viral silencing suppressors: Tools forged to fine-tune host-pathogen coexistence

Abstract

RNA silencing is a homology-dependent gene inactivation mechanism that regulates a wide range of biological processes including antiviral defense. To deal with host antiviral responses viruses evolved mechanisms to avoid or counteract this, most notably through expression of viral suppressors of RNA silencing. Besides working as silencing suppressors, these proteins may also fulfill other functions during infection. In many cases the interplay between the suppressor function and other "unrelated" functions remains elusive. We will present host factors implicated in antiviral pathways and summarize the current status of knowledge about the diverse viral suppressors' strategies acting at various steps of antiviral silencing in plants. Besides, we will consider the multi-functionality of these versatile proteins and related biochemical processes in which they may be involved in fine-tuning the plant-virus interaction. Finally, we will present the current applications and discuss perspectives of the use of these proteins in molecular biology and biotechnology.

Functional and Genetic Analysis Identify a Role for Arabidopsis ARGONAUTE5 in Antiviral RNA Silencing

“…VSRs may affect different steps of RNA silencing by sequestering sRNAs or by inhibiting RISC assembly, sRNA methylation, or silencing-signal amplification (Pumplin and Voinnet, 2013). In addition, multiple VSR proteins have been shown to directly target AGO proteins, by either destabilizing AGO proteins directly or by inhibiting RISC formation (Csorba et al, 2015). The triple gene block protein 1 (TGB1) of Potato virus X (PVX), also known as P25, suppresses movement of a systemic silencing signal (Voinnet et al, , 2000.…”

Biochemical and genetic functional dissection of the P38 viral suppressor of RNA silencing

“…In plants and invertebrates, antiviral defense relies upon RNA silencing pathway components (Pumplin and Voinnet 2013) and, as a counter-defensive strategy, viral suppressors of RNA silencing (VSRs) have been evolved independently by diverse virus genera (Csorba et al 2015).…”

“…Potent substrates for vsiRNA production include viral fold-back RNA structures and long dsRNAs synthesized by viral RNA-dependent RNA polymerases (RDRs) as part of the replication process, and by additional activities of hostencoded RDRs including, chiefly, Arabidopsis RDR6, RDR1 and, to some extent, RDR2 (Garcia-Ruiz et al 2010;Wang et al 2010;Csorba et al 2015). Antiviral PTGS also requires the cooperative and distinctive functions of AGO1 and AGO2 in Arabidopsis, while AGO5, AGO7, and AGO10 have also been shown to play roles under some circumstances (Qu et al 2008;Wang et al 2011;Zhang et al 2012;Garcia-Ruiz et al 2015).…”

“…Our current understanding of viral suppression of RNA silencing shows that binding of virus-derived siRNA is a conserved property displayed by VSRs from diverse viral genera (Csorba et al 2015). As perhaps the best-characterized VSR, the tombusviral P19 protein acts as dimeric molecular caliper that measures the length of dsRNAs and sequesters siRNA duplexes in a size-selective and sequence-independent manner (Vargason et al 2003;Ye et al 2003).…”

Antiviral Silencing and Suppression of Gene Silencing in Plants

“…vsiRNA-binding VSRs do not compromise dicing, but efficiently inhibit antiviral silencing (Csorba et al 2015). The downstream AGO-dependent effector step is therefore necessary to restrict virus replication and spread of both RNA and DNA viruses (Azevedo et al 2010;Carbonell et al 2012;Harvey et al 2011;Pantaleo et al 2007;Qu et al 2008;Raja et al 2014;Raja et al 2008;Wang et al 2011).…”