BSK1, a receptor-like cytoplasmic kinase, involved in both BR signaling and innate immunity in Arabidopsis.

Molecular interaction between powdery mildew fungi and Arabidopsis has been widely used as a model system to study plant immunity. Arabidopsis EDR2 (enhanced disease resistance 2) is a well characterized negative regulator in powdery mildew resistance and mildew-induced cell death. Recently, we showed that a mutation in BSK1 (br-signaling kinase 1), suppressed edr2-mediated disease resistance. (1) And the bsk1-1 single mutant displayed enhanced susceptibility to multiple pathogens, indicating that BSK1 plays important roles in plant immunity. BSK1 is a receptor-like cytoplasmic kinase and localizes on plasma membrane; loss of the membrane localization signaling disrupts BSK1 functions in edr2-mediated resistance. Significantly, BSK1 physically associates with the PAMP receptor FLS2 (flagellin sensing 2) and is required by FLS2-mediated ROS burst. (1) Here we show that disruption of BSK1 membrane localization affects the BSK1-FLS2 interactions, suggesting the membrane association of BSK1 is important for both edr2-mediated signaling and the BSK1-FLS2 complex formation. Previously, it was shown that BSK1 is a substrate of the brassinosteroid (BR) receptor BRI1 (brassinosteroid insensitive 1) and plays critical roles in BR signaling. (2) Further exploration of signaling transductions downstream of BSK1-FLS2 complex will not only shed new light on how BSK1 regulates plant immunity, but may also help to dissect the connections between plant growth and defense.