Regulation of receptor kinase-mediated immune signalling by dynamic phosphorylation
Plants recognize pathogen-associated molecular patterns (PAMPs) via cell surface-localized pattern recognition receptors (PRRs), initiating a broad-spectrum defense response against pathogens, known as PRR-triggered immunity (PTI). However, immunity comes at a cost; and immune responses need to be tightly regulated. How PTI signalling is negatively regulated in plants is not fully understood. PRRs are present at the plasma membrane in dynamic kinase complexes that heavily rely on trans-phosphorylation to initiate signaling. The Arabidopsis cytoplasmic kinase BIK1 associates with different PRRs and plays a central role in the activation of downstream immune signaling. In this study, we identify the protein phosphatase PP2C38 as a negative regulator of BIK1 activity and BIK1-mediated immunity. PP2C38 dynamically associates with BIK1, as well as with the PRRs FLS2 and EFR, but not with the regulatory receptor kinase (RK) BAK1. PP2C38 regulates PAMP-induced BIK1 phosphorylation and impairs the phosphorylation of the NADPH oxidase RBOHD by BIK1, leading to reduced oxidative burst and stomatal immunity. Notably, upon PAMP perception, PP2C38 is phosphorylated on serine 77, most likely by BIK1, and dissociates from the PRR-BIK1 complex. We suggest that this mechanism relieves the negative regulation imposed by PP2C38 to enable efficient BIK1 activation. This study uncovers an important regulatory mechanism of this central immune component, and extends our knowledge on how plant immunity is appropriately controlled.
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