Darkness causes seedling hypocotyls to elongate, an event that is characteristic of etiolation. As soon as a dark-grown seedling is exposed to light, a dramatic reduction of elongation rate occurs. Photoreceptors such as phytochrome (phy) and cryptochrome (cry) are involved in this growth suppression process. Specific photoreceptors act at distinct wavelengths and fluence levels. It is possible to identify mutants affected in components of the photocontrol machinery by measuring hypocotyl elongation rates in darkness. Studies on such mutants demonstrated that phyB acts to inhibit hypocotyl elongation in red light, while phyA acts during growth inhibition in far-red (FR) light. The photoreceptor cry1 inhibits growth under high-fluence blue light conditions, while the related photoreceptor cry2 also inhibits growth in blue light but only at low fluence levels.
In a review article published in the October 2001 issue of Current Opinion in Plant Biology (4(5):436-440), Edgar Spalding, Kevin Folta and Brian Parks at the University of Wisconsin, Madison explain the state of knowledge about mechanisms of stem growth responses to light. The authors describe conclusions drawn from recent growth kinetic studies designed to identify components of light-regulated growth responses. These studies also have determined the timing and action of these components.
Phy A and phyB are involved in photocontrol of stem growth by red and far-red light, but each photoreceptor has a distinct function. PhyA-regulated growth suppression in red light takes place during the first three hours of illumination. The active form of phyA is Pfr, the far-red absorbing form. Pfr moves to the nucleus to initiate a photocontrol response. After three hours, the influence of phyA decreases and there is an increasing effect of phyB. Red light also induces the movement of phyB into the nucleus to regulate transcription directly. In these manners, phyA and phyB act as signaling components during light-regulated growth responses.
However, phyA and phyB are not the only regulatory components yet described. Kinetic analyses have also identified additional factors involved in photocontrol of growth. One such factor is SPA1, a WD repeat protein that may act as a transcription factor. Kinetic analysis experiments on SPA1 have resulted in a revised view of its possible function. Though originally proposed to serve as a negative regulator of phyA signaling, the authors have proposed that SPA1 promotes hypocotyl growth and that it counteracts inhibition of growth initiated by phytochrome. If this is indeed the case, growth responses would occur through the balance of two opposing processes: the retardation of growth by phytochrome activation and the promotion of growth by SPA1. Other examples of this type of “push-pull” regulation exist throughout nature.
The authors point out that it is possible that such opposing mechanisms may regulate a common parameter, such as hormone levels. There are some data to indicate that auxin activity and light signaling may be interrelated. The AUX and IAA proteins are auxin-induced transcription factors that are phosphorylated by phytochrome in vitro, indicating close association between phytochrome and hormone signaling pathways. Further studies are needed to determine whether these or other components are involved an interrelationship between hormone levels and hypocotyl growth control in vivo.
The growth rate of etiolated hypocotyls is also dramatically affected by blue light. This inhibition occurs within a mere 30 seconds in several species, including Arabidopsis. Photoreceptors involved in blue light growth responses are Cry1, Cry2, and phototropin. In cry1 mutants, growth is inhibited normally during the first 30 minutes after their exposure to blue or UV-A radiation. After sixty minutes, cry1 mutants can overcome growth inhibition by blue light and return to growth rates typical of dark conditions. Blue light-induced membrane depolarization is thought to be involved in this process, a postulation supported by the fact that wild-type plants treated with NPPB 5-nitro-2[3-phenyl] benzoic acid (an anion-channel blocker) also show continuation of growth in blue light. Whereas cry1 mutant phenotypes may be observed in high fluence blue light, the phenotypic effects of mutations in cry2 can only be observed in low-fluence blue light. Thus, both Cry photoreceptors respond to blue or UV-A radiation, but Cry2 is labile and only functions in low light conditions. While blue light growth responses occurring after illumination for 30 minutes or more are controlled by Cry photoreceptors, another photoreceptor called phototropin (phot1) controls rapid growth inhibition within seconds of illumination. This photoreceptor also mediates phototropism responses.
Studies of early growth kinetics in blue light photoreceptor mutants indicate that there are three phases of growth regulation in this process. The first phase lasts 30 minutes and consists of Phot1-mediated growth inhibition. In the second phase, Cry1 and Cry 2 are required for anion channel activation and to maintain growth retardation initiated by Phot1. Action of Cry1 dominates growth suppression during the third and final phase, which occurs after the first several hours of blue light illumination.
The authors also describe studies that suggest interactions exist between members of different families of photoreceptors, such as between phytochromes and cryptochromes. For example, although phytochromes are thought function during responses to red or far-red light, phyA or phyB mutants that are grown for days in continuous blue light display a long hypocotyl phenotype, indicating that normal blue light growth inhibition depends on functional phytochromes as well. Kinetic studies of blue-light responses of phytochrome mutants have led to speculation about the mechanism involved in this co-action. The authors suggest that phyA and phyB have opposing roles in high-fluence blue light conditions, with phyA, cry1, and cry2 all participating in a common growth suppression process involving membrane depolarization. In contrast, phyB may serve to promote growth under high fluence blue light conditions, a positive growth effect that is thought to be transient. The overall growth response in high levels of blue light would thus be a function of the balance between these opposing events.
The authors conclude that two opposing forces, inhibition and promotion, determine the rate of stem elongation in light. These physiological responses to light conditions are mediated by endogenous rhythms and hormones. The authors feel that the use of kinetic analysis to study the hypocotyl growth will further elucidate the contributions made by individual receptors and signaling components in the early phases of light signaling.