BIOLOGICALLY INDUCED SYSTEMIC ACQUIRED-RESISTANCE IN ARABIDOPSIS-THALIANA

Local infection with a necrotizing pathogen can render plants resistan t to subsequent infection by normally virulent pathogens. A system for biological induction of such systemic acquired resistance (SAR) in Ar abidopsis thaliana is reported. When plants were immunized by local in oculation of a single leaf with avirulent Pseudomonas syringae pv. tom ato (Pst) carrying the avrRpt2 avirulence gene, after 2 days other lea ves became resistant, as measured symptomatically and by in planta bac terial growth, to challenge with a virulent Pst strain lacking this av irulence gene. Resistance was systemic and protected the plants agains t infection by other virulent pathogens including P. syringae pv. macu licola. Low-dose inoculation induced a strong SAR and double immunizat ions did not increase the level of protection indicating that the resp onse of only a few cells to the immunizing bacteria is required. SAR w as not induced by the virulent strain of Pst tacking avrRpt2. However, experiments with the Arabidopsis RPS2 disease resistance gene mutant rps2-201, which does not exhibit a local hypersensitive response to Ps t 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 pa thway or separate parallel pathways for induction of localized hyperse nsitive resistance and SAR, with downstream potentiation of the system ic response by the local response. Using this system for the biologica l induction of SAR in Arabidopsis, it should be possible to dissect th e molecular genetics of SAR by the isolation of mutants affected in th e production, transmission, perception and transduction of the systemi c signal(s).