The tomato gene Sw-5 confers resistance against tospovirus species and codes for a nucleotide binding site and leucine rich protein. Tomatoes with the Sw-5 gene develop a necrotic reaction when mechanically inoculated with tospoviruses. The necrotic lesions on inoculated leaves of the resistant plants and the structure of the protein codified by Sw-5 suggests that the resistance depends on recognition of the pathogen and activation of hypersensitive response (HR). The capacity of the Sw-5 to confer resistance in wild tobacco (Nicotiana benthamiana) was evaluated in transgenic plants transformed by Agrobacterium tumefaciens. The Sw-5 ORF and its own 3' UTR region were placed under 35S promoter control. Plants of R1 progenies were inoculated with tospovirus and evaluated for local and systemic symptoms. In one progeny with 3:1 (resistant:susceptible) segregation ratio a homozygous plant was selected and the resistance spectrum of its progeny was evaluated. Transgenic plants showed hypersensitive response 48 h after inoculations and were resistant to tospovirus infection. The resistance was isolate-specific and a Tomato chlorotic spot virus (TCSV) isolate caused systemic necrosis in the transformed plants, while Groundnut ringspot virus (GRSV) isolates and one Chrysanthemum stem necrosis virus (CSNV) related isolate was restricted to the inoculation site. Comparisons of the resistance spectrum with that observed in other members of the Solanaceae suggest that the signal transduction pathways and resistance responses triggered by Sw-5 are conserved in Solanaceae, and the genetic polymorphism in the signal transduction pathways or defense response components may result in different resistance levels.
virus resistance; heterologous expression; signal transduction
