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Abstract
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.
Highlights
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
High levels of accumulation of Potato virus X (PVX) coat protein (CP) were detected in protein extracts from leaves inoculated with PVX 2.7, but not in leaves inoculated with PVX UK3 (Fig. 1B)
The Pepper ringspot virus (PepRSV) 12K Cys-rich protein is homologous to the Tobacco rattle virus (TRV) 16K protein, which has previously been shown to function as a viral suppressors of RNA silencing (VSRs) (Ghazala et al, 2008; Martin-Hernandez and Baulcombe, 2008; Martinez-Priego et al, 2008), and we find that PepRSV 12K functions efficiently as a VSR in N. benthamiana transient expression assays (Fig. 4)
Summary
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. Small RNAs are generated from dsRNA precursors by DICER-LIKE (DCL) RNase III-related enzymes and subsequently incorporated into RNAinduced silencing complexes (RISCs). The highly structured or dsRNA of viruses is targeted by DCL proteins to generate virus-derived small interfering RNAs (vsiRNAs). These vsiRNAs can be incorporated into virus-induced RISC complexes that target viral RNAs, making RNA silencing a doubly effective antiviral mechanism (Ding and Voinnet, 2007; Omarov et al, 2007). Small RNAs can serve as primers for host RNA-dependent RNA polymerases to generate additional dsRNA targets for DCL enzymes to amplify the silencing signal (Voinnet, 2008; Vaistij and Jones, 2009; Garcia-Ruiz et al, 2010)
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