Systemic resistance in Arabidopsis induced by rhizobacteria requires ethylene-dependent signaling at the site of application

Root colonization of Arabidopsis thaliana by the nonpathogenic, rhizosphere -colonizing, biocontrol bacterium Pseudomonas fluorescens WCS417r has been shown to elicit induced systemic resistance (ISR) against Pseudomonas syrin gae pv. tomato (Pst). The ISR response differs from the pathogen-inducible systemic acquired resistance (SAR) response in that ISR is independent of s alicylic acid and not associated with pathogenesis-related proteins. Severa l ethylene-response mutants were tested and showed essentially normal sympt oms of Pst infection. ISR was abolished in the ethylene-insensitive mutant etr1-1, whereas SAR was unaffected. Similar results were obtained with the ethylene-insensitive mutants ein2 through ein7, indicating that the express ion of ISR requires the complete signal-transduction pathway of ethylene kn own so far. The induction of ISR by WCS417r was not accompanied by increase d ethylene production in roots or leaves, nor by increases in the expressio n of the genes encoding the ethylene biosynthetic enzymes 2-aminocyclopropa ne-1-carboxylic (ACC) synthase and ACC oxidase. The eir1 mutant, displaying ethylene insensitivity in the roots only, did not express ISR upon applica tion of WCS417r to the roots, but did exhibit ISR when the inducing bacteri a were infiltrated into the leaves. These results demonstrate that, for the induction of ISR, ethylene responsiveness is required at the site of appli cation of inducing rhizobacteria.