Atypical immunity induced by extracellular water in plants

Microbial pathogens require nutrients and water to support their growth and proliferation. Pathogen-mediated resource acquisition is orchestrated by the secretion of virulence factors that have evolved a diversity of forms (from effector proteins to small toxins), but which have converged in function (resource acquisition). A key observable virulence mechanism employed by a large number of microbial pathogens to cause disease is to induce a water-rich niche in the extracellular space (i.e. the apoplast) of their host, known as ‘water-soaked lesions’. Given the ubiquity of this virulence mechanism, we asked whether plants respond to pathogen-induced extracellular water to activate immune programs. We find that inducing a water-soaked apoplast induces an atypical transcriptional response, yet results in a functional immune response that restricts pathogen growth as effectively as canonical pattern-triggered immunity. Genetic and functional analyses reveal a role for the immune phytohormone salicylic acid (SA) in mediating apoplastic water-induced immunity (AWII). These results indicate that plants engage in an immune-priming program in response to apoplastic water accumulation and suggest that the pathogenic niches induced during infection may be perceived as danger signals.