PHYTOCHROME INTERACTING FACTOR1 Enhances the E3 Ligase Activity of CONSTITUTIVE PHOTOMORPHOGENIC1 to Synergistically Repress Photomorphogenesis in Arabidopsis    

Abstract

CONSTITUTIVE PHOTOMORPHOGENIC1 (COP1) is a RING/WD40 repeat-containing ubiquitin E3 ligase that is conserved from plants to humans. COP1 forms complexes with SUPPRESSOR OF PHYTOCHROME A (SPA) proteins, and these complexes degrade positively acting transcription factors in the dark to repress photomorphogenesis. Phytochrome-interacting basic helixloop-helix transcription factors (PIFs) also repress photomorphogenesis in the dark. In response to light, the phytochrome family of sensory photoreceptors simultaneously inactivates COP1-SPA complexes and induces the rapid degradation of PIFs to promote photomorphogenesis. However, the functional relationship between PIFs and COP1-SPA complexes is still unknown. Here, we present genetic evidence that the pif and cop1/spa Arabidopsis thaliana mutants synergistically promote photomorphogenesis in the dark. LONG HYPOCOTYL5 (HY5) is stabilized in the cop1 pif1, spa123 pif1, and pif double, triple, and quadruple mutants in the dark. Moreover, the hy5 mutant suppresses the constitutive photomorphogenic phenotypes of the pifq mutant in the dark. PIF1 forms complexes with COP1, HY5, and SPA1 and enhances the substrate recruitment and autoubiquitylation and transubiquitylation activities of COP1. These data uncover a novel function of PIFs as the potential cofactors of COP1 and provide a genetic and biochemical model of how PIFs and COP1-SPA complexes synergistically repress photomorphogenesis in the dark.

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

“…3a-c). PIF1 also displayed interaction with COP1 in the dark as previously shown 7 . However, these assays showed that CUL4, COP1 and SPA1 interacted preferentially with a slow migrating form of PIF1 in response to light ( Fig.…”

“…Because PIF1 formed a complex with COP1 and SPA1, components of the E3 ubiquitin ligases in the dark 7 , we examined whether the CUL4 COP1-SPA E3 ubiquitin ligases also play a role in the light-induced degradation of PIF1 in Arabidopsis. We performed immunoblots for PIF1 levels in the individual components of the CUL4 COP1-SPA complex mutants both in the dark and light.…”

“…Moreover, the APB domain of PIF1 interacts with the WD40 domain of COP1 (ref. 7). We performed yeast two-hybrid assays using the full-length and various deletion fragments of PIF1 and SPA1.…”

“…4a,b; ref. 7). Because of the transient and early phyA-mediated degradation of PIF1 as well as exclusive role of PIF1, we focused on seed germination, a more light-sensitive response, under far-red and red light 26,27 .…”

“…These complexes, either by themselves or by forming CUL4 COP1-SPA E3 ubiquitin ligases, degrade the positively acting transcription factors through the ubiquitin proteasome system (UPS) to repress photomorphogenesis in the dark 6 . Strikingly, PIF1 directly interacts with COP1 and SPA complex and acts as a cofactor of COP1 E3 ubiquitin ligase to synergistically degrade the positively acting factor HY5 to repress photomorphogenesis in the dark 7 . PIFs also interact with DET1 and synergistically repress photomorphogenesis in the dark 8 .…”