Molecular determinants required for the avirulence function of AvrPphB in bean and other plants.

The avirulence gene avrPphB from Pseudomonas syringae pv. phaseolicola determines incompatibility, manifested as a hypersensitive reaction (HR), on bean cultivars carrying the R3 resistance gene and also confers avirulence on other plants. The AvrPphB protein carries an embedded consensus myristoylation motif and is cleaved in bacteria and certain plants to yield fragments of about 6 and 28 kDa. We investigated plant recognition and type III translocation determinants in AvrPphB by constructing three N-terminally truncated and two site-directed mutants carrying substitutions in the conserved G63 residue of the myristoylation motif, which lies adjacent to the proteolytic cleavage site. The peptides were either delivered to plant cells by pseudomonads or were expressed transiently in planta via the Agrobacterium tumefaciens or Potato virus X. The 63 amino terminal residues were required for type III-mediated translocation from Pseudomonas strains to the plant, but were partially dispensable for effector recognition following in planta expression. Substitution of the G63 residue resulted in differential HR phenotypes in two different R3 cultivars of bean and abolished effector processing in Pseudomonas strains. Agrobacterium-mediated expression of the mutant proteins elicited HR in resistant bean hosts and in tomato but elicited no reaction in Nicotiana species.