Changes in cultivar-specificity toward pea can result from transfer of plasmid RP4 and other incompatibility group P1 replicons to Pseudomonas syringae pv. pisi.

Abstract

Changes in cultivar-specificity toward pea can result from transfer of plasmid RP4 and other incompatibility group P1 replicons to Pseudomonas syringae pv. pisi. England, Frenchay Campus, Coldharbour Lane, Bristol BS16 IQY, UK 2Horticulture Research International, Wellesbourne, Warwick CV3.5 9EF, UK (Received 25 June 1993; revised 21 July 1993; accepted 23 July 1993) Transfer of RP4 and related replicons belonging to the Escherichia coli incompatibility group P (Pseudomonas aeruginosa IncP1) to races 2 and 6 of P. syringae pv. pisi was associated with the creation of two types of transconjugant, one resembling the parental race and the other showing an altered cultivar-specificity towards pea. The latter, irrespective of the parental race, exhibited a novel pattern of interaction with pea that corresponded to race 4; consequently such transconjugants were termed race 4-like. Curing of RP4 did not affect the phenotype, except in relation to the antibiotic resistances specified by RP4. The race 4-like strains were nonfluorescent when cultured on appropriate media (in contrast to the particular isolates of races 2 and 6 from which they were derived), showed an enhanced ability to inherit RP4 subsequently (at frequencies up to lo-' per recipient) and differed from their parental race in their pattern of plasmid profile. The plasmid profiles were similar for all race 4-like strains irrespective of origin. There was no evidence that RP4 had recombined with DNA in the recipient 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 plant pathogenic bacteria. Transfer of the IncW plasmids S-a and R388 did not cause any changes in the fluorescence or cultivar-specificity of races 2 or 6. The novel avirulence expressed by the race 4-like variants derived from races 2 and 6 provides evidence for the presence in races 2 and 6 of an inhibitorlsuppressor gene, which modulates the expression of the race 4-like avirulence gene. IntroductionPlant pathogenic bacteria often exhibit considerable specificity in their interactions with potential host plants. In pathovars of Pseudomonas syringae, this specificity appears to be based upon matching genes for recognition in the host and pathogen. The response of the host to invasion by the pathogen is determined by avirulence (am) genes in the pathogen and matched resistance (R) genes in the host. When a race of the pathogen carries an avr gene that is matched by the presence of an R gene in the host the interaction which occurs results in the induction of the host defence mechanism, often seen as a hypersensitive reaction. In the absence of the matching avr and R genes disease is the outcome (Keen, 1990 The cloning of avirulence genes in P. syringae pv. pisi and other pathovars (Vivian, 1992) has been based on the use of gene libraries constructed in vectors, such as...

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