HY5 Background Information
Arabidopsis development is mediated by several environmental stimuli. Light plays an important role in many developmental processes, including photosynthesis, chloroplast biogenesis, leaf initiation, and floral induction (1). Light-dependent development, called photomorphogenesis, relies heavily on the action of five phytochromes, PhyA, B, C, D, and E, which are involved in photoperiod sensing and the shade avoidance syndrome (2-4). These phytochromes are partially regulated by transcriptional repressors of photomorphogenic development, such as COP1 and COP9 (5,6). COP1 also negatively regulates the bZIP transcription factor, HY5, by targeting it for degradation via the 26S proteosome under dark conditions (7,8). However, upon light perception, COP1 is excluded from the nucleus, which permits HY5 accumulation (8,9). HY5 is able to promote photomorphogenesis by positively regulating the transcription of light-inducible genes involved in cell elongation, cell proliferation and chloroplast development (7-9).
