Plants are equipped with versatile pattern recognition receptors (PRRs), which monitor their external environment and elicit defensive measures upon detection of potential risk for disease. Inside the cell, receptor-like cytoplasmic kinases (RLCKs) are key components of PRR signalling, but their molecular functions and regulatory interactions are not yet fully understood. In tomato, two RLCKs, Pti1a and Pti1b, are important signalling components that relay early defence signals elicited by bacterial flagellin, a conserved pattern common to various pathogenic and non-pathogenic microbes. An important question to resolve is how plant immune reactions are regulated to prevent unnecessary defensive measures. A recent paper published in the Biochemical Journal by Giska and Martin [Biochem. J. (2019) 476, 1621–1635] reports the identification and biochemical characterization of a new tomato (Solanum lycopersicum) protein phosphatase that negatively controls early defence signalling. The phosphatase, termed pattern-triggered immunity inhibiting PP2C 1 (Pic1), negatively controls the signalling function of Pti1b and therefore holds a central position in the defence signalling network. The Pti1b–Pic1 kinase–phosphatase interaction provides mechanistic insights that forward our understanding of protein phosphatases and their importance in plant immunity.

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