Identification of Pns6, a putative movement protein of RRSV, as a silencing suppressor

Role of Plant Virus Movement Proteins in Suppression of Host RNAi Defense

One of the systems of plant defense against viral infection is RNA silencing, or RNA interference (RNAi), in which small RNAs derived from viral genomic RNAs and/or mRNAs serve as guides to target an Argonaute nuclease (AGO) to virus-specific RNAs. Complementary base pairing between the small interfering RNA incorporated into the AGO-based protein complex and viral RNA results in the target cleavage or translational repression. As a counter-defensive strategy, viruses have evolved to acquire viral silencing suppressors (VSRs) to inhibit the host plant RNAi pathway. Plant virus VSR proteins use multiple mechanisms to inhibit silencing. VSRs are often multifunctional proteins that perform additional functions in the virus infection cycle, particularly, cell-to-cell movement, genome encapsidation, or replication. This paper summarizes the available data on the proteins with dual VSR/movement protein activity used by plant viruses of nine orders to override the protective silencing response and reviews the different molecular mechanisms employed by these proteins to suppress RNAi.

Mycoviruses as a part of the global virome: Diversity, evolutionary links and lifestyle

Knowledge of mycovirus diversity, evolution, horizontal gene transfer and shared ancestry with viruses infecting distantly related hosts, such as plants and arthropods, has increased vastly during the last few years due to advances in the high throughput sequencing methodologies. This also has enabled the discovery of novel mycoviruses with previously unknown genome types, mainly new positive and negative single-stranded RNA mycoviruses ((+) ssRNA and (-) ssRNA) and single-stranded DNA mycoviruses (ssDNA), and has increased our knowledge of double-stranded RNA mycoviruses (dsRNA), which in the past were thought to be the most common viruses infecting fungi. Fungi and oomycetes (Stramenopila) share similar lifestyles and also have similar viromes. Hypothesis about the origin and cross-kingdom transmission events of viruses have been raised and are supported by phylogenetic analysis and by the discovery of natural exchange of viruses between different hosts during virus-fungus coinfection in planta. In this review we make a compilation of the current information on the genome organization, diversity and taxonomy of mycoviruses, discussing their possible origins. Our focus is in recent findings suggesting the expansion of the host range of many viral taxa previously considered to be exclusively fungal, but we also address factors affecting virus transmissibility and coexistence in single fungal or oomycete isolates, as well as the development of synthetic mycoviruses and their use in investigating mycovirus replication cycles and pathogenicity.

Optimized RNA-Silencing Strategies for Rice Ragged Stunt Virus Resistance in Rice

Rice ragged stunt virus (RRSV) is one of the most damaging viruses of the rice culture area in south and far-eastern Asia. To date, no genetic resistance has been identified and only expensive and non-environmentally friendly chemical treatments are deployed to fight this important disease. Non-chemical approaches based on RNA-silencing have been developed as resistance strategies against viruses. Here, we optimized classical miRNA and siRNA-based strategies to obtain efficient and durable resistance to RRSV. miRNA-based strategies are involved in producing artificial miRNA (amiR) targeting viral genomes in plants. Classically, only one amiR is produced from a single construct. We demonstrated for the first time that two amiRs targeting conserved regions of RRSV genomes could be transgenically produced in Nicotiana benthamiana and in rice for a single precursor. Transgenic rice plants producing either one or two amiR were produced. Despite efficient amiR accumulations, miRNA-based strategies with single or double amiRs failed to achieve efficient RRSV resistance in transformed rice plants. This suggests that this strategy may not be adapted to RRSV, which could rapidly evolve to counteract them. Another RNA-silencing-based method for viral resistance concerns producing several viral siRNAs targeting a viral fragment. These viral siRNAs are produced from an inverted repeat construct carrying the targeted viral fragment. Here, we optimized the inverted repeat construct using a chimeric fragment carrying conserved sequences of three different RRSV genes instead of one. Of the three selected homozygous transgenic plants, one failed to accumulate the expected siRNA. The two other ones accumulated siRNAs from either one or three fragments. A strong reduction of RRSV symptoms was observed only in transgenic plants expressing siRNAs. We consequently demonstrated, for the first time, an efficient and environmentally friendly resistance to RRSV in rice based on the siRNA-mediated strategy.

Variability, Functions and Interactions of Plant Virus Movement Proteins: What Do We Know So Far?

Of the various proteins encoded by plant viruses, one of the most interesting is the movement protein (MP). MPs are unique to plant viruses and show surprising structural and functional variability while maintaining their core function, which is to facilitate the intercellular transport of viruses or viral nucleoprotein complexes. MPs interact with components of the intercellular channels, the plasmodesmata (PD), modifying their size exclusion limits and thus allowing larger particles, including virions, to pass through. The interaction of MPs with the components of PD, the formation of transport complexes and the recruitment of host cellular components have all revealed different facets of their functions. Multitasking is an inherent property of most viral proteins, and MPs are no exception. Some MPs carry out multitasking, which includes gene silencing suppression, viral replication and modulation of host protein turnover machinery. This review brings together the current knowledge on MPs, focusing on their structural variability, various functions and interactions with host proteins.

Rice Stripe Mosaic Virus-Encoded P4 Is a Weak Suppressor of Viral RNA Silencing and Is Required for Disease Symptom Development

Viral suppressors of RNA silencing (VSRs) are a cluster of viral proteins that have evolved to counteract eukaryotic antiviral RNA silencing pathways, thereby contributing to viral pathogenicity. In this study, we revealed that the matrix protein P4 encoded by rice stripe mosaic virus (RSMV), which is an emerging cytoplasmic rhabdovirus, is a weak RNA silencing suppressor. By conducting yeast two-hybrid, bimolecular fluorescence complementation, and subcellular colocalization assays, we proved that P4 interacts with the rice endogenous suppressor of gene silencing 3 (OsSGS3). We also determined that P4 overexpression has no effect on OsSGS3 transcription. However, P4 can promote the degradation of OsSGS3 via ubiquitination and autophagy. Additionally, a potato virus X-based expression system was used to confirm that P4 enhances the development of mosaic symptoms on Nicotiana benthamiana leaves by promoting hydrogen peroxide accumulation but not cell death. To verify whether P4 is a pathogenicity factor in host plants, we generated transgenic P4-overexpressing rice plants that exhibited disease-related developmental defects including decreased plant height and excessive tillering. Our data suggest that RSMV-encoded P4 serves as a weak VSR that inhibits antiviral RNA silencing by targeting OsSGS3.

Manipulation of the Plant Host by the Geminivirus AC2/C2 Protein, a Central Player in the Infection Cycle

Geminiviruses are a significant group of emergent plant DNA viruses causing devastating diseases in food crops worldwide, including the Southern United States, Central America and the Caribbean. Crop failure due to geminivirus-related disease can be as high as 100%. Improved global transportation has enhanced the spread of geminiviruses and their vectors, supporting the emergence of new, more virulent recombinant strains. With limited coding capacity, geminiviruses encode multifunctional proteins, including the AC2/C2 gene that plays a central role in the viral replication-cycle through suppression of host defenses and transcriptional regulation of the late viral genes. The AC2/C2 proteins encoded by mono- and bipartite geminiviruses and the curtovirus C2 can be considered virulence factors, and are known to interact with both basal and inducible systems. This review highlights the role of AC2/C2 in affecting the jasmonic acid and salicylic acid (JA and SA) pathways, the ubiquitin/proteasome system (UPS), and RNA silencing pathways. In addition to suppressing host defenses, AC2/C2 play a critical role in regulating expression of the coat protein during the viral life cycle. It is important that the timing of CP expression is regulated to ensure that ssDNA is converted to dsDNA early during an infection and is sequestered late in the infection. How AC2 interacts with host transcription factors to regulate CP expression is discussed along with how computational approaches can help identify critical host networks targeted by geminivirus AC2 proteins. Thus, the role of AC2/C2 in the viral life-cycle is to prevent the host from mounting an efficient defense response to geminivirus infection and to ensure maximal amplification and encapsidation of the viral genome.

Dufulin Intervenes the Viroplasmic Proteins as the Mechanism of Action against Southern Rice Black-Streaked Dwarf Virus

Southern rice black-streaked dwarf virus (SRBSDV) causes disease in crops, which reduces the quality and yield. Several commercial antiviral agents are available to control the SRBSDV induced disease. However, the mechanism of antiviral agents controlling SRBSDV is largely unknown. Identifying targets in SRBSDV is a key step of antiviral agent discovery. Here, we investigated the potential protein target of the antiviral agent dufulin. We cloned and expressed a soluble viroplasmic P6 protein in the prokaryote Escherichia coli and the eukaryote Spodoptera frugiperda 9. The dissociation constants of dufulin with the purified P6 protein from E. coli and S. frugiperda 9 expression systems were 4.49 and 4.95 μM, respectively, indicating a strong binding affinity between dufulin and P6 protein. In vivo, dufulin significantly inhibited the expression of both P6 protein and P6 gene in the SRBSDV-infected rice leaves. This inhibition on P6 protein expression was also observed in transformed Nicotiana benthamiana where the P6 was overexpressed. Our data also showed that dufulin inhibited the duplication of SRBSDV in a dose-dependent manner in infected rice leaves with a half maximum effective concentration of 3.32 mM. It is therefore concluded that dufulin targets the viroplasmic protein P6 to inhibit the virulence of SRBSDV.

Co-infection of two reoviruses increases both viruses accumulation in rice by up-regulating of viroplasm components and movement proteins bilaterally and RNA silencing suppressor unilaterally

Background

Synergism between southern rice black-streaked dwarf virus (SRBSDV) and rice ragged stunt virus (RRSV) not only aggravates disease symptoms but also enhances their vector acquisition efficiencies by increasing both viruses’ titers in co-infected rice plants, which may exacerbate the epidemic of both viruses and cause significant damage to rice production. The molecular mechanism of viral synergism of these two viruses remains unexplored.

Methods

Single and double infection of SRBSDV and RRSV were obtained with the viruliferous white-backed planthopper and brown planthopper inoculation on four-leaf stage rice seedlings, respectively, under experimental condition. The second upper leaf from each inoculated rice plants were collected at 9, 15, and 20 days post inoculation (dpi) and used for relative quantification of 13 SRBSDV genes and 11 RRSV genes by the reverse-transcription quantitative PCR. Viral gene expression levels were compared between singly and doubly infected samples at the same stage.

Results

The movement protein and viroplasm matrix-related genes as well as the structural (capsid) protein genes of both viruses were remarkably up-regulated at different time points in the co-infected rice plants compared with the samples singly infected with SRBSDV or RRSV, however, the RNA silencing suppressor (P6) of only RRSV, but not of both the viruses, was up-regulated.

Conclusions

The SRBSDV-RRSV synergism promoted replication and movement of both viruses and inhibited the host immunity by enhancing the gene suppressing effect exerted by one of them (RRSV).

Electronic supplementary material

The online version of this article (doi:10.1186/s12985-017-0819-0) contains supplementary material, which is available to authorized users.

Evaluation of Rice Resistance to Southern Rice Black-Streaked Dwarf Virus and Rice Ragged Stunt Virus through Combined Field Tests, Quantitative Real-Time PCR, and Proteome Analysis

Diseases caused by southern rice black-streaked dwarf virus (SRBSDV) and rice ragged stunt virus (RRSV) considerably decrease grain yield. Therefore, determining rice cultivars with high resistance to SRBSDV and RRSV is necessary. In this study, rice cultivars with high resistance to SRBSDV and RRSV were evaluated through field trials in Shidian and Mangshi county, Yunnan province, China. SYBR Green I-based quantitative real-time polymerase chain reaction (qRT-PCR) analysis was used to quantitatively detect virus gene expression levels in different rice varieties. The following parameters were applied to evaluate rice resistance: acre yield (A.Y.), incidence of infected plants (I.I.P.), virus load (V.L.), disease index (D.I.), and insect quantity (I.Q.) per 100 clusters. Zhongzheyou1 (Z1) and Liangyou2186 (L2186) were considered the most suitable varieties with integrated higher A.Y., lower I.I.P., V.L., D.I. and I.Q.

FEATURES

In order to investigate the mechanism of rice resistance, comparative label-free shotgun liquid chromatography tandem-mass spectrometry (LC-MS/MS) proteomic approaches were applied to comprehensively describe the proteomics of rice varieties' SRBSDV tolerance. Systemic acquired resistance (SAR)-related proteins in Z1 and L2186 may result in the superior resistance of these varieties compared with Fengyouxiangzhan (FYXZ).

Virus altered rice attractiveness to planthoppers is mediated by volatiles and related to virus titre and expression of defence and volatile-biosynthesis genes

Viruses may induce changes in plant hosts and vectors to enhance their transmission. The white-backed planthopper (WBPH) and brown planthopper (BPH) are vectors of Southern rice black-streaked dwarf virus (SRBSDV) and Rice ragged stunt virus (RRSV), respectively, which cause serious rice diseases. We herein describe the effects of SRBSDV and RRSV infections on host-selection behaviour of vector and non-vector planthoppers at different disease stages. The Y-tube olfactometer choice and free-choice tests indicated that SRBSDV and RRSV infections altered the attractiveness of rice plants to vector and non-vector planthoppers. The attractiveness was mainly mediated by rice volatiles, and varied with disease progression. The attractiveness of the SRBSDV- or RRSV-infected rice plants to the virus-free WBPHs or BPHs initially decreased, then increased, and finally decreased again. For the viruliferous WBPHs and BPHs, SRBSDV or RRSV infection increased the attractiveness of plants more for the non-vector than for the vector planthoppers. Furthermore, we observed that the attractiveness of infected plants to planthoppers was positively correlated with the virus titres. The titre effects were greater for virus-free than for viruliferous planthoppers. Down-regulated defence genes OsAOS1, OsICS, and OsACS2 and up-regulated volatile-biosynthesis genes OsLIS, OsCAS, and OsHPL3 expression in infected plants may influence their attractiveness.

Suppression of Jasmonic Acid-Mediated Defense by Viral-Inducible MicroRNA319 Facilitates Virus Infection in Rice

MicroRNAs (miRNAs) are pivotal modulators of plant development and host-virus interactions. However, the roles and action modes of specific miRNAs involved in viral infection and host susceptibility remain largely unclear. In this study, we show that Rice ragged stunt virus (RRSV) infection caused increased accumulation of miR319 but decreased expression of miR319-regulated TCP (TEOSINTE BRANCHED/CYCLOIDEA/PCF) genes, especially TCP21, in rice plants. Transgenic rice plants overexpressing miR319 or downregulating TCP21 exhibited disease-like phenotypes and showed significantly higher susceptibility to RRSV in comparison with the wild-type plants. In contrast, only mild disease symptoms were observed in RRSV-infected lines overexpressing TCP21 and especially in the transgenic plants overexpressing miR319-resistant TCP21. Both RRSV infection and overexpression of miR319 caused the decreased endogenous jasmonic acid (JA) levels along with downregulated expression of JA biosynthesis and signaling-related genes in rice. However, treatment of rice plants with methyl jasmonate alleviated disease symptoms caused by RRSV and reduced virus accumulation. Taken together, our results suggest that the induction of miR319 by RRSV infection in rice suppresses JA-mediated defense to facilitate virus infection and symptom development.

miRNA: A Novel Link Between Rice Ragged Stunt Virus and Oryza sativa

Rice ragged stunt disease caused leads to severe loss of rice yield. Recently, rice ragged stunt virus (RRSV) were found to be increasingly common in rice-growing regions of China and Vietnam. RRSV may cause problem by interacting with microRNAs (miRNAs) of host cells and the mechanism is not clear yet. In this study we identified 11 miRNAs in response to RRSV infection and predicted their possible targets to viral RNA segments (S1-S10) through the bioinformatics analysis. Interestingly, we found that Osa-miR-168b might bind to both the CDS region and 3'UTR of S5 and S8 and target eEF-1A to inhibit the activity of host cells to facilitate RRSV replication. These results suggest that miRNAs may be a potential target for developing rice against RRSV infection.

P2 of Rice grassy stunt virus (RGSV) and p6 and p9 of Rice ragged stunt virus (RRSV) isolates from Vietnam exert suppressor activity on the RNA silencing pathway

In Vietnam, the two main viruses that cause disease in rice are the Rice grassy stunt virus (RGSV) and the Rice ragged stunt virus (RRSV). Outbreaks of these two viruses have dramatically decreased rice production in Vietnam. Because natural resistance genes are unknown, an RNAi strategy may be an alternative method to develop resistance to RGSV and RRSV. However, this strategy will be efficient only if putative silencing suppressors encoded by the two viruses are neutralized. To identify these suppressors, we used the classical green fluorescent protein (GFP) agroinfiltration method in Nicotiana benthamiana. Then, we investigated the effects of viral candidate proteins on GFP expression and GFP siRNA accumulation and their interference with the short- or long-range signal of silencing. RGSV genes s2gp1, s5gp2, and s6gp1 and RRSV genes s5gp1, s6gp1, s9gp1, and s10gp1 were selected for viral silencing suppressor investigation according to their small molecular weight, the presence of cysteines, or the presence of a GW motif in related protein products. We confirmed that protein p6 of RRSV displays mild silencing suppressor activity and affects long-range silencing by delaying the systemic silencing signal. In addition, we identified two new silencing suppressors that displayed mild activity: p2 of RGSV and p9 of RRSV.

Rice ragged stunt virus-induced apoptosis affects virus transmission from its insect vector, the brown planthopper to the rice plant

Most plant viruses that seriously damage agricultural crops are transmitted by insects. However, the mechanisms enabling virus transmission by insect vectors are poorly understood. The brown planthopper (Nilaparvata lugens) is one of the most serious rice pests, causing extensive damage to rice plants by sucking the phloem sap and transmitting viruses, including Rice ragged stunt virus (RRSV). In this study, we investigated the mechanisms of RRSV transmission from its insect vector to the rice plant in vivo using the terminal deoxynucleotidyl transferase dUTP nick-end labeling assay and RNA interference technology. RRSV induced apoptosis in the salivary gland cells of its insect vector, N. lugens. The RRSV-induced apoptosis was regulated through a caspase-dependent manner, and inhibition of the expression of N. lugens caspase-1 genes significantly interfered with virus transmission. Our findings establish a link between virus-associated apoptosis and virus transmission from the insect vector to the host plant.

Synergism between southern rice black-streaked dwarf virus and rice ragged stunt virus enhances their insect vector acquisition

Southern rice black-streaked dwarf virus (SRBSDV), a tentative species in the genus Fijivirus, family Reoviridae, is a novel rice virus transmitted by the white-backed planthopper (Sogatella furcifera). Since its discovery in 2001, SRBSDV has spread rapidly throughout eastern and southeastern Asia and caused large rice losses in China and Vietnam. Rice ragged stunt virus (RRSV) (genus Oryzavirus, family Reoviridae) is a common rice virus vectored by the brown planthopper (Nilaparvata lugens). RRSV is also widely distributed in eastern and southeastern Asia but has not previously caused serious problems in China owing to its low incidence. With SRBSDV's spread, however, RRSV has become increasingly common in China, and is frequently found in co-infection with SRBSDV. In this study, we show that SRBSDV and RRSV interact synergistically, the first example of synergism between plant viruses in the family Reoviridae. Rice plants co-infected with both viruses displayed enhanced stunting, earlier symptoms, and higher virus titers compared with singly infected plants. Furthermore, white-backed and brown planthoppers acquired SRBSDV and RRSV, respectively, from co-infected plants at higher rates. We propose that increased RRSV incidence in Chinese fields is partly due to synergism between SRBSDV and RRSV.

Cimodo virus belongs to a novel lineage of reoviruses isolated from African mosquitoes

A novel reovirus, designated Cimodo virus (CMDV), was isolated from mosquitoes collected in a rainforest region in Côte d’Ivoire. The entire genome comprised 24 835 bp divided into 12 segments ranging from 585 to 4080 bp. The icosahedral non-enveloped virions were 80 nm in diameter. Eight major viral proteins of about 150, 135, 120, 80, 66, 59, 42 and 30 kDa were identified and seven proteins were mapped to the corresponding genome segments by liquid chromatography mass spectrometry. Predicted protein genes diverged by >77 % encoded amino acids from their closest reovirus relatives. The deep phylogenetic branching suggests that CMDV defines an as-yet-unidentified genus within the subfamily Spinareovirinae.

Development of proteomic technology of shotgun and label free combined with multiple reaction monitoring to simultaneously detect southern rice black-streaked dwarf virus and rice ragged stunt virus

The co-infection of rice caused by southern rice black-streaked dwarf virus (SRBSDV) and rice ragged stunt virus (RRSV) was widely found at many regions, such as Yunnan Province, China, and North and Central Vietnam. These rice viral diseases lead to seriously yield loss of rice. In this study, the proteomics technology of shotgun and label free combined with multiple reaction monitoring (MRM) was developed to detect rice sample of a single or/and co-infection. The shotgun assay indicated that some proteins coded by SRBSDV and RRSV were detected via the mode of in-gel digestion, except for P5-2, P7-2 and P9-2 of SRBSDV and P4b, P5, P6, P8a and P8b of RRSV. The technology of label free combined with MRM indicated that P2, P5-1, P4, P8, P7-1, P6, P9-1 and P10 of SRBSDV and P1, P3 and P9 of RRSV were higher abundance in rice plant, and P5-2, P7-2 and P9-2 of SRBSDV and P4b and P5 of RRSV were lower abundance in viruliferous-rice plant. So SRBSDV P9-1 and RRSV P3 was selected as marker molecule to be used in detection technology, and the label free combined with MRM technology was established to detect two kinds of rice virus.

Development of continuous cell culture of brown planthopper to trace the early infection process of oryzaviruses in insect vector cells

Rice ragged stunt virus (RRSV), an oryzavirus in the family Reoviridae, is transmitted by the brown planthopper, Nilaparvata lugens, in a persistent-propagative manner. Here, we established a continuous cell line of brown planthopper to investigate the mechanism underlying the formation of the viroplasm, the putative site for viral replication and assembly, during infection of RRSV in its insect vector cells. Within 24 h of viral infection of cultured cells, the viroplasm had formed and contained the viral nonstructural proteins Pns6 and Pns10, known to be constituents of viroplasm. Core capsid protein P3, core particles, and newly synthesized viral RNAs were accumulated inside the viroplasm, while outer capsid protein P8 and virions were accumulated at the periphery of the viroplasm, confirming that the viroplasm induced by RRSV infection was the site for viral replication and assembly. Pns10 formed viroplasm-like inclusions in the absence of viral infection, suggesting that the viroplasm matrix was largely composed of Pns10. Pns6 was recruited in the viroplasm by direct interaction with Pns10. Core capsid protein P3 was recruited to the viroplasm through specific association with Pns6. Knockdown of Pns6 and Pns10 expression using RNA interference inhibited viroplasm formation, virion assembly, viral protein expression, and viral double-stranded RNA synthesis. Thus, the present study shows that both Pns6 and Pns10 of RRSV play important roles in the early stages of viral life cycle in its insect vector cells, by recruiting or retaining components necessary for viral replication and assembly.

IMPORTANCE

The brown planthopper, a commonly distributed pest of rice in Asia, is the host of numerous insect endosymbionts, and the major vector of two rice viruses (RRSV and rice grassy stunt virus). For the first time, we successfully established the continuous cell line of brown planthopper. The unique uniformity of brown planthopper cells in the monolayer can support a consistent, synchronous infection by endosymbionts or viral pathogens, improving our understanding of molecular insect-microbe interactions.

Identification and characterization of the interaction between viroplasm-associated proteins from two different plant-infecting reoviruses and eEF-1A of rice

A rice protein homologous to eukaryotic translation elongation factor 1A (eEF-1A) was found to interact with the Pns6 of rice ragged stunt virus (RRSV), the type member of the genus Oryzavirus, family Reoviridae, in yeast two-hybrid screening. The interaction between the rice protein, designated OseEF-1A, and RRSV Pns6 was confirmed by bimolecular fluorescence complementation. Besides Pns6, OseEF-1A also interacted with the viroplasm matrix protein, Pns10, of RRSV. When expressed together, OseEF-1A co-localized with RRSV Pns10 in epidermal cells of Nicotiana benthamiana. Pns6 of southern rice black-streaked dwarf virus (SRBSDV), a newly reported member of the genus Fijivirus, family Reoviridae, was the only non-structural SRBSDV protein studied here that also interacted with OseEF-1A. Like Pns6 of rice black-streaked dwarf virus (RBSDV), SRBSDV Pns6 interacted with itself and co-localized with Pns9-1 in N. benthamiana. In the presence of Pns6, OseEF-1A co-localized with Pns9-1, the putative viroplasm matrix protein of SRBSDV.

Assembly of the viroplasm by viral non-structural protein Pns10 is essential for persistent infection of rice ragged stunt virus in its insect vector

Rice ragged stunt virus (RRSV), an oryzavirus, is transmitted by brown planthopper in a persistent propagative manner. In this study, sequential infection of RRSV in the internal organs of its insect vector after ingestion of virus was investigated by immunofluorescence microscopy. RRSV was first detected in the epithelial cells of the midgut, from where it proceeded to the visceral muscles surrounding the midgut, then throughout the visceral muscles of the midgut and hindgut, and finally into the salivary glands. Viroplasms, the sites of virus replication and assembly of progeny virions, were formed in the midgut epithelium, visceral muscles and salivary glands of infected insects and contained the non-structural protein Pns10 of RRSV, which appeared to be the major constituent of the viroplasms. Viroplasm-like structures formed in non-host insect cells following expression of Pns10 in a baculovirus system, suggesting that the viroplasms observed in RRSV-infected cells were composed basically of Pns10. RNA interference induced by ingestion of dsRNA from the Pns10 gene of RRSV strongly inhibited such viroplasm formation, preventing efficient virus infection and spread in its insect vectors. These results show that Pns10 of RRSV is essential for viroplasm formation and virus replication in the vector insect.