Occurrence and diversity of Tomato spotted wilt virus isolates breaking the Tsw resistance gene of Capsicum chinense in Yunnan, southwest China

Widely used resistant peppers (Capsicum spp.) bearing the Tsw locus triggered the rapid emergence of resistance-breaking (RB) isolates of Tomato spotted wilt virus (TSWV) around the world. However, although TSWV-induced diseases have rapidly increased in Yunnan, southwest China, in recent years, no information is available about the diversity of TSWV isolates in this region. In this study, the occurrence of natural TSWV RB variants among isolates collected in Yunnan is reported. Initially, a TSWV isolate from asparagus lettuce (TSWV-LE) was collected in Yunnan in 2012. Surprisingly, this isolate of TSWV induced systemic necrosis on pepper carrying the Tsw resistance gene. Novel TSWV isolates, collected in 2015, included a tomato isolate (TSWV-YN18) and a tobacco isolate (TSWV-YN53) that also overcame Tsw-mediated resistance. TSWV-YN18 induced systemic ringspots, whereas TSWV-YN53 caused systemic chlorotic mottling. Variations in the TSWV nonstructural (NSs) protein are the key determinants associated with Tsw resistance-breaking isolates. It was found that TSWV-LE NSs retained the hypersensitive response (HR) induction, whereas TSWV-YN18 and TSWV-YN53 NSs were unable to induce HR. However, the NSs of all three RB isolates suppressed RNA silencing. Sequence analysis of the NSs revealed that RB isolates of Yunnan have no amino acid mutation sites common to other previously reported RB isolates. However, two amino acids (F74 and K272) on TSWV-LE NSs make it distinct from TSWV-YN18 and TSWV-YN53. The occurrence of different RB isolates and the failure of Tsw-mediated resistance control pose serious threats to domestic pepper crops in southwest China.