RNA silencing is a conserved defense mechanism against viruses and transposons (15, 46, 59, 60, 61, 62, 67). This mechanism, discovered in a wide variety of eukaryotic organisms, has been termed posttranscriptional gene silencing (PTGS) in plants, quelling in Neurospora, and RNA interference (RNAi) in Caenorhabditis elegans and Drosophila melanogaster (7, 8, 12, 14, 17, 22, 25, 27, 47, 59). The pathway is initially triggered by double-stranded RNAs (dsRNAs), which can be generated from exogenously introduced or endogenous transposons, transgenes, and replicating viral RNA intermediates (2, 13, 17, 20, 28). The dsRNA is recognized and cleaved into small interfering RNAs (siRNAs) of 20 to 25 nucleotides (nt) by an RNase III-like RNase called Dicer (4, 27, 56, 69, 71). The siRNAs are subsequently incorporated into an RNase complex called RNA-induced silencing complex (RISC). The siRNAs direct the RISC to target RNAs by sequence-specific base pairing. Cleavage by RISC results in elimination of the target mRNA (16, 19, 27, 40, 42, 49). In plants, fungi, and C. elegans, RNA silencing exhibits an intriguing feature: it is non-cell autonomous, which means that RNA silencing originated at one site can transmit to remote cells or tissues to cause systemic RNA silencing. In a plant, for instance, RNA silencing signals can spread between cells through the plasmodesmata and over long distances via the vascular system to silence expression of a target gene throughout the plant (23, 43, 62, 63, 65). The exact nature of RNA silencing signals remains to be elucidated. However, RNA is likely a key component to confer sequence specificity in RNA silencing (29, 41). Recent studies suggested that RNA silencing might play a more extended role in regulation of gene expression than expected. For instance, RNA silencing can down-regulate the expression of chalcone synthase (CHS) genes in the way of natural occurrence that results in the inhibition of seed coat pigmentation in Glycine max (soybean) (50, 57). Correspondingly, an RNA silencing suppressor can alter the phenotype of the seed coat color via suppression of RNA silencing (50, 57). Many plant viruses have evolved a suppressor or suppressors of RNA silencing to counteract RNA silencing (37, 45, 48, 52). RNA silencing is a multistep process. Correspondingly, suppressors identified so far can interfere with this process at different steps. For instance, the helper component-proteinase (HC-Pro) of potyvirus, which was one of the first suppressors identified, interferes with RNA silencing at a step upstream of the production of siRNA (6, 36, 38). Recent studies showed that HC-Pro also affects microRNA (miRNA) biogenesis and function (10, 31, 52, 68). These results suggest that the mechanisms of HC-Pro function are complex and remain to be fully understood (3). On the other hand, the 2b protein encoded by Cucumber mosaic virus (CMV) could prevent spread of RNA silencing signals by blocking their translocation (6, 23). The p25 protein of Potato virus X (PVX) interrupts transmission of RNA silencing signals by preventing their formation (64). The p21 protein of Beet yellow virus and p19 protein of tombusvirus bind to and presumably inactivate siRNA (10, 51). Recent studies showed that p69 encoded by Turnip yellow mosaic virus suppresses RNA silencing by targeting a step upstream of dsRNA formation in the cellular RNA polymerase-dependent branch of RNA silencing (11). Over 20 suppressors encoded by both plant and animal viruses have been identified to date (15, 33, 48, 52, 62). The diversity of currently known viral suppressors in their sequences and activities suggest that novel RNA silencing suppressors are yet to be identified, and that continuing studies on the functions of viral suppressors should contribute significantly to our understanding of the basic mechanisms of RNA silencing as well as virus-host interactions (45, 62, 67). Viruses with dsRNA genomes are of unique importance in terms of host defense and viral counterdefense. When they are not encapsidated, these dsRNA genomes are conceivably the immediate trigger and target of host RNA silencing pathways. Whether these viruses have evolved unique or common antisilencing strategies is an outstanding question in virology. The protein σ3, encoded by a mammalian reovirus, has been shown to function as an RNA silencing suppressor (35). σ3 is the outer shell protein and can bind to dsRNA. For plant dsRNA viruses, however, RNA silencing suppressors have not been identified. We use Rice dwarf phytoreovirus (RDV) as a model system to address this question. RDV is a member of the genus Phytoreovirus, belonging to the family Reoviridae (5, 70). It replicates in rice as well as in insect vectors (Nephotettix cincticeps or Resilia dorsalis) (53). The RDV genome consists of 12 dsRNAs (S1 to S12) which encode at least seven structural proteins, P1, P2, P3, P5, P7, P8, and P9, as well as five nonstructural proteins, Pns4, Pns6, Pns10, Pns11, and Pns12 (70, 72, 73, 74). The processes and viral proteins involved in viral particle assembly have been intensively studied (24, 73). For the functions of nonstructural proteins, recent evidence demonstrated that Pns11 is a nucleic acid binding protein and Pns6 is a cell-to-cell movement protein (34). Here we present data showing that RDV Pns10 is a suppressor of RNA silencing. It may function at a step upstream of dsRNA production and prevents the spread of systemic RNA silencing signals.