AGO2: A New Argonaute Compromising Plant Virus Accumulation

Abstract

Plant viruses use several strategies to transport their nucleic acid genomes throughout the plants. Regardless of the movement mechanism, a universal major block to uninterrupted viral trafficking is the induction of antiviral silencing that degrades viral RNA. To counteract this defense, viruses encode suppressors that block certain steps in the RNA silencing pathway, and consequently these proteins allow viral spread to proceed. There is a constant battle between plants and viruses and sometimes viruses will succeed and invade the plants and in other cases the RNA silencing mechanism will override the virus. A key role in the silencing versus suppression conflict between plants and viruses is played by one or more members of the Argonaute protein (AGO) family encoded by plants. Here we review the mechanisms and effects of antiviral silencing with an emphasis on the contribution of AGOs, especially the recently discovered role of AGO2.

Keywords: Argonaute; RNA silencing; movement; plant; suppressor; virus.

Figure 1

The antiviral RNA silencing model as presently understood. During viral replication or transcription of DNA or RNA viruses, highly structured RNA or double-stranded (ds) RNA is formed providing a substrate for cleavage by Dicer-like proteins (DCL) associated with dsRNA binding protein (DRB). This generates duplex short-interfering RNAs (siRNAs) and upon methylation by HUA enhancer 1 (HEN1) these associate with an antiviral RNA-induced silencing complex (vRISC), or in absence of methylation these will be polyuridylated and degraded. One siRNA strand remains bound to an Argonaute protein (AGO) in vRISC and this operates as a search-and-strike module to cleave single-stranded (ss) RNA complementary to the siRNA. The resulting RNA fragments are either degraded via XRN4 or re-amplified by concerted action of SGS3, SDE3, SDE5, and an RNA-dependent RNA polymerase (RDR). The vRISC (star-shaped) associated AGO is indicated in red, whereas other possible AGO-mediated interactions alluded to in the text are denoted by ?AGO? Figure modified from Alvarado and Scholthof (2009).

Figure 2

AGO2-mediated silencing of Tomato bushy stunt virus RNA in Nicotiana benthamiana. Plants were agroinfiltrated to express Tobacco rattle virus (TRV) as a virus-induced gene silencing vector containing either no insert (00-control) or an AGO2 fragment (AGO2-silenced). At ~40 days post-infiltration, upper leaves of these plants were infiltrated to express TBSV–GFP (TG) or TBSV–GFP devoid of P19 translation (TGdP19); images of leaves were captured 5 days later under UV illumination. Evidently when AGO2 expression is silenced, GFP expression from TGdP19 (only infiltrated in one half of the leaf) is maintained indicating the absence of effective silencing when compared to that in the 00-control. Details can be found in Scholthof et al. (2011), that also describes the eventual complete abolishment of GFP expression in TGdP19-infected 00-control plants at later time points, while expression in AGO2-silenced plants is maintained stably.