Escape of a plant virus from amplicon-mediated RNA silencing is associated with biotic or abiotic stress.

Strong RNA silencing was induced in plants transformed with an amplicon consisting of full-length cDNA of potato leafroll virus (PLRV) expressing green fluorescent protein (GFP), as shown by low levels of PLRV-GFP accumulation, lack of symptoms and accumulation of amplicon-specific short interfering RNAs (siRNAs). Inoculation of these plants with various viruses known to encode silencing suppressor proteins induced a striking synergistic effect leading to the enhanced accumulation of PLRV-GFP, suggesting that it had escaped from silencing. However, PLRV-GFP escape also occurred following inoculation with viruses that do not encode known silencing suppressors and treatment of silenced plants with biotic or abiotic stress agents. We propose that viruses can evade host RNA-silencing defences by a previously unrecognized mechanism that may be associated with a host response to some types of abiotic stress such as heat shock.