Southern Rice Black-streaked Dwarf Virus Infection Research Articles

A virus induces alterations in root morphology while exerting minimal effects on the rhizosphere and endosphere microorganisms in rice.

The highly destructive southern rice black-streaked dwarf virus (SRBSDV) causes significant losses in rice production. To understand its impact on rice root, we studied fibrous root development and root microbiota variation (rhizosphere and endosphere) after SRBSDV infection. SRBSDV infection reduced the number and length of fibrous roots in rice. Interestingly, the rhizosphere had higher bacterial diversity and abundance at the initial (0 days) and 30-day postinfection stages, while 30-day-old roots showed increased diversity and abundance. However, there were no significant differences in microbiota diversity between infected and noninfected rice plants. The major rhizosphere microbiota included Proteobacteria, Bacteroidota, Acidobacteriota, and Planctomycetota, comprising about 80% of the community. The endosphere was dominated by Proteobacteria and Cyanobacteria, constituting over 90%, with Bacteroidota as the next most prominent group. Further, we identified differentially expressed genes related to plant-pathogen interactions, plant hormone signal, and ABC transporters, potentially affecting root morphology. Notably, specific bacteria (e.g. Inquilinus and Actinoplanes) showed correlations with these pathways. In conclusion, SRBSDV primarily influences root growth through host metabolism, rather than exerting direct effects on the root microbiota. These insights into the interactions among the pathogen, rice plant, and associated microbiota could have implications for managing SRBSDV's detrimental effects on rice production.

A Rice Receptor-like Protein Negatively Regulates Rice Resistance to Southern Rice Black-Streaked Dwarf Virus Infection.

Plants rely on various receptor-like proteins and receptor-like kinases to recognize and defend against invading pathogens. However, research on the role of receptor-like proteins in plant antiviral defense, particularly in rice-virus interactions, is limited. In this study, we identified a receptor-like gene, OsBAP1, which was significantly induced upon infection with southern rice black-streaked dwarf virus (SRBSDV) infection. A viral inoculation assay showed that the OsBAP1 knockout mutant exhibited enhanced resistance to SRBSDV infection, indicating that OsBAP1 plays a negatively regulated role in rice resistance to viral infection. Transcriptome analysis revealed that the genes involved in plant-pathogen interactions, plant hormone signal transduction, oxidation-reduction reactions, and protein phosphorylation pathways were significantly enriched in OsBAP1 mutant plants (osbap1-cas). Quantitative real-time PCR (RT-qPCR) analysis further demonstrated that some defense-related genes were significantly induced during SRBSDV infection in osbap1-cas mutants. Our findings provide new insights into the role of receptor-like proteins in plant immune signaling pathways, and demonstrate that OsBAP1 negatively regulates rice resistance to SRBSDV infection.

Southern rice black-streaked dwarf virus induces incomplete autophagy for persistence in gut epithelial cells of its vector insect.

Autophagy plays an important role in virus infection of the host, because viral components and particles can be degraded by the host's autophagy and some viruses may be able to hijack and subvert autophagy for its benefit. However, details on the mechanisms that govern autophagy for immunity against viral infections or benefit viral survival remain largely unknown. Plant reoviruses such as southern rice black-streaked dwarf virus (SRBSDV), which seriously threaten crop yield, are only transmitted by vector insects. Here, we report a novel mechanism by which SRBSDV induces incomplete autophagy by blocking autophagosome-lysosome fusion, resulting in viral accumulation in gut epithelial cells of its vector, white-backed planthopper (Sogatella furcifera). SRBSDV infection leads to stimulation of the c-Jun N-terminal kinase (JNK) signaling pathway, which further activates autophagy. Mature and assembling virions were found close to the edge7 of the outer membrane of autophagosomes. Inhibition autophagy leads to the decrease of autophagosomes, which resulting in impaired maturation of virions and the decrease of virus titer, whereas activation of autophagy facilitated virus titer. Further, SRBSDV inhibited fusion of autophagosomes and lysosomes by interacting with lysosomal-associated membrane protein 1 (LAMP1) using viral P10. Thus, SRBSDV not only avoids being degrading by lysosomes, but also further hijacks these non-fusing autophagosomes for its subsistence. Our findings reveal a novel mechanism of reovirus persistence, which can explain why SRBSDV can be acquired and transmitted rapidly by its insect vector.

A plant nonenveloped double-stranded RNA virus activates and co-opts BNIP3-mediated mitophagy to promote persistent infection in its insect vector

Mitophagy that selectively eliminates damaged mitochondria is an essential mitochondrial quality control mechanism. Recently, mitophagy has been shown to be induced in host cells infected by a few animal viruses. Here, we report that southern rice black-streaked dwarf virus (SRBSDV), a plant nonenveloped double-stranded RNA virus, can also trigger mitophagy in its planthopper vector to prevent mitochondria-dependent apoptosis and promote persistent viral propagation. We find that the fibrillar structures constructed by the nonstructural protein P7-1 of SRBSDV directly target mitochondria via interaction with the mitophagy receptor BNIP3 (BCL2 interacting protein 3), and these mitochondria are then sequestered within autophagosomes to form mitophagosomes. Moreover, SRBSDV infection or P7-1 expression alone can promote BNIP3 dimerization on the mitochondria, and induce autophagy via the P7-1-ATG8 interaction. Furthermore, SRBSDV infection stimulates the phosphorylation of AMP-activated protein kinase (AMPK), resulting in BNIP3 phosphorylation via the AMPKα-BNIP3 interaction. Together, P7-1 induces BNIP3-mediated mitophagy by promoting the formation of phosphorylated BNIP3 dimers on the mitochondria. Silencing of ATG8, BNIP3, or AMPKα significantly reduces virus-induced mitophagy and viral propagation in insect vectors. These data suggest that in planthopper, SRBSDV-induced mitophagosomes are modified to accommodate virions and facilitate persistent viral propagation. In summary, our results demonstrate a previously unappreciated role of a viral protein in the induction of BNIP3-mediated mitophagy by bridging autophagosomes and mitochondria and reveal the functional importance of virus-induced mitophagy in maintaining persistent viral infection in insect vectors.

A virus-derived small RNA targets the rice transcription factor ROC1 to induce disease-like symptom

Virus-derived small interfering RNAs (vsiRNAs) in plants can target host transcripts to induce symptom development. Southern rice black-streaked dwarf virus (SRBSDV) is the most damaging rice-infecting virus, which causes severe stunting and poorly developed roots in rice plants, thereby posing a serious threat to rice production. In this study, we show that a vsiRNA (vsiR-S9-18), generated from SRBSDV genome segment 9, targets the transcription factor ROC1 in rice. SRBSDV infection triggered the production of vsiR-S9-18 and resulted in the downregulation of ROC1 in rice plants. Transient expression of vsiR-S9-18 in rice protoplasts and tobacco leaves further demonstrated the specific association of vsiR-S9-18 with ROC1. Moreover, the ROC1-knockout rice plants displayed shortened roots, similar to the phenotype of root growth inhibition caused by SRBSDV infection. We propose that vsiR-S9-18 inhibits root elongation via ROC1, which has been proven to regulate root growth. This finding extends our understanding of the role of vsiRNA in viral disease development and is helpful for the development of new antiviral approaches.

A viral protein orchestrates rice ethylene signaling to coordinate viral infection and insect vector-mediated transmission

Arthropod-borne viruses cause serious threats to human health and global agriculture by rapidly spreading via insect vectors. Southern rice black-streaked dwarf virus (SRBSDV) is the most damaging rice-infecting virus that is frequently transmitted by planthoppers. However, the molecular mechanisms underlying its propagation in the host plants and epidemics in the field are largely unknown. Here, we showed that the SRBSDV-encoded P6 protein is a key effector that regulates rice ethylene signaling to coordinate viral infection and transmission. In early SRBSDV infection, P6 interacts with OsRTH2 in the cytoplasm to activate ethylene signaling and enhance SRBSDV proliferation; this also repels the insect vector to reduce infestation. In late infection, P6 enters the nucleus, where it interacts with OsEIL2, a key transcription factor of ethylene signaling. The P6-OsEIL2 interaction suppresses ethylene signaling by preventing the dimerization of OsEIL2, thereby facilitating viral transmission by attracting the insect vector. Collectively, these findings reveal a novel molecular mechanism by which an arbovirus modulates the host defense system to promote viral infection and transmission.

Detection of Southern Rice Black-Streaked Dwarf Virus Using Western Blotting With P6

The southern rice black-streaked dwarf virus (SRBSDV) is a severe threat to the yield and quality of rice products worldwide. Traditional detection methods for diagnosing SRBSDV infection show several false positives and thus provide inaccurate findings. However, Western blotting (WB) can precisely solve this problem. In this study, P6—a viral RNA-silencing suppressor—was expressed and purified in vitro. Two polyclonal P6 antibodies were obtained and quantified by enzyme-linked immunosorbent assay and WB. Subsequently, WB was performed using the P6 antibodies to identify SRBSDV antigens derived from the suspected rice samples collected from nine districts in Guizhou, China. The assay results showed that Libo, Pingtang, Huishui, Dushan, and Anshun districts had experienced an SRBSDV outbreak. The virus content in the sampled rice tissues was quantified by WB. Our results revealed that SRBSDV mainly accumulated in rice stems rather than rice leaves. Thus, the findings of our study show that the SRBSDV P6 antibody can be used in WB for detecting and monitoring SRBSDV infection in infected rice plants.

Metabolomic Changes in Sogatella furcifera under Southern rice black-streaked dwarf virus Infection and Temperature Stress.

Southern rice black-streaked dwarf virus (SRBSDV) is a devastating newly emerged rice reovirus in Eastern and Southeastern Asia transmitted by a long-distance migratory pest, the white-backed planthopper (WBPH). We previously showed that SRBSDV infection decreased the cold tolerance but improved the heat tolerance of its vector, WBPH. Comparative metabolomic analysis was used to explore the potential mechanisms underlying these changes in temperature stress response. Fourth-generation WBPH nymphs were treated with SRBSDV and/or extreme temperature stress and were analyzed using gas chromatography-time of flight-mass spectrometry. A total of 605 distinguishable peaks were identified and 165, 207, and 202 differentially accumulated metabolites were identified in WBPH after virus infection, cold, or heat stress, respectively. The nucleic acids and fatty acids were the major categories of metabolites regulated by SRBSDV infection, whereas temperature stress regulated tricarboxylic acid cycle compounds, sugars, and polyols. For the WBPH samples infected with SRBSDV and subjected to temperature stress, amino acids, sugars, and polyols were the most significant regulated metabolites. The metabolomics study suggests that SRBSDV may influence the extreme temperature tolerance of WBPH by regulating the accumulation of amino acids, sugars, and polyols in the insect body.

Varietal differences in ovicidal response to the white-backed planthopper Sogatella furcifera (Hemiptera: Delphacidae) and susceptibility to Southern rice black-streaked dwarf virus in rice

The recent spread of Oryza sativa L. (rice) ssp. indica-derived cultivars in East and Southeast Asia increases the risk of white-backed planthopper, Sogatella furcifera, and Southern rice black-streaked dwarf virus (SRBSDV) outbreaks. We compared the degree of ovicidal response to S. furcifera and susceptibility to SRBSDV among two Oryza sativa ssp. japonica, two Oryza sativa ssp. indica and one Oryza sativa ssp. indica-derived rice cultivars. The ovicidal response of the O. sativa ssp. indica-derived cultivar was significantly higher than that of the tested O. sativa ssp. indica cultivars and was equivalent to that of the O. sativa ssp. japonica cultivars that we used, supporting previous findings that the ovicidal response is higher in O. sativa ssp. japonica cultivars than in O. sativa ssp. indica cultivars and is a quantitative trait controlled by a few relevant genes. Contrary to the varied ovicidal response, the SRBSDV infection rate among the cultivars and the height of SRBSDV-infected plants between most pairs of cultivars did not differ significantly, although twisting of the leaf tips, a typical symptom of SRBSDV, was not observed in the O. sativa ssp. japonica cultivars. These results indicate that damage by S. furcifera and SRBSDV can occur in O. sativa ssp. japonica as well as in O. sativa ssp. indica and O. sativa ssp. indica-derived cultivars; therefore, in the cultivation of O. sativa ssp. japonica, measures should also be taken against white-backed planthopper during high levels of emergence.

The Influence of Sogatella furcifera (Hemiptera: Delphacidae) Migratory Events on the Southern Rice Black-Streaked Dwarf Virus Epidemics.

Southern rice black-streaked dwarf virus (SRBSDV) caused serious rice losses. After the first outbreak in 2009 in northern Vietnam and southern China, the virus ravaged crops again on enormous scales in 2010, but infections have decreased sharply since 2011. We presumed that the sudden epidemics and fadeout of SRBSDV would be closely related to the migratory events of the insect vector, Sogatella furcifera. This study sought the source area of SRBSDV using the trajectory analysis method, and revealed the relationship between SRBSDV dynamics and migration of S. furcifera populations via an in-depth analysis of meteorological background of S. furcifera migration fields. The results showed that Northern Vietnam was the direct virus source area of the SRBSDV infection in China, and South Central Coast of Vietnam was the original source area of SRBSDV. Southwesterly winds were prevalent in spring of 2010 and carried large numbers of viruliferous S. furcifera to China from northern Vietnam. This infestation of S. furcifera was the direct cause of the SRBSDV outbreak in China in 2010. In 2011, the winter-spring temperatures were abnormally low and southeasterly and easterly winds dominated; therefore, the number of viruliferous S. furcifera that entered China was small, and consequently, the occurrence area of SRBSDV was rapidly reduced. The return of viruliferous S. furcifera to South Central Coast of Vietnam was an important factor that affected the occurrence scale of SRBSDV in the following year.

Interaction between southern rice black-streaked dwarf virus minor core protein P8 and a rice zinc finger transcription factor.

The fijivirus southern rice black-streaked dwarf virus (SRBSDV) causes one of the most serious viral diseases of rice in China and Vietnam. To better understand the molecular basis of SRBSDV infection, a yeast two-hybrid screen of a rice cDNA library was carried out using P8, a minor core protein of SRBSDV, as the bait. A rice Cys2His2-type zinc finger protein (OsZFP) was found to interact with SRBSDV P8. A strong interaction between SRBSDV P8 and OsZFP was then confirmed by pull-down assays, and bimolecular fluorescence complementation assays showed that the in vivo interaction was specifically localized in the nucleus of plant cells. Using a series of deletion mutants, it was shown that both the NTP-binding region of P8 and the first two zinc fingers of OsZFP were crucial for their interaction in plant cells. The localization in the nucleus and activation of transcription in yeast supports the notion that OsZFP is a transcription factor. SRBSDV P8 may play an important role in fijiviral infection and symptom development by interfering with the host transcription activity of OsZFP.

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.

Tolerance and responsive gene expression of Sogatella furcifera under extreme temperature stresses are altered by its vectored plant virus.

Southern rice black-streaked dwarf virus (SRBSDV), a newly emerged fijivirus causing great loss to rice production in eastern and southeastern Asian countries in recent years, is efficiently transmitted by a rice pest, white-backed planthopper (WBPH, Sogatella furcifera) in a persistent, circulative propagative manner and can be considered as an insect virus. In this study, SRBSDV infection in WBPH was found to increase the vector’s death rate under extreme cold stress but improve its survival rate under extreme heat stress. Digital gene expression profiling based on RNA-Seq revealed different gene regulation patterns in WBPH under viral and/or temperature stress. Under cold stress, the virus infection upregulated 1540 genes and downregulated 131 genes in the insect, most of which were related to membrane properties and biological processes of actin and cytoskeleton; whereas under heat stress, it upregulated 363 genes and downregulated 548 genes, most of which were associated to metabolism and intracellular organelles. Several types of stress-responsive genes involving intestinal mucin, cuticle protein, ubiquitin protease, immune response, RNA interference and heat shock response, were largely upregulated under cold stress, but largely downregulated under heat stress, by SRBSDV infection. Our results suggest two distinct mechanisms of virus-altered vector insect tolerance to temperature stress.

Identification and characterization of microRNAs in the white-backed planthopper,Sogatella furcifera

MicroRNAs (miRNAs) are a novel class of small, non-coding endogenous RNAs that play critical regulatory roles in many metabolic activities in eukaryotes. Reports of the identification of miRNAs in Sogatella furcifera (white-backed planthopper), the insect that acts as the only confirmed vector of the southern rice black-streaked dwarf virus (SRBSDV), are limited. In this study, a total of 382 miRNAs were identified in S. furcifera, including 106 conserved and 276 novel miRNAs, using high-throughput sequencing based on two small RNA libraries from viruliferous and non-viruliferous S. furcifera, and these miRNAs belonged to 52 conserved miRNA families and 58 S. furcifera-specific families, respectively. Comparison with miRNAs from 26 insect species and five other species in miRBase showed that more than half of the conserved miRNA families are highly conserved in Hexapoda, while other miRNAs are only conserved in non-dipterans. Furthermore, 4117 target genes predicted for the 382 identified miRNAs could be categorized into 45 functional groups annotated by Gene Ontology. Compared with non-viruliferous cells, eight up-regulated miRNAs and four down-regulated miRNAs were identified in cells inoculated with SRBSDV, among which miR-14 and miR-n98a may be involved in the immune response to SRBSDV infection. Analyses of the identified miRNAs will provide insights into the roles of these miRNAs in the regulation and expression of genes involved in the metabolism, development and viral infection of S. furcifera.

Small Interfering RNA Pathway Modulates Initial Viral Infection in Midgut Epithelium of Insect after Ingestion of Virus.

Numerous viruses are transmitted in a persistent manner by insect vectors. Persistent viruses establish their initial infection in the midgut epithelium, from where they disseminate to the midgut visceral muscles. Although propagation of viruses in insect vectors can be controlled by the small interfering RNA (siRNA) antiviral pathway, whether the siRNA pathway can control viral dissemination from the midgut epithelium is unknown. Infection by a rice virus (Southern rice black streaked dwarf virus [SRBSDV]) of its incompetent vector (the small brown planthopper [SBPH]) is restricted to the midgut epithelium. Here, we show that the siRNA pathway is triggered by SRBSDV infection in continuously cultured cells derived from the SBPH and in the midgut of the intact insect. Knockdown of the expression of the core component Dicer-2 of the siRNA pathway by RNA interference strongly increased the ability of SRBSDV to propagate in continuously cultured SBPH cells and in the midgut epithelium, allowing viral titers in the midgut epithelium to reach the threshold (1.99 × 10(9) copies of the SRBSDV P10 gene/μg of midgut RNA) needed for viral dissemination into the SBPH midgut muscles. Our results thus represent the first elucidation of the threshold for viral dissemination from the insect midgut epithelium. Silencing of Dicer-2 further facilitated the transmission of SRBSDV into rice plants by SBPHs. Taken together, our results reveal the new finding that the siRNA pathway can control the initial infection of the insect midgut epithelium by a virus, which finally affects the competence of the virus's vector. Many viral pathogens that cause significant global health and agricultural problems are transmitted via insect vectors. The first bottleneck in viral infection, the midgut epithelium, is a principal determinant of the ability of an insect species to transmit a virus. Southern rice black streaked dwarf virus (SRBSDV) is restricted exclusively to the midgut epithelium of an incompetent vector, the small brown planthopper (SBPH). Here, we show that silencing of the core component Dicer-2 of the small interfering RNA (siRNA) pathway increases viral titers in the midgut epithelium past the threshold (1.99 × 10(9) copies of the SRBSDV P10 gene/μg of midgut RNA) for viral dissemination into the midgut muscles and then into the salivary glands, allowing the SBPH to become a competent vector of SRBSDV. This result is the first evidence that the siRNA antiviral pathway has a direct role in the control of viral dissemination from the midgut epithelium and that it affects the competence of the virus's vector.

Virus-mediated chemical changes in rice plants impact the relationship between non-vector planthopper Nilaparvata lugens Stål and its egg parasitoid Anagrus nilaparvatae Pang et Wang.

In order to clarify the impacts of southern rice black-streaked dwarf virus (SRBSDV) infection on rice plants, rice planthoppers and natural enemies, differences in nutrients and volatile secondary metabolites between infected and healthy rice plants were examined. Furthermore, the impacts of virus-mediated changes in plants on the population growth of non-vector brown planthopper (BPH), Nilaparvata lugens, and the selectivity and parasitic capability of planthopper egg parasitoid Anagrus nilaparvatae were studied. The results showed that rice plants had no significant changes in amino acid and soluble sugar contents after SRBSDV infection, and SRBSDV-infected plants had no significant effect on population growth of non-vector BPH. A. nilaparvatae preferred BPH eggs both in infected and healthy rice plants, and tended to parasitize eggs on infected plants, but it had no significant preference for infected plants or healthy plants. GC-MS analysis showed that tridecylic aldehyde occurred only in rice plants infected with SRBSDV, whereas octanal, undecane, methyl salicylate and hexadecane occurred only in healthy rice plants. However, in tests of behavioral responses to these five volatile substances using a Y-tube olfactometer, A. nilaparvatae did not show obvious selectivity between single volatile substances at different concentrations and liquid paraffin in the control group. The parasitic capability of A. nilaparvatae did not differ between SRBSDV-infected plants and healthy plant seedlings. The results suggested that SRBSDV-infected plants have no significant impacts on the non-vector planthopper and its egg parasitoid, A. nilaparvatae.

MicroRNAs responding to southern rice black-streaked dwarf virus infection and their target genes associated with symptom development in rice

Southern rice black-streaked dwarf virus (SRBSDV) is a recently emerged rice virus that has spread across Asia. This devastating virus causes rice plants to produce a variety of symptoms during different growth stages. MicroRNAs (miRNAs) comprise a large group of 21–24-nt RNA molecules that are important regulators of plant development processes and stress responses. In this study, we used microarray profiling to investigate rice miRNAs responding to SRBSDV infection at 3, 9, 15, and 20days post-inoculation (dpi). Expression levels of 56 miRNAs were altered in SRBSDV-infected rice plants, with these changes classified into eight different regulation patterns according to their temporal expression dynamics. Fourteen miRNAs belonging to six families (miR164, R396, R530, R1846, R1858, and R2097) were significantly regulated at 20dpi. We used RT-qPCR to search for expression level correlations between members of these families and their putative targets at 3, 9, and 15dpi. Some members of the miR164, R396, R530, and R1846 families were found to be positively or negatively correlated with their respective targets during 3–15days after SRBSDV infection, whereas in more cases the rice miRNAs were not in correlation with their targets along the post-inoculation period, suggesting that some additional factors may be involved in rice miRNA-target interactions. The reported functions of rice genes targeted by the miR164, R396, R530, R1846, and R1858 families indicated that these genes are associated with symptom development. These results provide insights into miRNA-mediated SRBSDV-rice interactions.

Southern Rice Black-Streaked Dwarf Virus Infection Improves Host Suitability for Its Insect Vector, <I>Sogatella furcifera</I> (Hemiptera: Delphacidae)

The impact of southern rice black-streaked dwarf virus (SRBSDV) on the life cycle of the vector Sogatella furcifera (Horváth) (Hemiptera: Delphacidae) was investigated. The adult longevity was significantly shorter when the insect was reared on the SRBSDV-infected rice plants than those reared on the noninfected rice treatment. The fecundity of S. furcifera reared on the SRBSDV-infected rice plants was slightly higher than that of those reared on the noninfected rice plants both in macropterous and brachypterous cohorts. In addition, SRBSDV infection in rice increased the population size of macropterous adults from both macropterous and brachypterous cohorts. After a 10 or 15 d feeding period on the SRBSDV-infected rice plants, the amount of honeydew excreted by the insect increased significantly when compared with those reared on the noninfected plants. These results revealed SRBSDV infection improves host suitability for its vector, S. furcifera.

New model for the genesis and maturation of viroplasms induced by fijiviruses in insect vector cells.

Plant reoviruses are thought to replicate and assemble within cytoplasmic, nonmembranous structures called viroplasms. Here, we established continuous cell cultures of the white-backed planthopper (Sogatella furcifera Horváth) to investigate the mechanisms for the genesis and maturation of the viroplasm induced by Southern rice black-streaked dwarf virus (SRBSDV), a fijivirus in the family Reoviridae, during infection of its insect vector. Electron and confocal microscopy revealed that the viroplasm consisted of a granular region, where viral RNAs and nonstructural proteins P6 and P9-1 accumulated, and a filamentous region, where viral RNAs, progeny cores, viral particles, as well as nonstructural proteins P5 and P6 accumulated. Our results suggested that the filamentous viroplasm matrix was the site for the assembly of progeny virions. Because viral RNAs were produced by assembled core particles within the filamentous viroplasm matrix, we propose that these viral RNAs might be transported to the granular viroplasm matrix. P5 formed filamentous inclusions and P9-1 formed granular inclusions in the absence of viral infection, suggesting that the filamentous and granular viroplasm matrices were formed primarily by P5 and P9-1, respectively. P6 was apparently recruited in the whole viroplasm matrix by direct interaction with P9-1 and P5. Thus, the present results suggested that P5, P6, and P9-1 are collectively required for the genesis and maturation of the filamentous and granular viroplasm matrix induced by SRBSDV infection. Based on these results, we propose a new model to explain the genesis and maturation of the viroplasms induced by fijiviruses in insect vector cells.

The Complete Genome Sequence of Southern rice black-streaked dwarf virus Isolated from Vietnam

Faculty of Agriculture Forestry Fisheries, Vinh University, Vinh city, Nghe An province 42000, Vietnam(Received on March 21, 2012; Revised on July 3, 2012; Accepted on July 11, 2012)We determined the complete genome sequence of aVietnamese isolate of Southern rice black-streaked dwarfvirus (SRBSDV). Whole genome comparisons andphylogenetic analysis showed that the genome of theVietnamese isolate shared high nucleotide sequenceidentities of over 97.5% with those of the reportedChinese isolates, confirming a common origin of them.Moreover, the greatest divergence between differentSRBSDV isolates was found in the segments S1, S3, S4and S6, which differs from the sequence alignmentresults between SRBSDV and Rice black streaked dwarfvirus (RBSDV), implying that SRBSDV evolved in aunique way independent of RBSDV. This is the firstreport of a complete nucleotide sequence of SRBSDVfrom Vietnam and our data provides new clues forfurther understanding of molecular variation andepidemiology of SRBSDV in Southeast Asia.Keywords : phylogenetic analysis, sequence, Southern riceblack-streaked dwarf virus, vietnamese isolate Southern rice black-streaked dwarf virus (SRBSDV) isproposed to be a novel member of the Fijivirus group 2 inthe family Reoviridae (Attoui, 2011; Zhang et al., 2008;Zhou et al., 2008). The double-stranded (dsRNA) ofSRBSDV contains 10 segments (S1 to S10), which werenamed in order of decreasing molecular weight (Zhou et al.,2008). SRBSDV was first observed in Yangxi county,Guangdong province in China in 2001. From then on, thisdisease became one of the most serious viral diseases ofrice in southern and central China, causing severe losses inrice production (Wang et al., 2010; Zhou et al., 2010). In2008, SRBSDV was also detected in maize plants innorthern China (Yin et al., 2011). In 2010, the occurrence ofSRBSDV disease was firstly reported in rice fields in theKumamoto Prefecture of Japan (Choi, 2010). In 2009, seriousviral rice disease occurred in many northern provinces ofVietnam, expanding to a total of 5506 hectares. Of this area,the yields of approximately 3510 hectares were totally lost.The causal agent of the rice dwarf disease in the north ofVietnam has also been identified as SRBSDV (Cuong et al.,2009; Hoang et al., 2011). Further investigation suggestedthat the outbreak of SRBSDV in Vietnam was associatedwith rising populations of the white-backed plant hopper(Sogatella furcifera), which is the major vector of SRBSDV(Cuong et al., 2009; Zhou et al., 2008). As planthoppers areconstantly displaced by wind currents, the spread of thevirus diseases they carry will be inevitable.Symptoms of SRBSDV infection vary depending on thecrop age when infected. Characteristic symptoms includedark-green and wrinkled leaves, incomplete tassel, tumor-like protrusions ending in small enations, tiller formation onthe upper parts, up-growing rootlets, and in particular thepresence of white to black waxy galls along the major veinsof the leaves and culms (Wang et al., 2012; Zhou et al.,2008; Zhou et al., 2010). Up to now, the complete genome sequence of SRBSDVhad been obtained only for three isolates, which originatedfrom Guangdong (GD), Hainan (HN) and Hubei (HB)provinces of China, respectively (Wang et al., 2010). Asonly partial sequences of segments 4 and 10 from severalVietnamese isolates were investigated and the S10 segmentwas very conserved among different isolates (Cuong et al.,2009; Hoang et al., 2011), more information about thegenome sequence of an SRBSDV isolate from Vietnamwill further characterize the geographic and molecularvariation of this virus, thus facilitating control and preven-tion of this disease in Southeast Asia.Viral genomic dsRNAs were extracted directly fromnaturally infected field-grown rice plants from Thua ThienHue province in central Vietnam using the method describ-ed previously (Dodds et al., 1984). Primers for cloningsegments S1 to S10 of Vietnamese isolate were designed toobtain the full length sequence of each genome segmentbased on the previously reported SRBSDV sequencesavailable in GenBank of NCBI (Wang et al., 2010). Twostep RT-PCR was employed to clone cDNA fragments of