Allelic Tests and Sequence Analysis of Three Genes for Resistance to Xanthomonas perforans Race T3 in Tomato

Three crosses, Hawaii7981 × PI128216, Hawaii7981 × LA1589, and PI128216 × LA1589, were made to develop F2 populations for testing allelism among three genes Xv3, Rx4, and RxLA1589 conferring resistance to bacterial spot caused by Xanthomonas perforans race T3 in tomato. Each population consisted of 535–1 655 individuals. An infiltration method was used to inoculate the leaves of the parental and F2 plants as well as the susceptible control OH88119 for detecting hypersensitive resistance (HR). The results showed that all the tomato plants except OH88119 had HR to race T3, indicating that Xv3, Rx4, and RxLA1589 were allelic genes. Genomic DNA fragments of the Rx4 alleles from Hawaii7981, PI128216, and LA1589 were amplified using gene-specific primers and sequenced. No sequence variation was observed in the coding region of Rx4 in the three resistant lines. Based on the published map positions of these loci as well as the allelic tests and sequence data obtained in this study, we speculated that Xv3, Rx4, and RxLA1589 were the same gene. The results will provide useful information for understanding the mechanism of resistance to race T3 and developing resistant tomato varieties.