The Arabidopsis protein phosphatase PP2C38 negatively regulates the central immune kinase BIK1

Plants recognize pathogen-associated molecular patterns (PAMPs) via cell surface-localized pattern recognition receptors (PRRs), leading to PRR-triggered immunity (PTI). The Arabidopsis cytoplasmic kinase BIK1 is a downstream substrate of several PRR complexes. How plant PTI is negatively regulated is not fully understood. Here, 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 co-receptor 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. Upon PAMP perception, PP2C38 is phosphorylated on serine 77 and dissociates from the FLS2/EFR-BIK1 complexes, enabling full BIK1 activation. Together with our recent work on the control of BIK1 turnover, this study reveals another important regulatory mechanism of this central immune component

Calcium-dependent protein kinase/NADPH oxidase activation circuit is required for rapid defense signal propagation

In animals and plants, pathogen recognition triggers the local activation of intracellular signaling that is prerequisite for mounting systemic defenses in the whole organism. We identified that Arabidopsis thaliana isoform CPK5 of the plant calcium-dependent protein kinase family becomes rapidly biochemically activated in response to pathogen-associated molecular pattern (PAMP) stimulation. CPK5 signaling resulted in enhanced salicylic acid–mediated resistance to the bacterial pathogen Pst DC3000, differential plant defense gene expression, and synthesis of reactive oxygen species (ROS). Using selected reaction monitoring MS, we identified the plant NADPH oxidase, respiratory burst oxidase homolog D (RBOHD), as an in vivo phosphorylation target of CPK5. Remarkably, CPK5-dependent in vivo phosphorylation of RBOHD occurs on both PAMP- and ROS stimulation. Furthermore, rapid CPK5-dependent biochemical and transcriptional activation of defense reactions at distal sites is compromised in cpk5 and rbohd mutants. Our data not only identify CPK5 as a key regulator of innate immune responses in plants but also support a model of ROS-mediated cell-to-cell communication, where a self-propagating mutual activation circuit consisting of the protein kinase, CPK5, and the NADPH oxidase RBOHD facilitates rapid signal propagation as a prerequisite for defense response activation at distal sites within the plant