Differences in Virulence among PVY Isolates of Different Geographical Origins When Infecting an Experimental Host under Two Growing Environments Are Not Determined by HCPro

The contribution of the HCPro factors expressed by several PVY isolates of different geographical origins (one from Scotland, one from Spain, and several from Tunisia) to differences in their virulence in Nicotiana benthamiana plants was investigated under two growing conditions: standard (st; 26 °C and current ambient levels of CO(2)), and climate change-associated (cc; 31 °C and elevated levels of CO(2)). In all cases, relative infection symptoms and viral titers were determined. The viral HCPro cistrons were also sequenced and amino-acid features of the encoded proteins were established, as well as phylogenetic distances. Additionally, the abilities of the HCPros of several isolates to suppress silencing were assessed under either growing condition. Overall, viral titers and infection symptoms decreased under cc vs. st conditions. However, within each growing condition, relative titers and symptoms were found to be isolate-specific, with titers and symptom severities not always correlating. Crucially, isolates expressing identical HCPros displayed different symptoms. In addition, all HCPro variants tested displayed comparable silencing suppression strengths. Therefore, HCPro alone could not be the main determinant of the relative differences in pathogenicity observed among the PVY isolates tested in this host, under the environments considered.