CHANGES IN CULTIVAR-SPECIFICITY TOWARD PEA CAN RESULT FROM TRANSFER OF PLASMID RP4 AND OTHER INCOMPATIBILITY GROUP P1 REPLICONS TO PSEUDOMONAS-SYRINGAE PV PISI

Transfer of RP4 and related replicons belonging to the Escherichia col i incompatibility group P (Pseudomonas aeruginosa IncP1) to races 2 an d 6 of P. syringae pv. pisi was associated with the creation of two ty pes of transconjugant, one resembling the parental race and the other showing an altered cultivar-specificity towards pea. The latter, irres pective of the parental race, exhibited a novel pattern of interaction with pea that corresponded to race 4; consequently such transconjugan ts were termed race 4-like. Curing of RP4 did not affect the phenotype , except in relation to the antibiotic resistances specified by RP4. T he race 4-like strains were nonfluorescent when cultured on appropriat e media (in contrast to the particular isolates of races 2 and 6 from which they were derived), showed an enhanced ability to inherit RP4 su bsequently (at frequencies up to 10(-1) per recipient) and differed fr om their parental race in their pattern of plasmid profile. The plasmi d profiles were similar for all race 4-like strains irrespective of or igin. There was no evidence that RP4 had recombined with DNA in the re cipient and probing failed to detect the retention of any part of RP4 in cured strains. The inheritance of the related cosmid vector, pLAFR3 , had similar effects in races 2 and 6. This observation is important since this vector has been widely used to clone avirulence genes in pl ant pathogenic bacteria. Transfer of the IncW plasmids S-a and R388 di d not cause any changes in the fluorescence or cultivar-specificity of races 2 or 6. The novel avirulence expressed by the race 4-like varia nts derived from races 2 and 6 provides evidence for the presence in r aces 2 and 6 of an inhibitor/suppressor gene, which modulates the expr ession of the race 4-like avirulence gene.