Plants show a tremendous plasticity in their response to light. Negative responses essentially serve the purpose of preventing unnecessary investment of resources, but can also be conceptualised as positive growth responses to the absence of light, or to light conditions sub-optimal for photosynthesis. The default state in a germinated seedling in the absence of any light stimulus is elongation growth. The need for photosynthesis in order to obtain energy requires that all available energy is invested in reaching the light. Once in the light such investment is no longer advantageous and resources can be allocated more efficiently in promotion of photosynthetic development.
The inhibition of hypocotyl elongation was the basis of the first screen for mutants deficient in aspects of photoperception. Koornneef and collaborators (1980) grew mutagenised seed of Arabidopsis in white light and identified a number of mutants showing a long hypocotyl. In doing so they identified mutants in the two major photoreceptors responsible for this light-induced inhibition of elongation growth: the red/far-red light photoreceptor, phyB, and the blue light photoreceptor, cry1.
In fact, all of the phytochromes and cryptochromes act in the inhibition of elongation growth. The combined action of this battery of photoreceptors gives plants the ability to respond appropriately to a wide range of conditions. Although the majority of these photoreceptors have little effect on a wildtype seedling in white light, their contribution can be observed under specific conditions. In etiolated seedlings high levels of phyA accumulate making the seedlings very sensitive to small amounts of light. Any wavelength of light is capable of producing the small amount of phyA active phytochrome (Pfr) required to initiate a very low fluence response (VLFR). The VLFR triggers an early inhibition of hypocotyl elongation, but this response accounts for only a minor effect and is quickly saturated. In red or white light phyA is then rapidly degraded as the majority of the phyA pool will be converted to the labile Pfr form. PhyA appears to play an "antenna" role in de-etiolation under these conditions. The light-stable phytochromes play a major role in inhibit ing hypocotyl elongation in red or white light, with phyB the major player and lesser roles for phyC, D and E being revealed in the absence of phyB. By contrast, in far-red light phyA plays a major role. The maintenance of a large pool of phyA in the stable Pr form prevents degradation and allows a phenomenon known as the FR high irradiance response (HIR), a response to prolonged irradiation that causes a dramatic inhibition of elongation growth. PhyA is the only phytochrome capable of inhibiting elongation growth in far-red light.
The needs of the established plant remain the same as those seen during de-etiolation, the priority being to gain maximum advantage from the light environment. The shade avoidance response is a response to competition from neighbouring plants. Plants are able to detect light that has been reflected from a neighbouring plant by perceiving the change in light quality. Plants absorb strongly in the red and blue wavelengths due to absorption by chlorophyll, but they reflect far-red wavelengths. Plants are able to monitor the red/far-red (R: FR) ratio of incident light and can interpret a reduction in the ratio as evidence of a neighbour growing closely alongside that may overtop it in future. A low R : FR ratio triggers a pronounced elongation growth response, reduction in branching and reduction in leaf area. Prolonged shade eventually triggers a flowering response as the plant ensures production of offspring as a last resort. This system of using far-red as a reference wavelength against which to compare a reduction in red wavelengths is vastly superior to a simple response triggered by a reduction in photosynthetically active radiation (PAR). An inanimate object alongside a plant may slightly reduce the intensity of incident PAR to the same extent as a neighbouring plant, but would not pose a future threat of severe shading.
The reversible, photochromic nature of the phytochromes makes them ideal to detect this change in R : FR ratio. As in the control of de-etiolation via the LFR, the Pr conformation is inactive and the Pfr conformation is active, suppressing elongation growth and flowering. A low R: FR ratio results in a loss of Pfr triggering the shade-avoiding phenotype. PhyB, the major phytochrome responsible for the LFR in de-etiolation, is the main player in shade avoidance affecting all aspects of the response. phyB mutants in a range of species display a constitutively shade-avoiding phenotype, due to the constitutive absence of phyB Pfr and a greatly reduced response to shade. The majority of work characterising the phytochromes involved in the shade-avoidance response has been carried out in the model plant, Ara-bidopsis. Here phyD and phyE also play more minor roles which are most apparent when looked for in the absence of phyB. PhyD is involved in the control of petiole elongation and flowering, while phyE is involved in the regulation of internode elongation and flowering. PhyA also plays a key role, but here it acts as a moderator of shade avoidance. Remarkably, in a multiple phytochrome phyA phyB phyE mutant of the normally rosette-forming plant Arabidopsis, internodes are constitutively formed, implying that even the rosette growth habit, resulting from constitutive inhibition of internode elongation, can be reversed by environmental sensory pathways.
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