The small genomic segment of tomato spotted wilt virus isolate BR20 is necessary but not sufficient to induce lethal necrosis in Nicotiana benthamiana and local necrotic lesions in Nicotiana tabacum cv. white burley.

We observed a consistently different phenotype on Nicotiana benthamiana and Nicotiana tabacum cv. White Burley plants infected by the Brazilian Tomato spotted wilt virus (TSWV) isolate Br20 and two Italian isolates, p272 and p105/43.14. Based on the phenotype of reassortant strains derived in experimental conditions carrying the small (S) genomic segment of p272 or p105/43.14 and the medium (M) and large (L) segments of Br20, we concluded that the small segment of Br20 is required for eliciting lethal necrosis on N. benthamiana and local necrotic lesions on N. tabacum. To evaluate whether the Br20 nucleocapsid (N) or the non structural (NSs) protein encoded by the S segment was able to cause the distinctive symptoms, either alone or in combination, we transiently expressed the three allelic variants (Br20, p272 and p105/43.14) of each gene in planta through agroinfiltration. All the alleles of the N and NSs proteins abundantly accumulated in the infiltrated N. benthamiana and N. tabacum leaves. No specific necrotic reaction was observed in any of the single or combined agro-infiltrations of the Br20 alleles, suggesting that the S encoded proteins of isolate Br20 are not sufficient to elicit the specific symptomatic response and that the co-involvement of other regions of the genome might be necessary.