A single amino acid residue of RNA-dependent RNA polymerase in the Potato virus X genome determines the symptoms in Nicotiana plants

A Potato virus X (PVX) strain, PVX-OS, causes a necrotic mosaic in Nicotiana benthamiana and ring spot mosaic in N. tabacum cv. SamsunNN. By contrast, strain PVX-BS causes a mild mosaic in N. benthamiana and systemic asymptomatic infection in N. tabacum cv. SamsunNN. To investigate the viral determinant of this difference, we produced various infectious cDNA clones chimeric between these PVX genomes and clones with point mutations introduced by site-directed mutagenesis. Inoculation tests with these clones mapped the symptom determinant in Nicotiana plants to the 1422 amino acid residue in the region of the C-terminus of RNA-dependent RNA polymerase (RdRp). Western blot analysis and local lesion assay indicated that virus accumulation in the infected leaves was similar for these PVX strains, suggesting that the symptom difference was not due to virus accumulation.