was later disregarded, his proposition that etiolated growth and phototropism are somehow connected remains a topic of investigation.

Due to the previous work of Browne and the discovery of the sensitive Mimosa plant, the idea of plant sensitivity to light began to receive wider acceptance ( Webster, 1966 ). However, responsiveness to light was still considered to be mechanical rather than inductive. Although Charles Bonnet (1720–1793) attributed the process of photomorphogenesis to light, he failed to recognize the importance of light for phototropism. Instead, his phototropism experiments ( Figure 1 ) led him to believe that plants were turning toward the warmth of the sun ( Bonnet, 1754 , 1779 ; Sachs et al., 1890 ). Yet based on Bonnet's observations, Henri-Louis DuHamel (1700–1782) concluded that light rather than warmth is more important for the response ( DuHamel, 1758 ; Sachs et al., 1890 ).

Woodcut Depicting a Charles Bonnet Tropism Experiment from 1779.

Two etiolated bean seedlings ( a and b ) oppositely placed in a vase ( v ) of water were tied downward ( e ). With the shutter ( f ) closed, each seedling reoriented upward toward the nearest wall (seedling a toward wall q and seedling b toward wall p ). When the shutter was raised, both seedlings reoriented toward the opening ( o ). Reprinted from Bonnet (1779) .

The Romantic period of the late 18th and early 19th centuries was characterized by a philosophical backlash against the mechanistic view of life that had dominated the enlightenment, and the concept of an endogenous “vital force” served as a common explanation of plant phenomena ( Sachs et al., 1890 ). As this outlook began losing popularity between 1820 and 1840, Henri Dutrochet (1776–1843) proposed that phototropism was an inductive response to light ( Dutrochet, 1824 , 1826 , 1828 , 1837 ). However, Dutrochet's contemporary, Augustin Pyramus de Candolle (1778–1841), thought that phototropism was simply a mechanical consequence of greater etiolated growth on the shaded side of the plant ( de Candolle, 1832 ). This explanation of phototropism was challenged by Albert Bernard Frank (1839–1900), who was the first to propose that phototropism and gravitropism are inductive responses sharing a common underlying process ( Frank, 1868 ). The inductive nature of the response was finally confirmed when Julius von Wiesner (1838–1919) showed that plants continue to bend toward a light source even after the light is turned off ( von Wiesner, 1878 ).

Charles Darwin (1809–1882) further explored the inductive nature and mechanistic connection between phototropism and gravitropism. He proposed that the back and forth circumnutation associated with plant growth could be directed by a stimulus such as light or gravity ( Darwin, 1880 ). Although Darwin's circumnutation theory of tropism served to propose a common mechanism underlying gravitropism and phototropism, the most significant discovery from his studies of plant movements was his demonstration that the site of photoperception at the shoot tip and the location of curvature are separable. From his observations, Darwin was able to propose that a transmissible substance produced in the tip is responsible for inducing curvature in lower regions of the plant ( Darwin, 1880 ). This insightful discovery eventually lead to the discovery of the first plant hormone, auxin.

THE DISCOVERY OF AUXIN AND UNDERSTANDING ITS ROLE IN PHOTOTROPISM

Darwin's ideas were initially dismissed by other plant physiologists (reviewed in Heslop-Harrison, 1980 ). Nevertheless, evidence in favor of Darwin's transmissible substance began to accumulate when Rothert (1894) also showed that light sensitivity is greatest near the tip of maize coleoptiles. Subsequent results of Fitting (1907) , Boysen-Jensen (1911) , and Paal (1918) provided more direct evidence that a transmissible substance produced in the tip participates in the response. This research culminated in a model put forth independently by Cholodny (1927) and Went (1926 , 1928 ), which proposed that light-mediated lateral redistribution of a plant growth hormone to the shaded side of the seedling causes the differential growth associated with phototropic curvature. This growth substance was shortly identified from human urine by Kogl and Haagen-Smit (1931) , who named the hormone auxin, derived from the Greek verb auxein, meaning “to grow.”

Although the Cholodny-Went model has remained the dominant explanation of phototropism, other models have challenged its validity. Historically, the model advocated by A.H. Blaauw has been one of the most common alternatives to the Cholodony-Went theory. Similar to Candolle, Blaauw proposed that phototropism is a secondary consequence of differential growth inhibition associated with photomorphogenesis ( Blaauw, 1919 ). While arguing against the Blaauw theory as the sole basis for phototropism, Boysen-Jensen et al. (1936) and Went and Thimann (1937) held out the possibility that both models might function during the phototropic response of dicotyledonous plants. Evidence for this complementary Blaauw/Cholodony-Went model was based on the work of Overbeek, who demonstrated that although unilateral light stimulates the movement of auxin to the shaded side of the hypocotyl, up to half the differential growth associated with phototropism can be attributed to light-mediated growth inhibition ( Overbeek, 1932 , 1933 ). More detailed measurements of growth during the 1980s supported this view by indicating that growth inhibition on the illuminated side is accompanied by growth promotion on the shaded side of seedlings ( Iino and Briggs, 1984 ; Macleod et al., 1985 ; Rich et al., 1985 ; Baskin, 1986 ). However, Cosgrove (1985) demonstrated that light-mediated growth inhibition occurs much sooner than the onset of phototropic curvature, a finding that would not be predicted by the Blaauw model. Additional evidence against the Blaauw model came from a study by Liscum et al. (1992) , which demonstrated that growth inhibition and phototropism are separable.

Another major challenge to the Cholodny-Went model came from an experiment indicating that carotenoids (proposed phototropism photoreceptors) participate in photoinactivation of auxin ( Kogl and Schuringa, 1944 ). This observation led to the hypothesis that phototropism is caused by differential carotenoid-mediated auxin inactivation (reviewed in Shank, 1950 ). Evidence against the auxin inactivation model came when Briggs et al. (1957) failed to see any in vivo change in total auxin concentration following treatment with light. Instead, their results showed that a barrier inserted between the illuminated and dark sides of the coleoptile prevents the development of an auxin gradient under unilateral illumination. Further support of the Cholodony-Went model came when Briggs (1963a) reported a correlation between an auxin gradient and the magnitude of phototropic response and when Pickard and Thimann (1964) showed that unilateral blue light causes greater radiolabeled auxin accumulation on the shaded flank of coleoptiles. Although these key studies demonstrated that a differential auxin gradient is associated with phototropism, they could not resolve how this gradient develops.

According to the classical Cholodny-Went model, lateral auxin transport gives rise to phototropism. However, experiments by Shen-Miller and Gordon ( Shen-Miller and Gordon, 1966 ; Shen-Miller et al., 1969 ) indicated that light inhibits polar auxin transport, which led them to propose that phototropism is caused by light-mediated inhibition of polar auxin transport on the illuminated flank of a seedling. Related to this model, Naqvi (1972) proposed that light-induced production of abscisic acid on the illuminated flank of the seedling causes unequal polar auxin transport. To test the idea that unequal polar auxin transport gives rise to phototropism, Gardner et al. (1974) followed radiolabeled auxin that had been asymmetrically applied to coleoptiles. Although they were not able to completely exclude a role for light-mediated inhibition of polar auxin transport, their results further demonstrated that unilateral blue light induced lateral auxin transport. At the same time, Kang and Burg (1974) reported that the enhancement of pea epicotyl phototropism by red light or gibberellins did not correlate with an increase in lateral auxin transport and proposed that the magnitude of the phototropic response is determined by adjusting auxin sensitivity. Altogether, these studies supported the involvement of auxin in phototropism, but the precise mechanism of how auxin transport and signaling cause phototropism remained unknown.

Many more details about how changes in auxin transport influence phototropism are now emerging from research using Arabidopsis as a model system. One report claims that mutations in PIN3 , which encodes an auxin efflux carrier involved in lateral auxin transport, can disrupt phototropism ( Friml et al., 2002 ). However the relationship between blue light signaling and PIN3 is uncertain. Work by Blakeslee et al. (2004) suggests that blue light does not effect the localization of PIN3 as it does for PIN1, an auxin efflux carrier thought to be important for polar auxin transport. Light-mediated relocalization of PIN1 appears to play a role in phototropism, as mutants in MDR1 , a gene encoding a P-glycoprotein ABC transporter, show less PIN1 localization to the basal end of hypocotyl cells and an enhanced phototropic response ( Noh et al., 2003 ). With less PIN1 localized to the basal end of hypocotyl cells, it was proposed that by decreasing polar auxin transport, phototropism may be enhanced by increasing the amount of auxin available for lateral transport ( Noh et al., 2003 ). However, some polar auxin transport appears to be important for the normal progression of phototropism as big mutants, which have diminished polar auxin transport, display longer phototropic latent periods ( Whippo and Hangarter, 2005 ). Clearly, more research is needed to determine how changes in both lateral and polar auxin transport impact phototropism. In particular, it will be important to understand how the dynamics of auxin transport are regulated in the context of the differential growth responses that lead to curvature.

Although our knowledge about auxin transport has advanced significantly, we have an even better understanding of auxin signaling during phototropism and a compelling model is developing. An important study by Harper et al. (2000) showed that the nonphototropic hypocotyl4 ( nph4 ) locus encodes Auxin Response Factor7 (ARF7), a member of the auxin response factor family. ARFs function as transcriptional regulators whose activity is inhibited by binding to AUX/IAA proteins. Auxin facilitates ARF activity by promoting the targeting of AUX/IAA proteins for degradation via the ubiquitin-proteosome pathway ( Liscum and Reed, 2002 ). With respect to phototropism, the degradation of IAA19, which binds to and inactivates ARF7, participates in phototropism under low light conditions ( Tatematsu et al., 2004 ). However, ARF7 involvement in phototropism does not seem absolutely necessary, since nph4/arf7 mutants display a phototropic response when treated with ethylene or given a red light pretreatment ( Stowe-Evans et al., 2001 ). This suggests that red light or ethylene activates another ARF that functions during phototropism ( Harper et al., 2000 ). A possible scenario is that the ARFs promote phototropism by controlling the expression of genes containing auxin response elements (AuxREs) ( Liscum and Reed, 2002 ). In support of this scenario, a recent study found that eight genes with differential transcript accumulation across phototropic-stimulated Brassica oleracea hypocotyls have one or more AuxREs ( Esmon et al., 2005 ). Interestingly, two of these genes encode expansins, which are involved in cell wall extension ( Esmon et al., 2005 ). Therefore, this study provides a plausible mechanism linking the differential growth underlying phototropism to the auxin regulation of expansin activity ( Esmon et al., 2005 ). Since the genes encoding for AUX/IAA proteins also contain this AuxRE, a negative feedback loop involving the upregulation of AUX/IAA proteins may participate in the progression of normal phototropism ( Tatematsu et al., 2004 ). For example, the reversal in the differential growth gradient that prevents the hypocotyl from curling around upon itself as the position of curvature migrates down the length of the hypocotyl ( Silk, 1984 ; Whippo and Hangarter, 2003 ) may be a partial consequence of ARF-mediated upregulation of AUX/IAA proteins.

THE SEARCH FOR A PHOTOTROPISM PHOTORECEPTOR

In parallel with research on the role of auxin in phototropism, another important area concerns how plants perceive a unilateral light source. As soon as it became more widely accepted that phototropism is stimulated by light in the 1800s, the focus turned toward identifying the property of light responsible. As early as 1817, Sebastiano Poggioli reported that blue wavelengths of light are more effective at orienting plant growth ( Poggioli, 1817 ). After several conflicting studies by Payer (1842) , Zantedeschi (1843) , Guillemin (1858) , and Sachs (1864) , Julius von Wiesner published the first methodical examination of the phototropism action spectra at specific wavelengths of light ( Figure 2 ) and confirmed that phototropism is strongest toward blue/violet light ( von Wiesner, 1878 ). Blaauw (1909) later identified a peak wavelength around 450 nm. Johnston elaborated on Blaauw's action spectra and identified dual peaks at 480 and 440 nm ( Johnston, 1934 ).

Julius von Wiesner's 1878 Diagram of the Action Spectra for Phototropism.

von Wiesner observed the phototropic response of pea (Wicke), cress (Kresse), and grass (Weide) seedlings to various light qualities. Letters along the x axis represent Fraunhofer lines demarking light quality between 759 nm (A) and 396 nm (H). Reprinted from von Wiesner (1878) .

When the action spectra for phototropism became better defined, attention turned toward identification of the blue light photoreceptor responsible for the response. Because the phototropism action spectra resemble the absorption spectra of carotenoids ( Haig, 1935 ; Wald and Du Buy, 1936 ) and carotenoid concentration is greatest in the tips where phototropism sensitivity is greatest, carotenoids were originally considered to be the pigments responsible for phototropism (reviewed in Shank, 1950 ). This hypothesis went relatively unchallenged until Galston and Baker (1949) proposed the involvement of a flavin or flavoprotein photoreceptor during phototropism. The flavin hypothesis was primarily based on observations that flavins have a peak absorbance around 450 and that flavonoids can inactivate auxin in vitro ( Galston, 1950 ).

For nearly the next 50 years, scientists would continue to debate the identity of the phototropism photoreceptor without convincing proof for or against either a flavonoid or carotenoid photoreceptor. One unfruitful line of research proposed that the blue light photoreceptor would be uncovered via studies of light-induced absorbance changes ( Berns and Vaughn, 1970 ; Munoz and Bulter, 1975 ; Brain et al., 1977 ). Another line of research used chemical inhibitors to try to elucidate the identity of the blue light photoreceptor. Schmidt et al. (1977) and Vierstra and Poff (1981a) showed that maize coleoptiles treated with flavin inhibitors fail to display a phototropic response. However, carotenoids also appeared to play a role, perhaps as screening pigments, because coleoptiles treated with the carotenoid biosynthesis inhibitor norflurazon also show a reduced phototropic response ( Vierstra and Poff, 1981b ). In another study, Leong and Briggs (1982) hypothesized the involvement of a flavin cytochrome complex in the perception of light for phototropism because treatment with the electron transport chain inhibitor acriflourfen increased the sensitivity to unilateral blue light.

Eventually, Briggs and associates turned to a biochemical approach in an attempt to identify the blue light photoreceptor. Their work led to the identification of a 120-kD membrane-bound protein whose phosphorylation state and activation is altered by blue light in a fashion that correlated with phototropism ( Gallanger et al., 1988 ; Short and Briggs, 1990 ; Reymond et al., 1992a ; Short et al., 1992 ). Stronger evidence that this blue light phosporylated protein is involved in phototropism came when Reymond et al. (1992b) reported that microsomes from the phototropism mutant JK224 contain lower levels of this 120-kD protein. The JK224 mutant was originally isolated by Khurana and Poff (1989) , who showed that it had a higher threshold requirement for the induction of phototropism and hypothesized that the mutant gene represented a photoreceptor involved in phototropism. Liscum and Briggs (1995) later identified other alleles of JK224 in their screen for nph mutants, strengthening this hypothesis.

Cloning of nph1 confirmed the prediction that this locus encodes the 120-kD protein ( Huala et al., 1997 ), and subsequent biochemical evidence indicated that two domains of this protein, LOV1 and LOV2, bind to flavin chromophores with spectral properties consistent with phototropism ( Huala et al., 1997 ; Christie et al., 1998 , 1999 ). Hence, this protein was renamed phototropin ( Christie et al., 1999 ), and the carotenoid-based photoreceptor hypothesis lost considerable ground. Still another photoreceptor was predicted to be involved in the induction of high-light phototropism because etiolated phot1 mutants retain a strong phototropic response to long-term irradiation with high-intensity blue light ( Sakai et al., 2000 ). A subsequent study showed that another member of the phototropin family, phot2, functions redundantly to phot1 in the induction of high-light phototropism ( Sakai et al., 2001 ).

With the identification of the phototropins as the phototropism photoreceptors, focus has turned to understanding their mechanism of light perception. Localization experiments reveled that more phot1 is located near the tip of etiolated seedlings than basally ( Sakamoto and Briggs, 2002 ; Knieb et al., 2004 ), thus providing a possible reason for why Darwin observed greater sensitivity toward the tip ( Darwin, 1880 ; Knieb et al., 2004 ). Salomon et al. (2000) presented evidence that light causes the formation of an adduct between a Cys residue located in the LOV domain and the flavin chromophore. In terms of the induction of phototropism and the light-mediated autophosphorylation of phot1, the LOV2 domain appears more critical than the LOV1 domain ( Christie et al., 2002 ). However, how light-mediated autophosporylation leads to a phototropic response remains to be seen. One possibility is that a gradient of autophosphorylated phototropin across the seedling precedes the development of phototropic curvature ( Salomon et al., 1997 ).

At this point, we know very little about the signaling components immediately downstream of the phototropins. NPH3 and RPT2, two related proteins with unknown function, bind to phot1 ( Motchoulski and Liscum, 1999 ; Inada et al., 2004 ) and function very early in phototropin signaling. These proteins are clearly important for phototropism since of all the characterized phototropism mutants, only nph3 mutants fail to show a response under any light condition ( Liscum and Briggs, 1996 ; Sakai et al., 2000 ). While RPT2 is not necessary for phototropism, it participates in the promotion of high-light phototropism ( Sakai et al., 2000 ). Future studies of NPH3, RPT2, and possibly other members of this family of proteins may become critical in uncovering how phototropin signaling modulates auxin transport. Calcium signaling has also been implicated as an early component of phototropin signaling ( Gehring et al., 1990 ; Babourina et al., 2004 ). Although phot1 is required for a rapid blue light–mediated increase in cytosolic calcium ( Baum et al., 1999 ) cytosolic calcium may not be necessary for phototropism ( Folta et al., 2003 ). Instead, the phot1-induced increase in calcium seems to be more directly related to rapid phot1-mediated growth inhibition in hypocotyls ( Folta et al., 2003 ).

PHYTOCHROME AND CRYPTOCHROME SIGNALING IN THE PROMOTION OF PHOTOTROPISM

The phototropins are not the only photoreceptors involved in phototropism. Although red light does not typically induce phototropism, a series of studies by Curry (1957) , Blaauw-Jansen (1959) , Asomaning and Galston (1961) , and Briggs (1963b) showed that pretreating seedlings with red light modulates phototropic sensitivity to unilateral blue light. Implicating the phytochrome red/far-red reversible photoreceptors in phototropism, Briggs (1963b) found that the red light enhancement of phototropism can be reversed by far-red light. This was later supported by a spectral correlation between phytochrome and the red light modification of phototropism ( Chon and Briggs, 1966 ). Although these studies implicated a phytochrome role in phototropism, they could not address the relative importance of phytochrome activity in the promotion of phototropism.

Several studies indicated that phytochromes can play more than just a secondary role in phototropism under some circumstances. For example, Iino et al. (1984) proposed that phytochromes may be essential for phototropism in some cases because they observed a phototropic response of maize mesocotyls to unilateral red light. Since phytochromes also absorb blue light and saturating red or far-red light from above could inhibit the phototropic response of pea epicotyls, Parker et al. (1989) also concluded that pea epicotyl phototropism is primarily induced by phytochromes in conjunction with an unknown blue light photoreceptor, then referred to as cryptochrome, playing a secondary role. However, red light is generally much less effective at inducing phototropism ( von Wiesner, 1878 ; Blaauw, 1909 ), so the phytochromes are not thought to be the directional photoreceptors in phototropism.

Several studies using Arabidopsis confirmed a significant role for the phytochromes, not only in the red light enhancement of phototropism ( Parks et al., 1996 ; Hangarter, 1997 ; Janoudi et al., 1997a , 1997b ; Stowe-Evans et al., 2001 ) but also in the absence of a red light pretreatment ( Hangarter, 1997 ; Whippo and Hangarter, 2004 ). Likewise, other studies indicate that the cryptochrome blue light photoreceptors can enhance the development of phototropic curvature ( Ahmad et al., 1998 ; Lasceve et al., 1999 ; Whippo and Hangarter, 2003 ). Since the phytochromes and cryptochromes function in parallel and somewhat redundantly during other light-mediated responses, they may be affecting phototropism in a similar fashion. Indeed, phytochrome cryptochrome double mutants display a severely reduced phototropic response under light conditions where phytochrome and cryptochrome single mutants show normal responses (C.W. Whippo and R.P. Hangarter, unpublished results). It is possible that the regulation of HY5, a transcriptional activator mainly associated with photomorphogenesis, by the phytochromes and cryptochromes participates in the promotion of phototropism because hy5 mutants display a significantly slower phototropic response to very low light treatments ( Whippo and Hangarter, 2005 ).

TOWARD UNDERSTANDING PHOTOTROPISM SENSITIVITY AND RESPONSIVENESS

The focus of phototropism research over the last 150 years was primarily concerned with the mechanistic aspects of the response. However, the degree to which a plant or plant part responds to unilateral light can vary widely. In some cases, different phototropic responses are a trivial result of mechanics: a large diameter shoot requires more differential growth than a small diameter shoot to reach the same angle of curvature. In other cases, differences in phototropism are more connected to the molecular physiology associated with changes in sensitivity and acclimation to prevailing light. Regulation of phototropism sensitivity/responsiveness can manifest itself in several different ways under long-term exposure to light or brief pulses of light.

The observation that etiolated seedlings exposed to continuous unilateral bright light have a slower response than seedlings exposed to continuous dim light was first observed by von Wiesner (1878) . As already discussed, von Wiesner was investigating whether phototropism is a passive mechanical consequence of light or an inductive response. According to the hypothesis that phototropism is a passive mechanical phenomenon, brighter light was expected to cause a faster, stronger response. When testing this predication of the mechanical phototropism model, von Wiesner (1878) observed that increasing the light intensity past a certain threshold actually retarded the phototropic response of etiolated seedlings. However, von Wiesner also believed that the induction of phototropism was closely related to the induction of photomorphogenesis and concluded that the regulation of phototropic sensitivity and responsiveness coincides with light-mediated growth inhibition ( von Wiesner, 1878 ). During the next century, Pringsheim (1912) and Ellis (1987) reconfirmed von Wiesner's initial observation, but a clear explanation of the phenomenon was not evident.

In a series of articles, Whippo and Hangarter (2003 , 2004 , 2005 ) studied the attenuating effect of continuous high light on the phototropic response of etiolated hypocotyls. From the results of these studies, it appears that there are at least two light-signaling pathways contributing to the attenuation of high-light phototropism. First, high light was found to cause a rapid phototropin-mediated decrease in phototropic responsiveness, as phot1 mutants display a fairly rapid high-light response ( Whippo and Hangarter, 2003 ). This may be due to high light–mediated desensitization of the phototropin photoreceptors or downstream effectors. Interestingly, RPT2 appears to partially mitigate the phot1-mediated attenuation of phototropism because RPT2 is not required for a high-light response in the absence of PHOT1 ( Sakai et al., 2000 ). Secondly, after the rapid phot1-mediated attenuation, the cryptochromes and phytochrome A were found to help maintain a slower high-light phototropic response ( Whippo and Hangarter, 2003 , 2004 ). The cryptochromes and phytochrome A probably function somewhat redundantly in this process by regulating the photomorphogenic regulation of auxin signaling and transport ( Whippo and Hangarter, 2005 ).

Although signaling elements associated with photomorphogenesis participate in the attenuation of high-light phototropism in etiolated seedlings, the relationship between development and phototropism is complex. In contrast with etiolated seedlings, Pringsheim (1912) observed that light-grown pea seedlings are more responsive to brighter light than to dimmer light. Similar behavior was observed in buckwheat ( Ellis, 1987 ) and Arabidopsis ( Whippo and Hangarter, 2005 ). However, how this developmental shift in phototropic sensitivity occurs is currently unknown.

Additional discoveries during the first decades of the 20th century also demonstrated that phototropic responsiveness to brief pulses of light is not as straightforward as might be expected. Using light pulses of varying duration or intensity, Blaauw (1909) determined that the magnitude of the response can be proportional to light dosage. Yet, Pringsheim (1909) and Clark (1913) soon reported that the reciprocity described by Blaauw is only valid under fairly low dosages of light (first positive phototropism). When further increasing the light dosage, Pringsheim (1909) and Clark (1913) stimulated a weak negative response (first negative phototropism), while even higher amounts of light restored a positive phototropic response (second positive phototropism).

Following the adoption of Arabidopsis as a model organism for plant biology research during the 1980s, Poff and associates conducted a series of detailed studies characterizing how light wavelength and dosage affect the first and second positive phototropic responses of etiolated Arabidopsis hypocotyls ( Steinitz and Poff, 1986 ; Konjevic et al., 1989 ; Janoudi and Poff, 1990 , 1991 , 1992 , 1993 ; Janoudi et al., 1992 ). These important physiological studies demonstrated that light signaling involved in pulse-induced phototropism is complex, with both additive and opposing effects on the development of hypocotyl curvature ( Janoudi and Poff, 1993 ). Salomon et al. (1997) have proposed that the complex fluence response curve associated with pulse-induced phototropism is related to the localization and magnitude of phototropin phosphorylation. However, this model awaits testing, and more research is needed to understand the underlying basis for the complex fluence curve of pulse-induced phototropism.

CONCLUSIONS

The history of phototropism is long and rich. Our current understanding of the response has its roots in ancient Greek philosophy and stems from the early physiological studies of the enlightenment. Recent research with Arabidopsis has tremendously expanded our mechanistic understanding of phototropism. We can no longer view the response as a simple or linear physiological response. Instead, phototropism must be viewed as a complex biological response involving interactions of multiple photoreceptors, multiple hormones, and multiple signaling pathways that together orchestrate the establishment of coordinated differential growth gradients. Given its complexity, much phototropism research remains to be done before we can understand all of the underlying mechanisms and know the full account of its biological significance.

Acknowledgments

We thank Nancy Eckardt for her excellent and thoughtful editing. We are supported by grants from the National Science Foundation (IBN-0080783), the Department of Energy (DE-FG02-01ER15223) (R.P.H.), and the Indiana University Briggs Developmental Biology Fellowship (C.W.W.).

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PHOTOTROPISM EXPERIMENTS

Branch botany (plants).

Phototropism in Plants

Phototropism in plants is a process where a plant bends in response to sunlight or any other light source. Plants sense light stimuli through their photoreceptors proteins. These specialized protein molecules or photoreceptors form a complex with the light-absorbing pigment (chromophore). When a chromophore absorbs light radiations, it brings changes in the photoreceptors’ structural configuration and initiates a signalling pathway. The signal then receives a light stimulus and responds accordingly.

We all know that the plants are the living organisms that remain fixed at one place by their roots. Thus, the plants cannot move, but they actually do. Now, you must wonder how the plants move? So, the answer is that the plants cannot walk like us, but their parts like the shoot system and root system tend to respond to external stimuli like light, water, wind, and chemical sources called tropism. Tropism merely refers to the response of living organisms, either similar or opposite to the direction of the external stimulus.

Phototropism merely refers to the plant’s directional movement (either towards or away) in response to a light stimulus. The plants mediate activities like energy production (photosynthesis), plant growth, and hormone production after receiving light stimuli. Some parts of plants (stems, leaves and flowers) show positive phototropism (bends towards the light). Oppositely, roots show negative phototropism (moves away from the light source).

In this context, definition, discoveries that led to the introduction of phototropism theory, mechanism, types and a brief overview of the phototropism phenomena. You will also get to know the role of plant hormones (phototropins and auxins) in the process of phototropism.

Content: Phototropism in Plants

Definition of phototropism, history of phototropism, mechanism of phototropism in plants, types of phototropism, overview of phototropism in plants, role of phototropins and auxins in phototropism.

• Positive Phototropism : The movement or bending of plant parts (shoot system) in response to light, is the phenomena termed positive phototropism.
• Negative Phototropism : The movement or bending of plant part (root system) in response to light, is the phenomena termed negative phototropism.

Let us take a quick overview of the phototropism process by taking an example. We must have seen the bending of plantlets placed near a window towards the light. It is because plants need the exposure of sunlight to produce energy and food. The roots under the pot grow upward, and once they break through the surface, the shoot system starts bending towards the light.

The whole phenomena of bending of the plant can be understood by the signalling pathway concept that is explained below. Plant senses light, by the specific molecules called “ Photoreceptors ”. The photoreceptors are the protein molecules, which link with the chromophore pigment. Chromophore absorbs light and distorts the configuration and activity of the protein.

These alternations bring a signal in response to light, and the plants respond to promote gene expression, growth and hormone production. The pathway of the response production is called “ Signalling pathway ”. Therefore, phototropism is a directional response where a plant bends towards the stimulus (light).

The Phototropins are the photoreceptors, which detect the light and absorb the blue range of the spectrum. The plant’s tip ( coleoptile ) possesses a high concentration of auxin (plant growth hormone). An exposure of coleoptile to the sunlight causes an unequal distribution of phototropins.

Thus, an unequal transport of a plant hormone (auxin) towards the darker side of the coleoptile is due to the different level of phototropins. Auxin will shift more towards the darker side than the illuminated side. The auxin on the darker side of the stem will promote cell elongation and more growth, which results in bending of the plant in response to a light source.

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Investigating Phototropism & Geotropism ( OCR A Level Biology )

Revision note.

Biology & Environmental Systems and Societies

Investigating Phototropism & Geotropism

Phototropism in plant shoots.

• Plant shoots are positively phototropic , meaning they grow towards light
• This ensures they maximise the amount of light they can absorb for photosynthesis
• Many of the experiments were conducted using coleoptiles (a sheath that surrounds the young growing shoot of grass plants)
• Darwin discovered that removing the tip of a coleoptile stopped the phototropic response to a unidirectional light source (light coming from one side) from occurring
• To ensure this was not simply due to the wounding caused to the plant, he covered the tip of a coleoptile with an opaque cover or 'cap' instead, to block out the light. This also stopped the phototropic response from occurring, showing that the tip of the coleoptile was responsible for detecting light
• Boysen-Jensen found that if he replaced the cut tip back on top of the coleoptile and inserted a gelatin block as a barrier in between, the phototropic response was restored
• This showed that the stimulus for growth was a chemical (hormone) , which was able to travel through the gelatin block
• Bosen-Jensen then inserted a mica barrier (mica is impermeable to chemicals) halfway through the coleoptile just below the tip, first on the lit side and then on the shaded side
• When the mica barrier was inserted into the lit side , the phototropic response occurred
• When the mica barrier was inserted into the shaded side , the phototropic response did not occur
• This confirmed that the stimulus for growth was a chemical (hormone) and showed that it was produced at the tip , before travelling down the coleoptile on the side opposite to the stimulus (i.e. the shaded side)
• It also showed that the stimulus acted by causing growth on the shaded side (rather than inhibiting growth on the lit side)
• Paál cut off the tip of a coleoptile and then replaced it off-centre in the dark
• The side of the coleoptile that the tip was placed on grew more than the other side, causing the coleoptile to curve (similar to a phototropic response)
• This showed that, in the light, the phototropic response was caused by a hormone diffusing through the plant tissue and stimulating the growth of the tissue
• Went placed the cut tip of a coleoptile on a gelatin block, allowing the hormones from the tip to diffuse into the block
• The block was then placed on the coleoptile, off-centre and in the dark
• As in Paál's experiment, the side of the coleoptile that the block was placed on grew more than the other side, causing the coleoptile to curve
• The greater the concentration of hormone present in the block, the more the coleoptile curved

Four historical phototropism experiments were conducted to investigate the process by which phototropism occurs

Controlling growth by elongation

• Indole-3-acetic acid (IAA), which is an auxin , is a specific growth factor found in plants
• IAA is synthesised in the growing tips of roots and shoots (i.e. in the meristems , where cells are dividing )
• IAA coordinates phototropisms in plants by controlling growth by elongation
• IAA molecules are synthesised in the meristem and pass down the stem to stimulate elongation growth
• The IAA molecules activate proteins in the cell wall known as expansins , which loosen the bonds between cellulose microfibrils , making cell walls more flexible
• The cell can then elongate

The phototropic mechanism

• Phototropism affects shoots and the top of stems
• The concentration of IAA determines the rate of cell elongation within the region of elongation
• If the concentration of IAA is not uniform on either side of a root or shoot then uneven growth can occur
• It is described as positive because growth occurs towards the stimulus
• Experiments have shown that IAA moves from the illuminated side of a shoot to the shaded side
• The higher concentration of IAA on the shaded side of the shoot causes a faster rate of cell elongation
• This causes the shoot to bend towards the light

Higher concentrations of IAA on the shaded side increases the rate of cell elongation so that the shaded side grows faster than the illuminated side

Geotropism in plant shoots and roots

• Gravity affects both plant shoots and roots , but in different ways
• Gravity modifies the distribution of IAA so that it accumulates on the lower side of the shoot
• As seen in the phototropic response, IAA increases the rate of growth in shoots , causing the shoot to grow upwards
• In roots, higher concentrations of IAA results in a lower rate of cell elongation
• The IAA that accumulates at the lower side of the root inhibits cell elongation
• As a result, the lower side grows at a slower rate than the upper side of the root
• This causes the root to bend downwards

Practical: investigating the effect of IAA on root growth

• Experiments can be carried out to investigate the effect of IAA on root growth in seedlings
• Seedlings (of the same age and plant species)
• Cutting tile
• Light source
• Lightproof container
• Blocks of agar (all the same volume)
• Use the scalpel to cut a 1cm section from the root tip of each seedling
• Mark the root tips at 2mm marks
• The water helps to keep the plant tissue alive
• Remove the ends of the root tips using the scalpel
• Transfer root cuttings with the end removed to an agar block
• A uniform light source is present
• Transfer intact root tips to an agar block
• A light-proof container is placed over the seedlings to prevent light from entering
• Apply a directional light source to one side of the root tips
• Leave all the roots in their treatment conditions for 3 hours
• Use the 2mm marker lines to determine if growth has occurred
• Note if the growth has been even on both sides

Results and analysis

• IAA is synthesised in the root tips so removing them means that no IAA is produced
• There is no inhibition of cell elongation
• There is an equal concentration of IAA on both sides of the root tip
• The inhibition of cell elongation is equal on both sides of the root tip
• The roots do not grow as long as those in group A due to the presence of IAA
• There is a greater concentration of IAA on the shaded side
• This results in greater inhibition of cell elongation on the shaded side
• So the illuminated side grows at a faster rate
• The roots bend away from the light – negative phototropism

Limitations

• Certain genotypes may be more prone to bending or have slightly different sensitivities to IAA
• If the root is mishandled the marks can be altered, which will affect the results
• Only general comments can be made about whether there has been even growth on both sides of the roots

The different treatments produce different levels of growth in the root tips. The IAA molecules inhibit cell elongation in roots

You may sometimes see IAA simply referred to as auxin. IAA is a particular type of auxin, which is a more general term for a particular group of plant hormones.

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Author: Alistair

Alistair graduated from Oxford University with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems & Societies. Alistair has continued to pursue his interests in ecology and environmental science, recently gaining an MSc in Wildlife Biology & Conservation with Edinburgh Napier University.

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Phototropism Experiments

Science Experiments With Plants for Kids

Phototropism in plants refers to the direction of plant growth in relation to a light source. Positive phototropism relates to plant growth towards a light source and includes most plant parts, such as leaves and stems. Negative phototropism is observed in roots and refers to growth in the opposite direction to the light source. These phenomena are easily observable through phototropism experiments that anybody can do at home or for a phototropism lab lesson for school.

The Dark Box

Any small pot plant can be used in this experiment. However, you can create your own seedling by planting bean, sunflower or corn seeds a week before starting your experiment. You will need a cardbord or wood box where you will put your pot plant or seedling. Allow at least 10 inches of free space on the top and the sides of the plant. Make a small hole, about two square inches, in one of the box's top corners. Place your plant inside the box, keeping it closed. Water regularly and observe the plant's growth towards the opening on the top.

Worshiping the Light

Plant some corn seed in a pot. Keep the pot in a dark room for three days, and then place a light bulb above the seedlings, in the center of the pot. After 14 hours of exposure to light, the seedlings bend towards the bulb as if they were worshiping the light. When the light bulb is turned off and natural light is allowed into the room, the seedlings go back to their vertical position.

Bending Beans

Plant three beans in separate small pots, place them in a well-lit area, and water them regularly. When the seedlings are about three inches tall, place a pot on its side in a horizontal position. Take a second pot and place on a windowsill and keep the third pot in its original position. Observe positive phototropism in the way leaves and stems lean toward the light during growth.

Negative Phototropism in Climbers

Some climbers, such as the English ivy (Hedera helix), show negative phototropism in their leaves, as they grow away from the light source and towards a dark substrate or wall. Buy two small ivies, but be aware the plant is an invasive species, which is illegal to sell or buy in Oregon. Place one plant inside the dark box described in the first experiment and put the other one in a well-lit place, but as far as you can from walls or other plants. Compare the growth direction in both plants. Destroy the plants after the experiment.

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• Plants-In-Motion at University of Indiana: Worshiping the light
• Photomorphogenesis in Plants"; Richard E. Kendrick; 1999
• "Structure And Functioning Of Cluster Roots ..."; Hans Lambers; 2003

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Student Sheet 8 – Phototropism: the Response of Seedlings to Light

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Seedlings growing on a windowsill will often bend towards the window as they respond to light – phototropism. But what exactly are the seedlings responding to? Which wavelengths of light stimulate the phototropic response?

The technique for this experiment helps students design an investigation to find out more about this tropic response. Students germinate seedlings in enclosed containers, with a coloured filter over the small hole allowing light in. Students predict which seedlings will demonstrate phototropism, and which will remain unaffected.

The students’ sheet contains a number of suggestions for further experiments, while the  worksheet outlines the basic technique.

Download the student sheets and teachers’ notes from the links on the right.

What's included?

• SAPS Sheet 8 - The response of seedlings to light - Student Notes
• SAPS Sheet 8 - The response of seedlings to light - Student Sheet
• SAPS Sheet 8 - The response of seedlings to light - Technical and Teaching Notes
• Plant growth
• Plant reproduction
• Plant responses

Related content

Teaching resources.

• Investigating Thigmotropism
• Tackling tropisms: gravitropism and phototropism

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Science project, how does phototropism affect the growth in green plants.

Grade Level: 6th - 7th; Type: Life Science

To determine whether plants are phototropic and provide evidence to substantiate the project`s results. 

Research Questions:

• What are tropisms and how do they affect the life of green plants?
•  What is heliotropism? How does it differ from phototropism?
•  Who was Frits W.Went and what did he discover that was of great significance in plant growth?
•  What role did Charles Darwin play in the area of plant growth?
•  What are auxins? What role do they play in plant growth?
•  What does the unequal concentration of auxins produce? What does that result In?

On the information level, this experiment serves to acquaint students with basic information on one of the key processes involved in and crucial to the growth and development of green plants. One of the processes involved is photosynthesis which is key in producing food for the plant. Photosynthesis is defined as producing something in the presence of light. We know that plants respond to many different stimuli in their environment. These stimuli are gravity, water, touch and light. These responses are called tropisms. The plant’s response to light is called phototropism. All of these tropisms are crucial to the plants survival. From our own experience, we know that the roots of plants respond to gravity by moving toward the ground. Also from experience we know that these roots are hydrotropic, namely the roots search for water. If we were to look closely at some plants, we may see the tendrils of vines fastening themselves around trees and bushes. This is called thigmotropism, namely, a response to touch. In this project we are searching for clear cut evidence as to the effect of light on green plants. How do green plants respond to light? We know that they do, but how do they do it? In sum, although we may think that plants do not move, they do move, all the time, but very, very, slowly responding to various stimuli. Plants are on the move! We call it growth. If there is unequal growth on one side of a plant, will it bend toward the other side? What precisely has phototropism to do with this? What is required is evidence to substantiate this uneven growth, specifically the key role of light in this process. The task is yours. Your experimentation coupled with your research on auxins will serve to further explain the results you obtain and provide a clear picture of what is actually occurring. 

This science fair experiment also serves to acquaint students with the essential processes of sciencing such as the importance of the use of a control when required, of identifying dependent and independent variables, of selecting a large enough sample of subjects when applicable to the objective, of accurate and organized data collection, of pictorial and or graphic presentation of data and of being able to make better judgments as to the validity and reliability of their findings. They take on the role of scientists and in the process they learn to act as scientist.

• 6 geranium plants of the same size with almost the same leaf configuration
• 4 cardboard boxes to hold some of the plants
•  razor blades
• a light source
• namely a window receiving direct sunlight,
• water, a measuring cup
• a watering can
• and a camera which is optional should you want to take photos for display purposes for the Science Fair.

Experimental Procedure:

• Gather all of the materials you will need for this project. These include the 6 geranium plants of the same size with almost the same leaf configuration,4 cardboard boxes to hold some of the plants, razor blades, a light source, namely a window receiving direct sunlight, water , a measuring cup, a watering can, a pen and labels.( a camera is optional)
• Copy the Chart provided on the next page so that you can readily record your observations.
• For both practical and dramatic value, you may wish to photograph the preparation stages as well as your plants starting with the before stages and continuing to the final stage.
• Arrange the plants into 3 groups of two. Label the sets of plants as follows: 1FL and 2FL (exposed to full light, no barriers), 1 PL and 2 PL (exposed to partial light, light only on one side) and 1NL and 2NL (no light, completely enclosed by the boxes).
• Prepare the boxes. In exposed to full light you have no need for boxes, in partial light you will. Remove one side of two of the boxes and in the no light category, keep the boxes intact.
• On your data chart record the appearance of each group of plants as identical or almost identical. Note any variation at the start before changing the conditions.
• Water all of the plants using the same amount of water. Continue watering each day. If you are taking photos, start now. Place group 1 of exposed to full light in the direct sunlight on a window sill, place group 2 in their individual boxes on the window sill with the closed side blocking the sun and place group 3 on the sill, completely enclosed in their boxes.
• Keep the plants in this location for four days. On day 4, observe each of the plants and record your observations in your chart. Note the position of the leaves in each of the three conditions.
• On day 6 record your observations. Now take the plants in group 2 and place them in full sunlight .Take the plants in group 3 and place them in partial sunlight.
• On day 9, view all of the plants and record your observations. Note any changes. Place all of the plants in full sunlight.
• Review all of the data. What did you conclude? Given your research on auxins, how do you explain your results? Were the results reversible or was their irrevocable damage? Why was it wise to use two plants in each group? Had you used a greater number of plants, how would it have affected your results? More reliable? More valid? Were you to extend your research in this area, what might you investigate the next time around? In the area of growth in animals are there “auxins”? Something to think about?

                                                                                         Chart of Data

Terms/Concepts:   Photosynthesis, chlorophyll, tropisms, hydrotropism, gravitropism, thigmotropism,phototropism, germination, auxins, indole-3-acetic acid, differential growth, natural light, artificial light and plant growth.

References:

Towle, A. (1993) Modern Biology Harcourt, Brace, Jovanovich

http:www.scienceclarified.com/Oi-Ph/Phototropism.html

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• Biology Article

Phototropism

Phototropism definition.

“Photropism is the ability of a plant to grow in the direction of light.”

What is Phototropism?

“Photo” means light, and “tropism” means turning. Phototropism is the phenomenon by which the plant bends in the direction of light. Light is required by the plants to stimulate energy production by the process of photosynthesis .

The hormone called auxins that are present in the cells, respond to photosynthesis and react to produce more protein and generate energy for the plant.

Almost all plants respond to photosynthesis in order to get more nutrition and energy. The stem and shoots usually react to positive phototropism by turning towards the sunlight, while negative phototropism takes place in the roots which turn away from the source of light. Here gravitropism or geotropism plays a major role in their growth.

Also Read:  What is Photosynthesis

Phototropic Response

Phototropism Discovery – Early Experiments

The following experiments were performed by different scientists in order to explain the mechanism of phototropism.

Charles Darwin’s Experiment

In 1880, Charles Darwin, with the help of his son researched phototropism on canary grass and oat coleoptiles and recorded his observations in the book ‘The Power of Movement in Plants’. They observed the bending of seedlings towards sunlight.

They demonstrated by covering the tips of the oat coleoptiles which prevented it from photosynthesis. Simultaneously, when they covered the lower portion of these coleoptiles, they turned phototropic. He concluded from this and several other experiments that the tip of the grass (called coleoptiles) have a strong sense of light and thus bend towards its source, while the middle section activates protons, thus decreasing the pH in the cells. This act acidifies the cell wall, activating an enzyme known as expansions which break down the cell wall thus making it less rigid.

Boysen Jensen’s Experiment

Phytochrome

Reflecting on Darwin’s experiment with oat coleoptiles, Boysen Jensen in 1913 experimented on seedlings by cutting off the tip and replacing it with a thin layer of gelatin between the tip and the cut stem. This did not prevent the stem to curve towards the source of light.

Next, he placed a small mica sheet below the tip of coleoptiles on the shaded side. The new way did not prevent developing a curvature. When the scientist placed the same mica sheet on the illuminated side, there was no curvature.

He concluded that a flow of material substance (later named as auxins) which passes through gelatin and mica sheet was the cause behind the curvature.

Phototropism Mechanism

The mechanism of phototropism is as follows:

• Light of wavelength 450 nm illuminates the plant.
• The photoreceptor receives the light, reacts to it, and initiates a response.
• Phototropins are the proteins that receive blue light during phototropism.
• Auxin moves to a darker side in the stem when exposed to light.
• Auxin releases hydrogen ions in the shaded region of the stem which causes a decrease in the pH. This decrease in pH activates expansin that causes the cells to swell and forces the stem to bend towards the light.

Phototropism Examples

Sunflower is a highly phototropic plant. They grow towards the sun and are also seen tracking the movement of the sun throughout the day. That is, the flower keeps changing its direction with the movement of the sun. Sunflower requires more light for its growth and survival.

Also Read:  Photosynthesis Concept Map

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Plant Phototropism Experiment For Students

• Spray bottle
• 8 bean seeds
• 8 small pots
• Two 1-foot-tall cardboard boxes with lids
• Masking tape
• Box cutter knife (with an adult to help)
• Piece of cardboard
• 2 small lamps
• 2 full spectrum light bulbs
• One 3” x 3” piece of clear, red green and blue cellophane
• Plant two of the bean seeds in two of the pots, give them water and let them grow. Wait until you can see them start to poke out of the soil.
• Use the waiting time to prepare your boxes. With each box, cut a hole 2” in diameter about 3 in. from the bottom. Take the clear cellophane and put it over the hole on one box. On the other box, place red cellophane over the hole. Now, one box will let every kind of light in and one box will only let red light in.
• Place your plants in the boxes, one in each. Use the ruler to place them 2 in. away from the hole. Use your camera to take a picture of the plants, looking down from above.
• Put the boxes on different sides of one room.
• Place the lamps next to the boxes, one for each, on the side with the cellophane covered hole. Make sure the lamps are equal distance from the holes, so everything is the same.
• Cover the boxes.
• In the morning, turn on the lamp. In the night, turn off the lamp. Repeat for a week.
• After the week, uncover the boxes and take a picture from above again. Then take the plants out and take a picture from the front. Did the plants grow differently?
• Repeat the experiment, but this time, change the red cellophane to blue cellophane. Then, repeat with green cellophane.
• Look at the pictures you’ve taken of each plants. Did the plants turn towards one color more than another? Did they not turn towards one color at all?

Results The plants in the box with the white cellophane, or the control plants, will have done better. The plants growing in red and blue light will have done better than the one in green light, the plants will grow towards both red and blue but will not have grown towards the green light.

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Phototropism and Geotropism

To study the phenomenon of phototropism and geotropism in plants

The experiment on phototropism and geotropism focuses on understanding how plants respond to external stimuli, specifically light and gravity, and how these responses impact their growth and orientation. Phototropism investigates how plants bend or grow towards a source of light, emphasising their ability to optimise photosynthesis for energy production. Geotropism, on the other hand, explores how plants respond to gravity by either growing downward (positive geotropism) or upward (negative geotropism), which is crucial for proper root development and nutrient absorption.

This experiment sheds light on the remarkable adaptive mechanisms of plants, demonstrating their capacity to sense and react to their environment, ultimately influencing their overall growth and orientation. 

Here are the steps of the experiment on phototropism and geotropism:

• Take two test tubes and fill them up to about two-thirds of their height with water. Label one as “Test Tube A” and the other as “Test Tube B.”
• Insert one plant into each test tube, ensuring that the roots are submerged in the water, while the stems and leaves extend out of the test tubes. Secure the openings of the test tubes with cotton balls to hold the plants in place.
• Seal the mouth of both test tubes tightly using additional cotton and adhesive tape to prevent any water leakage, even when the test tubes are turned upside down.
• Set up Test Tube A in an upright position using a laboratory stand, and fix Test Tube B upside down in another laboratory stand, ensuring that no water spills out.
• Place the laboratory stands near a window to expose the plants to direct sunlight.
• Over the course of the experiment, which includes days 2, 3, and 4, carefully observe both plants. Record your observations, paying attention to the direction in which the stems and primary roots are growing. Note any features that exhibit positive or negative phototropism and positive or negative geotropism.

In summary, this experiment revealed how plants adapt to their surroundings. We observed how they respond to light and gravity. Stems showed positive phototropism by leaning towards light for better photosynthesis, while roots exhibited positive geotropism by growing downward for stability and nutrient uptake. Stems also displayed negative geotropism, growing away from gravity to find optimal light. Overall, this experiment highlighted plant’s remarkable ability to sense and adapt to their environment, shedding light on their intricate growth mechanisms.

FAQs on Phototropism and Geotropism

Q.1 what is phototropism.

Ans. Phototropism is a plant’s growth response to light. It involves plants bending or growing towards a source of light to optimise photosynthesis.

Q.2 What is geotropism?

Ans. Geotropism, also known as gravitropism, is a plant’s growth response to gravity. It involves the orientation of plant roots downward (positive geotropism) and stems upwards (negative geotropism) due to the influence of gravity.

Q.3 What is the role of auxin in phototropism?

Ans. Auxin is a plant hormone that plays a crucial role in phototropism by promoting cell elongation on the shaded side of a plant, causing it to bend towards the light source.

Q.4 What are the practical applications of studying phototropism and geotropism?

Ans. Understanding these plant responses is essential for agriculture, as it helps optimise plant growth and crop yield. It also aids in designing controlled environments for plant cultivation.

Q.5 Can phototropism and geotropism occur simultaneously in the same plant?

Ans. Yes, phototropism and geotropism can occur simultaneously in a plant. For instance, the stem may bend towards the light source (phototropism), while the roots grow downward into the soil (geotropism).

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