The E3 ubiquitin ligases RING DOMAIN OF UNKNOWN FUNCTION 1117 1 (RDUF1) and RDUF2 control seedling photomorphogenesis in Arabidopsis.

The CONSTITUTIVE PHOTOMORPHOGENIC1 (COP1)-ELONGATED HYPOCOTYL5 (HY5) circuit controls plant seedling photomorphogenesis. Regulation of HY5 protein levels is key for optimal photomorphogenic growth. This study identified the E3 ubiquitin ligases Really Interesting New Gene (RING) DOMAIN OF UNKNOWN FUNCTION 1117 1 (RDUF1) and RDUF2 as novel components of the COP1-HY5 pathway in Arabidopsis. The RDUF1 and RDUF2 knockout mutants exhibited longer hypocotyls with reduced photopigment accumulation than the wild-type. In comparison, the overexpression transgenic lines showed shorter hypocotyls with enhanced photomorphogenic responses in a wavelength-independent manner. HY5 directly binds to the RDUF1 and RDUF2 promoters through the G-box, activating their expression in response to light. Epistatic analysis and biochemical data showed that RDUF1 and RDUF2 genetically interact with and stabilize the HY5 protein, plausibly engaging the N77 part of HY5 and preventing COP1-mediated ubiquitination and degradation. In the dark, COP1 physically interacted with and ubiquitinated RDUF1 and RDUF2, subjecting them to degradation to keep HY5 levels low and promote skotomorphogenesis. However, light-mediated inhibition of COP1 activity mediated by photoreceptors increased RDUF1 and RDUF2 accumulation, enhancing HY5 protein stability and photomorphogenesis. This study establishes COP1-RDUF1/RDUF2-HY5 as a regulatory module of seedling photomorphogenesis under dynamic light cues.