CUL4 forms an E3 ligase with COP1 and SPA to promote light-induced degradation of PIF1.

Plants undergo contrasting developmental programs in dark and light. Photomorphogenesis, a light-adapted programme is repressed in the dark by the synergistic actions of CUL4(COP1-SPA) E3 ubiquitin ligase and a subset of basic helix-loop-helix transcription factors called phytochrome interacting factors (PIFs). To promote photomorphogenesis, light activates the phytochrome family of sensory photoreceptors, which inhibits these repressors by poorly understood mechanisms. Here, we show that the CUL4(COP1-SPA) E3 ubiquitin ligase is necessary for the light-induced degradation of PIF1 in Arabidopsis. The light-induced ubiquitylation and subsequent degradation of PIF1 is reduced in the cop1, spaQ and cul4 backgrounds. COP1, SPA1 and CUL4 preferentially form complexes with the phosphorylated forms of PIF1 in response to light. The cop1 and spaQ seeds display strong hyposensitive response to far-red light-mediated seed germination and light-regulated gene expression. These data show a mechanism by which an E3 ligase attenuates its activity by degrading its cofactor in response to light.