In this article we will discuss about Phototropism. After reading this article you will learn about: 1. Fluence-Response Relations of Phototropism 2. Action Spectra and Photoreceptors 3. Auxin and Phototropism.
Fluence-Response Relations of Phototropism:
Darwin in his book, ‘The Power of Movement in Plants’ described that the extreme tip of a grass seedling controls the ability of the whole seedling to curve towards light. Covering the tip by an opaque cap made it insensitive towards the stimulus.
He further described that the oat coleoptile bent somewhat towards unilateral light even when the tip was covered. This means that some phototropic sensitivity occurs below the tips, which is called the base response. But the tip response is thousand times greater than the base response.
This differential response is dependent on the intensity of light. In dim light, the response is localized in the tip, whereas in higher light intensities, the sensitivity is distributed throughout the entire coleoptile.
So, the obvious question may be raised in terms of the Bunsen-Roscoe reciprocity law which determines whether the response is proportional to the duration of response, to the energy level of exposure, or to the product of both. Blaauw (1909), found that in Avena coleoptiles, energy and duration bore a reciprocal relationship with each other.
This simplified view, however, no longer prevails. It is found that with increase in light intensity there is an increase in the degree of curvature until a maximum is reached, and above which with further increase in stimulus quantity, the degree of curvature decreases and ultimately, a negative curvature occurs away from the light source. With further increase in intensity the curvature again becomes positive.
These fluence (dose)-response curves were obtained by Zimmermann and Briggs (1963), by plotting the degrees of curvature of Avena coleoptile as a function of the product of exposure duration multiplied by the energy level.
The curve showed the initial rising peak termed the first-positive curvature, the subsequent reverse peak is termed the first negative curvature, and the second increase is called the second positive curvature.
It is also possible to have another minimum and another peak, called the second-negative and third-positive curvatures, respectively. The first-positive region obeys the reciprocity law, but this is not true for other regions. These findings of Zimmermann and Briggs were confirmed experimentally by Blaauw and Blaauw-Jensen (1970a and 1970b) (Fig. 15.1).
It has been suggested that light has two-fold effects in phototropism—the triggering effect for the bending response and the non-directional tonic effect which changes the state of the organ so that its sensitivity to a subsequent energy increment is decreased. This is probably due to the effect of blue light that alters the sensitivity of the system. This tonic effect is time-limited instead of energy-limited.
If a coleoptile is kept in dark and then exposed to unilateral blue light for one second, positive curvature occurs and the degree of curvature decreases if the coleoptile is pretreated for 10 seconds with the same irradiance. It is easily detectable in vertical exposure instead of unilateral.
The second and third positive curvatures are not independent but appear to be due to desensitization of the first-positive response system.
Zimmermann and Briggs also showed the shifting (10-fold) of the regions of first-positive and first-negative curvatures towards higher level by a pre-treatment with red light prior to blue-light exposure.
There was a three-fold shift of the second-positive region towards the lower energy level by the same red-light pre-treatment. This red-light effect is reversed by far-red light suggesting the involvement of phytochrome in determining the sensitivity of coleoptile to the blue light that causes bending.
Action Spectra and Photoreceptors of Phototropism:
The action spectrum of phototropism indicates that blue light is most effective in producing phototropic bending. The absorption spectra of photoreceptor pigments suggest that the flavins and the carotenoids might be involved in phototropism. The strong peaks in UV region favour riboflavin as the photoreceptor, while two peaks in the blue-violet region favour β-carotene as the photoreceptor.
It has been observed that certain higher plant mutants with very low carotene content show phototropism, and that blockage of the formation of carotenoid pigments by certain herbicides cannot remove the phototropic bending activity.
In Avena and Phycomyces (a photo tropically active fungus) the action spectra for phototropism are identical. In both the systems, a flavoprotein appears to be involved in the process.
In the plasma membrane, the flavoprotein becomes oxidized by a b-type cytochrome after light absorption. Thus in both fungi and higher plants, the probable photoreceptor appears to be a flavin but not a carotene. But recently it has been found that the blue-light photoreceptor is a 75kDa protein called crypto chrome that mediates the inhibition of stem elongation.
It is a pigment protein complex containing FAD and a pterin, which is a light-absorbing pteridine derivative often functioning as pigment in insects, fishes and birds. Isolation of HY4 gene from Arabidopsis showed that it encodes the protein. But it has been shown that crypto chrome does not play a primary role in inhibiting stem elongation.
Some recently isolated Arabidopsis mutants are impaired in blue light-dependent phototropism of the hypocotyl. This provides valuable information about cellular events leading to phototropic bending. One of these mutants is nhp 1 that lacks a phototropic response in the hypocotyl but shows normal blue light-stimulated inhibition of hypocotyl elongation. The nph 1 gene encodes a protein called phototropin.
This gene is recently renamed as phot 1. The C-terminal half of phototropin is a serine/threonine kinase and its N-terminal half contains two repeated domains of about 100 amino acids each. This N-terminal half binds flavin mononucleotide (FMN) and shows a blue light- dependent auto phosphorylation reaction.
The Arabidopsis genome contains another gene, phot 2. The phot 2 mutants show normal phototropic response. But the phot 1 /phot 2 double mutant is severely impaired at both low and high intensities. The phot 1 mutant lacks hypocotyl phototropism in response to low intensity blue light whereas phot 2 mutant lacks phototropism at high intensity of blue light.
Auxin and Phototropism:
The earlier experiments regarding phototropism led to the conclusion that there is an asymmetry in the concentration of auxin from the tip to the base of the coleoptile. Auxin concentration becomes high on the shaded side. Several theories have been proposed about the role of auxin in phototropism.
These are:
(a) Cholodny-Went Theory:
This theorizes a lateral redistribution of auxin across the coleoptile tip leading to a greater concentration in the shaded half. This greater concentration of auxin promotes more growth on the darker side, producing a positive phototropic curvature. They could not, however, explain how lateral translocation of auxin took place.
(b) Photo-Destruction of Auxin:
There is a concept that an auxin concentration gradient across the tissue towards the darker side is established owing to the photo-destruction of auxin molecules in the illuminated side. Thus, inequality in auxin concentration leads to asymmetric growth, producing positive curvature.
(c) Differential Auxin Synthesis:
It is also suggested that the rate of auxin production is less on the illuminated side as compared to the darker side and consequently growth is also less on the illuminated side.
(d) Light-Growth Reaction:
This is the fourth concept which is devoid of auxin involvement. It is simply a light-growth reaction in which extension growth is inhibited by light at the exposed half. But this is achieved by red light which is not primarily important in inducing phototropism. Now there is a general agreement that blue light in some way causes differential auxin concentration, but in what way it brings about this change is questionable.
The Cholodny-Went theory has been supported by many experimental data. Went (1926) quantified the auxin amount collected in agar blocks placed below the illuminated and shaded side of cut coleoptile tips. He found that auxin quantities recovered in the basal agar blocks was in the ratio of 27: 57 for the exposed and shaded halves respectively, and there was loss of 16 per cent of total auxin quantity (Fig. 15.2).
This 16 per cent loss was regarded as insignificant by Went, but a number of other workers favoured the idea of photo-destruction of auxin in the illuminated side. Galston and his associates observed that riboflavin absorbed blue light and that absorbed energy was utilized in the photo-oxidation of IAA.
Briggs (1963), however, performed a series of experiments and established that no significant photo-destruction of the total diffusible auxin occurs. He did not find any auxin difference when the unilaterally illuminated Avena tips were completely split and separated by a thin glass barrier into two vertical halves.
But when the coleoptile tips were longitudinally separated by a thin glass barrier excepting the extreme apex, it was found that the agar block beneath the shaded half of the tip received more auxin, thus indicating the light-induced lateral redistribution of auxin from the illuminated side to the shaded side.
Pickard and Thimann (1964), demonstrated that the applied l4C-IAA to the extreme tip of the Zea coleoptile became asymmetrically distributed, establishing a concentration gradient in the tissue when illuminated laterally.
So, it is evident that when a coleoptile tip is unilaterally illuminated, a lateral migration of auxin molecules within the tip towards the shaded side takes place after the perception of the stimulus. But the actual mechanism of light-induced lateral redistribution of auxin molecule is not yet known.
Several workers, however, observed variable results. Kang and Burg (1974), showed by 3H-IAA application in Pisum sativum seedlings, the degree of curvature did not correlate quantitatively with IAA gradients if the seedlings were pre-treated with red light.
The phototropic mechanism may be different in leafy dicotyledons from that in coleoptiles. According to Bruinsma etal. (1975), IAA is probably not responsible for differential growth in Helianthus seedlings.
Phillips (1972) has found an asymmetric distribution of gibberellins in unilaterally illuminated excised hypocotyls of Helianthus annuus, indicating the probable involvement of gibberellins in phototropism at least in this species.
It has been found that in unilaterally illuminated Avena coleoptiles, a transverse bi-electric potential is developed. Johnsson (1965), is of the opinion that this potential may be a result of the auxin redistribution rather than its cause.