AVIRULENCE GENES FROM PSEUDOMONAS-SYRINGAE PATHOVARS PHASEOLICOLA AND PISI CONFER SPECIFICITY TOWARDS BOTH HOST AND NONHOST SPECIES

Avirulence genes from Pseudomonas syringae pathovars phaseolicola and pisi which determine specificity towards cultivars of their respective host species (bean and pea), were found also to determine specificity towards the non-host species. Transconjugants of Ps.s. pv. pisi harbouring clones containing avrPph3, previously isolated from race 3 of Ps.s. pv. phaseolicola, induced a hypersensitive reaction (HR) in all pea cultivars tested. Clones containing avrPpi2, previously isolated from Ps.s. pv. pisi race 2, caused Ps.s. pv. phaseolicola to induce a rapid HR in most cultivars of bean but in cv. Seafarer a markedly slower incompatible response was observed. Transconjugants of the heterologous pathovar harbouring avrPpi2 or avrPph3, therefore, induced resistance patterns in non-host cultivars which were unlike those of any known race of the homologous pathogen. The quantitatively different resistance responses of some bean cultivars were further characterized by analyses of phytoalexin accumulation and bacterial multiplication in pod tissue. Responses of F2 progeny of crosses between bean cultivars towards transconjugants harbouring avrPpi2 suggested the presence of a dominant allele at a single locus regulating the rapid HR characteristic of resistance in cv. Canadian Wonder and a dominant allele at an independent second locus regulating the slower resistance reaction of cv. Seafarer. It appears that a one gene for two genes relationship is involved in the avrPpi2-bean interaction. Clones carrying two other avirulence genes, avrPph2 and avrPpi3, from Ps.s. pv. phaseolicola and Ps.s. pv. pisi, respectively, had no effect on the virulence of heterologous pathovars towards any of the cultivars tested. © 1992.