The Luteovirus P4 Movement Protein Is a Suppressor of Systemic RNA Silencing.

Adriana Fusaro,Deborah Barton,Maite Vaslin,Kenlee Nakasugi,Melanie Kalischuk,Lawrence Kawchuk,Regis Correa,Peter Waterhouse,Craig Jackson

doi:10.3390/v9100294

Abstract

The plant viral family Luteoviridae is divided into three genera: Luteovirus, Polerovirus and Enamovirus. Without assistance from another virus, members of the family are confined to the cells of the host plant’s vascular system. The first open reading frame (ORF) of poleroviruses and enamoviruses encodes P0 proteins which act as silencing suppressor proteins (VSRs) against the plant’s viral defense-mediating RNA silencing machinery. Luteoviruses, such as barley yellow dwarf virus-PAV (BYDV-PAV), however, have no P0 to carry out the VSR role, so we investigated whether other proteins or RNAs encoded by BYDV-PAV confer protection against the plant’s silencing machinery. Deep-sequencing of small RNAs from plants infected with BYDV-PAV revealed that the virus is subjected to RNA silencing in the phloem tissues and there was no evidence of protection afforded by a possible decoy effect of the highly abundant subgenomic RNA3. However, analysis of VSR activity among the BYDV-PAV ORFs revealed systemic silencing suppression by the P4 movement protein, and a similar, but weaker, activity by P6. The closely related BYDV-PAS P4, but not the polerovirus potato leafroll virus P4, also displayed systemic VSR activity. Both luteovirus and the polerovirus P4 proteins also showed transient, weak local silencing suppression. This suggests that systemic silencing suppression is the principal mechanism by which the luteoviruses BYDV-PAV and BYDV-PAS minimize the effects of the plant’s anti-viral defense.

Highlights

Plants fight against viral infection with their intrinsic RNA degradation mechanism, often termed the RNA silencing machinery [1]
There is considerable sequence diversity in the 5' portion of the genomes (ORFs 0, 1 and 2) of the luteovirids, but the P0 proteins, encoded by open reading frame (ORF) 0 of the poleroviruses and enamovirus, have the conserved function of being viral suppressors of RNA silencing (VSRs) that target AGO1 for degradation [11,20]. Luteoviruses, such as barley yellow dwarf virus-PAV (BYDV-PAV), have no P0 to carry out the VSR role, so we investigated whether other proteins or RNAs encoded by BYDV-PAV confer protection against the plant’s silencing machinery
The viruses of the family Luteoviridae are strongly associated with the phloem tissues of their host plants [51], and the possession of strong locally-acting VSRs by the enamovirus and the poleroviruses might suggest that cells of the vasculature possess highly active

Summary

Introduction

Plants fight against viral infection with their intrinsic RNA degradation mechanism, often termed the RNA silencing machinery [1]. Most RNA viruses are preferentially degraded by the DCL4 protein, which explains the greater accumulation of 21-nt siRNAs in most of the plants analyzed. Proteins to single-stranded RNA molecules, promoting their destabilization [8] The efficiency of this antiviral defense system is largely dependent on a siRNA amplification step promoted by host RNA-dependent RNA polymerases (RDRs). Following AGO-mediated cleavage, RDRs, in particular RDR1 and RDR6, use target viral RNAs to make new dsRNA molecules which are, in turn, processed by DCLs, generating secondary virus-derived siRNAs [6,9]. Secondary siRNAs may presumably act as systemic silencing signals, priming antiviral responses in uninfected cells [10]. On the other hand, are able to counteract this defense system by expressing proteins that act as viral suppressors of RNA silencing (VSRs). Two major strategies used by VSRs are known: inhibiting the production or accumulation of locally-acting siRNAs in the infected tissue (local suppression) or preventing the spread of silencing signals to systemic leaves (systemic silencing suppression) [11]

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Conclusion