Phytochrome signaling mechanism.

As sessile organisms, plants are unable to move actively towards favorable or away from unfavorable environmental conditions. Therefore, through their evolution, plants have adapted a high degree of developmental plasticity to optimize their growth and reproduction in response to their ambient environments. Light is one of the major environmental signals that influence plant growth and development. Not only is light the primary energy source for plants, but it also provides them with positional information to modulate their developmental processes such as seed germination, seedling de-etiolation, gravitropism and phototropism, chloroplast movement, shade avoidance, circadian rhythms, and flowering time. Plants can detect almost all facets of light such as direction, duration, quantity, and wavelength by using three major classes of photoreceptors: the red (R)/far-red (FR) light (600-750 nm) absorbing phytochromes (phys), the blue (B)/UV-A (320-500 nm) absorbing cryptochromes (crys) and phototropins (phots), and the UV-B (282-320 nm) sensing UV-B receptors (Kendrick and Kronenberg, 1994; Briggs and Olney, 2001; Briggs et al., 2001). These photoreceptors perceive, interpret, and transduce light signals, via distinct intracellular signaling pathways, to photoresponsive nuclear genes, which modulates plant growth and development. The phenotypic changes associated with the seedling photomorphogenic development are among the most dramatic events mediated by these photoreceptors. Seedlings grown in the dark undergo skotomorphogenesis (etiolation) and are characterized by long hypocotyls, closed cotyledons and apical hooks, and development of the proplastids into etioplasts. Light-grown seedlings undergo photomorphogenesis (de-etiolation) and are characterized by short hypocotyls, open and expanded cotyledons, and development of the proplastids into green mature chloroplasts (thus a process considered “de-etiolation” of the etioplasts, McNellis and Deng, 1995, Figure 1). The past decade has seen dramatic advances in our knowledge of plant photoreceptors and in our understanding of their signal transduction pathways that lead to various physiological responses. Here, we briefly review the most recent progress that has provided new insights into constructing an emerging integrated picture of phytochrome signaling in Arabidopsis, the model plant for molecular genetic studies. However, results derived from other model organisms or plant species which provide unique insights into phytochrome signaling mechanism are also briefly discussed in this review where deemed appropriate. The interested readers are referred to the accompanying reviews on other subjects related to phytochrome signaling, such as photomorphogenesis (reviewed by Joanne Chory), blue light signaling (reviewed by Winslow Briggs), circadian rhythms (reviewed by C Robertson McClung and Steve Kay), phototropism (reviewed by Mannie Liscum), flowering (reviewed by George Coupland, Caroline Dean and Detlef Phytochrome Signaling Mechanism