Molecular Mechanism Of The Arabidopsis Red Photoreceptor PhyB Interacting With BIN2 To Regulate Brassinolide Signaling And Photomorphogenesis

Light is one of the most important environmental signals for plant growth and development.Plants have evolved photoreceptors with different functions to adapt to various light signal in environments.Among them,phytochrome B(phy B)is red light photoreceptor to sense red light.phy B mediates red light signaling to promote photomorphogenesis.Brassinosteroid(BR)is the sixth class of phytohormone involved in diverse physiological and biochemical activities and plays an irreplaceable role in Arabidopsis.BES1 and BZR1 are crucial transcription factors in the BR signaling pathway,which can regulate the expression of thousands of BR target genes.BIN2(Brassinosteroid-Insensitive 2),a class of highly conserved multifunctional protein serine/threonine kinases,which is a constitutively negative regulator in the BR signaling pathway,and can repress BR signaling by phosphorylating BES1 and BZR1,finally inhibit their DNA-binding ability,and promote de-etiolation.The BIN2 gain-offunction mutant bin2-1 exhibits an enhanced photomorphogenesis phenotype under red light.Previous study demonstrated that phy B can inhibit BES1's DNA binding ability by interacting with dephosphorylated BES1,and finally inhibit the expression of BR response genes.Another study showed that activated CRY1 enhanced the interaction between BIN2 and BZR1 by interacting with BIN2,prompting BIN2 to phosphorylate BZR1,thereby inhibiting BR signaling and skotomorphogenesis.However,whether phy B promotes BIN2-BES1 interaction by interacting with BIN2 protein,thus regulating the phosphorylated form of BES1,inhibiting BR signaling and promoting photomorphogenesis has not been explored.Our study found red light-activated phy B can directly interact with BIN2,thereby promoting the interaction between BIN2 and BES1,and inducing the phosphorylation of BES1.We show through a series of biochemical studies,such as in vitro/sem-in vivo Pull-down,tobacco Split-LUC,and found that Arabidopsis phy B physically interacts with BIN2 protein in a light-dependent manner.We further found that phy B interacts with BIN2 protein under red light to increase the interaction of BIN2 with BES1 to promote the phosphorylation of BES1,and ultimately promotes photomorphogenesis.Our fundings indicate that red light-activated phy B inhibits BR signaling response by interacting with BIN2 protein to promote BES1 phosphorylation,which defines a new layer of regulatory mechanism that allows plants to coordinate light and BR signaling to optimize photomorphogenesis.