Plant Light Signaling Mediated by Phytochrome Photoreceptors

Light is essential for plant growth and development, which serves as an energy source for photosynthesis and as environmental cues for photomorphogenesis (i.e., light-mediated development). Plants continually adapt to light environments to optimize their growth and development by monitoring with help of various photoreceptors including phytochromes. As red (R) and far-red (FR) light-absorbing photoreceptors, they function as a molecular switch with R-absorbing Pr and FR-absorbing Pfr forms. In the dark, phytochromes are biosynthesized as the inactive Pr form, which can be phototransformed into the physiologically active Pfr form upon exposure to light. The Pr-to-Pfr photoactivation of phytochromes induces a highly regulated signaling network for photomorphogenesis in plants, which includes translocation of phytochromes into the nucleus, interaction of phytochromes with a wide array of downstream signal transducers, regulated proteolysis of signaling targets, and transcriptional regulation of various photoresponsive genes. Moreover, several key components in the phytochrome-mediated light signaling have been identified from extensive research over several decades, which includes phytochrome-interacting factor (PIF), constitutively photomorphogenic 1 (COP1), suppressor of phyA-105 (SPA), and elongated hypocotyl 5 (HY5). In this review, we provide a highlighted view for the molecular mechanisms of phytochromes in plant light signaling based on recent advances.