Introduction

Phototropism is to be defined as the tropic effect of the plant in response to the factor light. Let us break the word into two, ‘photo’ means light and’ tropism’ means turning to a direction in response to the stimulus. This process works in accordance with the other five different types of tropisms which are: geotropism, chemotropism, hydrotropism, thigmotropism, and electrotropism.

There is a bit of confusion between the two words; phototropism and photoperiodism. Photoperiodism regulates flower development and growth regarding the length of day and night. In phototropism, a plant’s shoot grows towards the light imparting ‘positive phototropism’ while a plant’s root grows away from light causing ‘negative phototropism’. The negative phototropism is also called skototropism. In the positive case, the plant arranges its chloroplasts in such a way to attain maximum light to carry out the greatest photosynthesizing effect. This induces growth within the plant’s structure. 

The plants undergo a process of photosynthesis. We all know plants are autotrophs and are the food chain starter in an ecosystem. They produce their foods in the form of starch which gets stored in the leaves, stems, shoots, and roots. This is how we get energy and carbohydrates when we eat sugarcane (shoot) and reddish (root) of the plants. These materials serve to be the building block of the plant’s existence. The plants during the day when they are photosynthesizing take in carbon dioxide from the atmosphere and water from the soil. In the presence of light, the plant can produce glucose when polymerized stores in the form of starch.

Photosensitivity

This is a process whereby light energy is filtered, absorbed, and transduced to the final step. There are certain photoreceptors to detect such changes integrated with a complex structure of the protein. There are several pigment molecules that serve the major function of transduction. A few of the major pigments are listed below:

• Chlorophyll
• Cytochromes
• Phycobilin
• phytochrome

The role of light on seedlings

The light energy serves to be a starter pack for germination for a certain variety of seeds. The seeds only germinate when they receive a certain amount of light. The plant can effectively respond to light. For the detection of every stimulus, nature has designed certain receptors to detect it. These produce physiological changes when undergoing signal transduction. The plants have photoreceptors that detect light via the help of a pigment called a chromophore. The conformational change in protein’s structure is how the external stimulus is translated into internal changes.

Experimental phototropism analysis

Charles Darwin along with his son named Francis in 1880 worked to discover phototropism. They study young grass seedlings and monitor their bending towards the light. The seedlings which they studied have just sprouted radicle and plumule. Yet, these were covered with a protective covering known as coleoptile.

What the analysts did was cover the coleoptile at two points, at the tip and the lower part. The light was perceived at the tip. At the same time, it induced unequal elongation below the tip. This brings about bending as stated above.  Later, another physiologist Peter-Boysen-Jensen carried on the legacy and came forward with signal transduction theory which was meant to bring about bending to a certain angle. He repeated the experiment by covering the tip with gelatin and a mica plate. It was able to bend when exposed to light with gelatin while with the latter the coleoptile was not found responsive to light.

The Role of Phototropism in Plants

We take plants not moving at all, but they move slowly and evenly to accommodate growth. The even growth in the plant is all the way supported via the light and certain growth regulators named auxin. The unequal plant growth may cause it to bend to either side not substantial for the plant to maintain its balance and the pressure which it experiences from with itself. The pressure is called turgor pressure. Now to evaluate such scientific pieces of evidence we must carry out some experiments. This experiment will eventually help you understand the actual role played by every single constituent in the role and development act.

Experiment on Phototropism

It is important to identify the dependent and independent variables before proceeding with an experiment. Here we are studying the effect of light so the light will be a dependent variable.

The steps for the experiment are as follow:

• Take a total of 6 germinating plants keeping in mind that each one of them is identical.
• Keep them in the cardboard boxes
• Gather each plant in a group of 3. This means there be a pair of plant groups to be evaluated.
• Label both the boards separately to make sure you note down appropriate results on the evaluation chart.
• To expose the plant groups one to full light and others to no light.
• Water all the plants equally.
• Evaluate the plants under above-provided conditions for consecutive four days.
• Record all your observations and analyse the results to see the effect of light.

Role of Auxin in Phototropism

Here again, Cholodony-Went Hypothesis plays a vital role likewise it played in gravitropism. He brings about the point of auxin’s role in plant growth in about the 20th century. He told that an asymmetric light will cause the auxin to move towards the part of the plant which is unexposed to light. It gets concentrated over there. The auxin causes elongation and expansion of the cells allowing them to tilt towards the light side.

What auxin do, is work by acting on H ion pumps also called proton pumps. The auxin causes intracellular reserves of H ions to increase decreasing the pH of the cell in the darker part of the plant. This shifting of pH from neutral to acidic side will eventually activate the acidic enzymes known as expansins.

The role of expansins is not known but they disrupt Hydrogen crosslinks in the cell wall region. The greater osmotic gradient develops due to excessive intracellular movements of the solutes. The water is a polar and a small molecule, so it enters the cell. As the bond breaks and the continuous water entry is sustained, the turgor pressure exceeds the threshold. This leads to swelling of the cells and the phototropic movements to take place.

PIN genes are also known to play a vital role in phototropism. They polarize auxin to receive light and to inhibit certain kinases that activate these genes at a further higher pace. The uneven distribution of PIN3 also leads to uneven distribution of auxin which leads to asymmetric elongation. PIN7 aids by inducing pulse induce phototropism.

Types of phototropin

Phototropins are blue light-activated kinases. The blue light ranges somewhere between 300-500nm on the wavelength scale of the electromagnetic spectrum. The coleoptiles, as discussed earlier have two types of phototropin. Phototropins in actual are proteins that induce phototropism. These are flavoproteins, working together with cytochromes of the plants. These are also known as blue light receptors because of the light they perceive. They also aid in the stomatal opening which allows the exchange of gases to occur. This will eventually mediate the process of photosynthesis.

The phototrophic system in plants is made up of phototropin. There are numerously found in the coleoptiles. The two types of phototropin are phot1 and phot2. Phot1 is downregulated when there is more exposure to light while phot2 is upregulated. The phot1 is almost homologous to phot2. These act to be blue light acceptors and transducers and in response the open stomata to carry out the further action. Cytochrome 1 and cytochrome 2 also assist phots in this case. Cytochromes tend to maintain circadian rhythms in the plant cycle.

Auxin distribution models in phototropism

The variable auxin concentration notifies the plant’s growth in response to light. The concentration of auxin mainly increases in shaded regions of the plant. The models were demonstrated on the plant species called Arabidopsis thaliana.

First model

This is known to deactivate auxin in the part of the plant which is exposed to light thereby causing the shaded part to grow evenly to bend towards the light.

Second Model

This explains the inhibition of auxin synthesis in the part of the plant exposed to light. This will ultimately decrease the concentration of the auxin in the lighted part.

Third Model

This is the movement of auxin from the area of higher concentration to lower concentration. This will increase auxin in the shaded part hence inducing rapid growth in the region.

Fourth Model

The auxin moves to the shaded sides only.

Fifth Model

It is a combination of Model 3 and Model 4. It states that the auxin moves in a vertical direction gravitating down basally. A small amount of auxin also moves horizontally. Light inhibits auxin to be distributed horizontally to the auxiliary plant’s parts.

Examples of Phototropism

• The highly phototropic plant is the sunflower. Its actions are sound and compatible with its name. These flowers grow towards the sun and face it all day long. It moves following the sun when the sun rises from the east to its setting in the west. They face the sun for growth and to produce better flowers out of them.
• Few species of fungi are also known to show phototropism. Fungi are microbes. The Pilobolus crystallins are saprobic and feed on the decayed matter. They reproduce via forming spores. Their phototropic ability allows spores to be well orientated towards the face of light. This will cause them to disperse well.

Phototaxis and Photokinesis

Phototaxis is the directional movement of an organism in response to light. The movement can be either positive or negative. Phototropic organism shows positive phototaxis like phytoflagellates while negative phototaxis is to be shown in insects. Photokinesis is known to show the random movement of organisms in response to light. These days Photokinesis is the main principle to generate photons which in turn are used to produce illusions at certain levels. The elementary particle of light or electromagnetic radiation is called a photon. These can act both as a wave and a particle and travels at the speed of 3 hundred million meters per second.

Phototropic Response in the case of hypocotyl

Along the embryonic axis, the part which is above the cotyledon is called epicotyl while a part below is hypocotyl. The same phenomena exhibit growth as stated earlier in the introductory paragraph. A positive phototropism accounts for the plant growth towards the light and the negative one away from the light. Auxin synthesized is transported to the apical region predominantly.

Phytochrome and cytochrome promoting Phototropism

It is known that blue light induces phototropism. Neither red nor yellow has such abilities. The seedlings, when treated with red light, show sensitivity towards blue light. This was primarily explained in a series of studies by Curry, Asomaning, and Galston and a few other scientists to give a thought to it.

It is known that far-red light implicates a better understanding in comparison to existing red lights. The phototropic response is mainly seen to be directed to give a unilateral response. Phytochromes and cryptochromes mediate phototropism. Cytochrome even plays a vital role in inducing photosynthesis. The work is done by the Electron Transport Chain. They act by accepting and donating a series of electrons inducing a chain reaction to occur. These are divided into three classes A, B, and C depending on the absorption spectra.

Read more about Tropism

Summary

• The history of phototropism dates to some centuries ago. It is somewhat derived from Greek physiology and physiological studies.
• Arabidopsis are major plant species that are used to study phototropism in this regard.
• Phototropism is the response of the interaction of photoreceptors to external stimuli which is the light.
• This process is accompanied by the action of multiple enzymes and hormones undergoing a number of the signaling pathway.
• Consequently, a true combination of phototropism and gravitropism causes the plant to grow well on the right path.

Frequently Asked Questions

What is phototropism?

Phototropism is a mechanism because of which different parts of plants show differential growth in response to the stimulus of light. It is the differential growth of different parts of a plant in response to light.

How does phototropism work?

Phototropism happens due to plant hormones called auxins. This hormone is present in the tips of the plant’s leaves and stem. Auxins are sensitive to light and thus, cause the growth in the direction of light.

Which light is best for phototropism?

Blue light is best for phototropism. Experiments have shown that plants show the best response to blue light.

How phototropism is beneficial for plants?

Phototropism allows the maximum growth of stem and leaves in the direction of light so that these parts can get maximum light from the environment. As light is needed for photosynthesis, thus phototropism causes plants to get maximum light and prepare their food.

References

1. Goyal, A., Szarzynska, B., Fankhauser C. (2012). Phototropism: at the crossroads of light-signaling pathways. Cell 1-9.
2. Sakai, T.; Kagawa, T.; Kasahara, M.; Swartz, T.E.; Christie, J.M.; Briggs, W.R.; Wada, M.; Okada, K. (2001). “Arabidopsis nph1 and npl1: Blue light receptors that mediate both phototropism and chloroplast relocation”. PNAS. 98 (12): 6969–6974. doi:10.1073/pnas.101137598. PMC 34462. PMID 11371609.
3. Liscum, E. (2002). Phototropism: Mechanisms and Outcomes. Arabidopsis Book 1-21.
4. Christie, J.M.; Murphy, A.S. (2013). “Shoot phototropism in higher plants: New light through old concepts”. American Journal of Botany. 100 (1): 35–46. doi:10.3732/ajb.1200340. PMID 23048016.
5. Hager, Achim (2003-12-01). “Role of the plasma membrane H+-ATPase in auxin-induced elongation growth: historical and new aspects”. Journal of Plant Research. 116 (6): 483–505. doi:10.1007/s10265-003-0110-x. ISSN 1618-0860. PMID 12937999.