BRASSINOSTEROID-SIGNALING KINASE5 Associates with Immune Receptors and Is Required for Immune Responses

Plants utilize cell surface-localized pattern recognition receptors (PRRs) to detect pathogen- or damage-associated molecular patterns (PAMP/DAMPs) and initiate pattern-triggered immunity (PTI). Here, we investigated the role of Arabidopsis (Arabidopsis thaliana) BRASSINOSTEROID-SIGNALING KINASE5 (BSK5), a member of the receptor-like cytoplasmic kinase subfamily XII, in PRR-initiated immunity. BSK5 localized to the plant cell periphery, interacted in yeast and in planta with multiple receptor-like kinases, including the ELONGATION FACTOR-TU RECEPTOR (EFR) and PEP1 RECEPTOR1 (PEPR1) PRRs, and was phosphorylated in vitro by PEPR1 and EFR in the kinase activation loop. Consistent with a role in PTI, bsk5 mutant plants displayed enhanced susceptibility to the bacterial pathogen Pseudomonas syringae and to the fungus Botrytis cinerea Furthermore, bsk5 mutant plants were impaired in several immune responses induced by the elf18, pep1, and flg22 PAMP/DAMPs, including resistance to P. syringae and B. cinerea, production of reactive oxygen species, callose deposition at the cell wall, and enhanced PATHOGENESIS-RELATED1 gene expression. However, bsk5 plants were not affected in PAMP/DAMP activation of mitogen-activated protein kinases and expression of the FLG22-INDUCED RECEPTOR-LIKE KINASE1 or the WRKY domain-containing gene WRKY29 BSK5 variants mutated in the BSK5 myristoylation site, ATP-binding site, and kinase activation loop failed to complement defective PTI phenotypes of bsk5 mutant plants, suggesting that localization to the cell periphery, kinase activity, and phosphorylation by PRRs are critical for the function of BSK5 in PTI. These findings demonstrate that BSK5 plays a role in PTI by interacting with multiple immune receptors.