Dicer functions transcriptionally and posttranscriptionally in a multilayer antiviral defense.

Abstract

In antiviral RNA interference (RNAi), Dicer plays a primary role in processing double-stranded RNA (dsRNA) molecules into small-interfering RNAs (siRNAs) that guide Argonaute effectors to posttranscriptional suppression of target viral genes. Here, we show a distinct role for Dicer in the siRNA-independent transcriptional induction of certain host genes upon viral infection in a filamentous fungus. Previous studies have shown that the two key players, dicer-like 2 (<i>dcl2</i>) and argonaute-like 2 (<i>agl2</i>), of antiviral RNAi in a phytopathogenic ascomycete, <i>Cryphonectria parasitica</i>, are highly transcriptionally induced upon infection with certain RNA mycoviruses, including the positive-stranded RNA hypovirus mutant lacking the RNAi suppressor (<i>Cryphonectria</i> hypovirus 1-Δp69, CHV1-Δp69). This induction is regulated by the Spt-Ada-Gcn5 acetyltransferase (SAGA) complex, a well-known transcriptional coactivator. The present study shows that diverse host genes, in addition to <i>dcl2</i> and <i>agl2</i>, were up-regulated more than 10-fold by SAGA upon infection with CHV1-Δp69. Interestingly, DCL2, but not AGL2, was essential for SAGA-mediated global gene up-regulation. Moreover, deletion of certain virus-induced genes enhanced a CHV1-Δp69 symptom (growth rate) but not its accumulation. Constitutive, modest levels of <i>dcl2</i> expression drastically reduced viral siRNA accumulation but were sufficient for full-scale up-regulation of host genes, suggesting that high induction of <i>dcl2</i> and siRNA production are not essential for the transcriptional up-regulation function of DCL2. These data clearly demonstrate the dual functionality of DCL2: as a dsRNA-specific nuclease in posttranscriptional antiviral RNA silencing and as a key player in SAGA-mediated host gene induction, which independently represses viral replication and alleviates virus-induced symptom expression.