Enhanced resistance and neutralization of defense responses by suppressors of RNA silencing

The effects of transgenic expression of the potato virus Y (PVY) HCPro silencing suppressor in tobacco were examined on infection by several viruses. Infection by tobacco mosaic virus (TMV) was reduced at 25 °C, but not at 33 °C. By contrast, systemic infection at 33 °C by the TMV expressing green fluorescent protein was promoted by the HCPro. Infection by tobacco rattle virus (TRV) was restricted to local necrotic lesions by the PVY HCPro. However, this resistance was neutralized by expression of the cucumber mosaic virus (CMV) 2b protein from TRV. By contrast, infection by either wild-type CMV or CMV with a deletion of the 2b gene was not affected. Similarly, infection by cauliflower mosaic virus, red clover necrotic mosaic virus (both limited to infection of the inoculated leaves of tobacco) or tomato bushy stunt virus (systemically infecting tobacco) was not altered by the expression of PVY HCPro. Therefore, it appeared that the PVY HCPro was able to induce defense responses at 25 °C, but not at 33 °C, where it actually neutralized a pre-existing defense response. Moreover, the CMV 2b protein was able to neutralize a defense response activated by HCPro in combination with TRV.