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BIOLOGICAL SCIENCES / PLANT BIOLOGY
RNA silencing of host transcripts by cauliflower mosaic virus requires coordinated action of the four Arabidopsis Dicer-like proteins

Guillaume Moissiard, and Olivier Voinnet^*

Institut de Biologie Moléculaire des Plantes, Centre National de la Recherche Scientifique, Unité Propre de Recherche 2357, 12 Rue du Général Zimmer, 67084 Strasbourg Cedex, France

Edited by David Baulcombe, The Sainsbury Laboratory, Norwich, United Kingdom, and approved September 20, 2006 (received for review June 3, 2006)

RNA silencing is an ancient mechanism of gene regulation with important antiviral roles in plants and insects. Although induction of RNA silencing by RNA viruses has been well documented in plants, the interactions between DNA viruses and the host silencing machinery remain poorly understood. We investigate this question with cauliflower mosaic virus (CaMV), a dsDNA virus that expresses its genome through the polycistronic 35S RNA, which carries an unusually extensive secondary structure known as translational leader. We show that CaMV-derived siRNAs accumulate in turnip- and Arabidopsis-infected plants and that the leader is a major, albeit not exclusive, source for those molecules. Biogenesis of leader-derived siRNA requires the coordinated and hierarchical action of the four Arabidopsis Dicer-like (DCL) proteins. Our study also uncovers a "facilitating" role exerted by the microRNA biosynthetic enzyme DCL1 on accumulation of DCL2-, DCL3-, and DCL4-dependent siRNAs derived from the 35S leader. This feature of DCL1 defines a small RNA biosynthetic pathway that might have relevance for endogenous gene regulation. Several leader-derived siRNAs were found to bear near-perfect sequence complementarity to Arabidopsis transcripts, and, using a sensor transgene, we provide direct evidence that at least one of those molecules acts as a bona fide siRNA in infected turnip. Extensive bioinformatics searches identified >100 transcripts potentially targeted by CaMV-derived siRNAs, several of which are effectively down-regulated during infection. The implications of virus-directed silencing of host gene expression are discussed.

Author contributions: O.V. designed research; G.M. performed research; G.M. and O.V. analyzed data; and O.V. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS direct submission.

*To whom correspondence should be addressed. E-mail: olivier.voinnet{at}ibmp-ulp.u-strasbg.fr

© 2006 by The National Academy of Sciences of the USA

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