Molecular Characterization of the Pseudomonas syringae pv. pisi Plasmid-Borne Avirulence Gene avrPpiB Which Matches the R3 Resistance Locus in Pea. Benoit Cournoyer. Department of Biological Sciences, University of the West of England, Bristol BS16 1QY. Julie D. Sharp(1), Annamaria Astuto(1), Marjorie J. Gibbon(1), John D.Taylor(2), and Alan Vivian(1) (1)Department of Biological Sciences, University of the West of England, Bristol BS16 1QY, and (2)Horticulture Research International, Wellesbourne CV35 9EF, United Kingdom. MPMI 8:700-708. Accepted 13 June 1995. Copyright 1995 The American Phytopathological Society.

An avirulence gene (designated avrPpiB) from race 3 of Pseudomonas syringae pv. pisi was cloned and sequenced. The gene corresponded to a single open reading frame of 831 nt identified by transposon mutagenesis and subcloning. This ORF encodes a predicted hydrophilic protein of 276 amino acids (MW 31,300). It effects the expression of a resistance mechanism governed by a single genetic locus in pea. Cosegregation of resistance at the R3 locus of pea was observed towards race 3 and a transconjugant carrying the cloned avrPpiB gene according to the predicted 3:1 ratio of resistant:susceptible F2 progeny from a cross between Jade (R3 R3) and Kelvedon Wonder (rr) cultivars. DNA hybridization studies showed avrPpiB to be plasmidborne in race 3 and suggested the presence of other alleles on one of the endogenous plasmids of races 1 and 7. Disruption of the avrPpiB allele of race 1 and its complementation confirmed its behavior towards pea cultivars expressing the R3 locus. Homologs of avrPpiB were detected in P. syringae pv. phaseolicola, P. syringae pv. maculicola, and P. syringae pv. tomato. The presence of avrPpiB homologs in P. syringae pv. phaseolicola does not match any gene-for-gene pattern of interaction with bean cultivars. Un gene d'avirulence (nomme avrPpiB) provenant de la race 3 de Pseudomonas syringae pathovar pisi a ete clone et sequence. Ce gene correspond a un cadre de lecture ou-vert (ORF) de 831 nt identifie par sous-clonage et mutagenese par transposition. Cet ORF code pour une proteine hydrophilique de 276 acides amines (poids mole-culaire 31,300). Elle est impliquee dans I'expression d'un mecanisme de resistance gouverne par un seul locus gene-tique chez le pois. Selon un ratio 3:1 de resistants:susceptibles vis a vis de la race 3 et un transconjugant arborant un gene avrPpiB clone pour une generation F2 provenant d'un croisement entre les cultivars Jade (R3 R3) et Kelvedon Wonder (rr), la co-segregation de la resistance du locus R3 du pois a ete etablie. Des etudes d'hybridation ont montre qu'avrPpiB est porte par un plasmide chez la race 3 et que des alleles se retrouvent sur un des plasmides endogenes des races 1 et 7. La dislocation du gene avrPpiB de la race 1 et sa complementation ont confirme son implication au niveau de I'expression du locus R3 des cultivars de pois. Des genes homologues d'avrPpiB ont ete de-tectes chez P. syringae pv. phaseolicola, P. syringae pv. maculicola and P. syringae pv. tomato. La presence d'homologues chez P. syringae pv. phaseolicola ne correspond a aucun patron connu d'interaclion gene-pour-gene avec le haricot.