RECOGNITION SPECIFICITY FOR THE BACTERIAL AVIRULENCE PROTEIN AVRPTO IS DETERMINED BY THR-204 IN THE ACTIVATION LOOP OF THE TOMATO PTO KINASE

The Pto kinase confers resistance in tomato to P. syringae pv. tomato strains expressing the AvrPto protein. Physical interaction of the Pto kinase and AvrPto protein in the plant cell initiates host defense re sponses. The recognition event between these two proteins is very spec ific; AvrPto does not interact with other closely related kinases, inc luding the Fen kinase, which shares 80% amino acid identity with Pto. By using Pto-Fen chimeric proteins and site-directed mutagenesis, we f ound that Thr-204 is required for Pto interaction with AvrPto in a yea st two-hybrid system and for recognition specificity in a tobacco leaf transient assay. Substitution of Thr-204 into the Fen kinase allowed that kinase to interact with AvrPto and to confer an AvrPto-specific d efense response in tobacco leaves. Thus, simple mutations appear capab le of giving rise to new resistance gene specificities.