Promotion of photomorphogenesis by COP1

CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1) represses photomorphogenesis in darkness by targeting nuclear-localized transcription factors to proteasome-mediated degradation. Upon light exposureCOP1 migrates to the cytosol allowing photomorphogenesis to proceed but the residual nuclear pool down-regulates light signaling mediated by phytochrome A. Here we show that weak alleles of cop1 exhibit reverse photomorphogenic responsesi.e. reduced rather than enhanced cotyledon unfoldingunder red light compared to darkness. ConverselyCOP1 overexpressorswhich de-etiolate poorly under blue or far-red lightshowed enhanced photomorphogenesis under red light. The positive relationship between COP1 and photomorphogenic response required phytochrome B. Thusgenetic manipulation of COP1 levels differentially affects phytochrome A- compared to phytochrome B-mediated responses. We hypothesize that COP1 could be involved in degradation of negative regulators of photomorphogenesis or in transcriptional activationas observed for some E3 ligases in mammalian development.