Rice dwarf phytoreovirus segment S11 encodes a nucleic acid binding protein

A non-structural protein encoded by Rice Dwarf Virus targets to the nucleus and chloroplast and inhibits local RNA silencing

RNA silencing is a potent antiviral mechanism in plants and animals. As a counter-defense, many viruses studied to date encode one or more viral suppressors of RNA silencing (VSR). In the latter case, how different VSRs encoded by a virus function in silencing remains to be fully understood. We previously showed that the nonstructural protein Pns10 of a Phytoreovirus, Rice dwarf virus (RDV), functions as a VSR. Here we present evidence that another nonstructural protein, Pns11, also functions as a VSR. While Pns10 was localized in the cytoplasm, Pns11 was localized both in the nucleus and chloroplasts. Pns11 has two bipartite nuclear localization signals (NLSs), which were required for nuclear as well as chloroplastic localization. The NLSs were also required for the silencing activities of Pns11. This is the first report that multiple VSRs encoded by a virus are localized in different subcellular compartments, and that a viral protein can be targeted to both the nucleus and chloroplast. These findings may have broad significance in studying the subcellular targeting of VSRs and other viral proteins in viral-host interactions.

Nonstructural protein Pns12 of rice dwarf virus is a principal regulator for viral replication and infection in its insect vector

Plant reoviruses are thought to replicate and assemble within cytoplasmic structures called viroplasms. The molecular mechanisms underling the formation of the viroplasm during infection of rice dwarf virus (RDV), a plant reovirus, in its leafhopper vector cells remain poorly understood. Viral nonstructural protein Pns12 forms viroplasm-like inclusions in the absence of viral infection, suggesting that the viroplasm matrix is basically composed of Pns12. Here, we demonstrated that core capsid protein P3 and nonstructural protein Pns11 were recruited in the viroplasm by direct interaction with Pns12, whereas nonstructural protein Pns6 was recruited through interaction with Pns11. The introduction of dsRNA from Pns12 gene into cultured insect vector cells or intact insect strongly inhibited such viroplasm formation, preventing efficient viral spread in the leafhopper in vitro and in vivo. Thus, nonstructural protein Pns12 of RDV is a principal regulator for viral replication and infection in its insect vector.

OsRDR6 plays role in host defense against double-stranded RNA virus, Rice Dwarf Phytoreovirus

RNAi is a major antiviral defense response in plant and animal model systems. RNA-dependent RNA polymerase 6 (RDR6) is an essential component of RNAi, which plays an important role in the resistance against viruses in the model plants. We found previously that rice RDR6 (OsRDR6) functioned in the defense against Rice stripe virus (RSV), and Rice Dwarf Phytoreovirus (RDV) infection resulted in down-regulation of expression of RDR6. Here we report our new findings on the function of OsRDR6 against RDV. Our result showed that down-regulation of OsRDR6 through the antisense (OsRDR6AS) strategy increased rice susceptibility to RDV infection while over-expression of OsRDR6 had no effect on RDV infection. The accumulation of RDV vsiRNAs was reduced in the OsRDR6AS plants. In the OsRDR6 over-expressed plants, the levels of OsRDR6 RNA transcript and protein were much higher than that in the control plants. Interestingly, the accumulation level of OsRDR6 protein became undetectable after RDV infection. This finding indicated that the translation and/or stability of OsRDR6 protein were negatively impacted upon RDV infection. This new finding provides a new light on the function of RDR6 in plant defense response and the cross-talking between factors encoded by host plant and double-stranded RNA viruses.

[Plant-infecting reoviruses]

The family Reoviridae separates two subfamilies and consists of 15 genera. Fourteen viruses in three genera (Phytoreovirus, Oryzavirus, and Fijivirus) infect plants. The outbreaks of the plant-infecting reoviruses cause sometime the serious yield loss of rice and maize, and are a menace to safe and efficient food production in the Southeast Asia.　The plant-infecting reoviruses are double-shelled icosahedral particles, from 50 to 80nm in diameter, and include from 10 to 12 segmented double-stranded genomic RNAs depending on the viruses. These viruses are transmitted in a persistent manner by the vector insects and replicated in both plants and in their vectors. This review provides a brief overview of the plant-infecting reoviruses and their recent research progresses including the strategy for viral controls using transgenic rice plants.

Life cycle of phytoreoviruses visualized by electron microscopy and tomography

Rice dwarf virus and Rice gall dwarf virus, members of the genus Phytoreovirus in the family Reoviridae,are known as agents of rice disease, because their spread results in substantial economic damage in many Asian countries. These viruses are transmitted via insect vectors, and they multiply both in the plants and in the insect vectors. Structural information about the viruses and their interactions with cellular components in the life cycle are essential for understanding viral infection and replication mechanisms. The life cycle of the viruses involves various cellular events such as cell entry, synthesis of viral genome and proteins, assembly of viral components, viral egress from infected cells, and intra- and intercellular transports. This review focuses on the major events underlying the life cycle of phytoreoviruses, which has been visualized by various electron microscopy (EM) imaging techniques, including cryo-electron microscopy and tomography, and demonstrates the advantage of the advanced EM imaging techniques to investigate the viral infection and replication mechanisms.

Non-structural protein P6 encoded by rice black-streaked dwarf virus is recruited to viral inclusion bodies by binding to the viroplasm matrix protein P9-1

Like other members of the family Reoviridae, rice black-streaked dwarf virus (RBSDV, genus Fijivirus) is thought to replicate and assemble within cytoplasmic viral inclusion bodies, commonly called viroplasms. RBSDV P9-1 is the key protein for the formation of viroplasms, but little is known about the other proteins of the viroplasm or the molecular interactions amongst its components. RBSDV non-structural proteins were screened for their association with P9-1 using a co-immunoprecipitation assay. Only P6 was found to directly interact with P9-1, an interaction that was confirmed by bimolecular fluorescence complementation assay in Spodoptera frugiperda (Sf9) cells. Immunoelectron microscopy showed that P6 and P9-1 co-localized in electron-dense inclusion bodies, indicating that P6 is a constituent of the viroplasm. In addition, non-structural protein P5 also localized to viroplasms and interacted with P6. In Sf9 cells, P6 was diffusely distributed throughout the cytoplasm when expressed alone, but localized to inclusions when co-expressed with P9-1, suggesting that P6 is recruited to viral inclusion bodies by binding to P9-1. P5 localized to the inclusions formed by P9-1 when co-expressed with P6 but did not when P6 was absent, suggesting that P5 is recruited to viroplasms by binding to P6. This study provides a model by which viral non-structural proteins are recruited to RBSDV viroplasms.

Movement protein Pns6 of rice dwarf phytoreovirus has both ATPase and RNA binding activities

Cell-to-cell movement is essential for plant viruses to systemically infect host plants. Plant viruses encode movement proteins (MP) to facilitate such movement. Unlike the well-characterized MPs of DNA viruses and single-stranded RNA (ssRNA) viruses, knowledge of the functional mechanisms of MPs encoded by double-stranded RNA (dsRNA) viruses is very limited. In particular, many studied MPs of DNA and ssRNA viruses bind non-specifically ssRNAs, leading to models in which ribonucleoprotein complexes (RNPs) move from cell to cell. Thus, it will be of special interest to determine whether MPs of dsRNA viruses interact with genomic dsRNAs or their derivative sRNAs. To this end, we studied the biochemical functions of MP Pns6 of Rice dwarf phytoreovirus (RDV), a member of Phytoreovirus that contains a 12-segmented dsRNA genome. We report here that Pns6 binds both dsRNAs and ssRNAs. Intriguingly, Pns6 exhibits non-sequence specificity for dsRNA but shows preference for ssRNA sequences derived from the conserved genomic 5'- and 3'-terminal consensus sequences of RDV. Furthermore, Pns6 exhibits magnesium-dependent ATPase activities. Mutagenesis identified the RNA binding and ATPase activity sites of Pns6 at the N- and C-termini, respectively. Our results uncovered the novel property of a viral MP in differentially recognizing dsRNA and ssRNA and establish a biochemical basis to enable further studies on the mechanisms of dsRNA viral MP functions.

Multiple functions of Rice dwarf phytoreovirus Pns10 in suppressing systemic RNA silencing

RNA silencing is a potent mechanism of antiviral defense response in plants and other organisms. For counterdefense, viruses have evolved a variety of suppressors of RNA silencing (VSRs) that can inhibit distinct steps of a silencing pathway. We previously identified Pns10 encoded by Rice dwarf phytoreovirus (RDV) as a VSR, the first of its kind from double-stranded RNA (dsRNA) viruses. In this study we investigated the mechanisms of Pns10 function in suppressing systemic RNA silencing in the widely used Nicotiana benthamiana model plant. We report that Pns10 suppresses local and systemic RNA silencing triggered by sense mRNA, enhances viral replication and/or viral RNA stability in inoculated leaves, accelerates the systemic spread of viral infection, and enables viral invasion of shoot apices. Mechanistically, Pns10 interferes with the perception of silencing signals in recipient tissues, binds double-stranded small interfering RNA (siRNAs) with two-nucleotide 3' overhangs, and causes the downregulated expression of RDR6. These results significantly deepen our mechanistic understanding of the VSR functions encoded by a dsRNA virus and contribute additional evidence that binding siRNAs and interfering with RDR6 expression are broad mechanisms of VSR functions encoded by diverse groups of viruses.

Functional and biochemical properties of Mal de Río Cuarto virus (Fijivirus, Reoviridae) P9-1 viroplasm protein show further similarities to animal reovirus counterparts

Mal de Río Cuarto virus (MRCV) is a plant virus of the genus Fijivirus within the family Reoviridae that infects several monocotyledonous species and is transmitted by planthoppers in a persistent and propagative manner. Other members of the family replicate in viral inclusion bodies (VIBs) termed viroplasms that are formed in the cytoplasm of infected plant and insect cells. In this study, the protein coded by the first ORF of MRCV segment S9 (P9-1) was shown to establish cytoplasmic inclusion bodies resembling viroplasms after transfection of Spodoptera frugiperda insect cells. In accordance, MRCV P9-1 self-associates giving rise to high molecular weight complexes when expressed in bacteria. Strong self-interaction was also evidenced by yeast two-hybrid assays. Furthermore, biochemical characterization showed that MRCV P9-1 bound single stranded RNA and had ATPase activity. Finally, the MRCV P9-1 region required for the formation of VIB-like structures was mapped to the protein carboxy-terminal half. This extensive functional and biochemical characterization of MRCV P9-1 revealed further similarities between plant and animal reovirus viroplasm proteins.

Expression of rice dwarf phytoreovirus Pns6 and the specificity analysis of its monoclonal antibodies

The genome of rice dwarf phytoreovirus (RDV) is composed of 12 double-stranded RNA segments, of which segment S6 encodes a non-structural protein Pns6 identified as the movement protein. In this report, Pns6 with a 6-histidine tag at the N-terminal was expressed in E. coli after induction under low temperature (18 degrees C) and low concentration (0.4 mmol/L and 0.2 mmol/L) of IPTG, and then purified by Ni-chelated affinity chromatography. Stability analysis indicated that the expressed HisPns6 protein was stable at 37 degrees C after 24 h treatment. This recombinant protein was then used to make monoclonal antibody. Total 18 hybridoma clones were obtained. The specificity of antibodies was tested by Western blot using native Pns6 extracted from RDV-infected rice leaves, and 15 positive clones were confirmed. Mapping of the antigenic sites of Pns6 using antibodies showed that the most sensitive antigen determinant is located in the C-terminal region (the 296th-509th amino acids) of Pns6, which is confirms bioinformatics analysis.

Disruption of a novel gene for a NAC-domain protein in rice confers resistance to Rice dwarf virus

Rice dwarf virus (RDV) is a serious viral pest that is transmitted to rice plants (Oryza sativa L.) by leafhoppers and causes a dwarfism in infected plants. To identify host factors involved in the multiplication of RDV, we screened Tos17 insertion mutant lines of rice for mutants with reduced susceptibility to RDV. One mutant, designated rim1-1, did not show typical disease symptoms upon infection with RDV. The accumulation of RDV capsid proteins was also drastically reduced in inoculated rim1-1 mutant plants. Co-segregation and complementation analyses revealed that the rim1-1 mutation had been caused by insertion of Tos17 in an intron of a novel NAC gene. The rim1-1 mutant remained susceptible to the two other viruses tested, one of which is also transmitted by leafhoppers, suggesting that the multiplication rather than transmission of RDV is specifically impaired in this mutant. We propose that RIM1 functions as a host factor that is required for multiplication of RDV in rice.

Molecular analysis of six segments of tobacco leaf enation virus, a novel phytoreovirus from tobacco

Tobacco leaf enation virus (TLEV) is a putative member of the genus Phytoreovirus within the family Reoviridae. Previous western blot analysis of structural viral proteins (apparent molecular weights of 93 kDa; 58 kDa; 48 kDa; 39 kDa and 36 kDa) associated with TLEV, isolated from infected tobacco in South Africa, suggested that these proteins may correspond to structural Wound tumor virus (WTV) proteins. To further establish the nature of this novel virus disease phenotype in tobacco, molecular characterization of six dsRNA components was undertaken. Full-length cDNA clones were obtained by an optimized modified single-primer amplification sequence-independent dsRNA cloning method. Results of this study revealed the conserved terminal sequence: 5'GG(U/C)...UGAU 3' of segments S6-S12, while adjacent to these conserved terminal sequences are imperfect inverted repeats (7-15 bp in length), both features being common to reoviruses. The complete nucleotide sequences of segments S5 (2,610 bp), S7 (1,740 bp), S8 (1,439 bp), S10 (1,252 bp), S11 (1,187 bp) and S12 (836 bp) were determined. Comparison of full-length nucleotide sequences with corresponding segments of other phytoreoviruses, Rice gall dwarf virus (RGDV), Rice dwarf virus (RDV) and WTV has shown nucleotide and predicted amino acid identities within the range of 30-60%. TLEV consistently shows a higher identity to WTV than to other phytoreovirus species where sequence data is available. Each segment had a single predicted open reading frame encoding proteins with calculated molecular weights of S5 (90.6 kDa); S7 (58.1 kDa); S8 (47.7 kDa); S10 (39.8 kDa); S11 (35 kDa) and S12 (19.5 kDa). The relatively low nucleotide and amino acid identity to other members of the genus demonstrates that TLEV is a novel phytoreovirus, distinct from the only other reported dicotyledenous-infecting WTV and is the first phytoreovirus reported to emerge in Africa.

Molecular characterization of the largest and smallest genome segments, S1 and S12, of Rice gall dwarf virus

The nucleotide sequences of segments S1 and S12 of a Chinese isolate of Rice gall dwarf virus (RGDV) were determined. This provides the first complete sequences of these segments. The complete sequence of S1, the largest genome segment of RGDV, was 4,505 nucleotides in length and was predicted to encode a large protein of 1,458 amino acids with a calculated molecular mass of nearly 166.2 kDa. The protein was related to that encoded by S1 of Rice dwarf virus (RDV; 50% identity and 67% similarity) and (to a lesser extent) to some large proteins of other reoviruses. It appears to be an RNA-dependent RNA polymerase (RdRp) and is probably present in particles as a minor core protein. S12, the smallest genome segment of RGDV, was 853 nucleotides in length, encoding a single major protein of 206 amino acids with a calculated molecular mass of nearly 23.6 kDa. This protein, though a little larger than those of RDV S11 and Wound tumor virus (WTV) S12 in size, showed some similarity to them, especially in the conserved N-terminal region and may have RNA-binding properties. Despite having a common host plant, RDV and RGDV were not more closely related to one another than either of them was to WTV. Phylogenetic analysis of the RdRp showed that members of the genus Phytoreovirus were more closely related to those of the genus Rotavirus than to any other genus within the family Reoviridae.

Sequence and function of canine herpesvirus alpha-transinducing factor and its interaction with an immediate early promoter

The sequence of the alpha-transinducing factor (alpha-TIF) of canine herpesvirus (CHV-l) was determined. Alignment of the predicted CHV-1 alpha-TIF amino acid sequence with other alpha-TIF homologues reveals a core region of similarity with divergent amino and carboxyl termini. Analysis of the CHV-1 infected cell protein 4 promoter region identified a region containing nine copies of a 52 bp repeat that showed significant up-regulation of transcription by alpha-TIF. This region contained an imperfect 'TAATGARAT' motif, the binding site for herpes simplex virus 1 alpha-TIF, with an imperfect Oct-1 binding site immediately following. The infectious laryngotracheitis virus alpha-TIF was also shown to up-regulate transcription through this region of the promoter. Transfection of CHV-1 genomic DNA failed to yield infectious virus in canine kidney cell lines. Co-transfection of genomic DNA and an alpha-TIF expression plasmid resulted in virus plaques, indicating a potential essential role for alpha-TIF in CHV-1 infection.

The spread of Rice dwarf virus among cells of its insect vector exploits virus-induced tubular structures

Various cytopathological structures, known as inclusion bodies, are formed upon infection of cultured leafhopper cells by Rice dwarf virus, a member of the family Reoviridae. These structures include tubules of approximately 85 nm in diameter which are composed of the nonstructural viral protein Pns10 and contain viral particles. Such tubular structures were produced in heterologous non-host insect cells that expressed Pns10 of the virus. These tubules, when associated with actin-based filopodia, were able to protrude from the surface of cells and to penetrate neighboring cells. A binding assay in vitro revealed the specific binding of Pns10 to actin. Infection of clusters of cells was readily apparent 5 days after inoculation at a low multiplicity of infection with the virus, even in the presence of neutralizing antibodies. However, treatment of host cells with drugs that inhibited the elongation of actin filaments abolished the extension of Pns10 tubules from the surface of cells, with a significant simultaneous decrease in the extent of infection of neighboring cells. These results together revealed a previously undescribed aspect of the intercellular spread of Rice dwarf virus, wherein the virus exploits tubules composed of a nonstructural viral protein and actin-based filopodia to move into neighboring cells.

Pns4 of rice dwarf virus is a phosphoprotein, is localized around the viroplasm matrix, and forms minitubules

Rice dwarf virus (RDV), a member of the family Reoviridae, has a 12-segmented dsRNA genome. Seven segments, designated S1, S2, S3, S5, S7, S8, and S9, encode structural proteins, while the remainder encode nonstructural proteins. One of the nonstructural proteins, Pns4, which is encoded by S4, was characterized. Pns4 was a phosphorylatable substrate in a phosphorylation assay in vivo; it associated with large cytoplasmic fibrils and formed novel minitubules in infected cultured cells of its leafhopper insect vector, as revealed by immunofluorescence and immunoelectron microscopy. Early in infection, Pns4 was detected at the periphery of the viroplasm, and it was then observed on amorphous or fibrillar inclusions, which were identified as bundles of minitubules, at later stages of infection. Since viroplasms are believed to be the site of RDV replication, the intracellular location of Pns4 suggests that this protein might be involved in the process of assembly of the RDV virion.

Pns12 protein of Rice dwarf virus is essential for formation of viroplasms and nucleation of viral-assembly complexes

Cytoplasmic inclusion bodies, known as viroplasms or viral factories, are assumed to be the sites of replication of members of the family Reoviridae. Immunocytochemical and biochemical analyses were carried out to characterize the poorly understood viroplasms of the phytoreovirus Rice dwarf virus (RDV). Within 6 h of inoculation of cells, viroplasms, namely discrete cytoplasmic inclusions, were formed that contained the non-structural proteins Pns6, Pns11 and Pns12 of RDV, which appeared to be the constituents of the inclusions. Formation of similar inclusions in non-host insect cells upon expression of Pns12 in a baculovirus system and the association of molecules of Pns12 in vitro suggested that the inclusions observed in RDV-infected cells were composed basically of Pns12. Core proteins P1, P3, P5 and P7 and core virus particles were identified in the interior region of the inclusions. In contrast, accumulation of the outer capsid proteins P2, P8 and P9 and of intact virus particles was evident in the peripheral regions of the inclusions. These observations suggest that core particles were constructed inside the inclusions, whereas outer capsid proteins were assembled at the periphery of the inclusions. Viral inclusions were shown to be the sites of viral RNA synthesis by labelling infected cells with 5-bromouridine 5'-triphosphate. The number of viroplasms decreased with time post-inoculation as their sizes increased, suggesting that inclusions might fuse with one another during the virus-propagation process. Our results are consistent with a model, proposed for vertebrate reoviruses, in which viroplasms play a pivotal role in virus assembly.

The rice dwarf virus P2 protein interacts with ent-kaurene oxidases in vivo, leading to reduced biosynthesis of gibberellins and rice dwarf symptoms

The mechanisms of viral diseases are a major focus of biology. Despite intensive investigations, how a plant virus interacts with host factors to cause diseases remains poorly understood. The Rice dwarf virus (RDV), a member of the genus Phytoreovirus, causes dwarfed growth phenotypes in infected rice (Oryza sativa) plants. The outer capsid protein P2 is essential during RDV infection of insects and thus influences transmission of RDV by the insect vector. However, its role during RDV infection within the rice host is unknown. By yeast two-hybrid and coimmunoprecipitation assays, we report that P2 of RDV interacts with ent-kaurene oxidases, which play a key role in the biosynthesis of plant growth hormones gibberellins, in infected plants. Furthermore, the expression of ent-kaurene oxidases was reduced in the infected plants. The level of endogenous GA1 (a major active gibberellin in rice vegetative tissues) in the RDV-infected plants was lower than that in healthy plants. Exogenous application of GA3 to RDV-infected rice plants restored the normal growth phenotypes. These results provide evidence that the P2 protein of RDV interferes with the function of a cellular factor, through direct physical interactions, that is important for the biosynthesis of a growth hormone leading to symptom expression. In addition, the interaction between P2 and rice ent-kaurene oxidase-like proteins may decrease phytoalexin biosynthesis and make plants more competent for virus replication. Moreover, P2 may provide a novel tool to investigate the regulation of GA metabolism for plant growth and development.

Identification of an RNA silencing suppressor from a plant double-stranded RNA virus

RNA silencing is a mechanism which higher plants and animals have evolved to defend against viral infection in addition to regulation of gene expression for growth and development. As a counterdefense, many plant and some animal viruses studied to date encode RNA silencing suppressors (RSS) that interfere with various steps of the silencing pathway. In this study, we report the first identification of an RSS from a plant double-stranded RNA (dsRNA) virus. Pns10, encoded by S10 of Rice dwarf phytoreovirus (RDV), exhibited RSS activity in coinfiltration assays with the reporter green fluorescent protein (GFP) in transgenic Nicotiana benthamiana line 16c carrying GFP. The other gene segments of the RDV genome did not have such a function. Pns10 suppressed local and systemic silencing induced by sense RNA but did not interfere with local and systemic silencing induced by dsRNA. Expression of Pns10 also increased the expression of beta-glucuronidase in transient assays and enhanced Potato virus X pathogenicity in N. benthamiana. Collectively, our results establish Pns10 as an RSS encoded by a plant dsRNA virus and further suggest that Pns10 targets an upstream step of dsRNA formation in the RNA silencing pathway.

Rice dwarf phytoreovirus segment S6-encoded nonstructural protein has a cell-to-cell movement function

Rice dwarf virus (RDV) is a member of the genus Phytoreovirus, which is composed of viruses with segmented double-stranded RNA genomes. Proteins that support the intercellular movement of these viruses in the host have not been identified. Microprojectile bombardment was used to determine which open reading frames (ORFs) support intercellular movement of a heterologous virus. A plasmid containing an infectious clone of Potato virus X (PVX) defective in cell-to-cell movement and expressing either beta-glucuronidase or green fluorescent protein (GFP) was used for cobombardment with plasmids containing ORFs from RDV gene segments S1 through S12 onto leaves of Nicotiana benthamiana. Cell-to-cell movement of the movement-defective PVX was restored by cobombardment with a plasmid containing S6. In the absence of S6, no other gene segment supported movement. Identical results were obtained with Nicotiana tabacum, a host that allows fewer viruses to infect and spread within its tissue. S6 supported the cell-to-cell movement of the movement-defective PVX in sink and source leaves of N. benthamiana. A mutant S6 lacking the translation start codon did not complement the cell-to-cell movement of the movement-defective PVX. An S6 protein product (Pns6)-enhanced GFP fusion was observed near or within cell walls of epidermal cells from N. tabacum. By immunocytochemistry, unfused Pns6 was localized to plasmodesmata in rice leaves infected with RDV. S6 thus encodes a protein with characteristics identical to those of other viral proteins required for the cell-to-cell movement of their genome and therefore is likely required for the cell-to-cell movement of RDV.

Assembly of double-shelled, virus-like particles in transgenic rice plants expressing two major structural proteins of rice dwarf virus

Rice dwarf virus (RDV) is a double-shelled particle that contains a major capsid protein (P8), a major core protein (P3), several minor core proteins, and viral genomic double-stranded RNA. Coexpression of P8 and P3 in transgenic rice plants resulted in formation of double-shelled, virus-like particles (VLPs) similar to the authentic RDV particles. The VLPs were not detected in transgenic rice plant cells expressing P8 alone. This in vivo result suggests that P8 interacted with P3 and that these two proteins provide the structural integrity required for the formation of VLPs in rice cells independently of other structural proteins, nonstructural proteins, or viral genomic double-stranded RNAs.