ARGONAUTE2 Mediates RNA-Silencing Antiviral Defenses againstPotato virus Xin Arabidopsis    

Abstract

2R1 (P.M.)RNA-silencing mechanisms control many aspects of gene regulation including the detection and degradation of viral RNA through the action of, among others, Dicer-like and Argonaute (AGO) proteins. However, the extent to which RNA silencing restricts virus host range has been difficult to separate from other factors that can affect virus-plant compatibility. Here we show that Potato virus X (PVX) can infect Arabidopsis (Arabidopsis thaliana), which is normally a nonhost for PVX, if coinfected with a second virus, Pepper ringspot virus. Here we show that the pepper ringspot virus 12K protein functions as a suppressor of silencing that appears to enable PVX to infect Arabidopsis. We also show that PVX is able to infect Arabidopsis Dicer-like mutants, indicating that RNA silencing is responsible for Arabidopsis nonhost resistance to PVX. Furthermore, we find that restriction of PVX on Arabidopsis also depends on AGO2, suggesting that this AGO protein has evolved to specialize in antiviral defenses.

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

“…AGO2 antiviral activity has been demonstrated for several positivesense ssRNA viruses (Garcia-Ruiz et al, 2010;Harvey et al, 2011;Jaubert et al, 2011;Scholthof et al, 2011;Zhang et al, 2012;Carbonell et al, 2012;Ma et al, 2015) but a major antiviral role for AGO5 has not previously been shown. Although AGO5 has been shown to bind CMV-and PSTVd-derived siRNAs, such an observation is not informative as most AGO proteins tested bind such sRNAs (Takeda et al, 2008;Minoia et al, 2014).…”

“…In addition, an ago7 mutant has increased susceptibility to specific derivatives of VSR-defective TCV (Qu et al, 2008), and ago4 mutants are more susceptible to TRV (Ma et al, 2015). By contrast, AGO2 appears to be broadly required for antiviral defenses, having been shown to be involved in defense against a wide range of viruses, including CMV, TCV, TRV, PVX, Turnip mosaic virus (TuMV), and Tomato bushy stunt virus (Harvey et al, 2011;Jaubert et al, 2011;Scholthof et al, 2011;Wang et al, 2011;Carbonell et al, 2012;Zhang et al, 2012;Ma et al, 2015). RISC complexes containing AGO1, 2, 3, or 5 act on viruses in in vitro Tombuvirus replication assays, and AGO1, 2, 3, 4, 5, and 9 can all bind to sRNAs derived from viruses or viroids (Takeda et al, 2008;Wang et al, 2011;Schuck et al, 2013;Minoia et al, 2014).…”

“…Although the PVX VSR, P25, destabilizes Arabidopsis AGO1, this virus does not normally systemically infect Arabidopsis Columbia-0 (Col-0) (Chiu et al, 2010;Jaubert et al, 2011). However, PVX can effectively infect Arabidopsis if a VSR is supplied in trans from a second virus or if RNA silencing is attenuated by mutation of DCL-encoding genes or AGO2 Andika et al, 2015).…”

RNAi in Plants: An Argonaute-Centered View

“…Arabidopsis AGO2 plays a major role in antiviral defense and has been shown to be required for resistance to a broad spectrum of plant viruses (Harvey et al, 2011;Jaubert et al, 2011;Wang et al, 2011b). AGO2 binds vsiRNAs with 59 terminal A (Wang et al, 2011b), and the catalytic activity of AGO2 is essential for its role in antiviral defense (Carbonell et al, 2012).…”

“…Arabidopsis AGO7 (also known as ZIP) specifically binds miR390 (Montgomery et al, 2008), which targets TAS3 transcripts (Bohmert et al, 1998;Morel et al, 2002;Vaucheret et al, 2004;Baumberger and Baulcombe, 2005;Qi et al, 2005;Zhang et al, 2006) ta-siRNAs vsiRNAs OsAGO1a Os02g45070 miRNAs Plant development, antiviral defense Os04g47870 miRNAs Plant development, antiviral defense (Wu et al, , 2015 (Takeda et al, 2008;Tucker et al, 2012;Brosseau and Moffett, 2015) vsiRNAs OsAGO5c Os03g58600 phasiRNAs Germ cell division (Nonomura et al, 2007;Komiya et al, 2014) AtAGO10 AT5G43810 miR165/166 SAM development, antiviral defense (Moussian et al, 1998;Lynn et al, 1999;Liu et al, 2009;Ji et al, 2011;Zhu et al, 2011;Garcia-Ruiz et al, 2015;Zhou et al, 2015) miR172 vsiRNAs (Harvey et al, 2011;Jaubert et al, 2011;Wang et al, 2011b;Zhang et al, 2011;Wei et al, 2012) vsiRNAs miR393* AtAGO3 AT1G31290 n.d. n.d. AtAGO7 AT1G69440 miR390 ta-siRNA biogenesis, phase transition, antiviral defense (Hunter et al, 2003;Adenot et al, 2006;Montgomery et al, 2008;Qu et al, 2008) OsAGO7 Os03g33650 miR390 ta-siRNA biogenesis, SAM development (Nagasaki et al, 2007) ZmAGO7 GRMZM5G892991 miR390 ta-siRNA biogenesis, leaf development (Douglas et al, 2010) AtAGO4 AT2G27040 hc-siRNAs DNA methylation, antibacterial immunity, defense against DNA viruses (Zilberman et al, 2003;Li et al, 2006;Pontes et al, 2006;…”