A bacterial exotoxin-triggered plant immune response restricts pathogen growth.

For optimal growth and development, hosts must promote healthy symbiotic interactions while restricting pathogens. To ask whether hosts can distinguish phylogenetically similar pathogens and beneficial bacteria, we used two closely related plant root-associated strains within the Pseudomonas fluorescens species complex. Despite having similar immunogenic microbe-associated molecular patterns, one strain is beneficial and the other exhibits exotoxin-dependent virulence. We show that the two strains co-exist in vitro, but the beneficial strain outcompetes the pathogen in the rhizosphere. We find that plants respond to the pathogen, but not the beneficial strain, predominantly via an exotoxin-triggered defense response in roots. The purified exotoxin is sufficient to induce immunity and restrict bacterial growth in a BAK1/BKK1/CERK1-dependent manner. We show that these immune components are also required for balancing the growth between the beneficial and pathogenic strains. We conclude that plant immunity can distinguish phylogenetically similar microbes with distinct lifestyles, in part, through perception of exotoxins.