HY5-COP1: the central module of light signaling pathway

Light acts catalytically to initiate a cascade of events to eventually regulate different aspects of plant development. The cascade of light signal transduction involves several components that can be broadly grouped as photoreceptors, early signaling factors, central integrators and downstream effectors. ELONGATED HYPOCOTYL 5 (HY5), a bZIP transcription factor, is the most well characterized downstream effector in Arabidopsis, which acts as a positive regulator of photomorphogenesis. HY5 acts as a master regulator and binds to promoters of close to 4000 genes, thereby, regulating wide diversity of photomorphogenic responses. It physically interacts with CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1), a negative regulator of photomorphogenesis. HY5-COP1 interaction mediates cross-talk among multiple pathways, thereby, enhancing the phenotypic plasticity of plants. This interaction regulates not only the upstream components of light signaling pathway like photoreceptors but also regulates a number of downstream effectors and early signaling factors, which help to regulate a number of developmental processes in plants. This review highlights the key interaction between HY5 and COP1 and how it regulates seedling photomorphogenesis, shade avoidance, circadian clock, root architecture, flowering, thermomorphogenesis, hormone signaling and stress responses, in conjunction with other signaling components and transcription factors.