[Stress effects of imidacloprid on RSV in rice plants]

The rice stripe disease is a viral disease transmitted by small brown planthopper, Laodelphax striatellus, which outbroke a few years ago in the Yangtze River basin, especially Jiangsu region, China. To study the effects of imidacloprid stress on rice stripe virus (RSV) in rice plants, the rice seedlings were treated with imidacloprid 1, 2, 3 and 4 times (B1, B2, B3 and B4), respectively, after artificial inoculation by L. striatellus for 48 h, and the expression levels of relative genes including RSV NS3, CP, SP and NSvc4, as well as the protein concentrations of CP and SP were detected at different stages by real-time PCR and Western blotting. The results showed that the effects of imidacloprid treatment on the expression levels of four genes were gene-specific and correlated with application frequencies of imidacloprid. The expression levels of NS3 gene were upregulated in three treatments, and the highest expression level (10.86) was observed 16 days after inoculation in B4 treatment, but a significant down-regulation of NS3 gene was found in all other treatments. The expression levels of CP, SP and NSvc4 genes were down-regulated significantly (0-0.74) in almost all B2 and B3 treatments, while a significant up-regulation was found in half of B1 and B4 treatments, and the highest expression levels of SP gene were observed 16 days after inoculation in B1 (258.89) and B4 (730.54) treatment, respectively. On the other hand, the effects of imidacloprid stress on the expression patterns of CP and SP genes were different from those of CP and SP proteins. For example, the expression level of CP gene was almost no expression (0) 19 days after inoculation in B1 treatment, while significantly up-regulated (23.08) was observdd for CP protein.

Heat shock protein 70 is necessary for Rice stripe virus infection in plants

Heat shock proteins 70 (HSP70s) are a highly conserved family of genes in eukaryotes, and are involved in a remarkable variety of cellular processes. In many plant positive-stranded RNA viruses, HSP70 participates in the construction of a viral replication complex and plays various roles during viral infection. Here, we found increased expression of HSP70 following infection by Rice stripe virus (RSV), a negative-stranded RNA virus, in both rice (the natural host) and Nicotiana benthamiana (an experimental host). Heat treatment of N. benthamiana (Nb) plants enhanced viral infection, whereas RSV infection was retarded and viral RNAs accumulated at a low level when HSP70 was silenced. In both bimolecular fluorescence complement and in vitro pull-down assays, the N-terminus of RSV RNA-dependent RNA polymerase (RdRp) interacted and co-localized with the HSP70s of both plants (OsHSP70 and NbHSP70). The localization of the N-terminus of RdRp when expressed alone was not obviously different from when it was co-expressed with OsHSP or NbHSP, and vice versa. RSV infection also had no effect on the localization of host HSP70. These results demonstrate that host HSP70 is necessary for RSV infection and probably plays a role in viral replication by interacting with viral RdRp, which provides the first evidence of an interacting host protein related to RSV replication, which has been little studied to date.

[Construction of rice stripe virus NS2 and NS3 Co-RNAi transgenic rice and disease-resistance analysis]

NS2 and NS3 are two post-transcriptional gene silencing suppressors that are encoded by Rice stripe virus. Gene silencing suppressors are always related to the pathogenicity of viruses. In this study, the cDNA of NS2 and NS3 were recombined by overlapping PCR assays, ligated to the RNAi vector, and inserted into the PXQ expression vector using Pst I; the expressed vector was transferred into calluses induced from seeds of the japonica rice cultivar, 'Nipponbare', using an Agrobacterium-mediated method. Thirty-one T0 transgenic plants were selected by G418 screening. PCR and southern blot analyses confirmed that the target gene was transformed into transgenic rice successfully, and different transgenic plants contained various copies of the gene. The disease resistance assay revealed that T0 transgenic rice had a delayed onset of RSV for approximately 10-20 d, and the accumulation of virus in the transgenic plants was reduced by 30%-50%. This was related to the delayed onset of disease.

Interactive Effects of Southern Rice Black-Streaked Dwarf Virus Infection of Host Plant and Vector on Performance of the Vector, Sogatella furcifera (Homoptera: Delphacidae)

Performance of insect vectors can be influenced by the viruses they transmit, either directly by infection of the vectors or indirectly via infection of the host plants. Southern rice black-streaked dwarf virus (SRBSDV) is a propagative virus transmitted by the white-backed planthopper, Sogatella furcifera (Hovath). To elucidate the influence of SRBSDV on the performance of white-backed planthopper, life parameters of viruliferous and nonviruliferous white-backed planthopper fed rice seedlings infected or noninfected with SRBSDV were measured using a factorial design. Regardless of the infection status of the rice plant host, viruliferous white-backed planthopper nymphs took longer to develop from nymph to adult than did nonviruliferous nymphs. Viruliferous white-backed planthopper females deposited fewer eggs than nonviruliferous females and both viruliferous and nonviruliferous white-backed planthopper females laid fewer eggs on infected than on noninfected plants. Longevity of white-backed planthopper females was also affected by the infection status of the rice plant and white-backed planthopper. Nonviruliferous white-backed planthopper females that fed on infected rice plants lived longer than the other three treatment groups. These results indicate that the performance of white-backed planthopper is affected by SRBSDV either directly (by infection of white-backed planthopper) or indirectly (by infection of rice plant). The extended development of viruliferous nymphs and the prolonged life span of nonviruliferous adults on infected plants may increase their likelihood of transmitting virus, which would increase virus spread.

Infection of rice plants by rice black streaked dwarf virus improves an egg parasitoid, Anagrus nilaparvatae (Hymenoptera: Mymaridae), of rice planthoppers

The effects of rice plants infected by rice black streaked dwarf virus (RBSDV) on the host preference, duration of immature stages, sex ratio, and adult longevity and parasitic capacity of an egg parasitoid, Anagrus nilaparvatae Pang et Wang, of rice brown planthopper, Nilaparvata lugens Stål, were evaluated. Tests of response to plant volatiles using an olfactometer showed that A. nilaparvatae preferred rice plants harboring rice brown planthopper eggs over plants free of rice brown planthopper eggs. However, both the response to plant volatiles and the host selectivity test showed no significant differences in host preference between RBSDV-infected plants and healthy plants when both contained rice brown planthopper eggs. The developmental duration at immature stage of the male A. nilaparvatae in rice brown planthopper eggs on RBSDV-infected rice plants was significantly prolonged, and the parasitic capacity of rice brown planthopper eggs was significantly increased in comparison with the A. nilaparvatae parasite in rice brown planthopper eggs on healthy rice plants. There were no significant differences between RBSDV-infected rice plants and healthy rice plants in other ecological fitness parameters, including the developmental duration of female adults, female percentage, and adult longevity of A. nilaparvatae.

STV11 encodes a sulphotransferase and confers durable resistance to rice stripe virus

Rice stripe virus (RSV) causes one of the most serious viral diseases of rice (Oryza sativa L.), but the molecular basis of RSV resistance has remained elusive. Here we show that the resistant allele of rice STV11 (STV11-R) encodes a sulfotransferase (OsSOT1) catalysing the conversion of salicylic acid (SA) into sulphonated SA (SSA), whereas the gene product encoded by the susceptible allele STV11-S loses this activity. Sequence analyses suggest that the STV11-R and STV11-S alleles were predifferentiated in different geographic populations of wild rice, Oryza rufipogon, and remained prevalent in cultivated indica and japonica rice varieties, respectively. Introgression of the STV11-R allele into susceptible cultivars or heterologous transfer of STV11-R into tobacco plants confers effective resistance against RSV. Our results shed new insights into plant viral defense mechanisms and suggest effective means of breeding RSV-resistant crops using molecular marker-assisted selection or genetic engineering.

Tumours induced by a plant virus are derived from vascular tissue and have multiple intercellular gateways that facilitate virus movement

Structural studies showed that tumours induced by Southern rice black-streaked dwarf virus (SRBSDV; genus Fijivirus, family Reoviridae) were highly organized, modified phloem, composed of sclerenchyma, vessels, hyperplastic phloem parenchyma and sieve elements (SEs). Only parenchyma and SEs were invaded by the virus. There was a special region that consisted exclusively of SEs without the usual companion cells and a new flexible type of intercellular gateway was observed on all SE-SE interfaces in this region. These flexible gateways significantly increased the intercellular contacts and thus enhanced potential symplastic transport in the tumour. Flexible gateways were structurally similar to compressed plasmodesmata but were able to accommodate complete SRBSDV virions (~80 nm diameter). Virions were also found in sieve-pore gateways, providing strong evidence for the movement of a virus with large virions within phloem tissue and suggesting that the unusual neovascularization of plant virus-induced tumours facilitated virus spread. A working model for the spread of tumour-inducing reoviruses in plants is presented.

Virus-induced tubule: a vehicle for rapid spread of virions through basal lamina from midgut epithelium in the insect vector

The plant reoviruses, plant rhabdoviruses, tospoviruses, and tenuiviruses are transmitted by insect vectors in a persistent propagative manner. These viruses induce the formation of viral inclusions to facilitate viral propagation in insect vectors. The intestines of insect vectors are formed by epithelial cells that lie on the noncellular basal lamina surrounded by visceral muscle tissue. Here, we demonstrate that a recently identified plant reovirus, southern rice black-streaked dwarf virus (SRBSDV), exploits virus-containing tubules composed of virus-encoded nonstructural protein P7-1 to directly cross the basal lamina from the initially infected epithelium toward visceral muscle tissues in the intestine of its vector, the white-backed planthopper (Sogatella furcifera). Furthermore, such tubules spread along visceral muscle tissues through a direct interaction of P7-1 and actin. The destruction of tubule assembly by RNA interference with synthesized double-stranded RNA targeting the P7-1 gene inhibited viral spread in the insect vector in vitro and in vivo. All these results show for the first time that a virus employs virus-induced tubule as a vehicle for viral spread from the initially infected midgut epithelium through the basal lamina, facilitating the rapid dissemination of virus from the intestine of the insect vector.

IMPORTANCE

Numerous plant viruses are transmitted in a persistent manner by sap-sucking insects, including thrips, aphids, planthoppers, and leafhoppers. These viruses, ingested by the insects, establish their primary infection in the intestinal epithelium of the insect vector. Subsequently, the invading virus manages to transverse the basal lamina, a noncellular layer lining the intestine, a barrier that may theoretically hinder viral spread. The mechanism by which plant viruses cross the basal lamina is unknown. Here, we report that a plant virus has evolved to exploit virus-induced tubules to pass through the basal lamina from the initially infected midgut epithelium of the insect vector, thus revealing the previously undescribed pathway adapted by the virus for rapid dissemination of virions from the intestine of the insect 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.

Influence of rice black streaked dwarf virus on the ecological fitness of non-vector planthopper Nilaparvata lugens (Hemiptera: Delphacidae)

Rice black streak dwarf virus (RBSDV) is transmitted by the small brown planthopper (SBPH), Laodelphax striatellus (Fallen). Non-vector rice brown planthopper (BPH), Nilaparvata lugens (Stål), shares the same host rice plants with SBPH in paddy fields. The changes in nutritional composition of rice plants infected by RBSDV and the ecological fitness of BPH feeding on the infected plants were studied under both artificial climate chamber and field conditions. Contents of 16 detected amino acids and soluble sugar in RBSDV infected rice plants were higher than those in the healthy ones. On the diseased plants BPH had significantly higher nymphal survival rates, nymphal duration of the males, weight of the female adults, as well as egg hatchability compared to BPH being fed on healthy plants. However, there was no obvious difference in female nymph duration, longevity and fecundity. Defense enzymes (superoxidase dismutase, SOD and catalase, CAT) and detoxifying enzymes (carboxylesterase, CAE and glutathione S-transferase, GST) in BPH adults fed on diseased plants had markedly higher activities. The results indicate rice plants infected by RBSDV improved the ecological fitness of the brown planthopper, a serious pest but not a transmitter of the RBSDV virus.

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.

Interaction between Rice stripe virus disease-specific protein and host PsbP enhances virus symptoms

Rice stripe virus (RSV) causes severe diseases in Oryza sativa (rice) in many Eastern Asian countries. Disease-specific protein (SP) of RSV is a non-structural protein and its accumulation level in rice plant was shown to determine the severity of RSV symptoms. Here, we present evidence that expression of RSV SP alone in rice or Nicotiana benthamiana did not produce visible symptoms. Expression of SP in these two plants, however, enhanced RSV- or Potato virus X (PVX)-induced symptoms. Through yeast two-hybrid screening, GST pull-down, and bimolecular fluorescence complementation assays, we demonstrated that RSV SP interacted with PsbP, a 23-kDa oxygen-evolving complex protein, in both rice and N. benthamiana. Furthermore, our investigation showed that silencing of PsbP expression in both plants increased disease symptom severity and virus accumulation. Confocal microscopy using N. benthamiana protoplast showed that PsbP accumulated predominantly in chloroplast in wild-type N. benthamiana cells. In the presence of RSV SP, most PsbP was recruited into cytoplasm of the assayed cells. In addition, accumulation of SP during RSV infection resulted in alterations of chloroplast structure and function. Our findings shed light on the molecular mechanism underlying RSV disease symptom development.

Transovarial transmission of a plant virus is mediated by vitellogenin of its insect vector

Most plant viruses are transmitted by hemipteroid insects. Some viruses can be transmitted from female parent to offspring usually through eggs, but the mechanism of this transovarial transmission remains unclear. Rice stripe virus (RSV), a Tenuivirus, transmitted mainly by the small brown planthopper (Laodelphax striatellus), is also spread to the offspring through the eggs. Here, we used the RSV–planthopper system as a model to investigate the mechanism of transovarial transmission and demonstrated the central role of vitellogenin (Vg) of L. striatellus in the process of virus transmission into the eggs. Our data showed Vg can bind to pc3 in vivo and in vitro and colocalize in the germarium. RSV filamentous ribonucleoprotein particles (RNPs) only accumulated in the terminal filaments and pedicel areas prior to Vg expression and was not present in the germarium until Vg was expressed, where RSV RNPs and Vg had colocalized. Observations by immunoelectron microscopy (IEM) also indicated that these two proteins colocalized in nurse cells. Knockdown of Vg expression due to RNA interference resulted in inhibition of the invasion of ovarioles by RSV. Together, the data obtained indicated that RSV RNPs may enter the nurse cell of the germarium via endocytosis through binding with Vg. Finally, the virus enters the oocytes through nutritive cords, using the same route as for Vg transport. Our results show that the Vg of L. striatellus played a critical role in transovarial transmission of RSV and shows how viruses can use existing transovarial transportation systems in insect vectors for their own purposes.

Numerous parasites including viruses, bacteria, and microsporidia can be maternally transmitted, with the parasite passing from mother to offspring, usually through eggs. However, the process of the parasites spreading into eggs from primarily infected tissues and the factors that mediate this process in live hosts or vectors are unknown due to the lack of useful tools. Here, we used several techniques to investigate the molecular mechanisms of transovarial transmission of Rice stripe virus (RSV), a plant virus belonging to the genus Tenuivirus, by its insect vector (Laodelphax striatellus). We found that the nucleocapsid protein of RSV bound to insect's vitellogenin (Vg) in vitro and in vivo. We also found that RSV invaded the egg tubes of the ovariole until Vg is highly expressed, then colocalized with Vg in the germarium. When Vg expression was knocked down due to RNA interference, the invasion of ovarioles by RSV decreased largely. Our study provides new insights into the transovarial transmission of an important viral pathogen that uses existing transovarial transportation systems in insect vectors to invade eggs.

Southern rice black-streaked dwarf virus alters insect vectors' host orientation preferences to enhance spread and increase rice ragged stunt virus co-infection

In recent years, Southern rice black-streaked dwarf virus (SRBSDV), a tentative species in the genus Fijivirus (family Reoviridae), has spread rapidly and caused serious rice losses in eastern and southeastern Asia. With this virus spread, Rice ragged stunt virus (RRSV, genus Oryzavirus, family Reoviridae) became more common in southern China, usually in co-infection with the former. SRBSDV and RRSV are transmitted by two different species of planthoppers, white-backed planthopper (WBPH, Sogatella furcifera) and brown planthopper (BPH, Nilaparvata lugens), respectively, in a persistent, circulative, propagative manner. In this study, using a Y-shape olfactometer-based device, we tested the host preference of three types of macropterous WBPH adults for healthy or SRBSDV-infected rice plants. The results showed that virus-free WBPHs significantly preferred infected rice plants to healthy plants, whereas both the viruliferous and nonviruliferous WBPHs preferred healthy plants to infected plants. In additional tests, we found that the BPHs significantly preferred healthy plants when they were virus free, whereas RRSV-carrying BPHs preferred SRBSDV-infected rice plants. From these findings, we propose that plant viruses may alter host selection preference of vectors to enhance their spread and that of insects vectoring another virus to result in co-infection with more than one virus.

Comparative transcriptome analysis of two rice varieties in response to rice stripe virus and small brown planthoppers during early interaction

Rice stripe, a virus disease, transmitted by a small brown planthopper (SBPH), has greatly reduced production of japonica rice in East Asia, especially in China. Although we have made great progress in mapping resistance genes, little is known about the mechanism of resistance.

By de novo transcriptome assembling, we gained sufficient transcript data to analyze changes in gene expression of early interaction in response to SBPH and RSV infection in rice. Respectively 648 and 937 DEGs were detected from the disease-resistant (Liaonong 979) and the susceptible (Fengjin) varieties, most of which were up-regulated. We found 37 genes related to insect resistance, which mainly included genes for jasmonate-induced protein, TIFY protein, lipoxygenase, as well as trypsin inhibitor genes and transcription factor genes. In the interaction process between RSV and rice, 87 genes were thought to be related to RSV resistance; these primarily included 12 peroxidase biosynthesis genes, 12 LRR receptor-like protein kinase genes, 6 genes coding pathogenesis-related proteins, 4 glycine-rich cell wall structural protein genes, 2 xyloglucan hydrolase genes and a cellulose synthase. The results indicate that the rice-pathogen interaction happened both in disease-resistant and susceptible varieties, and some genes related to JA biosynthesis played key roles in the interaction between SBPHs and rice. When rice was infected by RSV a hypersensitive reaction (HR) in the disease-resistant variety was suppressed, which resulted from an increase in peroxidase expression and down-regulation of LRR receptor-like protein kinase and pathogenesis-related proteins, while, the changes of peroxidase biosynthesis, glycine-rich cell wall structural protein, cellulose synthase and xyloglucan endotransglucosylase/hydrolase could lead to the strengthening of physical barriers of rice, which may be an important resistance mechanism to RSV in rice.

A recessive resistance to rice yellow mottle virus is associated with a rice homolog of the CPR5 gene, a regulator of active defense mechanisms

RYMV2 is a major recessive resistance gene identified in cultivated African rice (Oryza glaberrima) which confers high resistance to the Rice yellow mottle virus (RYMV). We mapped RYMV2 in an approximately 30-kb interval in which four genes have been annotated. Sequencing of the candidate region in the resistant Tog7291 accession revealed a single mutation affecting a predicted gene, as compared with the RYMV-susceptible O. glaberrima CG14 reference sequence. This mutation was found to be a one-base deletion leading to a truncated and probably nonfunctional protein. It affected a gene homologous to the Arabidopsis thaliana CPR5 gene, known to be a defense mechanism regulator. Only seven O. glaberrima accessions showing this deletion were identified in a collection consisting of 417 accessions from three rice species. All seven accessions were resistant to RYMV, which is an additional argument in favor of the involvement of the deletion in resistance. In addition, fine mapping of a resistance quantitative trait locus in O. sativa advanced backcrossed lines pinpointed a 151-kb interval containing RYMV2, suggesting that allelic variants of the same gene may control both high and partial resistance.

Rice black-streaked dwarf virus P10 induces membranous structures at the ER and elicits the unfolded protein response in Nicotiana benthamiana

Endoplasmic reticular (ER) membrane modifications play an important role in viral RNA replication and virion assembly but little is known about the involvement of ER-membrane remodeling in the infection cycle of fijiviruses in plant cells. The subcellular localization of Rice black-streaked dwarf virus outer capsid P10 was therefore examined using live-cell imaging. P10 fused to eGFP formed vesicular structures associated with ER membranes in Nicotiana benthamiana epidermal cells and in rice protoplasts. Subcellular fractionation experiments confirmed that P10 is an integral membrane protein. Three predicted transmembrane domains and two less-well-defined domains were each able to target eGFP to the ER. Disruption of the actin cytoskeleton with LatB, indicated that the maintenance of P10-induced membrane structures required the intact actin cytoskeleton. P10 induced the expression of ER stress marker genes, including ER stress-related chaperones and transcription factor, indicating that RBSDV P10 triggers ER stress and the unfolded protein response.

Comparative proteomic analysis reveals the cross-talk between the responses induced by H2O2 and by long-term rice black-streaked dwarf virus infection in rice

Hydrogen peroxide (H₂O₂) could be produced during the plant-virus compatible interaction. However, the cell responses regulated by the enhanced H₂O₂ in virus infected plant are largely unknown. To make clear the influence of Rice black-streaked dwarf virus (RBSDV) infection on H₂O₂ accumulation, we measured the content of H₂O₂ and found the H₂O₂ level was increased in rice seedlings inoculated with RBSDV. To reveal the responses initiated by the enhanced H₂O₂ during plant-virus interaction, the present study investigated the global proteome changes of rice under long-term RBSDV infection. Approximately 1800 protein spots were detected on two-dimensional electrophoresis (2-DE) gels. Among them, 72 spots were found differently expressed, of which 69 spots were successfully identified by MALDI-TOF/TOF-MS. Furthermore, the differentially expressed proteins induced by RBSDV infection were compared to that induced by H₂O₂. 19 proteins corresponding to 37 spots, which were differentially expressed under RBSDV infection, were observed differentially expressed under H₂O₂ stress as well. These overlapping responsive proteins are mainly related to photosynthesis, redox homeostasis, metabolism, energy pathway, and cell wall modification. The increased H₂O₂ in RBSDV infected plant may produce an oxidative stress, impair photosynthesis, disturb the metabolism, and eventually result in abnormal growth. The data provide a new understanding of the pivotal role of H₂O₂ in rice-RBSDV compatible interaction.

Nonstructural protein P7-2 encoded by Rice black-streaked dwarf virus interacts with SKP1, a core subunit of SCF ubiquitin ligase

BACKGROUND

Rice black-streaked dwarf virus (RBSDV), a member of the genus Fijivirus within the family Reoviridae, causes severe damage to cereal crops in South East Asia. The protein P7-2, encoded by the second open reading frame of segment S7, is conserved among most plant-infecting fijiviruses, but its function is still obscure.

RESULTS

In this study, P7-2 was used as bait in two-hybrid screens of a cDNA library expressing Zea mays proteins. It was found that there is a strong interaction between P7-2 and Z. mays SKP1 (SKP1Maize), a core subunit of the multicomponent SCF (SKP1/Cullin1/F-box/Rbx1) E3 ubiquitin ligase. The interaction was then confirmed in leaf epidermal cells of Nicotiana benthamiana by bimolecular fluorescence complementation assay. Further investigations indicated that P7-2 also interacts with SKP1 proteins from other plants, including Arabidopsis thaliana, N. benthamiana,Oryza sativa and Saccharum sinense. The C-terminal fragment of SKP1Maize (residues 97-176) and the middle fragment of P7-2 (residues 79-214) are necessary to sustain the interaction, while the C-terminal putative α-helix domain spanning residues 214-295 of P7-2 greatly facilitates the interaction. Agrobacterium-mediated transient suppression assay showed that P7-2 has no obvious activity to suppress local RNA silencing.

CONCLUSIONS

Taken together, our results indicated that RBSDV P7-2 can interact with SKP1 proteins from different plants. This is the first report linking a Fijivirus protein to a component of the ubiquitin proteasome system. P7-2 might be a potential F-box protein encoded by RBSDV and involved in the plant-virus interaction through ubiquitination pathway.

Distribution and genetic diversity of Southern rice black-streaked dwarf virus in China

BACKGROUND

Rice and maize dwarf diseases caused by the newly introduced Southern rice black-streaked dwarf virus (SRBSDV) have led to severe economic losses in South China in recent years. The distribution and diversity of SRBSDV have not been investigated in the main rice and maize growing areas in China. In this study, the distribution of SRBSDV in China was determined by using reverse transcription-polymerase chain reaction (RT-PCR).

RESULTS

Between 2009 and 2010, 2404 plant samples (2294 rice, 110 maize samples, and more than 300 cultivars) with dwarf symptoms were collected from fields in 194 counties of 17 provinces in China and SRBSDV was detected. The results indicated that 1545 (64.27%) of samples (both rice and maize) were infected with SRBSDV. SRBSDV was detected widely in Hainan, Guangdong, Guangxi, Yunnan, Guizhou, Chongqing, Fujian, Jiangxi, Hunan, Hubei, Anhui, Jiangsu, and Zhejiang provinces, which suggests SRBSDV is an important pathogen causing rice dwarfing diseases in South China. Phylogenetic analysis of 15 representative virus isolates revealed that SRBSDV isolates in China had high levels of nucleotide and amino acid sequence identities (>97.8%).

CONCLUSIONS

SRBSDV spreads naturally in Yangtze River basin and south region, the location of the major rice production areas. In comparison, the virus rarely spreads north of Yangtze River in North China. Distribution of SRBSDV is consistent with the migrating and existing ranges of its vector WBPH, suggesting that SRBSDV might be introduced into South China along with the migration of viruliferous WBPH.

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.

Screening anti-southern rice black-streaked dwarf virus drugs based on S7-1 gene expression in rice suspension cells

Southern rice black-streaked dwarf virus (SRBSDV) is a rice pathogen that had an outbreak in southern China in 2010 and caused significant crop losses. Therefore, screening for effective antiviral drugs against SRBSDV is very important. This study used rice suspension cells infected with SRBSDV by polyethylene glycol-mediated uptake for screening antiviral drugs. SRBSDV P7-1, which is coded by the S7-1 gene, has an intrinsic ability to self-interact to form tubules that play an important role in viral infection. Therefore, relative expression level of the SRBSDV S7-1 gene in infected rice suspension cells was assayed by real-time quantitative polymerase chain reaction to evaluate the antiviral activities of various drugs. Dufulin displayed the highest inhibitory activity against SRBSDV S7-1 expression. In addition, changes in peroxidase (POD), polyphenol oxidase (PPO), and phenylalanine ammonia-lyase (PAL) activities were determined in inoculated and noninoculated cells. The results showed that both POD and PPO activities increased upon dufulin treatment. Furthermore, the validity of this approach was confirmed in an in vivo experiment in which dufulin was found to effectively inhibit SRBSDV.

The complete genome sequence of a south Indian isolate of Rice tungro spherical virus reveals evidence of genetic recombination between distinct isolates

In this study, complete genome of a south Indian isolate of Rice tungro spherical virus (RTSV) from Andhra Pradesh (AP) was sequenced, and the predicted amino acid sequence was analysed. The RTSV RNA genome consists of 12,171 nt without the poly(A) tail, encoding a putative typical polyprotein of 3,470 amino acids. Furthermore, cleavage sites and sequence motifs of the polyprotein were predicted. Multiple alignment with other RTSV isolates showed a nucleotide sequence identity of 95% to east Indian isolates and 90% to Philippines isolates. A phylogenetic tree based on complete genome sequence showed that Indian isolates clustered together, while Vt6 and PhilA isolates of Philippines formed two separate clusters. Twelve recombination events were detected in RNA genome of RTSV using the Recombination Detection Program version 3. Recombination analysis suggested significant role of 5' end and central region of genome in virus evolution. Further, AP and Odisha isolates appeared as important RTSV isolates involved in diversification of this virus in India through recombination phenomenon. The new addition of complete genome of first south Indian isolate provided an opportunity to establish the molecular evolution of RTSV through recombination analysis and phylogenetic relationship.

Transcription of ORFs on RNA2 and RNA4 of Rice stripe virus terminate at an AUCCGGAU sequence that is conserved in the genus Tenuivirus

Rice stripe virus, the type member of the genus Tenuivirus, has four genomic RNAs. RNAs 2-4 have an ambisense coding strategy and the noncoding intergenic regions (IRs) separating the two ORFs are thought to function in termination of transcription. Sequencing the 3'-untranslated region of transcripts from RNA2 and RNA4 in virus-infected Oryza sativa (the natural host), Nicotiana benthamiana (an experimental host) and Laodelphax striatellus (the vector), showed that the sequences of p2 and pc2 transcripts on RNA2, and p4 and pc4 transcripts on RNA4 terminated with high frequency at a palindromic sequence AUCCGGAU that was located in a region predicted to form a hairpin secondary structure. The AUCCGGAU sequence is highly conserved in RNA2 and RNA4 of different RSV isolates and is also conserved among the corresponding genomic RNAs of other tenuiviruses. p3 transcripts from the three hosts all had the same dominant termination site, while pc3 transcripts from different hosts terminated at different sites. All pc1 3'-UTR sequences ended at the 3'-end of the viral complementary strand of RNA1 (data not shown), indicating that the pc1 transcript may be synthesized by runoff of viral polymerase, but had no characteristic termination sequence. This is the first experimental report determining the exact transcription termination sites of a plant ambisense virus, and has implications for understanding the transcription of RSV as well as other plant viruses with an ambisense coding strategy.