Investigating the role of dicer 2 (dcr2) in gene silencing and the regulation of mycoviruses in Botrytis cinerea

Botrytis cinerea, the fungus causing gray mould disease, is usually controlled by cultural and chemical methods. It would be interesting to see if mycoviruses were a feasible method for reducing fungal virulence thus controlling the disease, but first more has to be understood of the RNA silencing mechanism and whether mycoviruses can combat such defences. Analysis of the B. cinerea genome data identified two Dicer genes: dcr1 and dcr2. In other fungi, mutation or deletion of dcr2 usually leads to impaired gene silencing. Targeted gene disruption created two independent B. cinerea Δdcr2 mutants in a ku70 background. When the Δdcr2 mutants were transformed with an argininosuccinate synthetase (bcass1) silencing cassette, many of these transformants displayed arginine auxotrophy, suggesting that silencing was still functional in a Δdcr2 mutant. Transfection of the wild-type and dcr2-disrupted B. cinerea lines with Botrytis virus F (BVF) gave no readily detectable alteration in fungal growth rate or virulence. Expression of dcr2, but not dcr1, was suppressed in the wild-type at 7 days post infection with BVF, whereas in a Δdcr2 mutant, dcr1 expression was suppressed. By 28 days post BVF-infection, dcr1 and dcr2 were expressed to the elevated levels typically observed when gene silencing is induced. This shows that whilst dcr2 is not essential for gene silencing or for controlling mycovirus such as BVF, it would appear that the mycovirus BVF is able to suppress the normal expression of genes involved in the silencing pathway, at least during early stages of infection of B. cinerea.