The COP1 Target SHI-RELATED SEQUENCE5 Directly Activates Photomorphogenesis-Promoting Genes

Plant seedlings undergo distinct developmental processes in the dark and in the light. Several genes, including ELONGATED HYPOCOTYL5 (HY5), B-BOX PROTEIN21 (BBX21), and BBX22, have been identified as photomorphogenesis-promoting factors in Arabidopsis thaliana; however, the overexpression of these genes does not induce photomorphogenesis in the dark. Using an activation-tagging approach, we identified SRS5ox, which overexpresses SHI-RELATED SEQUENCE5 (SRS5) following induction with estradiol. SRS5 overexpression in SRS5ox and Pro35S:SRS5-GFP seedlings results in a constitutive photomorphogenesis phenotype in the dark, whereas SRS5 loss of function in the srs5-2 mutant results in long hypocotyls in the light. This indicates that SRS5 is a positive regulator of photomorphogenesis. Furthermore, SRS5 promotes photomorphogenesis by directly binding to the promoters of photomorphogenesis-promoting genes, such as HY5, BBX21, and BBX22, and activating their expression, thus affecting the expression of downstream light-signaling genes. These data indicate that SRS5 acts in the upregulation of photomorphogenesis-promoting genes. In addition, CONSTITUTIVELY PHOTOMORPHOGENIC1 (COP1), which plays a central repressive role in seedling photomorphogenesis, directly ubiquitinates SRS5, promoting its degradation in the dark. Taken together, our results demonstrate that SRS5 directly activates the expression of downstream genes HY5, BBX21, and BBX22 and is a target of COP1-mediated degradation in Arabidopsis.