BSU1 family phosphatases mediate Flagellin-FLS2 signaling through a specific phosphocode

Among the hundreds of receptor-like kinases (RLKs) in plants, the brassinosteroid (BR) hormone receptor BR-INSENSITIVE 1 (BRI1) and the immunity receptor FLAGELLIN SENSING 2 (FLS2) share a common co-receptor kinase, but lead to distinct growth and immunity responses, respectively. Here we show that the BSU1 family of phosphatases, known to mediate BR inactivation of GSK3-like kinases, also mediate flagellin-FLS2 signaling to the MAP kinases, through different phosphocodes. Flagellin treatment induced phosphorylation of BSU1 phosphatase at serine-251 (S251) located in the N-terminal kelch-repeat domain. The quadruple mutant (), with loss or compromised function of all four BSU family members, showed defects in flagellin-triggered MAP kinase activation. The Botrytis-induced kinase 1 (BIK1), a substrate of FLS2, phosphorylated BSU1 at S251 in a flagellin-dependent manner. Mutation of S251 to alanine in BSU1 reduced its phosphorylation by BIK1, interaction with MEKK1 and ability to restore flagellin-induced MAP kinase activation of the quadruple mutant, without affecting its ability to activate BR-dependent growth. Our results demonstrate that BSU1 acts as a substrate of BIK1 downstream of FLS2 in the innate immunity pathway, in addition to its role in the BR pathway. Our results suggest that different RLKs sharing downstream components may maintain signaling specificity through phosphocodes in higher plants.