The Molecular Basis of Specificity in the Interaction Between Pseudomonas Syringae pv. pisi and Cultivars of Pisum sativum.

We review progress in the isolation and characterization of a avirulence genes from Pseudomonas syringae pv. pisi, which are responsible for the pattern of racecuitivar interactions with pea (Pisum sativum). The avirulence gene, avrPpiA1.R2 was obtained from the race 2 isolate 203 and was found to be chromosomally located in all race 2 strains examined, whereas homologues in races 5 and 7 were on plasmids. The gene, avrPpiB1.R3 was obtained from the race 3 isolate 870A and together with homologues from races 1 and 7 was shown to be plasmid-borne. Both genes have been sequenced and their products shown to share many features of other P. syringae avr genes. The recent isolation of a cosmid clone apparently harbouring the race 5 gene, avrPpiD, has confirmed the existence of the fifth avirulence gene postulated in the gene-for-gene scheme.