Ancient origin of pathogen recognition specificity conferred by the tomatodisease resistance gene Pto

We have investigated the origin of the Pto disease resistance (R) gene that was previously identified in the wild tomato species Lycopersicon pimpinel lifolium and isolated by map-based cloning. Pto encodes a serine-threonine protein kinase that specifically recognizes strains of Pseudomonas syringae pv. tomato (Pst) that express the avirulence gene avrPto. We examined an a ccession of the distantly related wild species Lycopersicon hirsutum var. g labratum that exhibits avrPto-specific resistance to Pst. The Pst resistanc e of L, hirsutum was introgressed into a susceptible Lycopersicon esculentu m background to create the near-isogenic line 96T133-3, Resistance to Pst(a vrPto) in 96T133-3 was inherited as a single dominant locus end cosegregate d with a restriction fragment length polymorphism detected by the Pto gene. This observation suggested that a member of the Pto gene family confers Ps t(avrPto) resistance in this L. hirsutum line. Here we report the cloning a nd characterization of four members of the Pto family from 96T133-3, One ge ne (LhirPto) is 97% identical to Pto and encodes a catalytically active pro tein kinase that elicits a hypersensitive response when coexpressed with av rPto in leaves of Nicotiana benthamiana, In common with the Pto kinase, the LhirPto protein physically interacts with AvrPto and downstream members of the Pto signaling pathway. Our studies indicate that R genes of the protei n kinase class may not evolve rapidly in response to pathogen pressure and rather that their ability to recognize specific Avr proteins can be highly conserved.