A Phosphorelay Circuit Drives Extracellular Alkalinization in Plant Receptor Kinase Signaling

Extracellular alkalinization has long been recognized as a hallmark of plant cell-surface receptor activation, including during pattern-triggered immunity (PTI); yet the mechanisms driving elicitor-induced alkalinization and its role in immune signaling remain unclear. Here, we demonstrate that inhibition of autoinhibited H⁺-ATPases (AHAs) is required for elicitor-induced extracellular alkalinization. This alkalinization is essential for immune signaling mediated by diverse plasma membrane-localized receptor kinases (RKs) through modulation of ligand-receptor interactions. Notably, RKs transduce elicitor-triggered signaling via BOTRYTIS-INDUCED KINASE 1 (BIK1), which inhibits AHA activity by disrupting AHA-GENERAL REGULATORY FACTOR (GRF) interactions through a conserved phosphorylation event. Interestingly, this pathway is crucial for cell wall damage (CWD) responses involving the RK MALE DISCOVERER 1-INTERACTING RECEPTOR LIKE KINASE 2 (MIK2) and its ligand, SERINE RICH ENDOGENOUS PEPTIDE 18 (SCOOP18). Our findings reveal a conserved phospho-regulatory pathway that governs extracellular alkalinization to coordinate plant immune signaling, offering new insights into plant stress resilience.