During the transition from darkness to light, the rate of hypocotyl elongation is determined from the integration of light signals sensed through the phototropin, cryptochrome, and phytochrome signalling pathways. In all light conditions studied, from UV to far-red, early hypocotyl growth is rapidly and robustly suppressed within minutes of illumination in a manner dependent upon light quality and quantity.
The response has a threshold between 10−1 and 100 μmol m−2, is saturated before 102 μmol m−2 and obeys reciprocity. Genetic analyses indicate that the cryptochrome or phototropin photoreceptors do not participate in the response. The major phytochrome receptors influence the normal amplitude and timing of the GL response, yet the GL response is normal in seedlings grown for hours under constant dim-red light. Therefore, phytochrome activation enhances, but is not required for, the GL response. Seedlings grown under green, red, and blue light together are longer than those grown under red and blue alone. This data indicates that a novel GL-activated light sensor promotes early stem elongation that antagonizes growth inhibition.
The first sensing of light transitions the etiolated seedling into a developmental program that prepares the plant for autotrophy. This process, photomorphogenesis, is typified by changes at the biochemical, molecular, and physiological levels that guide early plant morphology during establishment. One of the most conspicuous changes to occur during photomorphogenic development is an inhibition of hypocotyl (stem) growth rate. Ultraviolet, blue, red, and far-red light each rapidly inhibit stem growth within minutes of irradiation, making this rapid response an excellent reporter of light sensing and signal integration.
Monochromatic green light (GL) has been shown to act as a signal in regulating specific facets of plant physiology, inhibiting seedling mass, plant cell culture growth, and light-induced gravitropic root elongating. Recently it has been shown that GL can reverse blue light-induced stomatal opening. The GL response is mediated through a yet-to-be-defined photosensor, and genetic analyses suggest the response to be zeaxanthin based. Plant responses to GL may be initiated through known light sensors. Phytochromes and cryptochromes absorb GL and possibly influence light-induced events. However, the action/response spectra for GL-induced responses exhibit a peak between 540 to 550 nm and thus are incongruous with the absorption spectra for phytochromes, cryptochromes, and phototropins and the action spectra for the responses they govern. GL signals may also be a consequence of low-level coactivation of multiple sensory systems that together guide atypical physiological outcomes.
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