Targeted destabilization of HY5 during light-regulated development of Arabidopsis

Arabidopsis seedlings display contrasting developmental patterns depending on the ambient light. Seedlings grown in the light develop photomorphogenic ally, characterized by short hypocotyls and expanded green cotyledons. In c ontrast, seedlings grown in darkness become etiolated, with elongated hypoc otyls and closed cotyledons on an apical hook. Light signals, perceived by multiple photoreceptors and transduced to downstream regulators, dictate th e extent of photomorphogenic development in a quantitative manner. Two key downstream components, COP1 and HY5, act antagonistically in regulating see dling development(1). HY5 is a bZIP transcription factor that binds directl y to the promoters of light-inducible genes, promoting their expression and photomorphogenic development(2,3). COP1 is a RING-finger protein with WD-4 0 repeats whose nuclear abundance is negatively regulated by light(4,5). CO P1 interacts directly with HY5 in the nucleus to regulate its activity nega tively(1). Here we show that the abundance of HY5 is directly correlated wi th the extent of photomorphogenic development, and that the COP1-HY5 intera ction may specifically target HY5 for proteasome-mediated degradation in th e nucleus.