SALICYLIC ACID SENSOR1 reveals the propagation of an SA hormone surge during plant pathogen advance.

Salicylic acid (SA) is a key phytohormone that orchestrates immune responses against pathogens, including Pseudomonas syringae bacteria. The timing and extent of SA accumulation are tightly controlled by plants but can be suppressed by pathogens to overcome immunity. Understanding SA dynamics at high spatiotemporal resolution remains challenging owing to limitations in existing detection methods that are indirect, destructive, or lacking in cellular precision and temporal resolution. We developed SalicS1, a genetically encoded fluorescence resonance energy transfer (FRET) biosensor specific to SA. SalicS1 enables real-time, reversible monitoring of SA levels in vivo with minimal perturbation of endogenous signaling. We reveal the propagation of an SA surge spreading from bacterial infection sites with spatiotemporal fidelity. SalicS1 unlocks precise understanding of SA dynamics underpinning crop resilience to pathogens.