GENETIC-ANALYSIS OF A COMPLEX HYPERSENSITIVE REACTION TO BACTERIAL SPOT IN TOMATO

Lycopersicon esculentum accession Hawaii 7998 is the only identified source of resistance to Xanthomonas campestris pv. vesicatoria. This resistance is based on a hypersensitive reaction. Interspecific progenies with L. pennellii were generated to analyze the inheritance of a hypersensitive reaction. This wild species is susceptible to bacterial spot and has a large number of allelic differences with respect to Hawaii 7998. Eighteen isozymes and a morphological marker were used to probe about 30% of the tomato genome for hypersensitive reaction factors. A rating scale was developed to evaluate the inoculation responses of the parental genotypes and the segregating progenies; evaluations were performed every 8 h after inoculation. Hourly rates of change in score were used to analyze the inheritance of hypersensitive reaction. Linkage between marker loci and hypersensitive reaction factors was tested with two-way contingency tables. Significant heterogeneity between genotypic classes for the relative proportion of plants that changed the necrosis score was interpreted as linkage between the marker locus and a hypersensitive reaction factor. Linkage to the same chromosome 1 markers was detected in the F-2 and the BC1 to Hawaii 7998 but not in the reciprocal BC1; this region did not explain all the variation. These results indicated that the hypersensitive response in Hawaii 7998 is controlled by multiple nondominant factors.