The hormone salicylic acid (SA) plays a crucial role in plant immunity by activating responses that arrest pathogen ingress. Since SA accumulation also penalizes growth, the question remains why healthy plants synthesize this hormone. By overexpressing SA-inactivating hydroxylases in Arabidopsis thaliana, we reveal that basal SA levels in unchallenged plants are needed for expression of selected immune receptor and signaling genes, thereby enabling early pathogen detection and activation of immunity
