The pseudomonas AvrPto protein is differentially recognized by tomato and tobacco and is localized to the plant plasma membrane

The avrPto gene of Pseudomonas syringae pv tomato triggers race-specific re sistance in tomato plants carrying Pto, a resistance gene encoding a protei n kinase. When introduced into P. s. tabaci, avrPto triggers resistance in tobacco W38 plants that carry the corresponding R gene. The AvrPto protein is believed to be secreted into host cells through the bacterial type III s ecretion pathway, where it activates disease resistance in tomato by intera cting with Pto. We report here the identification of two distinct regions i n AvrPto that determine the recognition specificity of this protein in toma to and tobacco. Point mutations in the central region disrupted the avirule nce activity in tomato but not in tobacco. Conversely, point mutations in t he C-terminal region abolished the avirulence in tobacco but not in tomato. We further report that AvrPto was localized to the plasma membrane of plan t cells. Disrupting the membrane association by mutating a putative myristo ylation motif of AvrPto abolished the avirulence activity in both tomato an d tobacco. These findings demonstrate that AvrPto is recognized differently by the R genes in tomato and tobacco and that the recognition of AvrPto pr obably is associated with the plasma membrane.