Pathogen-associated molecular patterns (PAMPs) are recognized by plant pattern recognition receptors to activate PAMP-triggered immunity (PTI). Mitogen-activated protein kinases (MAPKs), as well as other cytoplasmic kinases, integrate upstream immune signals and, in turn, dissect PTI signaling via different substrates to regulate defense responses. However, only a few direct substrates of these signaling kinases have been identified. Here, we show that PAMP perception enhances phosphorylation of BRASSINOSTEROID INSENSITIVE1-ETHYL METHANESULFONATE-SUPPRESSOR1 (BES1), a transcription factor involved in brassinosteroid (BR) signaling pathway, through pathogen-induced MAPKs in Arabidopsis (Arabidopsis thaliana). BES1 interacts with MITOGEN-ACTIVATED PROTEIN KINASE6 (MPK6) and is phosphorylated by MPK6. bes1 loss-of-function mutants display compromised resistance to bacterial pathogen Pseudomonas syringae pv tomato DC3000. BES1 S286A/S137A double mutation (BES1(SSAA)) impairs PAMP-induced phosphorylation and fails to restore bacterial resistance in bes1 mutant, indicating a positive role of BES1 phosphorylation in plant immunity. BES1 is phosphorylated by glycogen synthase kinase3 (GSK3)-like kinase BR-insensitive2 (BIN2), a negative regulator of BR signaling. BR perception inhibits BIN2 activity, allowing dephosphorylation of BES1 to regulate plant development. However, BES1(SSAA) does not affect BR-mediated plant growth, suggesting differential residue requirements for the modulation of BES1 phosphorylation in PTI and BR signaling. Our study identifies BES1 as a unique direct substrate of MPK6 in PTI signaling. This finding reveals MAPK-mediated BES1 phosphorylation as another BES1 modulation mechanism in plant cell signaling, in addition to GSK3-like kinase-mediated BES1 phosphorylation and F box protein-mediated BES1 degradation.
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