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

Abstract

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.

Plant immune signaling: Advancing on two frontiers

“…These studies implicated that FER may play multiple roles in plant immunity and development by recognizing multiple RALFs via distinct LLGs. ANXUR1 (ANX1) and ANX2, also a Malectin domain RLK as FER, interacts with BAK1 and negatively regulates FLS2 signaling (Mang et al 2017). It seems that PRRs sharing the same co‐receptors have complex interactions, which fine‐tunes the output of immune signaling from different ligands.…”

A Novel Specialized Immune Player: BSK5 Is Required for Restricting Pathogen Progression

“…The bsk5 mutants are also compromised in resistance to pathogens otherwise induced by pathogen associated molecular patterns and fail to accumulate callose, reactive oxygen species, and PATHOGENESIS-RELATED 1 transcripts. Majhi et al (2019) further demonstrate that phosphorylation, kinase activity, and plasma membrane localization are essential for BSK5 immune function.…”

“…In this issue of Plant Physiology, Majhi et al (2019) present evidence supporting the role of BSK5 in PTI. The authors show that BSK5 interacts with multiple RLKs and PRRs such as PEPR1 and EFR in a direct and specific fashion.…”

Surface Sensor Systems in Plant Immunity

“…Like PBL27 and RLCK-VII-4s mentioned previously, AtBSK1 is also reported to phosphorylate AtMAPKKK5 and thereby initiate MAPK signaling (Yan et al, 2018). The related AtBSK5 was recently shown to be involved in AtFLS2, AtEFR, and AtPEPR-mediated responses (Majhi et al, 2019). In addition to RLCK-VII and XII RLCKs, other important kinases in early signaling include calcium-dependent kinases (CPK) such as AtCPK5 (Yip Delormel and Boudsocq, 2019), the MAP4Ks AtSIK1 and AtMAP4K4 Jiang et al, 2019), and cysteine-rich receptor-like kinases such as AtCRK2 (Kimura et al, 2019).…”