Molecular Mechanism Of The Arabidopsis Red Photoreceptor Phytochrome PhyB Promoting Photomorphogenesis By Regulating The Site Occupancy Of The Histone Variant H2A.Z

Light not only provides the energy for plant photosynthesis,but also acts as a crucial environmental cue for plant development and growth.Plants have evolved multiple photoreceptors to perceive light signal for monitoring and responding to dynamic changes in light.Among them,phytochrome B(phy B)is the primary red light receptor.Upon the red light irradiation,phy B translocates from the cytoplasm to the nucleus.On one hand,nucleus-localized phy B interacts with the negative regulators of photomorphogenesis COP1 and SPA1 to inhibit their activities,leading to promoted accumulation of b ZIP transcription factor HY5,the positive regulator of photomorphogenesis.On the other hand,red light-activated phy B interacts with negative regulators of photomorphogenesis b HLH transcription factors PIFs(PHYTOCHROME INTERACTING FACTORs),resulting in rapid phosphorylation and degradation of PIFs.H2 A.Z is a highly conserved histone variant in eukaryotic cells,which plays a critical role in developmental and physiological processes,such as gene expression regulation,DNA replication,DNA damage repair,and DNA recombination.The replacement of H2 A with H2 A.Z in the nucleosomes is catalyzed by chromatin remodeling ATP-dependent SWR1 complex.Here,we show that the Pfr form of phy B physically interacts with SWR1 complex subunits SWC6 and ARP6.Phy B and ARP6 co-regulate a large number of genes in the same direction,some of which are associated with auxin biosynthesis and response including YUC9,which encodes a rate-limiting enzyme in the tryptophan-dependent auxin biosynthesis pathway.Moreover,phy B and HY5/HYH act to inhibit hypocotyl elongation partially through repression of auxin biosynthesis.Based our findings and previous studies,we propose that phy B promotes H2 A.Z deposition at YUC9 to inhibit its expression through direct phy B-SWC6/ARP6 interactions,leading to repression of auxin biosynthesis,and thus inhibition of hypocotyl elongation in red light.