Direct phosphorylation of HY5 by SPA1 kinase to regulate photomorphogenesis in Arabidopsis

ELONGATED HYPOCOTYL5 (HY5) is a key transcription factor which promotes photomorphogenesis by regulating complex downstream growth programs. Previous studies suggest that the regulation of HY5 mainly depends on the CONSTITUTIVE PHOTOMORPHOGENIC1 (COP1) - SUPPRESSOR OF PHYTOCHROME A-105 (SPA) E3 ubiquitin ligase complex, which degrades positively acting transcription factors of light signaling to repress photomorphogenesis in the dark. SPA proteins function not only as a component of the E3 ubiquitin ligase complex but also as a kinase of PHYTOCHROME INTERACTING FACTOR1 (PIF1) through its N-terminal kinase domain. Here, we show that HY5 is a new substrate of SPA1 kinase. SPA1 can directly phosphorylate HY5 in vitro and in vivo. We also demonstrate that unphosphorylated HY5 strongly interacts with both COP1 and SPA1 than phosphorylated HY5, is the preferred substrate for degradation, whereas phosphorylated HY5 is more stable in the dark. In addition, unphosphorylated HY5 actively binds to the target promoters, and is physiologically more active form. Consistently, the transgenic plants expressing unphosphorylated mutant of HY5 displays enhanced photomorphogenesis. Collectively, our study revealed that SPA1 fine-tunes the stability and the activity of HY5 to regulate photomorphogenesis.