Argonaute 2 Controls Antiviral Activity against Sweet Potato Mild Mottle Virus in Nicotiana benthamiana.

Erzsébet Kenesi,Lóránt Lakatos,László Orosz,József Burgyán,Juan-Jose Lopez-Moya

doi:10.3390/plants10050867

Abstract

RNA silencing is a sequence specific post-transcriptional mechanism regulating important biological processes including antiviral defense in plants. Argonaute (AGO) proteins, the catalytic subunits of the silencing complexes, are loaded with small RNAs to execute the sequence specific RNA cleavage or translational inhibition. Plants encode several AGO proteins and a few of them, especially AGO1 and AGO2, have been shown to be required for antiviral silencing. Previously, we have shown that the P1 protein of the sweet potato mild mottle virus (SPMMV) suppresses the primary RNA silencing response by inhibiting AGO1. To analyze the role of AGO2 in antiviral defense against the SPMMV, we performed a comparative study using a wild type and ago2−/− mutant Nicotiana benthamiana. Here we show that the AGO2 of N. benthamiana attenuates the symptoms of SPMMV infection. Upon SPMMV infection the levels of AGO2 mRNA and protein are greatly increased. Moreover, we found that AGO2 proteins are loaded with SPMMV derived viral small RNAs as well as with miRNAs. Our results indicate that AGO2 protein takes over the place of AGO1 to confer antiviral silencing. Finally, we provide a plausible explanation for the AGO2 mediated recovery of an SPMMV-infected sweet potato.

Highlights

RNA silencing refers to one of the cellular pathways that regulates gene expression through specific mechanisms acting on RNA and is based on the sequence specificity of nucleic acids
To test whether AGO2 plays a role in antiviral defense against the sweet potato mild mottle virus (SPMMV), wild type and AGO2−/− mutant N. benthamiana plants were infected with the SPMMV
AGO2 was induced by SPMMV infection (Figure 3b,c) and the input and eluate fractions of SPMMV-infected wt plants contained viral siRNAs (vsRNAs) providing strong evidence for a physical interaction with the AGO2 protein (Figure 4a; lanes 4 and 5)

Summary

Introduction

RNA silencing refers to one of the cellular pathways that regulates gene expression through specific mechanisms acting on RNA and is based on the sequence specificity of nucleic acids. RNA silencing is considered to have evolved to regulate gene expression both in the nucleus and the cytoplasm of almost all eukaryotes. The replicative forms of ssRNA viruses or other sources of dsRNA such as regions with strong secondary structures result in double-stranded RNA that triggers RNA silencing. This viral dsRNA is recognized and processed into viral siRNAs (vsRNAs) by the Dicer-like (DCL) enzymes. Plant viruses evolved viral suppressors of RNA silencing (VSR) proteins to counteract This viral dsRNA is recognized and processed into viral siRNAs (vsRNAs) by the Dicer-like (DCL) enzymes. vsRNAs are associated with argonaute (AGO) proteins, which are the central molecule of the RNA induced silencing complex (RISC) and guide the RISC to the viral nucleic acids for target cleavage or translational inhibition [1].

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