High-Resolution Linkage Analysis and Physical Characterization of the Pto Bacterial Resistance Locus in Tomato. Gregory B. Martin. Department of Plant Breeding and Biometry, 252 Emerson Hall, Cornell University, Ithaca, NY 14853-1902, U.S.A. M. Carmen de Vicente, and Steven D. Tanksley. Department of Plant Breeding and Biometry, 252 Emerson Hall, Cornell University, Ithaca, NY 14853-1902, U.S.A. MPMI 6:26-34. Accepted 2 October 1992. Copyright 1993 The American Phytopathological Society.

Resistance to Pseudomonas syringae pv. tomato in tomato is conferred by a single dominant locus, Pto. We are attempting to isolate the Pto locus using a multistep positional cloning strategy. Towards this goal, a high-resolution linkage map containing 18 molecular markers and spanning 20 centiMorgans of tomato chromosome 5 was developed for the region containing Pto. Genome-wide mapping and two methods of targeting markers to specific regions were used to identify a total of 28 markers located in the Pto region. A subset of these markers was analyzed on an F₂ population derived from a cross between near-isogenic lines that were susceptible or resistant to P. s. pv. tomato. A total of 1,200 F₂ plants were prescreened using an organophosphate insecticide, fenthion, known to cause necrotic lesions on plants containing the dominant Pto allele. Only those plants showing insensitivity to the insecticide (251 total) were analyzed further. Individual plants carrying crossover events in the Pto region were identified using flanking restriction fragment length polymorphism markers, and a high-resolution map was constructed. One marker identified, TG538, cosegregated with Pto and therefore provides a starting point for chromosome walking. An estimate of physical distance between TG538 and another marker 0.4 cM away revealed that they lie no further than 435 kb apart. The existence of a relatively small physical distance in this region, the identification of a marker that cosegregates with Pto, and the availability of localized crossovers provide the foundation for the positional cloning of this locus.