Analysis of the molecular basis of Pseudomonas syringae pv. tomato resistance in tomato

Tomato (Lycopersicon esculentum) cultivars that contain the Pto bacterial resistance locus also exhibit sensitivity to fenthion, an organophosphorous insecticide. Resistance to Pseudomonas syringae pv. tomato (Pst) encoded by the Pto gene and sensitivity to fenthion cosegregate in large F₂ populations and were apparently introgressed together into tomato from the wild species Lycopersion pimpinellifolium. In order to isolate the genes responsible for these two phenotypes and to study their molecular basis, a multistep map-based cloning project was initiated. A closely linked RFLP marker was used to isolate a yeast artificial chromosome (YAC) clone that spanned the Pto region. Transcribed sequences within the Pto region were identified by isolating cDNA clones that hybridized to the YAC clone. Transformation of candidate cDNA clones into a Pst-susceptible, fenthion-insensitive tomato line succeeded in identifying a cDNA conferring Pst resistance and a separate cDNA conferring sensitivity to fenthion. The cDNA clones represent members of a tandemly repeated gene family and encode putative serine-threonine protein kinases. The role of these kinases in recognizing a signal from Pst or the fenthion molecule and in activating the plant response is currently being investigated.Abbreviations Pst Pseudomonas syringae pv. tomato - RAPD random amplified fragment polymorphism - RFLP restriction fragment length polymorphism - YAC yeast artifical chromosome