Resistance to Tomato spotted wilt virus in Tomato Conferred by Sw-5

Economic considerations have promoted the goal of TSWV-resistant tomato varieties in plant breeding programs for nearly 70 years. Early genetic studies reported five genes, Sw-1a, Sw-1b, sw-2, sw-3, and sw-5, from two species, Solanum pimpinellifolium and Solanum lycopersicum, all of which were overcome quickly. Sw-5, introgressed from Solanum peruvianum into tomato, has demonstrated broad and stable resistance. In resistant genotypes, local necrotic lesions develop on inoculated tissue, and systemic movement of the virus is restricted. The Sw-5 locus was isolated by positional cloning and sequenced, revealing that the resistance allele encodes a CC-NBS-LRR R protein. Sw-5 is remarkably similar to the tomato Mi gene for nematode resistance with the exception of four heptad amphipathic leucine zippers at the N terminus. This pronounced similarity suggests that Sw-5 and Mi may share a common signal transduction pathway. Sw-5 and its paralogs were mapped to tomato chromosome 9 and chromosome 12 with other fungal, viral, and bacterial R genes. A comparative analysis with the genus Capsicum, which is considerably diverged from Solanum within the tribe Solanae, indicated that paralog position was largely conserved between these genera. In Capsicum, monogenic dominant TSWV resistance conferred by Tsw showed identical resistance phenotype and strain-specificity to Sw5, but no cross-hybridization with Sw5 was detected. When resistance-breaking TSWV strains were analyzed, avirulence determinants mapped to different subgenomic RNAs.