Viral suppression of RNA silencing by destabilization of ARGONAUTE 1

Abstract

RNA silencing is a manifestation of a ubiquitous phenomenon that acts, at least in plants and some insects, as a natural defense mechanism against viruses. As a counter-strategy, viruses have evolved to encode silencing suppressor proteins (SSPs) that can block the defense response and evade the host immunity. Although numerous SSP have been identified, little information is available on the molecular basis of their mode of action. Among SSPs, the polerovirus protein P0 functions as an F-box protein that targets an essential actor of the silencing pathway. Our work demonstrates that one of the main targets is ARGONAUTE 1 (AGO1), a key component of the RISC effector complex. By a physical interaction with AGO1, P0 mediates AGO1 protein degradation in planta. This is the first report of a plant virus that exploits components of the host ubiquitination machinery to overcome RNA silencing. This unusual mode of action may provide some clues concerning the mechanism governing phloem tropism of poleroviruses.

Acknowledgements

We would like to thank Véronique Brault for the design of the aphid.

Figures and Tables

Figure 1 Model of the process of infection by Poleroviruses. Poleroviruses are introduced into the phloem by feeding aphids having acquired virus on infected cells. The virus multiplies in companion and phloem parenchyma cells and circulates within the sieve tubes. In these cell types P0 inhibits RNA silencing by degrading AGO1. As siRNAs are not targeted by the virus, they may spread ahead of the infection front to the neighbouring cells (e.g., mesophyll cells) through the connecting plasmodesmata (P) and thereby immunize them against subsequent invasion by the virus. This immunisation process could be implicated in phloem restriction of poleroviruses. Encapsidation into virions would provide protection to the virus entering new phloem cells via the sieve tubes. vRdRp, virus-encoded RNA dependant polymerase; DCL, dicer-like protein; RISC, RNA Induced Silencing Complex; the SCF^P0 complex contains an F-box protein (here P0), a SKP protein, a Cullin (CUL1) protein and a ring finger protein (RBX).

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