sRNA modification associated with suppression of RNA silencing by tobamovirus replicase protein

Department of Plant Physiology, Botanical Institute, University of Basel, Schoenbeinstr. 6, CH-4056 Basel, Switzerland, Institut Biologie Moléculaire des Plantes (IBMP), laboratoire propre du CNRS (UPR 2357) conventionné avec l'Université Louis Pasteur (Strasbourg 1), 12, rue du Général Zimmer, 67084 Strasbourg cedex, France

^* To whom correspondence should be addressed. Email: Manfred.heinlein{at}ibmp-ulp.u-strasbg.fr.

	Abstract

Plant viruses act as triggers and targets of RNA silencing and have evolved proteins to suppress this plant defense response during infection. Although Tobacco mosaic tobamovirus (TMV) triggers the production of virus-specific siRNAs, this does not lead to efficient silencing of TMV nor is a TMV-GFP hybrid able to induce silencing of a GFP-transgene in Nicotiana benthamiana, indicating that a TMV silencing suppressor is active and acts downstream of siRNA production. On the other hand, TMV-GFP is unable to spread into cells in which GFP silencing is established, suggesting that the viral silencing suppressor cannot revert already established silencing. Although previous evidence indicates that the tobamovirus silencing suppressing activity resides in the viral 126 kDa small replicase subunit, the mechanism of silencing suppression by this virus family is not known. Here, we connect the silencing suppressing activity of this protein with our previous finding that Oilseed rape mosaic tobamovirus infection leads to interference with HEN1-mediated methylation of siRNA and miRNA. We demonstrate that TMV infection similarly leads to interference with HEN1-mediated methylation of small RNAs and that this interference and the formation of virus-induced disease symptoms are linked to the silencing suppressor activity of the 126 kDa protein. Moreover, we show that also Turnip crinkle virus (TCV) interferes with methylation of siRNA but, in contrast to tobamoviruses, not with methylation of miRNA.