Overcoming host- and pathogen-mediated resistance in tomato and tobacco maps to the M RNA of Tomato spotted wilt virus

A viral genetic system was used to map the determinants of the ability of Tomato spotted wilt virus (TSWV) to overcome the R gene (Sw-5) in tomato and the resistance conferred by the nucleocapsid gene of TSWV (N gene) in tobacco. A complete set of reassortant genotypes was generated from TSWV isolates A and D. TSWV-A was able to overcome the Sw-5 gene in tomato and the TSWV N gene in tobacco, whereas TSWV-D was repressed by both forms of resistance. The ability to overcome both forms of resistance was associated with the M RNA segment of TSWV-A (M(A)). Overcoming the Sw-5 gene was linked solely to the presence of M(A), and the ability of M(A) to overcome the TSWV N gene was modified by the L RNA and the S RNA of TSWV-A, which is consistent with previous reports that suggest that the nucleocapsid gene is not the primary determinant for overcoming the nucleocapsid-mediated resistance. Sequence analysis of the M RNA segment of TSWV-A, -D, and the type isolate BR-01 revealed multiple differences in the coding and noncoding regions, which prevented identification of the resistance-breaking nucleotide sequences.