SEQUENCE VARIATIONS IN ALLELES OF THE AVIRULENCE GENE AVRPPHE.R2 FROMPSEUDOMONAS-SYRINGAE PV. PHASEOLICOLA LEAD TO LOSS OF RECOGNITION OF THE AVRPPHE PROTEIN WITHIN BEAN CELLS AND A GAIN IN CULTIVAR-SPECIFIC VIRULENCE

The bean halo blight pathogen, Pseudomonas syringae pv, phaseolicola ( Psph), is differentiated into nine races based on the presence or abse nce of five avirulence (avr) genes in the bacterium, which interact wi th corresponding resistance genes, R1-R5, in Phaseolus vulgaris. The r esistance gene R2 is matched by avrPphE, which is located adjacent to the cluster of hrp genes that are required for pathogenicity of Psph. Although only races 2, 4, 5 and 7 are avirulent on cultivars with R2 ( inducing the hypersensitive response; HR), homologues of avrPphE are p resent in all races of Psph. DNA sequencing of avrPphE alleles from ra ces of Psph has demonstrated two routes to virulence: via single basep air changes conferring amino acid substitutions in races 1, 3, 6 and 9 and an insertion of 104 bp in the allele in race 8. We have demonstra ted that these base changes are responsible for the difference between virulence and avirulence by generating transconjugants of a virulent race harbouring plasmids expressing the various alleles of avrPphE, Ag robacterium tumefaciens-directed expression of avrPphE from race 4 in bean leaves induced the HR in a resistance gene-specific manner, sugge sting that the AvrPphE protein is alone required for HR induction and is recognized within the plant cell. The allele from race 6, which is inactive if expressed in Psph, elicited a weak HR if expressed in plan ta, whereas the allele from race 1 did not. Our results suggest that t he affinity of interaction between AvrPphE homologues and an unknown p lant receptor mediates the severity of the plant's response. Mutation of avrPphE alleles did not affect the ability to colonize bean from a low level of inoculum. The avirulence gene avrPphB, which matches the R3 resistance gene, also caused a gene-specific HR following expressio n in the plant after delivery by A. tumefaciens.