Pathogen-associated molecular pattern (PAMP)-trigged immunity (PTI) is the first defensive line of plant innate immunity and is mediated by pattern recognition receptors. Here, we show that a mutation in BR-SIGNALING KINASE1 (BSK1), a substrate of the brassinosteroid (BR) receptor BRASSINOSTEROID INSENSITIVE1, suppressed the powdery mildew resistance caused by a mutation in ENHANCED DISEASE RESISTANCE2, which negatively regulates powdery mildew resistance and programmed cell death, in Arabidopsis thaliana. A loss-of-function bsk1 mutant displayed enhanced susceptibility to virulent and avirulent pathogens, including Golovinomyces cichoracearum, Pseudomonas syringae, and Hyaloperonospora arabidopsidis. The bsk1 mutant also accumulated lower levels of salicylic acid upon infection with G. cichoracearum and P. syringae. BSK1 belongs to a receptor-like cytoplasmic kinase family and displays kinase activity in vitro; this kinase activity is required for its function. BSK1 physically associates with the PAMP receptor FLAGELLIN SENSING2 and is required for a subset of flg22-induced responses, including the reactive oxygen burst, but not for mitogen-activated protein kinase activation. Our data demonstrate that BSK1 is involved in positive regulation of PTI. Together with previous findings, our work indicates that BSK1 represents a key component directly involved in both BR signaling and plant immunity.

Arabidopsis () immune receptor FLAGELLIN SENSING2 (FLS2) rapidly forms a complex to activate pathogen-associated molecular pattern-triggered immunity (PTI) upon perception of the bacterial protein flagellin. The receptor-like cytoplasmic kinase BRASSINOSTEROID-SIGNALINGKINASE1 (BSK1) interacts with FLS2 and is critical for the activation of PTI. However, it is unknown how BSK1 transduces signals to activate downstream immune responses. We identified MEK Kinase5 (MAPKKK5) as a potential substrate of BSK1 by whole-genome phosphorylation analysis. In addition, we demonstrated that BSK1 interacts with and phosphorylates MAPKKK5. In the - mutant, the Ser-289 residue of MAPKKK5 was not phosphorylated as it was in the wild type. Similar to the mutant, the mutant displayed enhanced susceptibility to virulent and avirulent strains of the bacterial pathogen pv DC3000, and to the fungal powdery mildew pathogen Phosphorylation of the Ser-289 residue is not involved in MAPKKK5-triggered cell death but is critical for MAPKKK5-mediated resistance to both bacterial and fungal pathogens. Furthermore, MAPKKK5 interacts with multiple MAPK kinases, including MKK1, MKK2, MKK4, MKK5, and MKK6. Overall, these results indicate that BSK1 regulates plant immunity by phosphorylating MAPKKK5 and suggest a direct regulatory mode of signaling from the immune complex to the MAPK cascade.