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

The salivary chaperone protein NlDNAJB9 of Nilaparvata lugens activates plant immune responses

The brown planthopper (BPH) Nilaparvata lugens (Stål) is a main pest on rice. It secretes saliva to regulate plant defense responses, when penetrating rice plant and sucking phloem sap through its stylet. However, the molecular mechanisms of BPH salivary proteins regulating plant defense responses remain poorly understood. A N. lugens DNAJ protein (NlDNAJB9) gene was highly expressed in salivary glands, and the knock down of NlDNAJB9 significantly enhanced honeydew excretion and fecundity of the BPH. NlDNAJB9 could induce plant cell death, and the overexpression of NlDNAJB9 gene in Nicotiana benthamiana induced calcium signaling, mitogen-activated protein kinase (MAPK) cascades, reactive oxygen species (ROS) accumulation, jasmonic acid (JA) hormone signaling and callose deposition. The results from different NlDNAJB9 deletion mutants indicated that the nuclear localization of NlDNAJB9 was not necessary to induce cell death. The DNAJ domain was the key region to induce cell death, and the overexpression of DNAJ domain in N. benthamiana significantly inhibited insect feeding and pathogenic infection. NlDNAJB9 might interact indirectly with NlHSC70-3 to regulate plant defense responses. NlDNAJB9 and its orthologs were highly conserved in three planthopper species, and could induce ROS burst and cell death in plants. Our study provides new insights into the molecular mechanisms of insect-plant interactions.

A Review of Vector-Borne Rice Viruses

Rice (Oryza sativa L.) is one of the major staple foods for global consumption. A major roadblock to global rice production is persistent loss of crops caused by plant diseases, including rice blast, sheath blight, bacterial blight, and particularly various vector-borne rice viral diseases. Since the late 19th century, 19 species of rice viruses have been recorded in rice-producing areas worldwide and cause varying degrees of damage on the rice production. Among them, southern rice black-streaked dwarf virus (SRBSDV) and rice black-streaked dwarf virus (RBSDV) in Asia, rice yellow mottle virus (RYMV) in Africa, and rice stripe necrosis virus (RSNV) in America currently pose serious threats to rice yields. This review systematizes the emergence and damage of rice viral diseases, the symptomatology and transmission biology of rice viruses, the arm races between viruses and rice plants as well as their insect vectors, and the strategies for the prevention and control of rice viral diseases.

Rice yellow stunt virus activates polyamine biosynthesis to promote viral propagation in insect vectors by disrupting ornithine decarboxylase antienzyme function

Intracellular polyamines (putrescine, spermidine, and spermine) have emerged as important molecules for viral infection; however, how viruses activate polyamines biosynthesis to promote viral infection remains unclear. Ornithine decarboxylase 1 (ODC1) and its antienzyme 1 (OAZ1) are major regulators of polyamine biosynthesis in animal cells. Here, we report that rice yellow stunt virus (RYSV), a plant rhabdovirus, could activate putrescine biosynthesis in leafhoppers to promote viral propagation by inhibiting OAZ1 expression. We observed that the reduction of putrescine biosynthesis by treatment with difluormethylornithine (DFMO), a specific nontoxic inhibitor of ODC1, or with in vitro synthesized dsRNAs targeting ODC1 mRNA could inhibit viral infection. In contrast, the supplement of putrescine or the increase of putrescine biosynthesis by treatment with dsRNAs targeting OAZ1 mRNA could facilitate viral infection. We further determined that both RYSV matrix protein M and ODC1 directly bind to the ODC-binding domain at the C-terminus of OAZ1. Thus, viral propagation in leafhoppers would decrease the ability of OAZ1 to target and mediate the degradation of ODC1, which finally activates putrescine production to benefit viral propagation. This work reveals that polyamine-metabolizing enzymes are directly exploited by a vector-borne virus to increase polyamine production, thereby facilitating viral infection in insect vectors.

Suppression of Rice Planthopper Populations by the Entomopathogenic Fungus Metarhizium anisopliae without Affecting the Rice Microbiota

Entomopathogenic fungi can regulate insect populations and function as crucial biological control agents against insect pests, but their impacts on nontarget microorganisms are poorly understood. In this study, we investigated the potential of the fungal strain Metarhizium anisopliae CQMa421 to control rice planthoppers under field conditions and its effects on rice microbiota. This fungus suppressed rice planthoppers during this period, and its control efficiency was more than 60% 7 days after application and did not significantly differ from that of the chemical treatment except in 2019. Both treatments showed a smaller population of rice planthoppers than the controls. After application, M. anisopliae was maintained on rice plants for approximately 14 days, showing a decreasing trend over time. Furthermore, the results showed that the bacterial and fungal richness (operational taxonomic units) and diversity (Shannon index) did not significantly differ between the fungal treatment and the controls after application. The major bacterial taxa of Proteobacteria (including Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, and Deltaproteobacteria), Actinobacteria, Bacteroidetes, and Cyanobacteria accounted for more than 80% of the bacterial community after fungal application, and the major fungal taxa Ascomycota (including Eurotiomycetes, Dothideomycetes, and Sordariomycetes) and Basidiomycota (including Ustilaginomycetes) represented more than 90% of the fungal community. However, the microbial communities of the rice phyllosphere did not significantly change after entomopathogenic-agent application, indicating that the indigenous microbial communities may adapt to fungal insecticide application. Taken together, the results suggest that this fungal agent has good potential for rice planthopper control with no substantial effects on rice microbial communities.IMPORTANCE Entomopathogenic fungi may be used as crucial biocontrol agents for the control of insect pests, but few effective fungal strains have been reported for the control of the rice planthopper, a major pest of rice. More importantly, the impacts of fungal insecticide application on nontarget microorganisms have not been well evaluated, especially under field conditions. Therefore, in this study, we investigated the effects of the fungal strain M. anisopliae CQMa421 on rice planthopper populations from 2017 to 2019 and evaluated its potential impacts on the microbiota of rice plants after application. The results suggested that this fungal agent has good potential for use in the control of rice planthoppers with no significant effects on rice microbial communities, representing an alternative strategy for the control of rice pests.

Stable Introduction of Plant-Virus-Inhibiting Wolbachia into Planthoppers for Rice Protection

Progress has been made in developing the maternally inherited endosymbiotic bacterium Wolbachia as a tool for protecting humans from mosquito-borne diseases. In contrast, Wolbachia-based approaches have not yet been developed for the protection of plants from insect pests and their associated diseases, with a major challenge being the establishment of artificial Wolbachia infections expressing desired characteristics in the hemipterans that transmit the majority of plant viruses. Here, we report stable introduction of Wolbachia into the brown planthopper, Nilaparvata lugens, the most destructive rice pest that annually destroys millions of hectares of staple crops. The Wolbachia strain wStri from the small brown planthopper, Laodelphax striatellus, was transferred to this new host, where it showed high levels of cytoplasmic incompatibility, enabling rapid invasion of laboratory populations. Furthermore, wStri inhibited infection and transmission of Rice ragged stunt virus and mitigated virus-induced symptoms in rice plants, opening up the development of Wolbachia-based strategies against major agricultural pests and their transmitted pathogens. VIDEO ABSTRACT.

The α-tubulin of Laodelphax striatellus mediates the passage of rice stripe virus (RSV) and enhances horizontal transmission

Rice stripe virus (RSV, genus Tenuivirus, family Phenuiviridae) is the causal agent of rice stripe disease transmitted by the small brown planthopper (SBPH, Laodelphax striatellus) in a persistent propagative manner. The midgut and salivary glands of SBPH are the first and last barriers to the viral circulation and transmission processes, respectively; however, the precise mechanisms used by RSV to cross these organs and transmit to rice plants have not been fully elucidated. We obtained the full-length cDNA sequence of L. striatellus α-tubulin 2 (LsTUB) and found that RSV infection increased the level of LsTUB in vivo. Furthermore, LsTUB was shown to co-localize with RSV nonstructural protein 3 (NS3) in vivo and bound NS3 at positions 74-76 and 80-82 in vitro. Transient gene silencing of LsTUB expression caused a significant reduction in detectable RSV loads and viral NS3 expression levels, but had no effect on NS3 silencing suppressor activity and viral replication in insect cells. However, suppression of LsTUB attenuated viral spread in the bodies of SBPHs and decreased RSV transmission rates to rice plants. Electrical penetration graphs (EPG) showed that LsTUB knockdown by RNAi did not impact SBPH feeding; therefore, the reduction in RSV transmission rates was likely caused by a decrease in viral loads inside the planthopper. These findings suggest that LsTUB mediates the passage of RSV through midgut and salivary glands and leads to successful horizontal transmission.

Rice Yellow Stunt Nucleorhabdovirus Matrix Protein Mediates Viral Axonal Transport in the Central Nervous System of Its Insect Vector

Persistently transmitted plant viruses encounter multiple membrane and tissue barriers in the process of completing their infection routes within their insect vectors. Some of these viruses have been reported to overcome the elaborate barriers of the central nervous system (CNS) to travel through the nervous tissues, but the specific mechanisms of this process remain unknown. Here, we report the axonal transport mechanism of rice yellow stunt virus (RYSV), a nucleorhabdovirus, in the CNS of the green rice leafhopper (Nephotettix cincticeps). The infection route of RYSV in the internal organs of its insect vector after ingestion of the virus was investigated by immunofluorescence microscopy. RYSV was first detected in the epithelial cells of midgut regions, from where it proceeded to the nervous system, and finally into the salivary glands. We then utilized immunofluorescence and electron microscopy to investigate the distribution of RYSV particles within the leafhopper CNS, demonstrating that non-enveloped viral particles distributed along the microtubule-based neurofilaments in the axon cytoplasm following the direct interaction of leafhopper α-tubulin with the RYSV M protein. Tubulin inhibitors inhibited the dissemination of RYSV to the CNS, then into the salivary glands in leafhoppers. We therefore describe a mechanism of plant virus transport through CNS axons as an alternative means of rapid viral dissemination in an insect vector.

Infection Characteristics of Rice Stripe Mosaic Virus in the Body of the Vector Leafhoppers

Rice stripe mosaic virus (RSMV), a novel species of Cytorhabdovirus, is transmitted by the leafhopper Recilia dorsalis in a persistent-propagative manner. In this study, we firstly confirmed that N protein of RSMV is a component of viroplasm and virion in vector culture cells of R. dorsalis. Confocal microscopy revealed that RSMV initially accumulated in epithelial cells of the filter chamber of R. dorsalis, from where it proceeded to the visceral muscles surrounding the filter chamber. Subsequently, RSMV spread quickly throughout the suspensory ligament to the salivary glands. Meanwhile, RSMV spread from the filter chamber to midgut, hindgut, esophagus, hemolymph, and central nervous system. We further observed that RSMV particles displayed as non-enveloped form when propagating in cytoplasm of different tissues, and became enveloped when spread within insect body by electron microscopy. Additionally, we found that the leafhopper Nephotettix virescens was also able to acquire and transmit RSMV. These results clarified the infection characteristics of RSMV in its leafhopper vectors, which will help guide the formulation of RSMV prevention and control strategies.

Genome sequence of the small brown planthopper, Laodelphax striatellus

Background

Laodelphax striatellus Fallén (Hemiptera: Delphacidae) is one of the most destructive rice pests. L. striatellus is different from 2 other rice planthoppers with a released genome sequence, Sogatella furcifera and Nilaparvata lugens, in many biological characteristics, such as host range, dispersal capacity, and vectoring plant viruses. Deciphering the genome of L. striatellus will further the understanding of the genetic basis of the biological differences among the 3 rice planthoppers.

Findings

A total of 190 Gb of Illumina data and 32.4 Gb of Pacbio data were generated and used to assemble a high-quality L. striatellus genome sequence, which is 541 Mb in length and has a contig N50 of 118 Kb and a scaffold N50 of 1.08 Mb. Annotated repetitive elements account for 25.7% of the genome. A total of 17 736 protein-coding genes were annotated, capturing 97.6% and 98% of the BUSCO eukaryote and arthropoda genes, respectively. Compared with N. lugens and S. furcifera, L. striatellus has the smallest genome and the lowest gene number. Gene family expansion and transcriptomic analyses provided hints to the genomic basis of the differences in important traits such as host range, migratory habit, and plant virus transmission between L. striatellus and the other 2 planthoppers.

Conclusions

We report a high-quality genome assembly of L. striatellus, which is an important genomic resource not only for the study of the biology of L. striatellus and its interactions with plant hosts and plant viruses, but also for comparison with other planthoppers.

Rice Ragged Stunt Virus Propagation and Infection on Rice Plants

Virus inoculation is a basic experimental procedure to evaluate the resistance of a rice variety or a transgenic material upon virus infection. We recently demonstrated that Rice ragged stunt virus (RRSV), an oryzavirus that is transmitted by brown planthopper (BPH), can suppress jasmonic acid-mediated antiviral defense through the induction of microRNA319 and facilitate virus infection in rice. To verify this, we performed virus inoculation experiments on wild-type rice plants and miR319-TCP21-associated transgenic rice plants through a modified group inoculation method. Here, we presented the detailed procedure of RRSV propagation and infection process on rice plants.

Transmission Characteristics of Barley Yellow Striate Mosaic Virus in Its Planthopper Vector Laodelphax striatellus

The most economically important plant viruses are specifically transmitted by phytophagous insects that significantly affect viral epidemiology. Barley yellow striate mosaic virus (BYSMV), a member of the genus Cytorhabdovirus, is transmitted by the small brown planthopper (SBPH, Laodelphax striatellus) in a persistent-propagative manner. However, the infection route of BYSMV in SBPHs is poorly understood. In this study, immunofluorescence confocal laser scanning microscopy (iCLSM) was performed to investigate the route of BYSMV in SBPHs. We unexpectedly found that BYSMV initially infected the hindgut epithelium of SBPHs, instead of the midgut epithelium initially infected by other persistent-propagative viruses. Subsequently, BYSMV disseminated to the hindgut visceral muscles and spread to other parts of alimentary canals, hemolymph, and salivary glands. Comparative analysis of gene expression on viral mRNAs and the BYSMV nucleoprotein by using different molecular detection and immunohistochemistry further demonstrated that BYSMV initially infected and replicated in the hindgut epithelial cells of SBPHs. Collectively, our study provides the first insight into that hindgut is initial infection site of BYSMV that represents a new dissemination route of persistent-propagative viruses.

Development of leafhopper cell culture to trace the early infection process of a nucleorhabdovirus, rice yellow stunt virus, in insect vector cells

Background

In China, the rice pathogen Rice yellow stunt virus (RYSV), a member of the genus Nucleorhabdovirus in the family Rhabdoviridae, was a severe threat to rice production during the1960s and1970s. Fundamental aspects of the biology of this virus such as protein localization and formation of the RYSV viroplasm during infection of insect vector cells are largely unexplored. The specific role(s) of the structural proteins nucleoprotein (N) and phosphoprotein (P) in the assembly of the viroplasm during RYSV infection in insect vector is also unclear.

Methods

In present study, we used continuous leafhopper cell culture, immunocytochemical techniques, and transmission electron microscopy to investigate the subcellular distributions of N and P during RYSV infection. Both GST pull-down assay and yeast two-hybrid assay were used to assess the in vitro interaction of N and P. The dsRNA interference assay was performed to study the functional roles of N and P in the assembly of RYSV viroplasm.

Results

Here we demonstrated that N and P colocalized in the nucleus of RYSV-infected Nephotettix cincticeps cell and formed viroplasm-like structures (VpLSs). The transiently expressed N and P are sufficient to form VpLSs in the Sf9 cells. In addition, the interactions of N/P, N/N and P/P were confirmed in vitro. More interestingly, the accumulation of RYSV was significantly reduced when the transcription of N gene or P gene was knocked down by dsRNA treatment.

Conclusions

In summary, our results suggest that N and P are the main viral factors responsible for the formation of viroplasm in RYSV-infected insect cells. Early during RYSV infection in the insect vector, N and P interacted with each other in the nucleus to form viroplasm-like structures, which are essential for the infection of RYSV.

Electronic supplementary material

The online version of this article (10.1186/s12985-018-0987-6) contains supplementary material, which is available to authorized users.

Analysis of Homologs of Cry-toxin Receptor-Related Proteins in the Midgut of a Non-Bt Target, Nilaparvata lugens (Stål) (Hemiptera: Delphacidae)

The brown planthopper (BPH) Nilaparvata lugens is one of the most destructive insect pests in the rice fields of Asia. Like other hemipteran insects, BPH is not susceptible to Cry toxins of Bacillus thuringiensis (Bt) or transgenic rice carrying Bt cry genes. Lack of Cry receptors in the midgut is one of the main reasons that BPH is not susceptible to the Cry toxins. The main Cry-binding proteins (CBPs) of the susceptible insects are cadherin, aminopeptidase N (APN), and alkaline phosphatase (ALP). In this study, we analyzed and validated de novo assembled transcripts from transcriptome sequencing data of BPH to identify and characterize homologs of cadherin, APN, and ALP. We then compared the cadherin-, APN-, and ALP-like proteins of BPH to previously reported CBPs to identify their homologs in BPH. The sequence analysis revealed that at least one cadherin, one APN, and two ALPs of BPH contained homologous functional domains identified from the Cry-binding cadherin, APN, and ALP, respectively. Quantitative real-time polymerase chain reaction used to verify the expression level of each putative Cry receptor homolog in the BPH midgut indicated that the CBPs homologous APN and ALP were expressed at high or medium-high levels while the cadherin was expressed at a low level. These results suggest that homologs of CBPs exist in the midgut of BPH. However, differences in key motifs of CBPs, which are functional in interacting with Cry toxins, may be responsible for insusceptibility of BPH to Cry toxins.

Phloem-limited reoviruses universally induce sieve element hyperplasia and more flexible gateways, providing more channels for their movement in plants

Virion distribution and ultrastructural changes induced by the infection of maize or rice with four different reoviruses were examined. Rice black streaked dwarf virus (RBSDV, genus Fijivirus), Rice ragged stunt virus (RRSV, genus Oryzavirus), and Rice gall dwarf virus (RGDV, genus Phytoreovirus) were all phloem-limited and caused cellular hyperplasia in the phloem resulting in tumors or vein swelling and modifying the cellular arrangement of sieve elements (SEs). In contrast, virions of Rice dwarf virus (RDV, genus Phytoreovirus) were observed in both phloem and mesophyll and the virus did not cause hyperplasia of SEs. The three phloem-limited reoviruses (but not RDV) all induced more flexible gateways at the SE-SE interfaces, especially the non-sieve plate interfaces. These flexible gateways were also observed for the first time at the cellular interfaces between SE and phloem parenchyma (PP). In plants infected with any of the reoviruses, virus-like particles could be seen within the flexible gateways, suggesting that these gateways may serve as channels for the movement of plant reoviruses with their large virions between SEs or between SEs and PP. SE hyperplasia and the increase in flexible gateways may be a universal strategy for the movement of phloem-limited reoviruses.

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

Background

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

Methods

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

Results

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

Conclusions

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

Electronic supplementary material

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

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

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

FEATURES

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

A mitochondrial membrane protein is a target for rice ragged stunt virus in its insect vector

Rice ragged stunt virus (RRSV; Reoviridae) is exclusively transmitted by the brown planthopper Nilaparvata lugens in a persistent-propagative manner. It is understood that RNA viral proliferation is associated with the intracellular membranes of the insect host cells. However, the molecular mechanisms of the interaction between the RRSV proliferation and the intracellular membranes remain essentially unknown. It will be of great interest to determine whether RRSV protein(s) directly interact with intracellular membrane components of its host cells. In this study, we identified a RRSV nonstructural protein Pns10 interacting with a host oligomycin-sensitivity conferral protein (OSCP) using yeast two-hybrid system. The interaction between RRSV Pns10 and N. lugens OSCP was verified by a glutathione S-transferase pull-down assay. Confocal miscopy revealed colocalization of these two proteins in the cytoplasm of the salivary gland cells during the viral infection. The virions were further detected in the mitochondria under confocal miscopy and transmission electron microscopy combined with western blotting assay. This is the first observation that RRSV protein has a direct link with mitochondria. Suppressing OSCP gene expression by RNA interference notably decreased the viral loads in RRSV-infected insects. These findings revealed novel aspects of a viral protein in targeting the host mitochondrial membrane and provide insights concerning the mitochondrial membrane protein-based virus proliferation mode in the insect vector.

The speed of tubule formation of two fijiviruses corresponds with their dissemination efficiency in their insect vectors

Background

Rice black-streaked dwarf virus (RBSDV) and Southern rice black-streaked dwarf virus (SRBSDV) are two closely related fijiviruses transmitted by the small brown planthopper (SBPH) and white-backed planthopper (WBPH), respectively. SRBSDV has a latent period 4 days shorter than that of RBSDV, implying a more efficient spread in insect vector. Currently, the mechanisms underlying this higher efficiency are poorly understood. However, our recent studies have implicated a role of virus induced tubular structures in the dissemination of fijiiruses within their insect vectors.

Methods

Immunofluorescence labeling was performed to visualize and compare the dynamics of P7-1 tubule formation of the RBSDV and SRBSDV in their own vector insects and nonhost Spodoptera frugiperda (Sf9) cells.

Results

Tubule formation of SRBSDV P7-1 was faster than that of RBSDV P7-1. For RBSDV, P7-1 formed tubules were observed at 3-days post-first access to diseased plants (padp) in SBPH. For SRBSDV, these structures were detected as early as 1 day padp in WBPH. Importantly, similar phenomena were observed when P7-1 proteins from the two viruses were expressed alone in Sf9 cells.

Conclusions

Our research revealed a relationship between the speed of P7-1 tubule formation and virus dissemination efficiency and also supports a role of such tubular structures in the spread of reoviruses within insect vectors.

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

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

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.

Viral receptors of the gut: insect-borne propagative plant viruses of agricultural importance

Insect-borne propagative plant viruses of agricultural importance are transmitted by sap-sucking insects. Although the infection routes of these viruses within the bodies of insect vectors are well established, cellular receptors on the microvilli, intercellular junctions, and basal lamina for mediating viral entry or spread in insect gut epithelium have not been well identified or characterized. Recent trends in the field are opening questions on how viruses exploit actin-based tubule motility to overcome insect gut epithelium barriers after viral entry in epithelium. Advances in insect cell lines, genome sequencing, reverse genetic systems and others not yet developed technologies are needed to find and characterize the counterpart receptors in vectors and to design strategies to interfere with viral transmission.

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

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

Assembly of viroplasms by viral nonstructural protein Pns9 is essential for persistent infection of rice gall dwarf virus in its insect vector

Rice gall dwarf virus (RGDV), a plant reovirus, is transmitted by leafhopper vector Recilia dorsalis in a persistent-propagative manner. In a sequential study of RGDV infection of its insect vector, the virus initially infected the filter chamber epithelium, then directly crossed the basal lamina into the visceral muscles, from where it spread throughout the entire midgut and hindgut. Finally, RGDV spread into the salivary glands. During RGDV infection of the continuous cultured cells of R. dorsalis, viroplasm that was mainly comprised of viral nonstructural protein Pns9 was formed and acted as the site of viral replication and assembly of progeny virions. Knockdown of Pns9 expression in cultured insect vector cells using synthesized dsRNAs from the Pns9 gene strongly inhibited viroplasm formation and viral infection. The microinjection of dsRNAs from the Pns9 gene strongly abolished viroplasm formation in the initially infected filter chamber epithelium and prevented viral spread into leafhopper visceral muscles. These results indicated that the assembly of viroplasms was essential for the persistent infection and spread of RGDV in its insect vector.

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.

Localization and distribution of wheat dwarf virus in its vector leafhopper, Psammotettix alienus

Numerous virus pathogens are transmitted by specific arthropod vectors. Understanding the mechanism of transmission is a critical step in the epidemiology of plant viruses and is crucial for the development of effective disease control strategies. In this study, we describe the localization and distribution of Wheat dwarf virus (WDV), an economically important and widespread single-stranded DNA virus, in its leafhopper vector, Psammotettix alienus. The results suggest that WDV not only can move to the salivary glands from the anterior and middle midgut via the hemocoel but also can pass directly through the sheath of the filter chamber and be readily transmitted to healthy wheat plants within 5 min of an acquisition access period on infected plants. When a bacterial-expressed recombinant capsid protein (CP) was incubated with the internal organs of leafhoppers, CP-immunoreactive antigens were found at the anterior and middle midgut. Furthermore, when leafhoppers were fed with an antiserum raised against the CP, the accumulation of WDV in the gut cells, hemocoel, and salivary glands was significantly reduced. These data provide evidence that transmission of WDV is determined by a CP-mediated virion-vector retention mechanism.

Infection route of rice grassy stunt virus, a tenuivirus, in the body of its brown planthopper vector, Nilaparvata lugens (Hemiptera: Delphacidae) after ingestion of virus

Rice grassy stunt virus (RGSV), a tenuivirus, is transmitted by the brown planthopper (BPH), Nilaparvata lugens (Hemiptera, Delphacidae), in a persistent-propagative manner. In this study, immunofluorescence microscopy was used to investigate the infection route of RGSV in the internal organs of BPH after acquiring the virus by feeding on RGSV-infected rice plants. The sequential infection study revealed that RGSV initially infected the midgut epithelium, then crossed the basal lamina into the midgut visceral muscles, from where RGSV apparently spread into the hemolymph, then into the salivary glands of its BPH vector. The mechanism underlying this infection route of RGSV in its BPH vector may confer an advantage for the direct spread of RGSV from the initially infected epithelium to the salivary glands in BPH, contributing to efficient transmission of RGSV by its insect vector.

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

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

Nonstructural protein NS4 of Rice Stripe Virus plays a critical role in viral spread in the body of vector insects

Rice stripe virus (RSV), a tenuivirus, is transmitted by small brown planthopper (SBPH) in a persistent-propagative manner. In this study, sequential infection of RSV in the internal organs of SBPH after ingestion of virus indicated that RSV initially infected the midgut epithelium, and then progressed to the visceral muscle tissues, through which RSV spread to the entire alimentary canal. Finally, RSV spread into the salivary glands and reproductive system. During viral infection, the nonstructural protein NS4 of RSV formed cytoplasmic inclusions in various tissues of viruliferous SBPH. We demonstrated that the ribonucleoprotein particles of RSV were closely associated with NS4-specific inclusions in the body of viruliferous SBPH through a direct interaction between NS4 and nucleoprotein of RSV. Moreover, the knockdown of NS4 expression due to RNA interference induced by dsRNA from NS4 gene significantly prevented the spread of RSV in the bodies of SBPHs without a significant effect on viral replication in continuous cell culture derived from SBPH. All these results suggest that the nonstructural protein NS4 of RSV plays a critical role in viral spread by the vector insects.

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

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

IMPORTANCE

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

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.

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.

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

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