Pathogenicity of Pepper mild mottle virus Is Controlled by the RNA Silencing Suppression Activity of Its Replication Protein but Not the Viral Accumulation.

ABSTRACT Pepper mild mottle virus (PMMoV) infects pepper plants, causing mosaic symptoms on the upper developing leaves. We investigated the relationship between a virus pathogenicity determinant domain and the appearance of mosaic symptoms. Genetically modified PMMoV mutants were constructed, which had a base substitution in the 130K replication protein gene causing an amino acid change or a truncation of the 3' terminal pseudoknot structure. Only one substitution mutant (at amino acid residue 349) failed to cause symptoms, although its accumulation was relatively high. Conversely, the pseudoknot mutants showed the lower accumulation, but they still caused mosaic symptoms as severe as the wild-type virus. Therefore, the level of virus accumulation in a plant does not necessarily correlate with the development of mosaic symptoms. The activity to suppress posttranscriptional gene silencing (PTGS) was impaired in the asymptomatic mutant. Consequently, pathogenicity causing mosaic symptoms should be controlled by combat between host PTGS and its suppression by the 130K replication protein rather than virus accumulation.