Recognition of pathogen-derived molecules by pattern recognition receptors (PRRs) is a common feature of both animal and plant innate immune systems. In plants, PRR signalling is initiated at the cell surface by kinase complexes, resulting in the activation of immune responses that ward off microorganisms. However, the activation and amplitude of innate immune responses must be tightly controlled. In this Review, we summarize our knowledge of the early signalling events that follow PRR activation and describe the mechanisms that fine-tune immune signalling to maintain immune homeostasis. We also illustrate the mechanisms used by pathogens to inhibit innate immune signalling and discuss how the innate ability of plant cells to monitor the integrity of key immune components can lead to autoimmune phenotypes following genetic or pathogen-induced perturbations of these components.

Multicellular organisms use cell-surface receptor kinases to sense and process extracellular signals. Many plant receptor kinases are activated by the formation of ligand-induced complexes with shape-complementary co-receptors. The best-characterized co-receptor is BRASSINOSTEROID INSENSITIVE 1-ASSOCIATED KINASE 1 (BAK1), which associates with numerous leucine-rich repeat receptor kinases (LRR-RKs) to control immunity, growth and development. Here we report key regulatory events that control the function of BAK1 and, more generally, LRR-RKs. Through a combination of phosphoproteomics and targeted mutagenesis, we identified conserved phosphosites that are required for the immune function of BAK1 in Arabidopsis thaliana. Notably, these phosphosites are not required for BAK1-dependent brassinosteroid-regulated growth. In addition to revealing a critical role for the phosphorylation of the BAK1 C-terminal tail, we identified a conserved tyrosine phosphosite that may be required for the function of the majority of Arabidopsis LRR-RKs, and which separates them into two distinct functional classes based on the presence or absence of this tyrosine. Our results suggest a phosphocode-based dichotomy of BAK1 function in plant signalling, and provide insights into receptor kinase activation that have broad implications for our understanding of how plants respond to their changing environment.

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.