A strain-specific segment of the RNA-dependent RNA polymerase of grapevine fanleaf virus determines symptoms in Nicotiana species.

Factors involved in symptom expression of viruses from the genus Nepovirus in the family Secoviridae such as grapevine fanleaf virus (GFLV) are poorly characterized. To identify symptom determinants encoded by GFLV, infectious cDNA clones of RNA1 and RNA2 of strain GHu were developed and used alongside existing infectious cDNA clones of strain F13 in a reverse genetics approach. In vitro transcripts of homologous combinations of RNA1 and RNA2 induced systemic infection in Nicotiana benthamiana and Nicotiana clevelandii with identical phenotypes to WT virus strains, i.e. vein clearing and chlorotic spots on N. benthamiana and N. clevelandii for GHu, respectively, and lack of symptoms on both hosts for F13. The use of assorted transcripts mapped symptom determinants on RNA1 of GFLV strain GHu, in particular within the distal 408 nt of the RNA-dependent RNA polymerase (1E(Pol)), as shown by RNA1 transcripts for which coding regions or fragments derived thereof were swapped. Semi-quantitative analyses indicated no significant differences in virus titre between symptomatic and asymptomatic plants infected with various recombinants. Also, unlike the nepovirus tomato ringspot virus, no apparent proteolytic cleavage of GFLV protein 1E(Pol) was detected upon virus infection or transient expression in N. benthamiana. In addition, GFLV protein 1E(Pol) failed to suppress silencing of EGFP in transgenic N. benthamiana expressing EGFP or to enhance GFP expression in patch assays in WT N. benthamiana. Together, our results suggest the existence of strain-specific functional domains, including a symptom determinant module, on the RNA-dependent RNA polymerase of GFLV.