A viral double-stranded RNA up regulates the fungal virulence of Nectria radicicola

Double-stranded RNAs (dsRNAs) are widespread in plant pathogenic fungi, but their functions in fungal hosts remain mostly unclear, with a few exceptio ns. We analyzed dsRNAs from Nectria radicicola, the causal fungus of ginsen g root rot. Four distinct sizes of dsRNAs, 6.0, 5.0, 2.5, and 1.5 kbp, were detected in 24 out of the 81 strains tested. Curing tests of individual ds RNAs suggested that the presence of 6.0-kbp dsRNA was associated with high levels of virulence, sporulation, laccase activity, and pigmentation in thi s fungus. The 6.0-kbp dsRNA-cured strains completely lost virulence-related phenotypes. This 6.0-kbp:dsRNA was reintroduced by hyphal anastomosis to a dsRNA-cured strain marked with hygromycin resistance, which resulted in th e restoration of virulence-related phenotypes, These results strongly sugge st that 6.0-kbp dsRNA: up regulates fungal virulence in N, radicicola, Sequ encing of several cDNA clones derived from 6.0-kbp dsRNA revealed the prese nce of a RNA-dependent RNA polymerase (RDRP) gene. Phylogenetic analysis sh owed that this gene is closely related to those of plant cryptic viruses. : Biochemical analyses suggested that the 6.0-kbp dsRNA may-regulate fungal v irulence through signal-transduction pathways involving cyclic AMP-dependen t protein kinase and protein kinase C.