New insights on the transmission mechanism of tenuiviruses by their vector insects

Complex interactions among insect viruses-insect vector-arboviruses

Insects are the host or vector of diverse viruses including those that infect vertebrates, plants, and fungi. Insect viruses reside inside their insect hosts and are vertically transmitted from parent to offspring. The insect virus-host relationship is intricate, as these viruses can impact various aspects of insect biology, such as development, reproduction, sex ratios, and immunity. Arthropod-borne viruses (arboviruses) that cause substantial global health or agricultural problems can also be vertically transmitted to insect vector progeny. Multiple infections with insect viruses and arboviruses are common in nature. Such coinfections involve complex interactions, including synergism, dependence, and antagonism. Recent studies have shed light on the influence of insect viruses on the competence of insect vectors for arboviruses. In this review, we focus on the biological effects of insect viruses on the transmission of arboviruses by insects. We also discuss the potential mechanisms by which insect viruses affect the ability of hosts to transmit arboviruses, as well as potential strategies for disease control through manipulation of insect viruses. Analyses of the interactions among insect vectors, insect viruses and arboviruses will provide new opportunities for development of innovative strategies to control arbovirus transmission.

Genetic diversity of RNA viruses infecting invertebrate pests of rice

Invertebrate species are a natural reservoir of viral genetic diversity, and invertebrate pests are widely distributed in crop fields. However, information on viruses infecting invertebrate pests of crops is limited. In this report, we describe the deep metatranscriptomic sequencing of 88 invertebrate samples covering all major invertebrate pests in rice fields. We identified 296 new RNA viruses and 13 known RNA viruses. These viruses clustered within 31 families, with many highly divergent viruses constituting potentially new families and genera. Of the identified viruses, 13 RNA viruses clustered within the Fiersviridae family of bacteriophages, and 48 RNA viruses clustered within families and genera of mycoviruses. We detected known rice viruses in novel invertebrate hosts at high abundances. Furthermore, some novel RNA viruses have genome structures closely matching to known plant viruses and clustered within genera of several plant virus species. Forty-five potential insect pathogenic RNA viruses were detected in invertebrate species. Our analysis revealed that host taxonomy plays a major role and geographical location plays an important role in structuring viral diversity. Cross-species transmission of RNA viruses was detected between invertebrate hosts. Newly identified viral genomes showed extensive variation for invertebrate viral families or genera. Together, the large-scale metatranscriptomic analysis greatly expands our understanding of RNA viruses in rice invertebrate species, the results provide valuable information for developing efficient strategies to manage insect pests and virus-mediated crop diseases.

Plant reoviruses hijack autophagy in insect vectors

Plant reoviruses, transmitted only by insect vectors, seriously threaten global cereal production. Understanding how insect vectors efficiently transmit the viruses is key to controlling the viral diseases. Autophagy commonly plays important roles in plant host defense against virus infection, but recent studies have shown that plant reoviruses can hijack the autophagy pathway in insect cells to enable their persistence in the insect and continued transmission to plants. Here, we summarize and discuss new insights on viral activation, evasion, regulation, and manipulation of autophagy within the insect vectors and the role of autophagy in virus survival in insect vectors. Deeper knowledge of the functions of autophagy in vectors may lead to novel strategies for blocking transmission of insect-borne plant viruses.

Mining Public Data to Investigate the Virome of Neglected Pollinators and Other Floral Visitors

This study reports the virome investigation of pollinator species and other floral visitors associated with plants from the south of Bahia: Aphis aurantii, Atrichopogon sp., Dasyhelea sp., Forcipomyia taiwana, and Trigona ventralis hoozana. Studying viruses in insects associated with economically important crops is vital to understand transmission dynamics and manage viral diseases that pose as threats for global food security. Using literature mining and public RNA next-generation sequencing data deposited in the NCBI SRA database, we identified potential vectors associated with Malvaceae plant species and characterized the microbial communities resident in these insects. Bacteria and Eukarya dominated the metagenomic analyses of all taxon groups. We also found sequences showing similarity to elements from several viral families, including Bunyavirales, Chuviridae, Iflaviridae, Narnaviridae, Orthomyxoviridae, Rhabdoviridae, Totiviridae, and Xinmoviridae. Phylogenetic analyses indicated the existence of at least 16 new viruses distributed among A. aurantii (3), Atrichopogon sp. (4), Dasyhelea sp. (3), and F. taiwana (6). No novel viruses were found for T. ventralis hoozana. For F. taiwana, the available libraries also allowed us to suggest possible vertical transmission, while for A. aurantii we followed the infection profile along the insect development. Our results highlight the importance of studying the virome of insect species associated with crop pollination, as they may play a crucial role in the transmission of viruses to economically important plants, such as those of the genus Theobroma, or they will reduce the pollination process. This information may be valuable in developing strategies to mitigate the spread of viruses and protect the global industry.

A rice plant expressing viral glycoprotein NSvc2-NS reduces the transmission of rice stripe virus by the small brown planthopper

BACKGROUND

Plant viruses transmitted by arthropod vectors threaten crop health worldwide. Rice stripe virus (RSV) is one of the most important rice viruses in East Asia and is transmitted by the small brown planthopper (SBPH). Previously, it was demonstrated that the viral glycoprotein NSvs2-N could mediate RSV infection of the vector midgut. Therefore, NSvc2-N protein could potentially be used to reduce RSV transmission by competitively blocking midgut receptors.

RESULTS

Here, we report that transgenic rice plants expressing viral glycoprotein can interfere with RSV acquisition and transmission by SBPH. The soluble fraction (30-268 amino acids, designated NSvs2-N_S ) of NSvs2-N was transformed into rice calli, which produced plants harboring the exogenous gene. When SBPH was fed on transgenic plants prior to RSV-infected rice (sequential feeding) and when insects were fed on RSV-infected transgenic plants (concomitant feeding), virus acquisition by the insect vector was inhibited, and subsequent viral titers were reduced. Immunofluorescence labeling also indicated that viral infection of the insect midgut was inhibited after SBPH was fed on transgenic plants. The system by which RSV infected insect cells in vitro was used to corroborate the role of NSvc2-N_S in reducing viral infection. After the cells were incubated with transgenic rice sap, the virus infection rate of the cells decreased significantly, and viral accumulation in the cells was lower than that in the control group.

CONCLUSION

These results demonstrated the negative effect of NSvs2-N_S transgenic plants on RSV transmission by insect vectors, which provides a novel and effective way to control plant viral diseases. © 2022 Society of Chemical Industry.

Insights Into the Effect of Rice Stripe Virus P2 on Rice Defense by Comparative Proteomic Analysis

Rice stripe virus (RSV) has a serious effect on rice production. Our previous research had shown that RSV P2 plays important roles in RSV infection, so in order to further understand the effect of P2 on rice, we used Tandem Mass Tag (TMT) quantitative proteomics experimental system to analyze the changes of protein in transgenic rice expressing P2 for the first time. The results of proteomics showed that a total of 4,767 proteins were identified, including 198 up-regulated proteins and 120 down-regulated proteins. Functional classification results showed that differentially expressed proteins (DEPs) were mainly localized in chloroplasts and mainly involved in the metabolic pathways. Functional enrichment results showed that DEPs are mainly involved in RNA processing and splicing. We also verified the expression of several DEPs at the mRNA level and the interaction of a transcription factor (B7EPB8) with RSV P2. This research is the first time to use proteomics technology to explore the mechanism of RSV infection in rice with the RSV P2 as breakthrough point. Our findings provide valuable information for the study of RSV P2 and RSV infection mechanism.

The Curious Strategy of Multipartite Viruses

Multipartite virus genomes are composed of several segments, each packaged in a distinct viral particle. Although this puzzling genome architecture is found in ∼17% of known viral species, its distribution among hosts or among distinct types of genome-composing nucleic acid remains poorly understood. No convincing advantage of multipartitism has been identified, yet the maintenance of genomic integrity appears problematic. Here we review recent studies shedding light on these issues. Multipartite viruses rapidly modify the copy number of each segment/gene from one host species to another, a putative benefit if host switches are common. One multipartite virus functions in a multicellular way: The segments do not all need to be present in the same cell and can functionally complement across cells, maintaining genome integrity within hosts. The genomic integrity maintenance during host-to-host transmission needs further elucidation. These features challenge several virology foundations and could apply to other multicomponent viral systems.

Membrane association of importin α facilitates viral entry into salivary gland cells of vector insects

The importin α family belongs to the conserved nuclear transport pathway in eukaryotes. However, the biological functions of importin α in the plasma membrane are still elusive. Here, we report that importin α, as a plasma membrane-associated protein, is exploited by the rice stripe virus (RSV) to enter vector insect cells, especially salivary gland cells. When the expression of three importin α genes was simultaneously knocked down, few virions entered the salivary glands of the small brown planthopper, Laodelphax striatellus Through hemocoel inoculation of virions, only importin α2 was found to efficiently regulate viral entry into insect salivary-gland cells. Importin α2 bound the nucleocapsid protein of RSV with a relatively high affinity through its importin β-binding (IBB) domain, with a dissociation constant K _D of 9.1 μM. Furthermore, importin α2 and its IBB domain showed a distinct distribution in the plasma membrane through binding to heparin in heparan sulfate proteoglycan. When the expression of importin α2 was knocked down in viruliferous planthoppers or in nonviruliferous planthoppers before they acquired virions, the viral transmission efficiency of the vector insects in terms of the viral amount and disease incidence in rice was dramatically decreased. These findings not only reveal the specific function of the importin α family in the plasma membrane utilized by viruses, but also provide a promising target gene in vector insects for manipulation to efficiently control outbreaks of rice stripe disease.

Gut Bacterial Diversity in Different Life Cycle Stages of Adelphocoris suturalis (Hemiptera: Miridae)

Bacteria and insects have a mutually beneficial symbiotic relationship. Bacteria participate in several physiological processes such as reproduction, metabolism, and detoxification of the host. Adelphocoris suturalis is considered a pest by the agricultural industry and is now a major pest in cotton, posing a serious threat to agricultural production. As with many insects, various microbes live inside A. suturalis. However, the microbial composition and diversity of its life cycle have not been well-studied. To identify the species and community structure of symbiotic bacteria in A. suturalis, we used the HiSeq platform to perform high-throughput sequencing of the V3-V4 region in the 16S rRNA of symbiotic bacteria found in A. suturalis throughout its life stages. Our results demonstrated that younger nymphs (1st and 2nd instar nymphs) have higher species richness. Proteobacteria (87.06%) and Firmicutes (9.43%) were the dominant phyla of A. suturalis. At the genus level, Erwinia (28.98%), Staphylococcus (5.69%), and Acinetobacter (4.54%) were the dominant bacteria. We found that the relative abundance of Erwinia was very stable during the whole developmental stage. On the contrary, the relative abundance of Staphylococcus, Acinetobacter, Pseudomonas, and Corynebacterium showed significant dynamic changes at different developmental stages. Functional prediction of symbiotic bacteria mainly focuses on metabolic pathways. Our findings document symbiotic bacteria across the life cycle of A. suturalis, as well as differences in both the composition and richness in nymph and adult symbiotic bacteria. Our analysis of the bacteria in A. suturalis provides important information for the development of novel biological control strategies.

Development of Rice Stripe Tenuivirus Minireplicon Reverse Genetics Systems Suitable for Analyses of Viral Replication and Intercellular Movement

Rice stripe virus (RSV), a tenuivirus with four negative-sense/ambisense genome segments, is one of the most devastating viral pathogens affecting rice production in many Asian countries. Despite extensive research, our understanding of RSV infection cycles and pathogenesis has been severely impaired by the lack of reverse genetics tools. In this study, we have engineered RSV minireplicon (MR)/minigenome cassettes with reporter genes substituted for the viral open reading frames in the negative-sense RNA1 or the ambisense RNA2-4 segments. After delivery to Nicotiana benthamiana leaves via agroinfiltration, MR reporter gene expression was detected only when the codon-optimized large viral RNA polymerase protein (L) was coexpressed with the nucleocapsid (N) protein. MR activity was also critically dependent on the coexpressed viral suppressors of RNA silencing, but ectopic expression of the RSV-encoded NS3 silencing suppressor drastically decreased reporter gene expression. We also developed intercellular movement-competent MR systems with the movement protein expressed either in cis from an RNA4-based MR or in trans from a binary plasmid. Finally, we generated multicomponent replicon systems by expressing the N and L proteins directly from complementary-sense RNA1 and RNA3 derivatives, which enhanced reporter gene expression, permitted autonomous replication and intercellular movement, and reduced the number of plasmids required for delivery. In summary, this work enables reverse genetics analyses of RSV replication, transcription, and cell-to-cell movement and provides a platform for engineering more complex recombinant systems.

Insights Into Insect Vector Transmission and Epidemiology of Plant-Infecting Fijiviruses

Viruses in genus Fijivirus (family Reoviridae) have caused serious damage to rice, maize and sugarcane in American, Asian, European and Oceanian countries, where seven plant-infecting and two insect-specific viruses have been reported. Because the planthopper vectors are the only means of virus spread in nature, their migration and efficient transmission of these viruses among different crops or gramineous weeds in a persistent propagative manner are obligatory for virus epidemics. Understanding the mechanisms of virus transmission by these insect vectors is thus key for managing the spread of virus. This review describes current understandings of main fijiviruses and their insect vectors, transmission characteristics, effects of viruses on the behavior and physiology of vector insects, molecular transmission mechanisms. The relationships among transmission, virus epidemics and management are also discussed. To better understand fijivirus-plant disease system, research needs to focus on the complex interactions among the virus, insect vector, insect microbes, and plants.

Ever-increasing viral diversity associated with the red imported fire ant Solenopsis invicta (Formicidae: Hymenoptera)

Background

Advances in sequencing and analysis tools have facilitated discovery of many new viruses from invertebrates, including ants. Solenopsis invicta is an invasive ant that has quickly spread worldwide causing significant ecological and economic impacts. Its virome has begun to be characterized pertaining to potential use of viruses as natural enemies. Although the S. invicta virome is the best characterized among ants, most studies have been performed in its native range, with less information from invaded areas.

Methods

Using a metatranscriptome approach, we further identified and molecularly characterized virus sequences associated with S. invicta, in two introduced areas, U.S and Taiwan. The data set used here was obtained from different stages (larvae, pupa, and adults) of S. invicta life cycle. Publicly available RNA sequences from GenBank’s Sequence Read Archive were downloaded and de novo assembled using CLC Genomics Workbench 20.0.1. Contigs were compared against the non-redundant protein sequences and those showing similarity to viral sequences were further analyzed.

Results

We characterized five putative new viruses associated with S. invicta transcriptomes. Sequence comparisons revealed extensive divergence across ORFs and genomic regions with most of them sharing less than 40% amino acid identity with those closest homologous sequences previously characterized. The first negative-sense single-stranded RNA virus genomic sequences included in the orders Bunyavirales and Mononegavirales are reported. In addition, two positive single-strand virus genome sequences and one single strand DNA virus genome sequence were also identified. While the presence of a putative tenuivirus associated with S. invicta was previously suggested to be a contamination, here we characterized and present strong evidence that Solenopsis invicta virus 14 (SINV-14) is a tenui-like virus that has a long-term association with the ant. Furthermore, based on virus sequence abundance compared to housekeeping genes, phylogenetic relationships, and completeness of viral coding sequences, our results suggest that four of five virus sequences reported, those being SINV-14, SINV-15, SINV-16 and SINV-17, may be associated to viruses actively replicating in the ant S. invicta.

Conclusions

The present study expands our knowledge about viral diversity associated with S. invicta in introduced areas with potential to be used as biological control agents, which will require further biological characterization.

The Epidemiology of Plant Virus Disease: Towards a New Synthesis

Epidemiology is the science of how disease develops in populations, with applications in human, animal and plant diseases. For plant diseases, epidemiology has developed as a quantitative science with the aims of describing, understanding and predicting epidemics, and intervening to mitigate their consequences in plant populations. Although the central focus of epidemiology is at the population level, it is often necessary to recognise the system hierarchies present by scaling down to the individual plant/cellular level and scaling up to the community/landscape level. This is particularly important for diseases caused by plant viruses, which in most cases are transmitted by arthropod vectors. This leads to range of virus-plant, virus-vector and vector-plant interactions giving a distinctive character to plant virus epidemiology (whilst recognising that some fungal, oomycete and bacterial pathogens are also vector-borne). These interactions have epidemiological, ecological and evolutionary consequences with implications for agronomic practices, pest and disease management, host resistance deployment, and the health of wild plant communities. Over the last two decades, there have been attempts to bring together these differing standpoints into a new synthesis, although this is more apparent for evolutionary and ecological approaches, perhaps reflecting the greater emphasis on shorter often annual time scales in epidemiological studies. It is argued here that incorporating an epidemiological perspective, specifically quantitative, into this developing synthesis will lead to new directions in plant virus research and disease management. This synthesis can serve to further consolidate and transform epidemiology as a key element in plant virus research.

Soybean Thrips (Thysanoptera: Thripidae) Harbor Highly Diverse Populations of Arthropod, Fungal and Plant Viruses

Soybean thrips (Neohydatothrips variabilis) are one of the most efficient vectors of soybean vein necrosis virus, which can cause severe necrotic symptoms in sensitive soybean plants. To determine which other viruses are associated with soybean thrips, the metatranscriptome of soybean thrips, collected by the Midwest Suction Trap Network during 2018, was analyzed. Contigs assembled from the data revealed a remarkable diversity of virus-like sequences. Of the 181 virus-like sequences identified, 155 were novel and associated primarily with taxa of arthropod-infecting viruses, but sequences similar to plant and fungus-infecting viruses were also identified. The novel viruses were predicted to have positive-sense RNA, negative-stranded RNA, double-stranded RNA, and single-stranded DNA genomes. The assembled sequences included 100 contigs that represented at least 95% coverage of a virus genome or genome segment. Sequences represented 12 previously described arthropod viruses including eight viruses reported from Hubei Province in China, and 12 plant virus sequences of which six have been previously described. The presence of diverse populations of plant viruses within soybean thrips suggests they feed on and acquire viruses from multiple host plant species that could be transmitted to soybean. Assessment of the virome of soybean thrips provides, for the first time, information on the diversity of viruses present in thrips.

Ribavirin targets sugar transporter 6 to suppress acquisition and transmission of rice stripe tenuivirus by its vector Laodelphax striatellus

BACKGROUND

Rice stripe tenuivirus (RSV) is one of the most destructive pathogens of rice and other cereal crops. The virus is transmitted by the small brown planthopper (SBPH, Laodelphax striatellus) in a circulative-propagative manner. Thus, blocking transmission by the insect vector would provide an effective strategy to prevent epidemic outbreaks of the disease.

RESULTS

In this study, we explored the effect of ribavirin on acquisition and transmission of the virus by specifically inhibiting the expression of sugar transporter 6 (LsSt-6), which was recently reported as a key vector component for RSV transmission. Ribavirin at the highest concentration tested (250 μmol L^-1 ) significantly reduced RSV acquisition and transmission efficiency by SBPHs through inhibiting LsSt-6 messenger RNA (mRNA) level. Survival of the model insect Spodoptera frugiperda cell line (Sf9) was 95.0 ± 2.2 and 85.6 ± 2.1% after exposure to 250 μmol L^-1 ribavirin or 8-azaguanine, respectively. Further study confirmed that 250 μmol L^-1 ribavirin also significantly reduced LsSt-6 mRNA and protein levels in Sf9 cells. However, 8-azaguanine did not significantly inhibit viral infectivity and LsSt-6 mRNA levels in SBPH or the Sf9 cell line.

CONCLUSION

This result provides evidence that ribavirin has the potential to disrupt LsSt-6 expression but not others like viral RNAs to prevent acquiring RSV, which leads to less viral accumulation in SBPH tissues and thereby lower transmission efficiency. © 2020 Society of Chemical Industry.

Deep Roots and Splendid Boughs of the Global Plant Virome

Land plants host a vast and diverse virome that is dominated by RNA viruses, with major additional contributions from reverse-transcribing and single-stranded (ss) DNA viruses. Here, we introduce the recently adopted comprehensive taxonomy of viruses based on phylogenomic analyses, as applied to the plant virome. We further trace the evolutionary ancestry of distinct plant virus lineages to primordial genetic mobile elements. We discuss the growing evidence of the pivotal role of horizontal virus transfer from invertebrates to plants during the terrestrialization of these organisms, which was enabled by the evolution of close ecological associations between these diverse organisms. It is our hope that the emerging big picture of the formation and global architecture of the plant virome will be of broad interest to plant biologists and virologists alike and will stimulate ever deeper inquiry into the fascinating field of virus-plant coevolution.

Rice black-streaked dwarf virus: From multiparty interactions among plant-virus-vector to intermittent epidemics

Rice black-streaked dwarf virus (RBSDV) (species Rice black-streaked dwarf virus, genus Fijivirus, family Reoviridae) is the causal agent of rice black-streaked dwarf and maize rough dwarf diseases, which occur in intermittent epidemics in East Asian countries and are responsible for considerable yield losses. Intermittency of epidemics make accurate forecasting and designing of effective management strategies difficult. However, recent insights into host-virus-vector insect interactions are now informing forecasting and disease control measures. Resistance genes are also being identified and mapped.

SYMPTOMATOLOGY AND HOST RANGE

RBSDV induces extreme stunting, darkened, and stiff leaves of crops and weeds only in the family Poaceae, including Oryza sativa, Zea mays, and Triticum aestivum. Infected plants produce totally or partially deformed panicles and remain alive through harvest.

GENOME AND GENE FUNCTION

The nonenveloped virus particles comprise a double-layered capsid, 50-nm core with genomic double-stranded RNA (dsRNA), and six proteins. The genome of RBSDV contains 10 segments of dsRNA, named S1 to S10 in decreasing order of molecular weight. Segments 1, 2, 3, 4, 6, 8, and 10 encode the RNA-dependent RNA polymerase (RdRp), the major core structural protein, a protein with guanylyltransferase activity, an outer-shell B-spike protein, viral RNA-silencing suppressor, the major capsid protein, and the outer capsid protein, respectively. Each of the segments 5, 7, and 9 encodes two proteins: P5-1, a component of viroplasms; P5-2 of unknown function; nonstructural protein P7-1, involved in forming the structural matrix of tubular structures in infected tissues; P7-2 of unknown function; P9-1, the main component of viroplasms in infected cells and involved in viral replication; and P9-2 of unknown function.

TRANSMISSION AND EPIDEMIOLOGY

RBSDV is transmitted by Laodelphax striatellus in a persistent propagative manner. The vector insect is the only means of virus spread in nature, so its migration and transmission efficiency are obligatory for disease epidemics to develop. Susceptible varieties are widely planted, but efficient transmission by vectors is the primary reason for the epidemics. Cultivation system, pesticide overuse, and climatic conditions also contribute to epidemics by affecting the development of the vector insects and their population dynamics.

DISEASE MANAGEMENT

In the absence of resistant varieties, integrated disease management aims at disrupting the cycle of virus transmission by the insect vector. Inheritance studies have indicated that resistance is mostly governed by quantitative trait loci or multiple genes. Genetic engineering through RNA-interference and gene-editing strategies are potential approaches for disease control.

The Diversity and Distribution of Viruses Associated with Culex annulirostris Mosquitoes from the Kimberley Region of Western Australia

Metagenomics revealed an impressive breadth of previously unrecognized viruses. Here, we report the virome of the Culex annulirostris Skuse mosquito, an important vector of pathogenic arboviruses in Australia. Mosquitoes were collected from three sites in the Kimberley region of Western Australia. Unbiased high-throughput sequencing (HTS) revealed the presence of 16 novel viral sequences that share less than 90% identity with known viruses. None were closely related to pathogenic arboviruses. Viruses were distributed unevenly across sites, indicating a heterogeneous Cx. annulirostris virome. Polymerase chain reaction assays confirmed HTS data and identified marked variation between the virus prevalence identified at each site.

Incidence and Distribution of Insect-Transmitted Cereal Viruses in Wheat in China from 2007 to 2019

Diseases caused by insect-transmitted viruses are the predominant constraint to wheat production worldwide. However, detailed knowledge of virus incidence and dynamics in China in recent years is very limited. Here, major wheat-growing regions of China were surveyed over 10 years for insect-transmitted viruses, and 2,143 samples were collected (in 2007 to 2015) and analyzed by molecular hybridization or multiplex reverse-transcription PCR for barley yellow dwarf viruses (BYDVs: BYDV-GAV, -GPV, and -PAV) and wheat dwarf virus (WDV). In a 4-year survey (2016 to 2019), the incidence of eight insect-transmitted viruses (BYDVs, WDV, wheat yellow striate virus [WYSV], barley yellow striate mosaic virus [BYSMV], northern cereal mosaic virus [NCMV], and rice black-streaked dwarf virus [RBSDV]) was investigated, and BYDVs and WDV were widely distributed across China. BYDV-GAV (29.0% of the tested sample) was the most abundant, followed by BYDV-PAV (23.2%) from 2007 to 2015. From 2016 to 2019, however, BYDV-PAV had become the predominant species (39.5% positive of 952 samples tested), while the incidence of BYDV-GAV (13.4%) had declined. During the entire survey, the incidence of BYDV-GPV was very low in some locations in northwestern and northern China, and all eight viruses caused only local epidemics, not large-scale outbreaks throughout China. Two new cereal-infecting rhabdoviruses, leafhopper-transmitted WYSV and planthopper-transmitted BYSMV, were also found in China in recent years.

Assessing the Potential Infection of Tagosodes orizicolus (Hemiptera: Delphacidae) by Rice Hoja Blanca Virus in Texas

Tagosodes orizicolus (Muir) is the most important pest of rice in Latin America. Besides causing direct damage called hopperburn from feeding on and ovipositing in rice leaves, this insect pest also transmits rice hoja blanca virus (RHBV, Family Phenuiviridae, Genus Tenuivirus) in a persistent-propagative manner. This pathosystem can cause up to 100% yield loss in Latin American rice fields. T. orizicolus and RHBV symptoms were detected in Louisiana, Mississippi, and Florida rice fields in the 1950s, 1960s, and 1980s. However, neither has been detected in the United States since. Two outbreaks of T. orizicolus on ratoon rice occurred in the fall of 2015 and 2018 in counties southwest and south of Houston, TX. Insects were collected from ratoon rice fields by sweep net methods. Insects from the 2015 and 2018 outbreaks were tested individually and in pools of 10, respectively, for RHBV infection and the cytochrome oxidase 1 (CO1) gene from Delphacidae. No insects were positive for RHBV, however, all samples yielded amplicons for the CO1 gene. Furthermore, the CO1 gene from five 2015 individuals was sequenced and found to have a 100% identity to the Fer26_Argentina and 99.81% identity to the DEL074 Venezuela isolates of T. orizicolus. Five new sequences from 2015 individuals have now been deposited in GenBank. It is imperative to stay up to date on the potential invasion and establishment of this exotic pest of rice in Texas and other rice-growing regions of the United States through continued monitoring and research.

Purification of Rice Stripe Virus

Although many spherical and rod-shaped plant virus purification protocols are now available, only a few protocols on filamentous plant virus purification have been published. Here, we report a protocol for large-scale purification of Rice stripe virus (RSV) from RSV-infected rice tissues. RSV virions with high infectivity were first precipitated with polyethylene glycol (PEG) followed by pelleting through primary ultracentrifugation, ultracentrifugation in a glycerol cushion and ultracentrifugation in density gradient. The purified RSV virions can not only be viewed as filamentous particles under an electron microscope, but can also be acquired by insect vector through direct injection into insect body or through membrane feeding prior to transmission to rice plants.

New Technologies for Studying Negative-Strand RNA Viruses in Plant and Arthropod Hosts

The plant viruses in the phylum Negarnaviricota, orders Bunyavirales and Mononegavirales, have common features of single-stranded, negative-sense RNA genomes and replication in the biological vector. Due to the similarities in biology, comparative functional analysis in plant and vector hosts is helpful for understanding host-virus interactions for negative-strand RNA viruses. In this review, we will highlight recent technological advances that are breaking new ground in the study of these recalcitrant virus systems. The development of infectious clones for plant rhabdoviruses and bunyaviruses is enabling unprecedented examination of gene function in plants and these advances are also being transferred to study virus biology in the vector. In addition, genome and transcriptome projects for critical nonmodel arthropods has enabled characterization of insect response to viruses and identification of interacting proteins. Functional analysis of genes using genome editing will provide future pathways for further study of the transmission cycle and new control strategies for these viruses and their vectors.

High Diversity and Functional Complementation of Alimentary Canal Microbiota Ensure Small Brown Planthopper to Adapt Different Biogeographic Environments

Almost all insects harbor commensal bacteria in the alimentary canal lumen or within cells and often play a pivotal role in their host’s development, evolution, and environmental adaptation. However, little is known about the alimentary canal microbiota and their functions in sap-sucking insect pests of crops, which can damage plants by removing plant sap and by transmitting various plant viruses, especially in the small brown planthopper, Laodelphax striatellus. In this study, we characterized the alimentary canal microbiota of L. striatellus collected from seven regions in China by sequencing 16S rDNA. The insects harbored a rich diversity of microbes, mainly consisted of bacteria from phyla Proteobacteria, Actinobacteria, Firmicutes, Bacteroidetes, and Tenericutes. The composition and abundance of microbiota were more similar as the geographic distance decreased between the populations and clustered by geographic location into three groups: temperate, subtropical, and tropical populations. Although the abundance and species of microbes differed among the populations, the various major microbes for each population performed similar functions based on a clusters of orthologous group analysis. Greater diversity in ecological factors in different regions might lead to higher microbial diversity, thus enabling L. striatellus to adapt or tolerate various extreme environments to avoid the cost of long-distance migration. Moreover, the abundance of various metabolic functions in the Kaifeng populations might contribute to higher fecundity in L. striatellus.

Complete genome sequence and characterization of a new iflavirus from the small brown planthopper (Laodelphax striatellus)

A novel iflavirus, tentatively named laodelphax striatellus iflavirus 1 (LsIV1), was identified in Laodelphax striatellus by total RNA-sequencing, and its genome sequence was confirmed by Sanger sequencing. The complete genome consisted of 10,831 nucleotides with a polyA tail and included one open reading frame, encoding a 361.7-kD polyprotein. Conserved motifs for structural proteins, helicase, protease, and RNA-dependent RNA polymerase were identified by aligning the deduced amino acid sequence of LsIV1 with several other iflaviruses. The genome has the highest identity with another planthopper iflavirus, nilaparvata lugens honeydew virus-3 (39.7%), under the species demarcation threshold (90%). Results of the identities and phylogenetic analysis based on the deduced amino acid sequences of the complete polyprotein and helicase of LsIV1 and other iflaviruses, indicated it is a new species belonging to the family Iflaviridae. Furthermore, we did not observe any differences of biological characterizations like development and reproduction between viruliferous and virus-free SBPH. Meanwhile, we found that female could transmit LsIV1 with higher transmission efficiency.

Entry of bunyaviruses into plants and vectors

The majority of plant-infecting viruses are transmitted by arthropod vectors that deliver them directly into a living plant cell. There are diverse mechanisms of transmission ranging from direct binding to the insect stylet (non-persistent transmission) to persistent-propagative transmission in which the virus replicates in the insect vector. Despite this diversity in interactions, most arthropods that serve as efficient vectors have feeding strategies that enable them to deliver the virus into the plant cell without extensive damage to the plant and thus effectively inoculate the plant. As such, the primary virus entry mechanism for plant viruses is mediated by the biological vector. Remarkably, viruses that are transmitted in a propagative manner (bunyaviruses, rhabdoviruses, and reoviruses) have developed an ability to replicate in hosts from two kingdoms. Viruses in the order Bunyavirales are of emerging importance and with the advent of new sequencing technologies, we are getting unprecedented glimpses into the diversity of these viruses. Plant-infecting bunyaviruses are transmitted in a persistent, propagative manner must enter two unique types of host cells, plant and insect. In the insect phase of the virus life cycle, the propagative viruses likely use typical cellular entry strategies to traverse cell membranes. In this review, we highlight the transmission and entry strategies of three genera of plant-infecting bunyaviruses: orthotospoviruses, tenuiviruses, and emaraviruses.

Higher Bacterial Diversity of Gut Microbiota in Different Natural Populations of Leafhopper Vector Does Not Influence WDV Transmission

The bacterial communities in the gut of an insect have important ecological and functional effects on the insect. However, the community composition and diversity of the gut microbiota in insects that vector plant viruses are poorly understood. As an important insect vector, Psammotettix alienus transmits various viruses including wheat dwarf virus (WDV). Here, we used the combination of leafhopper and WDV as model to survey the influence of gut microbiota on virus transmission characteristic of insect vector and vice versa. We have characterized 22 phyla and 249 genera of all gut bacterial communities in the leafhopper populations collected from six geographic regions in China. Community composition and diversity varied across different geographic populations. However, WDV transmission efficiencies of these six field populations were all greater than 80% with no significant difference. Interestingly, the transmission efficiency of WDV by laboratory reared insects with decreased gut bacterial diversity was similar to that of field populations. Furthermore, we found that the composition of the leafhopper gut bacteria was dynamic and could reversibly respond to WDV acquisition. Higher bacterial diversity and abundance of gut microbiota in different leafhopper populations did not influence their WDV transmission efficiency, while the acquisition of WDV changes gut microbiota by a dynamic and reversible manner. This report provides insight into the complex relationship between the gut microbiota, insect vector and virus.

The vitellogenin receptor has an essential role in vertical transmission of rice stripe virus during oogenesis in the small brown plant hopper

BACKGROUND

The small brown plant hopper (SBPH), Laodelphax striatellus Fallén, is one of the most destructive pests on rice. This pest transmits rice stripe virus (RSV) both horizontally and vertically, leading to major yield and economic losses in rice production. However, the way that RSV particles enter oocytes of SBPH remains largely unknown. Thus, identification of key factors involved in the interaction between SBPH and RSV in the ovary is crucial.

RESULTS

Transcriptome of non-viruliferous (NV) or high viruliferous (HV) SBPH ovaries at 24 and 48 h of emergence was sequenced. Differentially expressed genes analysis showed that vitellogenin receptor was significantly highly expressed in the ovary of the HV SBPH strains compared to NV strains. Quantitative real-time polymer chain reaction showed that the vitellogenin receptor in L. striatellus (LsVgR) was highly expressed in the ovaries of female adults and maintained a high level of expression at the early stage of ovary development. By using RNA interference, the expression of LsVgR in the ovaries of the HV strain was significantly decreased by 98.1%. RSV titer was reduced by 60.9% as quantified by viral RNA3 intergenic region and the transcripts of nucleocapsid protein gene (CP) reduced by 46.3%. The numbers of offspring hatched were significantly reduced in dsRNA-treated groups. The transcripts of CP were not affected by silencing LsVgR, whereas the abundance of RNA-dependent RNA polymerase increased by 15-fold in the member of surviving progenies.

CONCLUSION

Our results suggest that vitellogenin receptor participates in regulating RSV replication during oogenesis. © 2018 Society of Chemical Industry.