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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, no RB isolates have been reported so far in China. TSWV-induced diseases rapidly increased in Yunnan, Southwest China, in recent years. No information is available about the diversity of TSWV isolates in Yunnan. In this study, we report the occurrence of natural TSWV RB variants among isolates collected in Yunnan. We initially collected a TSWV isolate from asparagus lettuce (TSWV–LE) in Yunnan in 2012. Surprisingly, this isolate of TSWV induced systemic necrosis on pepper carrying Tsw-resistance gene. We then collected novel TSWV isolates in 2015 and found that a tomato-isolate (TSWV–YN18) and a tobacco-isolate (TSWV–YN53) also overcome the Tsw-mediated resistance. TSWV–YN18 induced systemic ringspots, whereas YN53 caused systemic chlorotic mottle. Variations in the TSWV NSs protein are the key determinants associated with Tsw resistance-breaking isolates. We found that TSWV-LE NSs retains the hypersensitive response (HR) induction, whereas TSWV–YN18 and –YN53 NSs are unable to induce HR. However, the NSs of all three RB isolates suppress RNA silencing. Sequence analysis of 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 LE-NSs make it distinct from YN18 and YN53 RB isolates. The occurrence of different RB isolates and the failure of Tsw-mediated resistance control pose serious threats to domestic pepper crops in Southwest China. 
 
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