Effects of dicer-like endoribonucleases 2 and 4 on infection of Arabidopsis thaliana by cucumber mosaic virus and a mutant virus lacking the 2b counter-defence protein gene

In tobacco and Nicotiana benthamiana, limited cross-protection against cucumber mosaic virus strain Fny (Fny-CMV) was provided by prior inoculation with a deletion mutant lacking the 2b silencing-suppressor protein gene (Fny-CMVΔ2b). Cross-protection by Fny-CMVΔ2b did not result from induction of systemic RNA silencing. We investigated whether protection occurs through induction of localized RNA silencing by using Arabidopsis thaliana plants harbouring mutations in genes encoding the dicer-like (DCL) endoribonucleases 2, 3 and 4 involved in antiviral silencing. In wild-type A. thaliana (Col-0) plants, Fny-CMVΔ2b was symptomless and cross-protected against Fny-CMV infection. Cross-protection by Fny-CMVΔ2b against Fny-CMV infection was not abolished in dcl2, dcl3 or dcl4 mutant plants and was strongest in dcl2/4 double mutants, although in these plants and in dcl4 mutants, Fny-CMVΔ2b replicated to high levels and induced strong symptoms. The results suggest that Fny-CMVΔ2b/Fny-CMV cross-protection is not completely dependent on RNA silencing and also involves competition between these viruses.