A cluster of mutations disrupt the avirulence but not the virulence function of AvrPto

avrPto in Pseudomonas syringae pv. tomato encodes an avirulence protein tha t triggers race-specific resistance in tomato plants carrying Pto. The AvrP to protein is secreted from P. syringae pv. tomato to plant cells through t he type III secretion pathway and activates race-specific resistance by a d irect interaction with the Pto protein, Here we report that avrPto enhances the virulence of P. syringae pv. tomato in a strain-dependent manner in to mato plants lacking Pto. To determine whether the virulence function can be structurally separated from the avirulence function, we examined the virul ence activity of a group of AvrPto mutants that carry single amino acid sub stitutions and lack the avirulence activity on tomato plants. Three mutants that were clustered in the center of AvrPto exhibited virulence activity i n tomato plants with or without Pro. The rest of the mutations abolished th e virulence. The identification of these mutants suggested that the avirule nce function of AvrPto can be structurally separated from the virulence fun ction.