A novel tomato spotted wilt virus isolate encoding a noncanonical NSm C118F substitution associated with Sw-5 tomato gene resistance breaking

The nonstructural protein NSm of tomato spotted wilt virus (TSWV) has been identified as the avirulence determinant of the tomato single dominant Sw-5 resistance gene. Although Sw-5 effectiveness has been shown for most TSWV isolates, the emergence of resistance-breaking (RB) isolates has been observed. It is strongly associated with two point mutations (C118Y or T120N) in the NSm viral protein. TSWV-like symptoms were observed in tomato crop cultivars (+Sw-5) in the Baja California peninsula, Mexico, and molecular methods confirmed the presence of TSWV. Sequence analysis of the NSm 118–120 motif and three-dimensional protein modelling exhibited a noncanonical C118F substitution in seven isolates, suggesting that this substitution could emulate the C118Y-related RB phenotype. Furthermore, phylogenetic and molecular analysis of the full-length genome (TSWV-MX) revealed its reassortment-related evolution and confirmed that putative RB-related features are restricted to the NSm protein. Biological and mutational NSm 118 residue assays in tomato (+Sw-5) confirmed the RB nature of TSWV-MX isolate, and the F118 residue plays a critical role in the RB phenotype. The discovery of a novel TSWV-RB Mexican isolate with the presence of C118F substitution highlights a not previously described viral adaptation in the genus Orthotospovirus, and hence, the necessity of further crop monitoring to alert the establishment of novel RB isolates in cultivated tomatoes.