The Molecular Mechanism Of Photomorphogenic Development Promoted By Red Light-dependent Interaction Of PhyB With SPA1 In Arabidopsis

Arabidopsis phytochromes(phyA-phyE) are photoreceptors dedicated to sensing red/far-red light. The photosensory signaling by phytochromes involves induction of rapid degradation of PIFs(PHYTOCHROME INTERACTING FACTORS), and suppression of nuclear abundance of COP1(CONSTITUTIVE PHOTOMORPHOGENIC 1) upon light irradiation through physical interactions with PIFs and COP1, resulting in promotion of photomorphogenic development. Among the phytochromes, phyB and phyA are the best characterized, which are predominantly responsible for promoting photomorphogenesis in red and far-red light, respectively. It has been demonstrated that SPA1(SUPPRESSOR OF PHYTOCHROME A 1) associates with COP1 to promote COP1 activity and suppress photomorphogenesis. Here, we report that the mechanism underlying phyB-promoted photomorphogenesis in red light involves direct and functional interactions of red-light-activated phyB with SPA1. SPA1 acts genetically downstream of PHYB to repress photomorphogenesis in red light. Protein interaction studies in both yeast and Arabidopsis demonstrate that photoactivated phyB represses the association of SPA1 with COP1, which is mediated, at least in part, through red-light-dependent interaction of phyB with SPA1. Moreover, we show that phyA physically interacts with SPA1 in a Pfr-form-dependent manner, and that SPA1 acts downstream of PHYA to regulate photomorphogenesis in far-red light. This study provides a genetic and biochemical model of how photoactivated phyB represses the activity of COP1–SPA1 complex to promote photomorphogenesis in red light through direct interaction with SPA1.