Inverse Modulation Of Innate Immunity And Brassinosteroid Signaling Pathway By The Receptor-like Cytoplasmic Kinase BIK1 In Arabidopsis Thaliana

Maintaining active growth and effective immune responses is often costly for a living organism to survive.Fine-tuning the shared cross-regulators is crucial for metazoans and plants to make a trade-off between growth and immunity.The Arabidopsis regulatory receptor-like kinase BAK1 complexes with the receptor kinase FLS2 in bacterial flagellin-triggered immunity and BRI1 in brassinosteroid-mediated growth.BR homeostasis and signaling unidirectionally modulate FLS2-mediated immune responses at multiple levels.We have shown previously that BIK1,a receptor-like cytoplasmic kinase,is directly phosphorylated by BAK1 and associates with FLS2/BAK1 complex in transducing flagellin signaling.In contrast to its positive role in plant immunity,we report here that BIK1 acts as a negative regulator in BR signaling.The bikl mutants display various BR hypersensitive phenotypes accompanied with increased accumulation of de-phosphorylated BES1 proteins and regulation of BZR1 and BES1 target genes.BIK1 associates with BRI1,and is released from the BRI1 receptor upon BR treatment,which is reminiscent of FLS2-BIK1 complex dynamics in flagellin signaling.The ligand-induced release of BIK1 from receptor complexes is associated with BIK1 phosphorylation.However,in contrast to BAK1-dependent FLS2-BIK1 dissociation.BAK1 is dispensable for BRI1-BIK1 dissociation.Consistently,unlike FLS2 signaling which depends on BAK1 to phosphorylate BIK1,BRI1 directly phosphorylates BIK1 to transduce BR signaling.Apparently,dissociation of BIK1 from BRI1 is a result of BR-induced BRI1 phosphorylation on BIK1 whereas flagellin-triggered BIK1 release from FLS2 complex is mediated through BAK1 phosphorylation on BIK1.Our studies indicate that BIK1 mediates distinct functions in plant immunity and development via association dynamics with specific receptor complexes and differential phosphorylation events.