Noncanonical role of Arabidopsis COP1/SPA complex in repressing BIN2-mediated PIF3 phosphorylation and degradation in darkness

Significance Initially discovered as a central repressor of the seedling photomorphogenesis in plants, and later characterized in animals, the E3 ligase CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1) has been shown to have conserved biochemical activities in eukaryotes. In this study, we show that to coordinately promote skotomorphogenesis, the COP1/ SUPPRESSOR of phyA-105 (SPA) complex represses the activity of the GSK3-like kinase BRASSINOSTEROID-INSENSITIVE 2 (BIN2) in a nonproteolytic manner to stabilize the key repressor of photomorphogenesis PHYTOCHROME INTERACTING FACTOR 3 (PIF3). The COP1/SPA-BIN2-PIF3 module represents an additional layer of COP1/SPA regulation in Arabidopsis and is essential for shaping the plants when grown in the dark.