CRYPTOCHROMES (CRYs) are photolyase-like ultraviolet-A/blue light photoreceptors that mediate various light responses in plants.The signaling mechanism of Arabidopsis CRYs (CRY1 and CRY2) involves direct CRY-COP1 interaction.Here,we report that CRY1G380R,which carries a Gly-to-Arg substitution of the highly conserved G380 in the photolyase-related (PHR) domain of Arabidopsis CRY1,shows constitutive CRY1 photoreceptor activity in Arabidopsis.Transgenic plants overexpressing CRY1G380R display a constitutively photomorphogenic (COP) phenotype in darkness,as well as a dramatic early flowering phenotype under short-day light conditions (SD).We further demonstrate that CRY1G380R expression driven by the native CRY1 promoter also results in a COP phenotype in darkness.Moreover,overexpression of either the Arabidopsis homolog CRY2G377R or the rice ortholog OsCRY1bG388R of CRY1G380R in Arabidopsis results in a COP phenotype in darkness.Cellular localization studies indicate that CRY1G380R co-localizes with COP1 in the same nuclear bodies (NBs) in vivo and inhibits the nuclear accumulation of COP1 in darkness.These results suggest that the conserved G380 may play a critical role in regulating the photoreceptor activity of plant CRYs and that CRY1G380R might constitutively phenocopy the photo-activated CRY1 in darkness and thus constitutively mediate CRY1 signaling.