Molecular characterization of the Pseudomonas syringae pv. pisi plasmid-borne avirulence gene avrPpiB which matches the R3 resistance locus in pea.

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 plasmid-borne 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.