Antiviral Defense Involves AGO4 in an Arabidopsis-Potexvirus Interaction

In plants, RNA silencing regulates gene expression through the action of Dicer-like (DCL) and Argonaute (AGO) proteins via micro RNAs and RNA-dependent DNA methylation (RdDM). In addition, RNA silencing functions as an antiviral defense mechanism by targeting virus-derived double-stranded RNA. Plants encode multiple AGO proteins with specialized functions, including AGO4-like proteins that affect RdDM and AGO2, AGO5, and AG01, which have antiviral activities. Here, we show that AGO4 is also required for defense against the potexvirus Plantago asiatica mosaic virus (PIAMV), most likely independent of RdDM components such as DCL3, Pol IV, and Pol V. Transient assays showed that AGO4 has direct antiviral activity on PIAMV and, unlike RdDM, this activity does not require nuclear localization of AGO4. Furthermore, although PIAMV infection causes a decrease in AGO4 expression, PIAMV causes a change in AGO4 localization from a largely nuclear to a largely cytoplasmic distribution. These results indicate an important role for AGO4 in targeting plant RNA viruses as well as demonstrating novel mechanisms of regulation of and by AGO4, independent of its canonical role in regulating gene expression by RdDM.