A black-streaked dwarf disease on rice in China is caused by a novel fijivirus

Gamma-aminobutyric acid treatment promotes resistance against Sogatella furcifera in rice

The Sogatella furcifera (Horváth) (Homoptera: Delphacidae) is a white-backed planthopper (WBPH) that causes "hopper burn" in rice, resulting in severe yield loss. Gamma-aminobutyric acid (GABA) is a well-known neurotransmitter that inhibits neurotransmission in insects by binding to specific receptors. In this study, we investigated the potential role of GABA in modulating rice resistance to WBPH and evaluated possible defense mechanisms. The experiment was conducted in green house in pots consist of four groups: control, GABA-treated, WBPH-infested, and WBPH-infested treated with GABA. Among the various tested concentration of GABA, 15 mM GABA was applied as a single treatment in water. The treatment was administered one week before WBPH infestation. The results revealed that 15 mM GABA treatment strongly increased WBPH resistance. A plate-based assay indicated that direct application of 15 mM GABA increased the mortality rate of WBPH and increased the damage recovery rate in rice plants. We found that GABA treatment increased the activation of antioxidant enzymes and reduced the reactive oxygen species content and malondialdehyde contents, and reduced the damage rate caused by WBPH. Interestingly, GABA-supplemented plants infested with WBPH exhibited increased phenylalanine ammonia-lyase and pathogenesis-related (PR) genes expression levels. GABA induced the accumulation of abscisic acid (ABA) and salicylic acid (SA) and enhanced the stomata closure and reduced leaf vessels to reduce water conductance during WBPH stress. Furthermore, we found that GABA application to the plant induced the expression of Jasmonic acid (JA) biosynthesis genes (LOX, AOS, AOC, and OPR) and melatonin biosynthesis-related genes (TDC, T5H, ASMT, and SNAT). Our study suggested that GABA increases resistance against WBPH infestation by regulating antioxidant defense system, TCA cycle regulation, phytohormonal signaling, and PR gene regulation.

A systematic review of southern rice black-streaked dwarf virus in the age of omics

Southern rice black-streaked dwarf virus (SRBSDV) is one of the most damaging rice viruses. The virus decreases rice quality and yield, and poses a serious threat to food security. From this perspective, this review performed a survey of published studies in recent years to understand the current status of SRBSDV and white-backed planthopper (WBPH, Sogatella furcifera) transmission processes in rice. Recent studies have shown that the interactions between viral virulence proteins and rice susceptibility factors shape the transmission of SRBSDV. Moreover, the transmission of SRBSDV is influenced by the interactions between viral virulence proteins and S. furcifera susceptibility factors. This review focused on the molecular mechanisms of key genes or proteins associated with SRBSDV infection in rice via the S. furcifera vector, and the host defense response mechanisms against viral infection. A sustainable control strategy using RNAi was summarized to address this pest. Finally, we also present a model for screening anti-SRBSDV inhibitors using viral proteins as targets. © 2023 Society of Chemical Industry.

A plant virus hijacks phosphatidylinositol-3,5-bisphosphate to escape autophagic degradation in its insect vector

Hosts can initiate macroautophagy/autophagy as an antiviral defense response, while viruses have developed multiple ways to evade the host autophagic degradation. However, little is known as to whether viruses can target lipids to subvert autophagic degradation. Here, we show that a low abundant signaling lipid, phosphatidylinositol 3,5-bisphosphate (PtdIns(3,5)P2), is required for rice black-streaked dwarf virus (RBSDV) to evade the autophagic degradation in the insect vector Laodelphax striatellus. RBSDV binds to PtdIns(3,5)P2 and elevates its level through its main capsid protein P10, leading to inhibited autophagy and promoted virus propagation. Furthermore, we show that PtdIns(3,5)P2 inhibits the autophagy pathway by preventing the fusion of autophagosomes and lysosomes through activation of Trpml (transient receptor potential cation channel, mucolipin), an effector of PtdIns(3,5)P2. These findings uncover a strategy whereby a plant virus hijacks PtdIns(3,5)P2 via its viral capsid protein to evade autophagic degradation and promote its survival in insects.

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 fijiviral nonstructural protein triggers cell death in plant and bacterial cells via its transmembrane domain

Southern rice black-streaked dwarf virus (SRBSDV; Fijivirus, Reoviridae) has become a threat to cereal production in East Asia in recent years. Our previous cytopathologic studies have suggested that SRBSDV induces a process resembling programmed cell death in infected tissues that results in distinctive growth abnormalities. The viral product responsible for the cell death, however, remains unknown. Here P9-2 protein, but not its RNA, was shown to induce cell death in Escherichia coli and plant cells when expressed either locally with a transient expression vector or systemically using a heterologous virus. Both computer prediction and fluorescent assays indicated that the viral nonstructural protein was targeted to the plasma membrane (PM) and further modification of its subcellular localization abolished its ability to induce cell death, indicating that its PM localization was required for the cell death induction. P9-2 was predicted to harbour two transmembrane helices within its central hydrophobic domain. A series of mutation assays further showed that its central transmembrane hydrophobic domain was crucial for cell death induction and that its conserved F90, Y101, and L103 amino acid residues could play synergistic roles in maintaining its ability to induce cell death. Its homologues in other fijiviruses also induced cell death in plant and bacterial cells, implying that the fijiviral nonstructural protein may trigger cell death by targeting conserved cellular factors or via a highly conserved mechanism.

The Role of Exogenous Gibberellic Acid and Methyl Jasmonate against White-Backed Planthopper (Sogatella furcifera) Stress in Rice (Oryza sativa L.)

Rice (Oryza sativa L.) is one of the essential staple foods for more than half of the world's population, and its production is affected by different environmental abiotic and biotic stress conditions. The white-backed planthopper (WBPH, Sogatella furcifera) causes significant damage to rice plants, leading to substantial economic losses due to reduced production. In this experiment, we applied exogenous hormones (gibberellic acid and methyl jasmonate) to WBPH-infested rice plants and examined the relative expression of related genes, antioxidant accumulation, the recovery rate of affected plants, endogenous hormones, the accumulation of H₂O₂, and the rate of cell death using DAB and trypan staining, respectively. The expression of the transcriptional regulator (OsGAI) and gibberellic-acid-mediated signaling regulator (OsGID2) was upregulated significantly in GA 50 µM + WBPH after 36 h. OsGAI was upregulated in the control, GA 50 µM + WBPH, GA 100 µM + WBPH, and MeJA 100 µM + WBPH. However, after 48 h, the OsGID2 was significantly highly expressed in all groups of plants. The glutathione (GSH) values were significantly enhanced by GA 100 µM and MeJA 50 µM treatment. Unlike glutathione (GSH), the catalase (CAT) and peroxidase (POD) values were significantly reduced in control + WBPH plants. However, a slight increase in CAT and POD values was observed in GA 50 + WBPH plants and a reduction in the POD value was observed in GA 100 µM + WBPH and MeJA 50 µM + WBPH plants. GA highly recovered the WBPH-affected rice plants, while no recovery was seen in MeJA-treated plants. MeJA was highly accumulated in control + WBPH, MeJA 50 µM + WBPH, and GA 100 µM + WBPH plants. The H₂O₂ accumulation was highly decreased in GA-treated plants, while extensive cell death was observed in MeJA-treated plants compared with GA-treated plants. From this study, we can conclude that the exogenous application of GA can overcome the effects of the WBPH and enhance resistance in rice.

Plant Viral Coat Proteins as Biochemical Targets for Antiviral Compounds

Coat proteins (CPs) of RNA plant viruses play a pivotal role in virus particle assembly, vector transmission, host identification, RNA replication, and intracellular and intercellular movement. Numerous compounds targeting CPs have been designed, synthesized, and screened for their antiviral activities. This review is intended to fill a knowledge gap where a comprehensive summary is needed for antiviral agent discovery based on plant viral CPs. In this review, major achievements are summarized with emphasis on plant viral CPs as biochemical targets and action mechanisms of antiviral agents. This review hopefully provides new insights and references for the further development of new safe and effective antiviral pesticides.

Genome-Wide Identification and Gene Expression Analysis of the OTU DUB Family in Oryza sativa

Ovarian tumor domain (OTU)-containing deubiquitinating enzymes (DUBs) are an essential DUB to maintain protein stability in plants and play important roles in plant growth development and stress response. However, there is little genome-wide identification and analysis of the OTU gene family in rice. In this study, we identified 20 genes of the OTU family in rice genome, which were classified into four groups based on the phylogenetic analysis. Their gene structures, conserved motifs and domains, chromosomal distribution, and cis elements in promoters were further studied. In addition, OTU gene expression patterns in response to plant hormone treatments, including SA, MeJA, NAA, BL, and ABA, were investigated by RT-qPCR analysis. The results showed that the expression profile of OsOTU genes exhibited plant hormone-specific expression. Expression levels of most of the rice OTU genes were significantly changed in response to rice stripe virus (RSV), rice black-streaked dwarf virus (RBSDV), Southern rice black-streaked dwarf virus (SRBSDV), and Rice stripe mosaic virus (RSMV). These results suggest that the rice OTU genes are involved in diverse hormone signaling pathways and in varied responses to virus infection, providing new insights for further functional study of OsOTU genes.

Analyses on the Infection Process of Rice Virus and the Spatiotemporal Expression Pattern of Host Defense Genes Based on a Determined-Part Inoculation Approach

Rice viral diseases adversely affect crop yield and quality. Most rice viruses are transmitted through insect vectors. However, the traditional whole-plant inoculation method cannot control the initial inoculation site in rice plants because the insect feeding sites in plants are random. To solve this problem, we established a determined-part inoculation approach in this study that restricted the insect feeding sites to specific parts of the rice plant. Rice stripe virus (RSV) was used as the model virus and was inoculated at the bottom of the stem using our method. Quantitative real-time PCR and Western blot analyses detected RSV only present at the bottom of the Nipponbare (NPB) stem at 1 day post-inoculation (dpi), indicating that our method successfully controlled the inoculation site. With time, RSV gradually moved from the bottom of the stem to the leaf in NPB rice plants, indicating that systemic viral spread can also be monitored using this method. In addition, a cultivar resistant to RSV, Zhendao 88 (ZD88), was inoculated using this method. We found that RSV accumulation in ZD88 was significantly lower than in NPB. Additionally, the expression level of the resistant gene STV11 in ZD88 was highly induced at the initial invasion stage of RSV (1 dpi) at the inoculation site, whereas it remained relatively stable at non-inoculated sites. This finding indicated that STV11 directly responded to RSV invasion to inhibit virus accumulation at the invasion site. We also proved that this approach is suitable for other rice viruses, such as Rice black-streaked dwarf virus (RBSDV). Interestingly, we determined that systemic infection with RSV was faster than that with RBSDV in NPB, which was consistent with findings in field trails. In summary, this approach is suitable for characterizing the viral infection process in rice plants, comparing the local viral accumulation and spread among different cultivars, analyzing the spatiotemporal expression pattern of resistance-associated genes, and monitoring the infection rate for different viruses.

Identification of Viruses Infecting Oats in Korea by Metatranscriptomics

Controlling infectious plant viruses presents a constant challenge in agriculture. As a source of valuable nutrients for human health, the cultivation of oats (Avena sativa L.) has recently been increased in Korea. To date, however, few studies have been undertaken to identify the viruses infecting oats in this country. In this study, we carried out RNA-sequencing followed by bioinformatics analyses to understand the virosphere in six different geographical locations in Korea where oats are cultivated. We identified three different virus species, namely, barley yellow dwarf virus (BYDV) (BYDV-PAV and BYDV-PAS), cereal yellow dwarf virus (CYDV) (CYDV-RPS and CYDV-RPV), and rice black-streaked dwarf virus (RBSDV). Based on the number of virus-associated reads and contigs, BYDV-PAV was a dominant virus infecting winter oats in Korea. Interestingly, RBSDV was identified in only a single region, and this is the first report of this virus infecting oats in Korea. Single nucleotide polymorphisms analyses indicated that most BYDV, CYDV, and RBSDV isolates show considerable genetic variations. Phylogenetic analyses indicated that BYDVs and CYDVs were largely grouped in isolates from Asia and USA, whereas RBSDV was genetically similar to isolates from China. Overall, the findings of this study provide a preliminary characterization of the types of plant viruses infecting oats in six geographical regions of Korea.

Recombinase polymerase amplification assay for sensitive and rapid detection of southern rice black-streaked dwarf virus in plants

Southern rice black-streaked dwarf virus (SRBSDV) naturally infects rice and maize plants through white-backed planthopper (Sogatella furcifera) causing significant crop losses in China and Vietnam. Thus, rapid and accurate detection methods for SRBSDV are urgently needed. Recombinase polymerase amplification (RPA) is a novel technique for rapid and sensitive detection of nucleic acids. In this research, a reverse transcription (RT)-RPA-based method was developed for the detection of SRBSDV. A pair of RPA primers targeting the conserved sequences within the SP10 (major coat protein) gene on genomic RNA S10 of SRBSDV were designed. The assay was performed in a single tube at 39 °C for 20 min and demonstrated that the RPA assay is an efficient alternative for rapid detection of SRBSDV.

The Quantitative Trait Loci Mapping of Rice Plant and the Components of Its Extract Confirmed the Anti-Inflammatory and Platelet Aggregation Effects In Vitro and In Vivo

Unpredictable climate change might cause serious lack of food in the world. Therefore, in the present world, it is urgent to prepare countermeasures to solve problems in terms of human survival. In this research, quantitative trait loci (QTLs) were analyzed when rice attacked by white backed planthopper (WBPH) were analyzed using 120 Cheongcheong/Nagdong double haploid lines. Moreover, from the detected QTLs, WBPH resistance-related genes were screened in large candidate genes. Among them, OsCM, a major gene in the synthesis of Cochlioquinone-9 (cq-9), was screened. OsCM has high homology with the sequence of chorismate mutase, and exists in various functional and structural forms in plants that produce aromatic amino acids. It also induces resistance to biotic stress through the synthesis of secondary metabolites in plants. The WBPH resistance was improved in rice overexpressed through map-based cloning of the WBPH resistance-related gene OsCM, which was finally detected by QTL mapping. In addition, cq-9 increased the survival rate of caecal ligation puncture (CLP)-surgery mice by 60%. Moreover, the aorta of rat treated with cq-9 was effective in vasodilation response and significantly reduced the aggregation of rat platelets induced by collagen treatment. A cq-9, which is strongly associated with resistance to WBPH in rice, is also associated with positive effect of CLP surgery mice survival rate, vasodilation, and significantly reduced rat platelet aggregation induced by collagen treatment. Therefore, cq-9 presents research possibilities as a substance in a new paradigm that can act on both Plant-Insect in response to the present unpredictable future.

Southern rice black-streaked dwarf virus hijacks SNARE complex of its insect vector for its effective transmission to rice

Vesicular trafficking is an important dynamic process that facilitates intracellular transport of biological macromolecules and their release into the extracellular environment. However, little is known about whether or how plant viruses utilize intracellular vesicles to their advantage. Here, we report that southern rice black-streaked dwarf virus (SRBSDV) enters intracellular vesicles in epithelial cells of its insect vector by engaging VAMP7 and Vti1a proteins in the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex. The major outer capsid protein P10 of SRBSDV was shown to interact with VAMP7 and Vti1a of the white-backed planthopper and promote the fusion of vesicles into a large vesicle, which finally fused with the plasma membrane to release virions from midgut epithelial cells. Downregulation of the expression of either VAMP7 or Vti1a did not affect viral entry and accumulation in the gut, but significantly reduced viral accumulation in the haemolymph. It also did not affect virus acquisition, but significantly reduced the virus transmission efficiency to rice. Our data reveal a critical mechanism by which a plant reovirus hijacks the vesicle transport system to overcome the midgut escape barrier in vector insects and provide new insights into the role of the SNARE complex in viral transmission and the potential for developing novel strategies of viral disease control.

Changes in Transcriptome and Gene Expression in Sogatella furcifera (Hemiptera: Delphacidae) in Response to Cycloxaprid

The white-backed planthopper, Sogatella furcifera (Horváth), causes substantial damage to crops by direct feeding or virus transmission, especially southern rice black-streaked dwarf virus, which poses a serious threat to rice production. Cycloxaprid, a novel cis-nitromethylene neonicotinoid insecticide, has high efficacy against rice planthoppers, including imidacloprid-resistant populations. However, information about the influence of cycloxaprid on S. furcifera (Hemiptera: Delphacidae) at the molecular level is limited. Here, by de novo transcriptome sequencing and assembly, we constructed two transcriptomes of S. furcifera and profiled the changes in gene expression in response to cycloxaprid at the transcription level. We identified 157,906,456 nucleotides and 131,601 unigenes using the Illumina technology from cycloxaprid-treated and untreated S. furcifera. In total, 38,534 unigenes matched known proteins in at least one database, accounting for 29.28% of the total unigenes. The number of coding DNA sequences was 28,546 and that of amino acid sequences in the coding region was 22,299. In total, 15,868 simple sequence repeats (SSRs) were identified. The trinucleotide repeats accounted for 45.1% (7,157) of the total SSRs and (AAG/CTT)n were the most frequent motif. There were 359 differentially expressed genes that might have been induced by cycloxaprid. There were 131 upregulated and 228 downregulated genes. Twenty-two unigenes might be involved in resistance against cycloxaprid, such as cytochrome P450, glutathione S-transferase (GST), acid phosphatase (ACP), and cadherin. Our study provides vital information on cycloxaprid-induced resistance mechanisms, which will be useful to analyze the molecular mechanisms of cycloxaprid resistance and may lead to the development of novel strategies to manage S. furcifera.

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.

Overexpression of OsF3H modulates WBPH stress by alteration of phenylpropanoid pathway at a transcriptomic and metabolomic level in Oryza sativa

The whitebacked planthopper (WBPH), has become a devastating pest for rice crops, causes serious yield losses each year, and urgently needs biological control. Here, we developed a WBPH-resistant rice cultivar by overexpressing the OsF3H gene. A genetic functional analysis of the OsF3H gene confirmed its role in facilitating flavonoid contents and have indicated that the expression of the OsF3H gene is involved in regulation of the downstream genes (OsDFR and OsFLS) of the flavonoid pathway and genes (OsSLR1 and OsWRKY13) involved in other physiological pathways. OxF3H (OsF3H transgenic) plants accumulated significant amounts of the flavonols kaempferol (Kr) and quercetin (Qu) and the anthocyanins delphinidin and cyanidin, compared to the wild type, in response to the stress induced by WBPH. Similarly, OsF3H-related proteins were significantly expressed in OxF3H lines after WBPH infestation. The present study, indicated that the regulation of JA in OxF3H plants was suppressed due the overexpression of the OsF3H gene, which induced the expression of downstream genes related to anthocyanin. Similarly, the OsWRKY13 transcriptional factor was significantly suppressed in OxF3H plants during WBPH infestation. Exogenous application of Kr and Qu increased the survival rates of susceptible TN1 lines in response to WBPH, while decreased the survival rate of first instar WBPHs, indicating that both flavonols exhibit pesticide activity. Phenotypic demonstration also affirms that OxF3H plants show strong resistance to WBPH compared with wild type. Collectively, our result suggested that OsF3H overexpression led to the up-regulation of defense related genes and enhanced rice resistance to WBPH infestation.

Emergence of Southern Rice Black-Streaked Dwarf Virus in the Centuries-Old Chinese Yuanyang Agrosystem of Rice Landraces

Southern rice black-streaked dwarf virus (SRBSDV), which causes severe disease symptoms in rice (Oriza sativa L.) has been emerging in the last decade throughout northern Vietnam, southern Japan and southern, central and eastern China. Here we attempt to quantify the prevalence of SRBSDV in the Honghe Hani rice terraces system (HHRTS)-a Chinese 1300-year-old traditional rice production system. We first confirm that genetically diverse rice varieties are still being cultivated in the HHRTS and categorize these varieties into three main genetic clusters, including the modern hybrid varieties group (MH), the Hongyang improved modern variety group (HY) and the traditional indica landraces group (TIL). We also show over a 2-year period that SRBSDV remains prevalent in the HHRTS (20.1% prevalence) and that both the TIL (17.9% prevalence) and the MH varieties (5.1% prevalence) were less affected by SRBSDV than were the HY varieties (30.2% prevalence). Collectively we suggest that SRBSDV isolates are freely moving within the HHRTS and that TIL, HY and MH rice genetic clusters are not being preferentially infected by particular SRBSDV lineages. Given that SRBSDV can cause 30-50% rice yield losses, our study emphasizes both the need to better monitor the disease in the HHRTS, and the need to start considering ways to reduce its burden on rice production.

The Dual Effect of the Brassinosteroid Pathway on Rice Black-Streaked Dwarf Virus Infection by Modulating the Peroxidase-Mediated Oxidative Burst and Plant Defense

The phytohormone brassinosteroid (BR) not only plays key roles in regulating plant growth and development but is also involved in modulating the plant defense system in response to pathogens. We previously found that BR application made rice plants more susceptible to the devastating pathogen rice black-streaked dwarf virus (RBSDV), but the mechanism of BR-mediated susceptibility remains unclear. We now show that both BR-deficient and -insensitive mutants are resistant to RBSDV infection. High-throughput sequencing showed that the defense hormone salicylic acid and jasmonic acid pathways were activated in the RBSDV-infected BR mutant. Meanwhile, a number of class III peroxidases (OsPrx) were significantly changed and basal reactive oxygen species (ROS) accumulated in BR mutants. Treatment with exogenous hormones and other chemicals demonstrated that the BR pathway could suppress the levels of OsPrx and the ROS burst by directly binding the promoters of OsPrx genes. Together, our findings indicate that BR-mediated susceptibility is at least partly caused by inhibition of the action of defense hormones, preventing the accumulation of the peroxidase-mediated oxidative burst.

Close evolutionary relationship between rice black-streaked dwarf virus and southern rice black-streaked dwarf virus based on analysis of their bicistronic RNAs

Background

Rice black-streaked dwarf virus (RBSDV) and Southern rice black-streaked dwarf virus (SRBSDV) seriously interfered in the production of rice and maize in China. These two viruses are members of the genus Fijivirus in the family Reoviridae and can cause similar dwarf symptoms in rice. Although some studies have reported the phylogenetic analysis on RBSDV or SRBSDV, the evolutionary relationship between these viruses is scarce.

Methods

In this study, we analyzed the evolutionary relationships between RBSDV and SRBSDV based on the data from the analysis of codon usage, RNA recombination and phylogenetic relationship, selection pressure and genetic characteristics of the bicistronic RNAs (S5, S7 and S9).

Results

RBSDV and SRBSDV showed similar patterns of codon preference: open reading frames (ORFs) in S7 and S5 had with higher and lower codon usage bias, respectively. Some isolates from RBSDV and SRBSDV formed a clade in the phylogenetic tree of S7 and S9. In addition, some recombination events in S9 occurred between RBSDV and SRBSDV.

Conclusions

Our results suggest close evolutionary relationships between RBSDV and SRBSDV. Selection pressure, gene flow, and neutrality tests also supported the evolutionary relationships.

Electronic supplementary material

The online version of this article (10.1186/s12985-019-1163-3) contains supplementary material, which is available to authorized users.

Differences in Molecular Characteristics of Segment 8 in Rice black-streaked dwarf virus and Southern rice black-streaked dwarf virus

Rice black-streaked dwarf virus (RBSDV) and Southern rice black-streaked dwarf virus (SRBSDV) cause maize rough dwarf disease (MRDD) and rice black-streaked dwarf disease (RBSDD) in China. RBSDV segment 8 (S8) contains the only deletion mutation in the genomes of these viruses, which are both members of the genus Fijivirus. To illuminate the molecular differences between the RBSDV and SRBSDV genomes and better understand the evolution of these viruses, and to determine which virus is specifically associated with MRDD and RBSDD in each region, S8 was analyzed in 66 virus isolates collected from 10 geographic locations in China and 14 S8 sequences obtained from the National Center for Biotechnology Information GenBank. Phylogenetic analysis showed that the pathogen associated with MRDD and RBSDD in the Yellow and Huai River valleys was RBSDV, whereas the pathogen associated with these diseases in Sanya was SRBSDV. Codon usage bias in S8 differed significantly between RBSDV and SRBSDV, as indicated by effective number of codons used by a gene (Nc) and GC values, Nc plots, and variation explained by the first axis in correspondence analysis. The nucleotide identities among these 66 RBSDV and SRBSDV isolates ranged from 66.2 to 68.2%, and were considerably lower than the nucleotide identities within RBSDV (from 94.1 to 99.9%) or SRBSDV (from 93.9 to 100%) isolates. Most S8 polymorphisms were identified in the region from 1,000 to 1,200 bp in RBSDV and in the region from 500 to 700 bp in SRBSDV. The difference in the lengths of RBSDV (1,936 bp) and SRBSDV (1,928 bp) was due to an 8-bp deletion in the 3'-untranslated region of SRBSDV. Six recombination events were detected in S8 in RBSDV and two recombination events were detected in S8 in SRBSDV. Recombination breakpoints were found within the region containing the deletion mutation in nine isolates. However, no recombination events were detected between RBSDV and SRBSDV. Both of these viruses were under negative and purifying selection, although the ratio of nonsynonymous mutations to synonymous mutations (Ka/Ks) for RBSDV S8 (0.0530) was not significantly lower than that of SRBSDV S8 (0.0823, P = 0.1550). We found that SRBSDV was more highly genetically differentiated (product of effective population size and the migration rate among populations < 1; values for the among-populations component of genetic variation or normalized variation > 0.33; and P values of the sequence statistic, the rank statistic, and the nearest-neighbor statistic < 0.01) than RBSDV. However, gene flow between RBSDV and SRBSDV was not frequent.

Label-free quantitative proteomics analysis of Cytosinpeptidemycin responses in southern rice black-streaked dwarf virus-infected rice

Southern rice black-streaked dwarf virus (SRBSDV), a genus Fijivirus of the family Reoviridae, could result in the significant crop losses because being short of an effective controlling measures. Cytosinpeptidemycin, a microbial pesticides developed by China, displayed a wide antiviral activity against many plant viruses. However, its underlying mechanism remains unclear. In this study, a total of 2321 proteins were identified using label-free proteomics technology. Compared with the treatment of SRBSDV-infected rice, 84 and 207 proteins were detected to be up-regulated and only presented in treatment group of SDBSDV-infected rice pre-treated by Cytosinpeptidemycin, which were partially enriched to stress and defense response, such as pathogenesis-related protein 5 (PR-5), pathogenesis-related protein 10 (PR-10) and heat shock protein (Hsp protein). Meanwhile, the real-time quantitative PCR (RT-qPCR) showed that Cytosinpeptidemycin could also up-regulate some resistance genes, and these results indicated a similar trends with the data of the label-free proteomics. Moreover, Cytosinpeptidemycin could enhance the defense enzymatic activities of peroxidase (POD), superoxide dismutase (SOD), and catalase (CAT). These data offer a more comprehensive view about the response of SRBSDV-infected rice triggered by Cytosinpeptidemycin in the level of the proteome, mRNA and enzymatic activity.

Selective toxicity of the mesoionic insecticide, triflumezopyrim, to rice planthoppers and beneficial arthropods

The novel mesoionic insecticide triflumezopyrim was highly effective in controlling both imidacloprid-susceptible and resistant planthopper populations in Malaysia. However, the toxicity of triflumezopyrim to planthopper populations and their natural enemies has been under-investigated in China. In this study, the median lethal concentrations (LC₅₀) of triflumezopyrim were determined in eight field populations of Nilaparvata lugens and one population of Sogatella furcifera from China under laboratory conditions. Triflumezopyrim showed higher toxicity to planthopper populations than the commonly-used insecticide, imidacloprid. Furthermore, the lethal effect of triflumezopyrim on eight beneficial arthropods of planthoppers was investigated in the laboratory and compared with three commonly-used insecticides, thiamethoxam, chlorpyrifos and abamectin. Triflumezopyrim was harmless to Anagrus nilaparvatae, Cyrtorhinus lividipennis and Paederus fuscipes, while thiamethoxam, chlorpyrifos and abamectin were moderately harmful or harmful to the insect parasitoid and predators. Triflumezopyrim and thiamethoxam were harmless to the predatory spiders Pirata subpiraticus, Ummeliata insecticeps, Hylyphantes graminicola and Pardosa pseudoannulata, and slightly harmful to Theridion octomaculatum. Chlorpyrifos caused slight to high toxicity to four spider species except U. insecticeps. Abamectin was moderately to highly toxic to all five spider species. Our results indicate that triflumezopyrim has high efficacy for rice planthoppers populations and is compatibile with their natural enemies in China.

Investigation of alimentary canal ultrastructure following knockdown of the Dicer-2 gene in planthoppers reveals the potential pathogenicity of southern rice black streaked dwarf virus to its insect vector

An increasing number of studies are suggesting that plant viruses, including southern rice black-streaked dwarf virus (SRBSDV), can adversely affect biological characteristics of insect vectors by unknown mechanisms. To study the adverse effect of SRBSDV at cellular level on the insect vector, we promoted viral infection by the disruption of the small interfering RNA (siRNA) pathway. The transmission electron microscopy was utilized to describe the ultrastructural changes that occurred in insects when the core component of the siRNA pathway, Dicer-2, was knocked down. The increasing accumulation of SRBSDV in virus-infected vector, the white-backed planthoppers, caused severe cytopathology in the alimentary canal. Similar cytopathology changes in the midgut ultrastructure were characterized in the virus-infected incompetent vector, the small brown planthopper. These results not only add support to the existing evidence suggesting that the siRNA pathway has an antiviral effect, but also reveal the universal and potential ability of SRBSDV to cause damage to the insect tissues of both the vector and non-vector.

Identification of virus-derived siRNAs and their targets in RBSDV-infected rice by deep sequencing

RNA interference (RNAi) is a conserved mechanism against viruses in plants and animals. It is thought to inactivate the viral genome by producing virus-derived small interfering RNAs (vsiRNAs). Rice black-streaked dwarf virus (RBSDV) is transmitted to plants by the small brown planthopper (Laodelphax striatellus), and seriously threatens production of rice in East Asia, particularly Oryza sativa japonica subspecies. Through deep sequencing, genome-wide comparisons of RBSDV-derived vsiRNAs were made between the japonica variety Nipponbare, and the indica variety 9311. Four small RNA libraries were constructed from the leaves and shoots of each variety. We found 659,756 unique vsiRNAs in the four samples, and only 43,485 reads were commonly shared. The size distributions of vsiRNAs were mostly 21- and 22-nt long, and A/U bias (66-68%) existed at the first nucleotide of vsiRNAs. Additionally, vsiRNAs were continuously but heterogeneously distributed along S1-S10 segments of the RBSDV genome. Distribution profiles of vsiRNA hotspots were similar in different hosts and tissues, and the 5'- and 3'-terminal regions of S4, S5, and S8 had more hotspots. Distribution and abundance of RBSDV vsiRNAs could be useful in designing efficient targets for exploiting RNA interference for virus resistance. Degradome analysis found 25 and 11 host genes appeared to be targeted by vsiRNAs in 9311 and Nipponbare. We report for the first time vsiRNAs derived from RBSDV-infected rice.

Migration Analysis of Sogatella furcifera (Hemiptera: Delphacidae) in the Northeastern Hunan Province in June

The Sogatella furcifera (Horváth) is a notorious and destructive insect pest, targeting >4 million hectares of rice cultivated in the Hunan Province. To understand the immigration dynamics, we collected S. furcifera light trap catches from 2006 to 2012 at Niangxiang, Linxiang, and Hanshou. We conducted a migration analysis to estimate the immigration source for the northeastern Hunan Province in June. Moreover, we dissected the ovaries of S. furcifera to classify population characteristics. We found that the first appearance of S. furcifera occurred from late April to early May, with June as the primary time for migrations into the northeastern Hunan Province. The majority of June ovaries caught in light traps were Grade I and Grade II, whereas those collected in paddy fields were Grade III and Grade IV, suggesting that the majority of S. furcifera immigrated into the northeastern Hunan Province. Our analysis points toward the northern and central Indo China Peninsula, the southern Hunan Province, the Guangxi Zhuang Autonomous Region Province, the Guangdong Province, and the Hainan Province as possible immigration sources of S. furcifera in June for the northeastern Hunan Province. We propose terrain, loss of forward flow, and wind shear as causes for the difference observed in light trap catches between Ningxiang, Linxiang, and Hanshou monitoring stations in the June of 2009. Thus, our results suggest that monitoring and forecasting of S. furcifera should be done with particular emphasis in June.

Expression Differences of Resistance-Related Genes Induced by Cycloxaprid Using qRT-PCR in the Female Adult of Sogatella furcifera (Hemiptera: Delphacidae)

As a newer cis-nitromethylene neonicotinoid pesticide at present, cycloxaprid has good industrialization prospects, including the management of imidacloprid-resistant populations, because this chemical have an excellent efficiency against rice planthoppers. Sogatella furcifera (Horváth) is the most economically important pest of rice worldwide and has developed resistance to many insecticides. This study focused on the expression change of these resistance genes, induced by cycloxaprid, involved in metabolic detoxification and receptor protein. Twenty-two differentially expressed genes (DEGs) that may be related with the insecticide resistance were found in the transcriptome of S. furcifera, including 2 cytochrome P450 genes, 2 glutathione S-transferase (GST) genes, 1 acid phosphatase (ACP) gene, 12 decarboxylase genes, 2 glycolipid genes, 1 cadherin gene, and 2 glycosyltransferase genes, which were up- or downregulated in response to an exposure of cycloxaprid. Furthermore, two P450 genes (CYP4 and CYP6 family, respectively), two decarboxylase genes, and one glycosyltransferase gene were validated by qRT-PCR. Expression differences of these genes verified successfully by qRT-PCR in response to different concentrations and times treated with cycloxaprid could explain the insecticide resistance mechanism under cycloxaprid stress in S. furcifera.

Effects of Transgenic Rice Infected with SRBSDV on Bt expression and the Ecological Fitness of Non-vector Brown Planthopper Nilaparvata lugens

The susceptibility of rice lines, T1C-19, T2A-1, and MH63 to SRBSDV infection are similar and the contents of cry protein in T2A-1 and T1C-19 do not change significantly. The survival rates of BPH nymphs feeding on SRBSDV-infected T1C-19, Bt T2A-1, or MH63 rice plants were not significantly different. The developmental stages of female BPH fed on T1C-19 plants infected with SRBSDV were significantly shorter than those fed on uninfected rice, while the males showed no significant difference. The duration of BPH feeding on SRBSDV-infected T2A-1 and MH63 also showed no significant difference in comparison with the respective control groups. Longevities of BPH adults feeding on SRBSDV-infected T1C-19, T2A-1 or MH63 were also not significant. However, the longevity of male adult BPH feeding on un-infected MH63 was significantly reduced in comparison with that of adult males feeding on un-infected T1C-19 and T2A-1 rice. In addition, the different rice lines and the rice plants infected and uninfected with SRBSDV did not significantly affect the sex ratio, female body weight, longevity, fecundity, or egg hatchability of BPH. In general, transgenic Bt rice infected with SRBSDV had little effect on the ecological adaptability of BPH.

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.

Label-Free Quantitative Proteomic Analysis of Chitosan Oligosaccharide-Treated Rice Infected with Southern Rice Black-Streaked Dwarf Virus

Southern rice black-streaked dwarf virus (SRBSDV) has spread from thesouth of China to the north of Vietnam in the past few years and severelyinfluenced rice production. Its long incubation period and early symptoms are not evident; thus, controlling it is difficult. Chitosan oligosaccharide (COS) is a green plant immunomodulator. Early studies showed that preventing and controlling SRBSDV have a certain effect and reduce disease infection rate, but its underlying controlling and preventing mechanism is unclear. In this study, label-free proteomics was used to analyze differentially expressed proteins in rice after COS treatment. The results showed that COS can up-regulate the plant defense-related proteins and down-regulate the protein expression levels of SRBSDV. Meanwhile, quantitative real-time PCR test results showed that COS can improve defense gene expression in rice. Moreover, COS can enhance the defense enzymatic activities of peroxidase, superoxide dismutase and catalase through mitogen-activated protein kinase signaling cascade pathway, and enhance the rice disease resistance.

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).

Analysis of Sogatella furcifera proteome that interact with P10 protein of Southern rice black-streaked dwarf virus

Southern rice black-streaked dwarf virus (SRBSDV) is transmitted efficiently only by white-backed planthopper (WBPH, Sogatella furcifera) in a persistent propagative manner. Here we used a yeast two-hybrid system to investigate the interactions between the SRBSDV- P10 and the cDNA library of WBPH. Of 130 proteins identified as putative interactors, 28 were further tested in a retransformation analysis and β-galactosidase assay to confirm the interaction. The full-length gene sequences of 5 candidate proteins: vesicle-associated membrane protein 7 (VAMP7), vesicle transport V-SNARE protein (Vti1A), growth hormone-inducible transmembrane protein (Ghitm), nascent polypeptide-associated complex subunit alpha, and ATP synthase lipid-binding protein) were amplified by 5' rapid amplification of cDNA ends (RACE) and used in a GST fusion protein pull-down assay. Three of these proteins interacted with SRBSDV-P10 in vitro experiment GST pull-down assay. In a gene expression analysis of 3 different growth stages and 6 different tissue organs of S. furcifera, the mRNA level of VAMP7 was high in adult males and gut. Vti1A was abundant in adult female, and malpighian tubule, gut and ovary. Ghitm was predominantly found in adult male and the malpighian tubule. These research findings are greatly helpful to understand the interaction between SRBSDV and insect vector.

Rice Reoviruses in Insect Vectors

Rice reoviruses, transmitted by leafhopper or planthopper vectors in a persistent propagative manner, seriously threaten the stability of rice production in Asia. Understanding the mechanisms that enable viral transmission by insect vectors is a key to controlling these viral diseases. This review describes current understanding of replication cycles of rice reoviruses in vector cell lines, transmission barriers, and molecular determinants of vector competence and persistent infection. Despite recent breakthroughs, such as the discoveries of actin-based tubule motility exploited by viruses to overcome transmission barriers and mutually beneficial relationships between viruses and bacterial symbionts, there are still many gaps in our knowledge of transmission mechanisms. Advances in genome sequencing, reverse genetics systems, and molecular technologies will help to address these problems. Investigating the multiple interaction systems among the virus, insect vector, insect symbiont, and plant during natural infection in the field is a central topic for future research on rice reoviruses.

Annual Fluctuations of Early Immigrant Populations of Sogatella furcifera (Hemiptera: Delphacidae) in Jiangxi Province, China

The white-backed planthopper, Sogatella furcifera (Horváth), is a destructive migratory pest in east and southeast Asia. Huge populations stemming from annual migrations by this insect have caused a series of devastating losses to rice production. There have been numerous early immigrations in five of the past 10 yr but few early immigrations in the others. The annual fluctuation in early immigration is evident, but the mechanism behind these annual fluctuations is unclear. This research aimed to determine the underlying causes for the annual fluctuations in early immigration. We used trajectory analysis to explore the source areas and investigated the meteorological conditions to determine the reason for the annual fluctuations. The results showed that 1) the source areas of S. furcifera are mainly located west of Guangdong and east of Guangxi; 2) the annual fluctuations of the immigrant population size is significantly correlated with the frequency of prevailing winds; and 3) early immigration is influenced by both winter and spring temperatures in the south central Indochina peninsula. These results indicated that an allopatric prediction and sustainable management of rice planthoppers would be difficult to implement within one country. International cooperation and information exchange about this pest between China and other countries in Southeast Asia should be implemented.

RNAi knockdown of acetyl-CoA carboxylase gene eliminates jinggangmycin-enhanced reproduction and population growth in the brown planthopper, Nilaparvata lugens

A major challenge in ecology lies in understanding the coexistence of intraguild species, well documented at the organismal level, but not at the molecular level. This study focused on the effects of the antibiotic, jinggangmycin (JGM), a fungicide widely used in Asian rice agroecosystems, on reproduction of insects within the planthopper guild, including the brown planthopper (BPH) Nilaparvata lugens and the white-backed planthopper (WBPH) Sogatella furcifera, both serious resurgence rice pests. JGM exposure significantly increased BPH fecundity and population growth, but suppressed both parameters in laboratory and field WBPH populations. We used digital gene expression and transcriptomic analyses to identify a panel of differentially expressed genes, including a set of up-regulated genes in JGM-treated BPH, which were down-regulated in JGM-treated WBPH. RNAi silencing of Acetyl Co-A carboxylase (ACC), highly expressed in JGM-treated BPH, reduced ACC expression (by > 60%) and eliminated JGM-induced fecundity increases in BPH. These findings support our hypothesis that differences in ACC expression separates intraguild species at the molecular level.

Relation Between the Viral Load Accumulation of Southern Rice Black-Streaked Dwarf Virus and the Different Developmental Stages of Sogatella furcifera (Hemiptera: Delphacidae)

The white-backed planthopper, Sogatella furcifera (Horvath), is currently the only confirmed vector of Southern rice black-streaked dwarf virus (SRBSDV), which causes severe rice production losses in China. In this study, an absolute quantification qPCR method was used to detect viral gene mRNA expression levels at different developmental stages of white-backed planthoppers fed SRBSDV-infected rice plants. A comparison of viral copy numbers of the SRBSDV S10 gene at the same developmental stage indicated that the white-backed planthopper had higher viral copy numbers when the virus was acquired at the earlier developmental stages. The adult-stage white-backed planthoppers that had acquired the virus at the first-second nymphal stage displayed significantly higher viral titers than white-backed planthoppers that acquired the virus at the third-fourth nymphal stage, at the fifth nymphal stage, and at the adult stage. The fifth nymphal stage white-backed planthoppers that acquired the virus at the first-second nymphal stage displayed higher viral copy numbers than fifth nymphal stage white-backed planthoppers that acquired the virus at the third-fourth nymphal stage and at the fifth nymphal stage. The highest viral load value appeared in the middle adult stage. The annual immigration characteristics of white-backed planthoppers would be beneficial for the dispersal of SRBSDV because this virus could be transmitted far away following the migration of vigorous planthoppers. Therefore, investigating the change in the viral load at different life stages of SRBSDV-positive individuals is required to develop more effective control of the spread of SRBSDV in the field.

Population Seasonality: Will They Stay or Will They Go? A Case Study of the Sogatella furcifera (Hemiptera: Delphacidae)

The whitebacked planthopper Sogatella furcifera (Horváth) (Hemiptera: Delphacidae) is one of the most destructive pests of rice in East and Southeast Asia. It is also a long-distance migratory insect and population size fluctuates frequently in these rice regions along the middle and lower Yangtze River. We analyzed the population seasonality of S. furcifera based on field surveys, light trap catching, and meteorological factors. We found that many S. furcifera were retained in local late rice in 2012, due to continuous rain and slightly windy weather conditions during the migration period. These results suggest that a new pattern of population fluctuation may occur where resident S. furcifera are dispersed into a single medium rice during harvest period, then rebound and thrive in late rice when there are suitable temperatures in September. Although the residency of S. furcifera in late rice fields in 2012 seems to be a special case, our findings suggest that S. furcifera exhibit a type of facultative migration. Our research also illuminates studies of the migration events of S. furcifera and benefits our understanding of the dynamics of S. furcifera in Hunan Province.

P5-2 of rice black-streaked dwarf virus is a non-structural protein targeted to chloroplasts

The genome segment S5 of rice black-streaked dwarf virus (genus Fijivirus, family Reoviridae) is functionally bicistronic in infected plants. It has a conserved second ORF (P5-2) partially overlapping the major ORF in a different reading frame, but its function remains unknown. P5-2 was detected in infected plants, but not in purified viral particles by Western blotting, indicating that it is a non-structural protein. In immunoelectron microscopy, polyclonal antibodies against P5-2 specifically labelled chloroplasts of infected rice plants. When P5-2 fused with green fluorescent protein was transiently expressed in leaves of Nicotiana benthamiana, fluorescence was also co-localized with chloroplasts. Experiments with deletion mutants of P5-2 showed that its N-terminal part was responsible for its targeting to chloroplasts.

Interaction research on the antiviral molecule dufulin targeting on southern rice black streaked dwarf virus p9-1 nonstructural protein

Southern rice black streaked dwarf virus (SRBSDV) causes severe harm to rice production. Unfortunately, studies on effective antiviral drugs against SRBSDV and interaction mechanism of antiviral molecule targeting on SRBSDV have not been reported. This study found dufulin (DFL), an ideal anti-SRBSDV molecule, and investigated the interactions of DFL targeting on the nonstructural protein P9-1. The biological sequence information and bonding characterization of DFL to four kinds of P9-1 protein were described with fluorescence titration (FT) and microscale thermophoresis (MST) assays. The sequence analysis indicated that P9-1 had highly-conserved C- and N-terminal amino acid residues and a hypervariable region that differed from 131 aa to 160 aa. Consequently, wild-type (WT-His-P9-1), 23 C-terminal residues truncated (TR-ΔC23-His-P9-1), 6 N-terminal residues truncated (TR-ΔN6-His-P9-1), and Ser138 site-directed (MU-138-His-P9-1) mutant proteins were expressed. The FT and MST assay results indicated that DFL bounded to WT-His-P9-1 with micromole affinity and the 23 C-terminal amino acids were the potential targeting site. This system, which combines a complete sequence analysis, mutant protein expression, and binding action evaluating system, could further advance the understanding of the interaction abilities between antiviral drugs and their targets.

Characterization of homologous and heterologous interactions between viroplasm proteins P6 and P9-1 of the fijivirus southern rice black-streaked dwarf virus

P6 of southern rice black-streaked dwarf virus (SRBSDV) is a multifunctional protein that is involved in the formation of viroplasms by interacting with P5-1. Here, we used yeast two-hybrid and bimolecular fluorescence complementation assays to show that there were homologous and heterologous interactions between SRBSDV P6 and P9-1 in yeast and plant cells. Mutational analysis showed that the N-terminal region (residues 1-93) of P6 was necessary for the interaction between P6 and P9-1. Self-interactions only occurred between the full-length P6 or P9-1. P9-1 was able to form viroplasm-like inclusion structures alone in the absence of other viral proteins.

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.

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-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.

Southern rice black-streaked dwarf virus (SRBSDV) directly affects the feeding and reproduction behavior of its vector, Sogatella furcifera (Horváth) (Hemiptera: Delphacidae)

BACKGROUND

Southern rice black-streaked dwarf virus (SRBSDV) is a recently discovered member of the genus Fijivirus and it is transmitted by the rice whitebacked planthopper (WBPH), Sogatella furcifera (Horváth). It was found that SRBSDV infected vectors might contribute negatively to the WBPH population, although the longer nymphal period might benefit viral acquisition, transmission and increase infection rate. The interaction between SRBSDV and its vector need to be further explored to gain better understanding of the dispersal of WBPH and the spread of virus disease, in particular the feeding and reproduction behavior of viruliferous WBPH.

METHODS

Newly hatched nymphs of WBPH were fed on healthy rice plant after feeding on SRBSDV-infected rice plants for 2 h, and newly emerged adults were numbered and tested. Feeding behaviors of WBPH adults were monitored electronically within a Faraday cage using a Giga-4 DC EPG amplifier. The newly emerged adults were paired, and the fecundity and egg hatchability were investigated. WBPH was molecularly identified for SRBSDV when they dead. According to the identification results, data on viruliferous and non-viruliferous WBPH were collected and analyzed.

RESULTS

Feeding behavior of viruliferous WBPH was different from those of non-viruliferous WBPH. Frequency of phloem sap ingestion of viruliferous WBPH increased significantly, however the total feeding duration did not increase markedly. When both WBPH parents were infected with SRBSDV, their fecundity and hatchability of the eggs produced were significant lower than those of normal WBPH parents. However, if only one of the parents was viruliferous, fecundity and egg hatchability were only slightly affected.

CONCLUSIONS

Viruliferous WBPH fed on the phloem more frequently than non-viruliferous WBPH and can thus contribute to virus transmission. When both vector parents are viruliferous fecundity and hatchability of the eggs were significantly reduced. However when only one of the parents WBPH was viruliferous, there were no significant effects.