LAUSR

Significance For the initial growth of seedlings, the transcription factor ELONGATED HYPOCOTYL5 (HY5) plays a critical role by initiating a series of light responses. Here, we demonstrated that besides transcription activity of HY5, the HY5 protein level, which is stringently regulated by deubiquitinating enzymes, is also vital for photomorphogenesis. We identified a ubiquitin-specific protease, Ub-SPECIFIC PROTEASE 14 (UBP14), which positively promotes plant photomorphogenesis in response to light. UBP14 directly interacts with HY5 and protects it from degradation by 26S proteasomes. Besides, HY5 promotes the expression and protein accumulation of UBP14 for positive feedback. The dynamic regulatory mechanism for UBP14 and HY5 provides an insight into the roles of posttranslational modifications of HY5 during photomorphogenesis in Arabidopsis thaliana.