P0 proteins of European beet-infecting poleroviruses display variable RNA silencing suppression activity

Post-transcriptional gene silencing (PTGS), or RNA silencing, is one of the key mechanisms of antiviral defence used by plants. To counter this defence response, viruses produce suppressor proteins that are able to inhibit the PTGS pathway or to interfere with some of its function. The aim of this study was to evaluate the RNA silencing suppressor (RSS) activity of PO proteins from selected European isolates of the beet-infecting poleroviruses beet chlorosis virus (BChV) and beet mild yellowing virus (BMYV) using two different experimental systems: (i) agro-infiltration of Nicotiana benthamiana green fluorescent protein-positive plants and (ii) mechanical inoculation of Chenopodium quinoa using a beet necrotic yellow vein virus (BNYVV, genus Benyvirus) RNA3-based replicon. The results demonstrated that PO of most BMYV isolates exhibited RSS activity, although at various efficiencies among isolates. Conversely, PO of BChV isolates displayed no RSS activity in either of the two systems under the experimental conditions used. These results are the first reported evidence that PO proteins of two closely related beet poleroviruses show strain-specific differences in their effects on RNA silencing.