Biologically induced systemic acquired resistance in Arabidopsis thaliana

Summary Local infection with a necrotizing pathogen can render plants resistant to subsequent infection by normally virulent pathogens. A system for biological induction of such systemic acquired resistance (SAR) in Arabidopsis thaliana is reported. When plants were immunized by local inoculation of a single leaf with avirulent Pseudomonas syringae pv. tomato (Pst) carrying the avrRpt2 avirulence gene, after 2 days other leaves became resistant, as measured symptomatically and by in planta bacterial growth, to challenge with a virulent Pst strain lacking this avirulence gene. Resistance was systemic and protected the plants against infection by other virulent pathogens including P. syringae pv. maculicola. Low‐dose inoculation induced a strong SAR and double immunizations did not increase the level of protection indicating that the response of only a few cells to the immunizing bacteria is required. SAR was not induced by the virulent strain of Pst lacking avrRpt2. However, experiments with the Arabidopsis RPS2 disease resistance gene mutant rps2‐201 , which does not exhibit a local hypersensitive response to Pst carrying the corresponding avirulence gene avrRpt2 , indicate that a hypersensitive response contributes to, but is not essential for, the induction of SAR. Thus, avrRpt2 activates either a branching signal pathway or separate parallel pathways for induction of localized hypersensitive resistance and SAR, with downstream potentiation of the systemic response by the local response. Using this system for the biological induction of SAR in Arabidopsis , it should be possible to dissect the molecular genetics of SAR by the isolation of mutants affected in the production, transmission, perception and transduction of the systemic signal(s).