A soluble form of the Tomato spotted wilt virus (TSWV) glycoprotein G(N) (G(N)-S) inhibits transmission of TSWV by Frankliniella occidentalis

Integrated Biological Experiments and Proteomic Analyses of Nicotiana tabacum Xylem Sap Revealed the Host Response to Tomato Spotted Wilt Orthotospovirus Infection

The plant vascular system is not only a transportation system for delivering nutrients but also a highway transport network for spreading viruses. Tomato spotted wilt orthotospovirus (TSWV) is among the most destructive viruses that cause serious losses in economically important crops worldwide. However, there is minimal information about the long-distance movements of TSWV in the host plant vascular system. In this this study, we confirm that TSWV virions are present in the xylem as observed by transmission electron microscopy (TEM). Further, a quantitative proteomic analysis based on label-free methods was conducted to reveal the uniqueness of protein expression in xylem sap during TSWV infection. Thus, this study identified and quantified 3305 proteins in two groups. Furthermore, TSWV infection induced three viral structural proteins, N, Gn and Gc, and 315 host proteins differentially expressed in xylem (163 up-regulated and 152 down-regulated). GO enrichment analysis showed up-regulated proteins significantly enriched in homeostasis, wounding, defense response, and DNA integration terms, while down-regulated proteins significantly enriched in cell wall biogenesis/xyloglucan metabolic process-related terms. KEGG enrichment analysis showed that the differentially expressed proteins (DEPs) were most strongly associated with plant-pathogen interaction, MAPK signaling pathway, and plant hormone signal transduction. Cluster analysis of DEPs function showed the DEPs can be categorized into cell wall metabolism-related proteins, antioxidant proteins, PCD-related proteins, host defense proteins such as receptor-like kinases (RLKs), salicylic acid binding protein (SABP), pathogenesis related proteins (PR), DNA methylation, and proteinase inhibitor (PI). Finally, parallel reaction monitoring (PRM) validated 20 DEPs, demonstrating that the protein abundances were consistent between label-free and PRM data. Finally, 11 genes were selected for RT-qPCR validation of the DEPs and label-free-based proteomic analysis concordant results. Our results contribute to existing knowledge on the complexity of host plant xylem system response to virus infection and provide a basis for further study of the mechanism underlying TSWV long-distance movement in host plant vascular system.

A call to arms: novel strategies for thrips and tospovirus control

Thrips and the tospoviruses they transmit are some of the most significant threats to food and ornamental crop production globally. Control of the insect and virus is challenging and new strategies are needed. Characterizing the thrips-virus interactome provides new targets for disrupting the transmission cycle. Viral and insect determinants of vector competence are being defined, including the viral attachment protein and its structure as well as thrips proteins that interact with and respond to tospovirus infection. Additional thrips control strategies such as RNA interference need further refinement and field-applicable delivery systems, but they show promise for the knockdown of essential genes for thrips survival and virus transmission. The identification of a toxin that acts to deter thrips oviposition on cotton also presents new opportunities for control of this important pest.

Two Alimentary Canal Proteins, Fo-GN and Fo-Cyp1, Act in Western Flower Thrips, Frankliniella occidentalis TSWV Infection

Tomato spotted wilt virus (TSWV) is a plant virus that causes massive economic damage to high-valued crops. This virus is transmitted by specific thrips, including the western flower thrips, Frankliniella occidentalis. TSWV is acquired by the young larvae during feeding on infected host plants. TSWV infects the gut epithelium through hypothetical receptor(s) and multiplies within the cells for subsequent horizontal transmission to other plant hosts via the salivary glands during feeding. Two alimentary canal proteins, glycoprotein (Fo-G_N) and cyclophilin (Fo-Cyp1), are thought to be associated with the TSWV entry into the gut epithelium of F. occidentalis. Fo-G_N possesses a chitin-binding domain, and its transcript was localized on the larval gut epithelium by fluorescence in situ hybridization (FISH) analysis. Phylogenetic analysis indicated that F. occidentalis encodes six cyclophilins, in which Fo-Cyp1 is closely related to a human cyclophilin A, an immune modulator. The Fo-Cyp1 transcript was also detected in the larval gut epithelium. Expression of these two genes was suppressed by feeding their cognate RNA interference (RNAi) to young larvae. The RNAi efficiencies were confirmed by the disappearance of the target gene transcripts from the gut epithelium by FISH analyses. The RNAi treatments directed to Fo-G_N or Fo-Cyp1 prevented the typical TSWV titer increase after the virus feeding, compared to control RNAi treatment. Our immunofluorescence assay using a specific antibody to TSWV documented the reduction of TSWV in the larval gut and adult salivary gland after the RNAi treatments. These results support our hypothesis that the candidate proteins Fo-G_N and Fo-Cyp1 act in TSWV entry and multiplication in F. occidentalis.

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.

Polerovirus N-terminal readthrough domain structures reveal molecular strategies for mitigating virus transmission by aphids

Poleroviruses, enamoviruses, and luteoviruses are icosahedral, positive sense RNA viruses that cause economically important diseases in food and fiber crops. They are transmitted by phloem-feeding aphids in a circulative manner that involves the movement across and within insect tissues. The N-terminal portion of the viral readthrough domain (NRTD) has been implicated as a key determinant of aphid transmission in each of these genera. Here, we report crystal structures of the NRTDs from the poleroviruses turnip yellow virus (TuYV) and potato leafroll virus (PLRV) at 1.53-Å and 2.22-Å resolution, respectively. These adopt a two-domain arrangement with a unique interdigitated topology and form highly conserved dimers that are stabilized by a C-terminal peptide that is critical for proper folding. We demonstrate that the PLRV NRTD can act as an inhibitor of virus transmission and identify NRTD mutant variants that are lethal to aphids. Sequence conservation argues that enamovirus and luteovirus NRTDs will follow the same structural blueprint, which affords a biological approach to block the spread of these agricultural pathogens in a generalizable manner.

HMG-Like DSP1 Mediates Immune Responses of the Western Flower Thrips (Frankliniella occidentalis) Against Beauveria bassiana, a Fungal Pathogen

Western flower thrips, Frankliella occidentalis, is a serious pest by directly infesting host crops. It can also give indirect damage to host crops by transmitting a plant virus called tomato spotted wilt virus. A fungal pathogen, Beauveria bassiana, can infect thrips. It has been used as a biopesticide. However, little is known on the defense of thrips against this fungal pathogen. This study assessed the defense of thrips against the fungal infection with respect to immunity by analyzing immune-associated genes of F. occidentalis in both larvae and adults. Immunity-associated genes of western flower thrips were selected from three immunity steps: nonself recognition, mediation, and immune responses. For the pathogen recognition step, dorsal switch protein 1 (DSP1) was chosen. For the immune mediation step, phospholipase A₂ (PLA₂) and prostaglandin E₂ synthase were also selected. For the step of immune responses, two phenoloxidases (PO) genes and four proPO-activating peptidase genes involved in melanization against pathogens were chosen. Dual oxidase gene involved in the production of reactive oxygen species and four antimicrobial peptide genes for executing humoral immune responses were selected. All immunity-associated genes were inducible to the fungal infection. Their expression levels were induced higher in adults than in larvae by the fungal infections. However, inhibitor treatments specific to DSP1 or PLA₂ significantly suppressed the inducible expression of these immune-associated genes, leading to significant enhancement of fungal pathogenicity. These results suggest that immunity is essential for thrips to defend against B. bassiana, in which DSP1 and eicosanoids play a crucial role in eliciting immune responses.

Probing the Application of OmpA-Derived Peptides to Disrupt the Acquisition of 'Candidatus Liberibacter asiaticus' by Diaphorina citri

Huanglongbing (HLB) is currently the most devastating disease of citrus worldwide. Both bacteria 'Candidatus Liberibacter asiaticus' (CLas) and 'Candidatus Liberibacter americanus' (CLam) are associated with HLB in Brazil but with a strong prevalence of CLas over CLam. Conventionally, HLB management focuses on controlling the insect vector population (Diaphorina citri; also known as Asian citrus psyllid [ACP]) by spraying insecticides, an approach demonstrated to be mostly ineffective. Thus, development of novel, more efficient HLB control strategies is required. The multifunctional bacterial outer membrane protein OmpA is involved in several molecular processes between bacteria and their hosts and has been suggested as a target for bacterial control. Curiously, OmpA is absent in CLam in comparison with CLas, suggesting a possible role in host interaction. Therefore, in the current study, we have treated ACPs with different OmpA-derived peptides, aiming to evaluate acquisition of CLas by the insect vector. Treatment of psyllids with 5 µM of Pep1, Pep3, Pep5, and Pep6 in artificial diet significantly reduced the acquisition of CLas, whereas increasing the concentration of Pep5 and Pep6 to 50 µM abolished this process. In addition, in planta treatment with 50 µM of Pep6 also significantly decreased the acquisition of CLas, and sweet orange plants stably absorbed and maintained this peptide for as long as 3 months post the final application. Together, our results demonstrate the promising use of OmpA-derived peptides as a novel biotechnological tool to control CLas.

Integration of transcriptomics and network analysis reveals co-expressed genes in Frankliniella occidentalis larval guts that respond to tomato spotted wilt virus infection

Background

The gut is the first barrier to infection by viruses that are internally borne and transmitted persistently by arthropod vectors to plant and animal hosts. Tomato spotted wilt virus (TSWV), a plant-pathogenic virus, is transmitted exclusively by thrips vectors in a circulative-propagative manner. Frankliniella occidentalis (western flower thrips), the principal thrips vector of TSWV, is transmission-competent only if the virus is acquired by young larvae. To begin to understand the larval gut response to TSWV infection and accumulation, a genome-assisted, transcriptomic analysis of F. occidentalis gut tissues of first (early L1) and second (early L2 and late L2) instar larvae was conducted using RNA-Seq to identify differentially-expressed transcripts (DETs) in response to TSWV compared to non-exposed cohorts.

Results

The larval gut responded in a developmental stage-dependent manner, with the majority of DETs (71%) associated with the early L1 stage at a time when virus infection is limited to the midgut epithelium. Provisional annotations of these DETs inferred roles in digestion and absorption, insect innate immunity, and detoxification. Weighted gene co-expression network analysis using all assembled transcripts of the gut transcriptome revealed eight gene modules that distinguish larval development. Intra-module interaction network analysis of the three most DET-enriched modules revealed ten central hub genes. Droplet digital PCR-expression analyses of select network hub and connecting genes revealed temporal changes in gut expression during and post exposure to TSWV.

Conclusions

These findings expand our understanding of the developmentally-mediated interaction between thrips vectors and orthotospoviruses, and provide opportunities for probing pathways for biomarkers of thrips vector competence.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-08100-4.

High Plains wheat mosaic virus: An enigmatic disease of wheat and corn causing the High Plains disease

BRIEF HISTORY

In 1993, severe mosaic and necrosis symptoms were observed on corn (maize) and wheat from several Great Plains states of the USA. Based on the geographical location of infections, the disease was named High Plains disease and the causal agent was tentatively named High Plains virus. Subsequently, researchers renamed this virus as maize red stripe virus and wheat mosaic virus to represent the host and symptom phenotype of the virus. After sequencing the genome of the pathogen, the causal agent of High Plains disease was officially named as High Plains wheat mosaic virus. Hence, High Plains virus, maize red stripe virus, wheat mosaic virus, and High Plains wheat mosaic virus (HPWMoV) are synonyms for the causal agent of High Plains disease.

TAXONOMY

High Plains wheat mosaic virus is one of the 21 definitive species in the genus Emaravirus in the family Fimoviridae.

VIRION

The genomic RNAs are encapsidated in thread-like nucleocapsids in double-membrane 80-200 nm spherical or ovoid virions.

GENOME CHARACTERIZATION

The HPWMoV genome consists of eight single-stranded negative-sense RNA segments encoding a single open reading frame (ORF) in each genomic RNA segment. RNA 1 is 6,981-nucleotide (nt) long, coding for a 2,272 amino acid protein of RNA-dependent RNA polymerase. RNA 2 is 2,211-nt long and codes for a 667 amino acid glycoprotein precursor. RNA 3 has two variants of 1,439- and 1,441-nt length that code for 286 and 289 amino acid nucleocapsid proteins, respectively. RNA 4 is 1,682-nt long, coding for a 364 amino acid protein. RNA 5 and RNA 6 are 1,715- and 1,752-nt long, respectively, and code for 478 and 492 amino acid proteins, respectively. RNA 7 and RNA 8 are 1,434- and 1,339-nt long, code for 305 and 176 amino acid proteins, respectively.

BIOLOGICAL PROPERTIES

HPWMoV can infect wheat, corn (maize), barley, rye brome, oat, rye, green foxtail, yellow foxtail, and foxtail barley. HPWMoV is transmitted by the wheat curl mite and through corn seed.

DISEASE MANAGEMENT

Genetic resistance against HPWMoV in wheat is not available, but most commercial corn hybrids are resistant while sweet corn varieties remain susceptible. Even though corn hybrids are resistant to virus, it still serves as a green bridge host that enables mites to carry the virus from corn to new crop wheat in the autumn. The main management strategy for High Plains disease in wheat relies on the management of green bridge hosts. Cultural practices such as avoiding early planting can be used to avoid mite buildup and virus infections.

The Bunyavirales: The Plant-Infecting Counterparts

Negative-strand (-) RNA viruses (NSVs) comprise a large and diverse group of viruses that are generally divided in those with non-segmented and those with segmented genomes. Whereas most NSVs infect animals and humans, the smaller group of the plant-infecting counterparts is expanding, with many causing devastating diseases worldwide, affecting a large number of major bulk and high-value food crops. In 2018, the taxonomy of segmented NSVs faced a major reorganization with the establishment of the order Bunyavirales. This article overviews the major plant viruses that are part of the order, i.e., orthospoviruses (Tospoviridae), tenuiviruses (Phenuiviridae), and emaraviruses (Fimoviridae), and provides updates on the more recent ongoing research. Features shared with the animal-infecting counterparts are mentioned, however, special attention is given to their adaptation to plant hosts and vector transmission, including intra/intercellular trafficking and viral counter defense to antiviral RNAi.

Relationships of virus titers and transmission rates among sympatric and allopatric virus isolates and thrips vectors support local adaptation

Plant viruses rely on insect vectors for transmission among plant hosts, but many of the specifics of virus-vector interactions are not fully understood. Thrips tabaci, which transmits Tomato spotted wilt virus (TSWV) in a persistent and propagative manner, varies greatly in its ability to transmit different isolates of TSWV. Similarly, TSWV isolates are transmitted at different efficiencies by different populations of T. tabaci. This study characterizes differences in virus titers in the vector among TSWV isolate-T. tabaci isoline pairings in relation to differences in transmission rates, and demonstrates that although transmission rates were higher for sympatric than allopatric TSWV isolate-T. tabaci isoline pairings, virus titers in the thrips vector were significantly lower in the sympatric pairings. Results further demonstrate that TSWV titers in the vector were unrelated to virus titers in the leaf tissue from which they acquired the virus and provide evidence for the importance of specific vector-virus interactions and local adaptation in determining transmission efficiency of TSWV by T. tabaci.

Thrips as the Transmission Bottleneck for Mixed Infection of Two Orthotospoviruses

Mixed infections provide opportunities for viruses to increase genetic diversity by facilitating genomic reassortment or recombination, and they may lead to the emergence of new virus species. Mixed infections of two economically important orthotospoviruses, Tomato spotted wilt orthotospovirus (TSWV) and Impatiens necrotic spot orthotospovirus (INSV), were found in recent years, but no natural reassortants between INSV and TSWV were ever reported. The goal of this study was to establish how vector preferences and the ability to transmit INSV and TSWV influence transmission and establishment of mixed infections. Our results demonstrate that thrips prefer to oviposit on TSWV and INSV mixed-infected plants over singly infected or healthy plants, providing young nymphs with the opportunity to acquire both viruses. Conversely, we observed that thrips served as a bottleneck during transmission and favored transmission of one of the two viruses over the second one, or over transmission of both viruses simultaneously. This constraint was relaxed in plants, when transmission of TSWV and INSV occurred sequentially, demonstrating that plants serve as orthotospovirus permissive hosts, while thrips serve as a bottleneck. Viral fitness, as measured by virus replication, transmission, and competition with other viral strains, is not well studied in mixed infection. Our study looks at the success of transmission during mixed infection of orthotopoviruses, enhancing the understanding of orthotospovirus epidemiology and evolution.

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.

A TMT-Based Quantitative Proteome Analysis to Elucidate the TSWV Induced Signaling Cascade in Susceptible and Resistant Cultivars of Solanum lycopersicum

Tomato spotted wilt virus (TSWV), transmitted by small insects known as thrips, is one of the major threats to tomato productivity across the globe. In addition to tomato, this virus infects more than 1000 other plants belonging to 85 families and is a cause of serious concern. Very little, however, is known about the molecular mechanism of TSWV induced signaling in plants. Here, we used a tandem mass tags (TMT)-based quantitative proteome approach to investigate the protein profiles of tomato leaves of two cultivars (cv 2621 and 2689; susceptible and resistant to TSWV infection, respectively) following TSWV inoculation. This approach resulted in the identification of 5112 proteins of which 1022 showed significant changes in response to TSWV. While the proteome of resistant cultivar majorly remains unaltered, the proteome of susceptible cultivar showed distinct differences following TSWV inoculation. TSWV modulated proteins in tomato included those with functions previously implicated in plant defense including secondary metabolism, reactive oxygen species (ROS) detoxification, mitogen-activated protein (MAP) kinase signaling, calcium signaling and jasmonate biosynthesis, among others. Taken together, results reported here provide new insights into the TSWV induced signaling in tomato leaves and may be useful in the future to manage this deadly disease of plants.

Invasion Biology, Ecology, and Management of Western Flower Thrips

Western flower thrips, Frankliniella occidentalis, first arose as an important invasive pest of many crops during the 1970s-1980s. The tremendous growth in international agricultural trade that developed then fostered the invasiveness of western flower thrips. We examine current knowledge regarding the biology of western flower thrips, with an emphasis on characteristics that contribute to its invasiveness and pest status. Efforts to control this pest and the tospoviruses that it vectors with intensive insecticide applications have been unsuccessful and have created significant problems because of the development of resistance to numerous insecticides and associated outbreaks of secondary pests. We synthesize information on effective integrated management approaches for western flower thrips that have developed through research on its biology, behavior, and ecology. We further highlight emerging topics regarding the species status of western flower thrips, as well as its genetics, biology, and ecology that facilitate its use as a model study organism and will guide development of appropriate management practices.

Transmission blockage of an orthotospovirus using synthetic peptides

Orthotospoviruses are acquired by thrips during feeding on infected tissue. Virions travel through the foregut and enter midgut epithelial cells through the interaction between the viral glycoproteins and cellular receptors. Glycoprotein RGD motifs and N-linked glycosylation sites have been predicted to mediate receptor binding or play important roles in virus entry into host cells, yet their function needs to be validated. In this study, peptides derived from the soybean vein necrosis virus N glycoprotein were utilized to identify critical regions in virus-vector interactions. Transmission mediated by single Neohydatothrips variabilis dropped by more than 2/3 when thrips were fed on peptide NASIAAAHEVSQE or the combination of NASIRGDHEVSQE and RLTGECNITKVSLTN when compared to the controls; indicating that this strategy could significantly reduce transmission efficiency, opening new avenues in the control of diseases caused by orthotospoviruses.

Discovery of Novel Thrips Vector Proteins That Bind to the Viral Attachment Protein of the Plant Bunyavirus Tomato Spotted Wilt Virus

The plant-pathogenic virus tomato spotted wilt virus (TSWV) encodes a structural glycoprotein (GN) that, like with other bunyavirus/vector interactions, serves a role in viral attachment and possibly in entry into arthropod vector host cells. It is well documented that Frankliniella occidentalis is one of nine competent thrips vectors of TSWV transmission to plant hosts. However, the insect molecules that interact with viral proteins, such as GN, during infection and dissemination in thrips vector tissues are unknown. The goals of this project were to identify TSWV-interacting proteins (TIPs) that interact directly with TSWV GN and to localize the expression of these proteins in relation to virus in thrips tissues of principal importance along the route of dissemination. We report here the identification of six TIPs from first-instar larvae (L1), the most acquisition-efficient developmental stage of the thrips vector. Sequence analyses of these TIPs revealed homology to proteins associated with the infection cycle of other vector-borne viruses. Immunolocalization of the TIPs in L1 revealed robust expression in the midgut and salivary glands of F. occidentalis, the tissues most important during virus infection, replication, and plant inoculation. The TIPs and GN interactions were validated using protein-protein interaction assays. Two of the thrips proteins, endocuticle structural glycoprotein and cyclophilin, were found to be consistent interactors with GN These newly discovered thrips protein-GN interactions are important for a better understanding of the transmission mechanism of persistent propagative plant viruses by their vectors, as well as for developing new strategies of insect pest management and virus resistance in plants.IMPORTANCE Thrips-transmitted viruses cause devastating losses to numerous food crops worldwide. For negative-sense RNA viruses that infect plants, the arthropod serves as a host as well by supporting virus replication in specific tissues and organs of the vector. The goal of this work was to identify thrips proteins that bind directly to the viral attachment protein and thus may play a role in the infection cycle in the insect. Using the model plant bunyavirus tomato spotted wilt virus (TSWV), and the most efficient thrips vector, we identified and validated six TSWV-interacting proteins from Frankliniella occidentalis first-instar larvae. Two proteins, an endocuticle structural glycoprotein and cyclophilin, were able to interact directly with the TSWV attachment protein, GN, in insect cells. The TSWV GN-interacting proteins provide new targets for disrupting the viral disease cycle in the arthropod vector and could be putative determinants of vector competence.

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.

Progress and challenges in identifying molecular mechanisms underlying host and vector manipulation by plant viruses

Plant virus infection fundamentally alters chemical and behavioral phenotypes of hosts and vectors. These alterations often enhance virus transmission, leading researchers to surmise that such effects are manipulations caused by virus adaptations and not just by-products of pathology. But identification of the virus components behind manipulation is missing from most studies performed to date. Here, we evaluate causative empirical evidence that virus components are the drivers of manipulated host and vector phenotypes. To do so, we link findings and methodologies on virus pathology with observational and functional genomics studies on virus manipulation. Our synthesis provides an overview of progress, areas of synergy, and new approaches that will lead to an improved mechanistic understanding of host and vector manipulation by plant viruses.

Molecular interactions between tospoviruses and thrips vectors

Thrips-transmitted tospoviruses are an emerging and re-emerging threat to crop production worldwide. Tospoviruses are transstadially transmitted from larval to pupal stages of development, with adults serving as the primary inoculators of plants. A unique feature of the transmission cycle is that adults-while they can acquire virus from plants directly-are competent as vectors only if they acquire virus as larvae. Thrips vectors also serve as hosts for the virus, supporting its replication in midgut tissues and salivary glands. There is a tight link between thrips development and virus dissemination in the insect, and recent transcriptome studies point to stage-specific responses that coincide with localization of the virus in the insect body. Transcriptome sequencing of thrips vectors is leading to identification of virus-responsive thrips genes and possibly new targets to disrupt the virus transmission cycle. Accumulation of thrips-omics resources and advancements in functional biology tools will propel new and exciting molecular studies of thrips-tospoviruses interactions.

Insect transmission of plant viruses: Multilayered interactions optimize viral propagation

By serving as vectors of transmission, insects play a key role in the infection cycle of many plant viruses. Viruses use sophisticated transmission strategies to overcome the spatial barrier separating plants and the impediment imposed by the plant cell wall. Interactions among insect vectors, viruses, and host plants mediate transmission by integrating all organizational levels, from molecules to populations. Best-examined on the molecular scale are two basic transmission modes wherein virus-vector interactions have been well characterized. Whereas association of virus particles with specific sites in the vector's mouthparts or in alimentary tract regions immediately posterior to them is required for noncirculative transmission, the cycle of particles through the vector body is necessary for circulative transmission. Virus transmission is also determined by interactions that are associated with changes in vector feeding behaviors and with alterations in plant host's morphology and/or metabolism that favor the attraction or deterrence of vectors. A recent concept in virus-host-vector interactions proposes that when vectors land on infected plants, vector elicitors and effectors "inform" the plants of the confluence of interacting entities and trigger signaling pathways and plant defenses. Simultaneously, the plant responses may also influence virus acquisition and inoculation by vectors. Overall, a picture is emerging where transmission depends on multilayered virus-vector-host interactions that define the route of a virus through the vector, and on the manipulation of the host and the vector. These interactions guarantee virus propagation until one or more of the interactants undergo changes through evolution or are halted by environmental interventions.

Differential proteomics analysis of Frankliniella occidentalis immune response after infection with Tomato spotted wilt virus (Tospovirus)

Tomato spotted wilt virus (TSWV) is mainly vectored by Frankliniella occidentalis Pergande, and it potentially activates the vector's immune response. However, molecular background of the altered immune response is not clearly understood. Therefore, using a proteomic approach, we investigated the immune pathways that are activated in F. occidentalis larvae after 24 h exposure to TSWV. Two-dimensional isoelectric focusing/sodium dodecyl sulfate polyacrylamide gel electrophoresis (2D-IEF/SDS/PAGE) combined with mass spectrometry (MS), were used to identify proteins that were differentially expressed upon viral infection. High numbers of proteins were abundantly expressed in F. occidentalis exposed to TSWV (73%) compared to the non-exposed (27%), with the majority functionally linked to the innate immune system such as: signaling, stress response, defense response, translation, cellular lipids and nucleotide metabolism. Key proteins included: 70 kDa heat shock proteins, Ubiquitin and Dermcidin, among others, indicative of a responsive pattern of the vector's innate immune system to viral infection.

Inheritance genetics of the trait vector competence in Frankliniella occidentalis (Western flower thrips) in the transmission of Tomato spotted wilt virus

The complexity of tospovirus–vector–host plant interaction is linked to a range of factors influencing vector's efficacy in virus transmission, leading to high variability in the transmission efficiency within vector populations. Main shortcomings of most studies are the missing information on the intrinsic potential of individual insects to serve as efficient vectors, both at phenotypic and at genotypic levels. Moreover, detailed analysis of vector competence heredity and monitoring the splitting of both genotypes and phenotypes in filial generations has not been reported. In this study, using the model system Frankliniella occidentalis and Tomato spotted wilt virus, we evaluated the inheritance and stability of the trait vector competence in a population through basic crossings of individually characterized partners, as well as virgin reproduction. We hypothesized that the trait is heritable in F. occidentalis and is controlled by a recessive allele. From the results, 83% and 94% of competent and noncompetent males respectively, inherited their status from their mothers. The trait was only expressed when females were homozygous for the corresponding allele. Furthermore, the allele frequencies were different between males and females, and the competent allele had the highest frequency in the population. These suggest that the trait vector competence is inherited in single recessive gene in F. occidentalis, for which the phenotype is determined by the haplodiploid mechanism. These findings are fundamental for our understanding of the temporal and spatial variability within vector populations with respect to the trait vector competence and at the same time offer an essential basis for further molecular studies.

The Genus Tospovirus: Emerging Bunyaviruses that Threaten Food Security

The genus Tospovirus is unique within the family Bunyaviridae in that it is made up of viruses that infect plants. Initially documented over 100 years ago, tospoviruses have become increasingly important worldwide since the 1980s due to the spread of the important insect vector Frankliniella occidentalis and the discovery of new viruses. As a result, tospoviruses are now recognized globally as emerging agricultural diseases. Tospoviruses and their vectors, thrips species in the order Thysanoptera, represent a major problem for agricultural and ornamental crops that must be managed to avoid devastating losses. In recent years, the number of recognized species in the genus has increased rapidly, and our knowledge of the molecular interactions of tospoviruses with their host plants and vectors has expanded. In this review, we present an overview of the genus Tospovirus with particular emphasis on new understandings of the molecular plant-virus and vector-virus interactions as well as relationships among genus members.

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.

Evaluation and identification of candidate genes for artificial microRNA-mediated resistance to tomato spotted wilt virus

Tomato spotted wilt virus (TSWV) is an economically important viral pathogen of a wide range of field and horticultural crops. We developed an artificial microRNA (amiRNA) strategy against TSWV, targeting the nucleoprotein (N) and silencing suppressor (NSs) genes. The amiRNA constructs replaced the natural miRNA in a shortened Arabidopsis 173-nucleotide (nt) miR159a precursor backbone (athmiR159a) without the stem base extending beyond the miR/miR* duplex. Further, each amiRNA was modified to contain a mismatch (wobble) sequence at nucleotide position 12 and 13 on the complementary strand amiRNA*, mimicking the endogenous miR159a sequence structure. Transient expression in Nicotiana benthamiana demonstrated that the introduction of a wobble sequence did not alter amiRNA expression levels. Following challenge inoculation with TSWV, plants expressing N-specific amiRNAs with or without the wobble remained asymptomatic and were negative for TSWV by ELISA. In contrast, plants expressing the NSs-specific amiRNAs were symptomatic and accumulated high levels of TSWV. Similar findings were obtained in stably transformed Nicotiana tabacum plants. Our results show that a shortened 173-nt athmiR159a backbone is sufficient to express amiRNAs and that the presence of mismatch at position 12-13 does not influence amiRNA expression or conferring of resistance. We also show that selection of target gene and positional effect are critical in amiRNA-based approach for introducing resistance. These findings open the possibility of employing the amiRNA approach for broad-spectrum resistance to tospoviruses as well as other viruses.

RNA interference tools for the western flower thrips, Frankliniella occidentalis

The insect order Thysanoptera is exclusively comprised of small insects commonly known as thrips. The western flower thrips, Frankliniella occidentalis, is an economically important pest amongst thysanopterans due to extensive feeding damage and tospovirus transmission to hundreds of plant species worldwide. Geographically-distinct populations of F. occidentalis have developed resistance against many types of traditional chemical insecticides, and as such, management of thrips and tospoviruses are a persistent challenge in agriculture. Molecular methods for defining the role(s) of specific genes in thrips-tospovirus interactions and for assessing their potential as gene targets in thrips management strategies is currently lacking. The goal of this work was to develop an RNA interference (RNAi) tool that enables functional genomic assays and to evaluate RNAi for its potential as a biologically-based approach for controlling F. occidentalis. Using a microinjection system, we delivered double-stranded RNA (dsRNA) directly to the hemocoel of female thrips to target the vacuolar ATP synthase subunit B (V-ATPase-B) gene of F. occidentalis. Gene expression analysis using real-time quantitative reverse transcriptase-PCR (qRT-PCR) revealed significant reductions of V-ATPase-B transcripts at 2 and 3 days post-injection (dpi) with dsRNA of V-ATPase-B compared to injection with dsRNA of GFP. Furthermore, the effect of knockdown of the V-ATPase-B gene in females at these two time points was mirrored by the decreased abundance of V-ATPase-B protein as determined by quantitative analysis of Western blots. Reduction in V-ATPase-B expression in thrips resulted in increased female mortality and reduced fertility, i.e., number of viable offspring produced. Survivorship decreased significantly by six dpi compared to the dsRNA-GFP control group, which continued decreasing significantly until the end of the bioassay. Surviving female thrips injected with dsRNA-V-ATPase-B produced significantly fewer offspring compared to those in the dsRNA-GFP control group. Our findings indicate that an RNAi-based strategy to study gene function in thrips is feasible, can result in quantifiable phenotypes, and provides a much-needed tool for investigating the molecular mechanisms of thrips-tospovirus interactions. To our knowledge, this represents the first report of RNAi for any member of the insect order Thysanoptera and demonstrates the potential for translational research in the area of thrips pest control.

Disruption of insect transmission of plant viruses

Plant-infecting viruses are transmitted by a diverse array of organisms including insects, mites, nematodes, fungi, and plasmodiophorids. Virus interactions with these vectors are diverse, but there are some commonalities. Generally the infection cycle begins with the vector encountering the virus in the plant and the virus is acquired by the vector. The virus must then persist in or on the vector long enough for the virus to be transported to a new host and delivered into the plant cell. Plant viruses rely on their vectors for breaching the plant cell wall to be delivered directly into the cytosol. In most cases, viral capsid or membrane glycoproteins are the specific viral proteins that are required for transmission and determinants of vector specificity. Specific molecules in vectors also interact with the virus and while there are few-identified to no-identified receptors, candidate recognition molecules are being further explored in these systems. Due to the specificity of virus transmission by vectors, there are defined steps that represent good targets for interdiction strategies to disrupt the disease cycle. This review focuses on new technologies that aim to disrupt the virus-vector interaction and focuses on a few of the well-characterized virus-vector interactions in the field. In closing, we discuss the importance of integration of these technologies with current methods for plant virus disease control.

Genetic and serological characterization of chrysanthemum stem necrosis virus, a member of the genus Tospovirus

Chrysanthemum stem necrosis virus (CSNV) is a member of a tentative tospovirus species. In this study, the complete genomic sequence of the Japanese CSNV isolate TcCh07A was determined. The L RNA is 8960 nt long and encodes the 331.0-kDa RNA-dependent RNA polymerase. The M RNA is 4828 nt long and encodes the 34.1-kDa movement protein (NSm) and the 127.7-kDa glycoprotein precursor (Gn/Gc). The S RNA is 2949 nt long and encodes the 52.4-kDa silencing suppressor protein (NSs) and the 29.3-kDa nucleocapsid (N) protein. The N protein of CSNV-TcCh07A was purified from virus-infected plant tissues and used for production of a rabbit polyclonal antiserum (RAs) and a monoclonal antibody (MAb). Results of serological tests by indirect ELISA and western blotting using the prepared RAs and MAb and a previously produced RAs against the N protein of tomato spotted wilt virus (TSWV) indicated that CSNV-TcCh07A, TSWV, tomato chlorotic spot virus, groundnut ringspot virus, alstroemeria necrotic streak virus and impatiens necrotic spot virus are serologically related.

Expression and functional characterisation of a soluble form of Tomato yellow leaf curl virus coat protein

BACKGROUND

Tomato yellow leaf curl virus (TYLCV), a member of the genus Begomovirus within the family Geminiviridae, is an important pathogen of tomato in many tropical, subtropical and temperate regions. TYLCV is exclusively transmitted by the whitefly Bemisia tabaci in a circulative manner. The viral coat protein (CP) has been assumed to play important roles in the entry of TYLCV into the insect midgut cells.

RESULTS

Testing the hypothesis that CP plays an important role in TYLCV acquisition by B. tabaci, a soluble form of the CP was expressed and purified. The purified recombinant CP made it possible to examine the function of TYLCV CP without other viral proteins. In an in vivo binding assay, specific binding of TYLCV CP to B. tabaci midguts was detected when purified CP was fed to B. tabaci. In addition, real-time polymerase chain reaction analysis of virus titre revealed that B. tabaci fed with purified CP had reduced the level of virus in their midgut compared with those fed with bovine serum albumin or maltose-binding protein. These results suggest that binding of TYLCV CP to the B. tabaci midgut specifically inhibits virus acquisition.

CONCLUSIONS

The findings that TYLCV CP binds to B. tabaci midguts and decreases virus acquisition provide direct evidence that CP mediates the attachment of TYLCV to receptors on the epithelial cells of the B. tabaci midgut.

The NSs protein of tomato spotted wilt virus is required for persistent infection and transmission by Frankliniella occidentalis

Tomato spotted wilt virus (TSWV) is the type member of tospoviruses (genus Tospovirus), plant-infecting viruses that cause severe damage to ornamental and vegetable crops. Tospoviruses are transmitted by thrips in the circulative propagative mode. We generated a collection of NSs-defective TSWV isolates and showed that TSWV coding for truncated NSs protein could not be transmitted by Frankliniella occidentalis. Quantitative reverse transcription (RT)-PCR and immunostaining of individual insects detected the mutant virus in second-instar larvae and adult insects, demonstrating that insects could acquire and accumulate the NSs-defective virus. Nevertheless, adults carried a significantly lower viral load, resulting in the absence of transmission. Genome sequencing and analyses of reassortant isolates showed genetic evidence of the association between the loss of competence in transmission and the mutation in the NSs coding sequence. Our findings offer new insight into the TSWV-thrips interaction and Tospovirus pathogenesis and highlight, for the first time in the Bunyaviridae family, a major role for the S segment, and specifically for the NSs protein, in virulence and efficient infection in insect vector individuals.

IMPORTANCE

Our work is the first to show a role for the NSs protein in virus accumulation in the insect vector in the Bunyaviridae family: demonstration was obtained for the system TSWV-F. occidentalis, arguably one of the most damaging combination for vegetable crops. Genetic evidence of the involvement of the NSs protein in vector transmission was provided with multiple approaches.

Disruption of vector transmission by a plant-expressed viral glycoprotein

Vector-borne viruses are a threat to human, animal, and plant health worldwide, requiring the development of novel strategies for their control. Tomato spotted wilt virus (TSWV) is one of the 10 most economically significant plant viruses and, together with other tospoviruses, is a threat to global food security. TSWV is transmitted by thrips, including the western flower thrips, Frankliniella occidentalis. Previously, we demonstrated that the TSWV glycoprotein GN binds to thrips vector midguts. We report here the development of transgenic plants that interfere with TSWV acquisition and transmission by the insect vector. Tomato plants expressing GN-S protein supported virus accumulation and symptom expression comparable with nontransgenic plants. However, virus titers in larval insects exposed to the infected transgenic plants were three-log lower than insects exposed to infected nontransgenic control plants. The negative effect of the GN-S transgenics on insect virus titers persisted to adulthood, as shown by four-log lower virus titers in adults and an average reduction of 87% in transmission efficiencies. These results demonstrate that an initial reduction in virus infection of the insect can result in a significant decrease in virus titer and transmission over the lifespan of the vector, supportive of a dose-dependent relationship in the virus-vector interaction. These findings demonstrate that plant expression of a viral protein can be an effective way to block virus transmission by insect vectors.

Localizing viruses in their insect vectors

The mechanisms and impacts of the transmission of plant viruses by insect vectors have been studied for more than a century. The virus route within the insect vector is amply documented in many cases, but the identity, the biochemical properties, and the structure of the actual molecules (or molecule domains) ensuring compatibility between them remain obscure. Increased efforts are required both to identify receptors of plant viruses at various sites in the vector body and to design competing compounds capable of hindering transmission. Recent trends in the field are opening questions on the diversity and sophistication of viral adaptations that optimize transmission, from the manipulation of plants and vectors ultimately increasing the chances of acquisition and inoculation, to specific "sensing" of the vector by the virus while still in the host plant and the subsequent transition to a transmission-enhanced state.

Tomato spotted wilt virus benefits a non-vector arthropod, Tetranychus urticae, by modulating different plant responses in tomato

The interaction between plant viruses and non-vector arthropod herbivores is poorly understood. However, there is accumulating evidence that plant viruses can impact fitness of non-vector herbivores. In this study, we used oligonucleotide microarrays, phytohormone, and total free amino acid analyses to characterize the molecular mechanisms underlying the interaction between Tomato spotted wilt virus (TSWV) and a non-vector arthropod, twospotted spider mite (Tetranychusurticae), on tomato plants, Solanumlycopersicum. Twospotted spider mites showed increased preference for and fecundity on TSWV-infected plants compared to mock-inoculated plants. Transcriptome profiles of TSWV-infected plants indicated significant up-regulation of salicylic acid (SA)-related genes, but no apparent down-regulation of jasmonic acid (JA)-related genes which could potentially confer induced resistance against TSM. This suggests that there was no antagonistic crosstalk between the signaling pathways to influence the interaction between TSWV and spider mites. In fact, SA- and JA-related genes were up-regulated when plants were challenged with both TSWV and the herbivore. TSWV infection resulted in down-regulation of cell wall-related genes and photosynthesis-associated genes, which may contribute to host plant susceptibility. There was a three-fold increase in total free amino acid content in virus-infected plants compared to mock-inoculated plants. Total free amino acid content is critical for arthropod nutrition and may, in part, explain the apparent positive indirect effect of TSWV on spider mites. Taken together, these data suggest that the mechanism(s) of increased host suitability of TSWV-infected plants to non-vector herbivores is complex and likely involves several plant biochemical processes.

Tubular structure induced by a plant virus facilitates viral spread in its vector insect

Rice dwarf virus (RDV) replicates in and is transmitted by a leafhopper vector in a persistent-propagative manner. Previous cytopathologic and genetic data revealed that tubular structures, constructed by the nonstructural viral protein Pns10, contain viral particles and are directly involved in the intercellular spread of RDV among cultured leafhopper cells. Here, we demonstrated that RDV exploited these virus-containing tubules to move along actin-based microvilli of the epithelial cells and muscle fibers of visceral muscle tissues in the alimentary canal, facilitating the spread of virus in the body of its insect vector leafhoppers. In cultured leafhopper cells, the knockdown of Pns10 expression due to RNA interference (RNAi) induced by synthesized dsRNA from Pns10 gene strongly inhibited tubule formation and prevented the spread of virus among insect vector cells. RNAi induced after ingestion of dsRNA from Pns10 gene strongly inhibited formation of tubules, preventing intercellular spread and transmission of the virus by the leafhopper. All these results, for the first time, show that a persistent-propagative virus exploits virus-containing tubules composed of a nonstructural viral protein to traffic along actin-based cellular protrusions, facilitating the intercellular spread of the virus in the vector insect. The RNAi strategy and the insect vector cell culture provide useful tools to investigate the molecular mechanisms enabling efficient transmission of persistent-propagative plant viruses by vector insects.

Disrupting the transmission of a vector-borne plant pathogen

Approaches to control vector-borne diseases rarely focus on the interface between vector and microbial pathogen, but strategies aimed at disrupting the interactions required for transmission may lead to reductions in disease spread. We tested if the vector transmission of the plant-pathogenic bacterium Xylella fastidiosa was affected by three groups of molecules: lectins, carbohydrates, and antibodies. Although not comprehensively characterized, it is known that X. fastidiosa adhesins bind to carbohydrates, and that these interactions are important for initial cell attachment to vectors, which is required for bacterial transmission from host to host. Lectins with affinity to substrates expected to occur on the cuticular surface of vectors colonized by X. fastidiosa, such as wheat germ agglutinin, resulted in statistically significant reductions in transmission rate, as did carbohydrates with N-acetylglucosamine residues. Presumably, lectins bound to receptors on the vector required for cell adhesion/colonization, while carbohydrate-saturated adhesins on X. fastidiosa's cell surface. Furthermore, antibodies against X. fastidiosa whole cells, gum, and afimbrial adhesins also resulted in transmission blockage. However, no treatment resulted in the complete abolishment of transmission, suggesting that this is a complex biological process. This work illustrates the potential to block the transmission of vector-borne pathogens without directly affecting either organism.

Tospoviruses in the Mediterranean area

Tospoviruses are among the most serious threats to vegetable crops in the Mediterranean basin. Tospovirus introduction, spread, and the diseases these viruses cause have been traced by epidemiological case studies. Recent research has centered on the close relationship between tospoviruses and their arthropod vectors (species of the Thripidae family). Here, we review several specific features of tospovirus-thrips associations in the Mediterranean. Since the introduction of Frankliniella occidentalis in Europe, Tomato spotted wilt virus (TSWV) has become one of the limiting factors for vegetable crops such as tomato, pepper, and lettuce. An increasing problem is the emergence of TSWV resistance-breaking strains that overcome the resistance genes in pepper and tomato. F. occidentalis is also a vector of Impatiens necrotic spot virus, which was first observed in the Mediterranean basin in the 1980s. Its importance as a cause of vegetable crop diseases is limited to occasional incidence in pepper and tomato fields. A recent introduction is Iris yellow spot virus, transmitted by the onion thrips Thrips tabaci, in onion and leek crops. Control measures in vegetable crops specific to Mediterranean conditions were examined in the context of their epidemiological features and tospovirus species which could pose a future potential risk for vegetable crops in the Mediterranean were discussed.

Analysis of expressed sequence tags for Frankliniella occidentalis, the western flower thrips

Thrips are members of the insect order Thysanoptera and Frankliniella occidentalis (the western flower thrips) is the most economically important pest within this order. F. occidentalis is both a direct pest of crops and an efficient vector of plant viruses, including Tomato spotted wilt virus (TSWV). Despite the world-wide importance of thrips in agriculture, there is little knowledge of the F. occidentalis genome or gene functions at this time. A normalized cDNA library was constructed from first instar thrips and 13 839 expressed sequence tags (ESTs) were obtained. Our EST data assembled into 894 contigs and 11 806 singletons (12 700 nonredundant sequences). We found that 31% of these sequences had significant similarity (E< or = 10(-10)) to protein sequences in the National Center for Biotechnology Information nonredundant (nr) protein database, and 25% were functionally annotated using Blast 2GO. We identified 74 sequences with putative homology to proteins associated with insect innate immunity. Sixteen sequences had significant similarity to proteins associated with small RNA-mediated gene silencing pathways (RNA interference; RNAi), including the antiviral pathway (short interfering RNA-mediated pathway). Our EST collection provides new sequence resources for characterizing gene functions in F. occidentalis and other thrips species with regards to vital biological processes, studying the mechanism of interactions with the viruses harboured and transmitted by the vector, and identifying new insect gene-centred targets for plant disease and insect control.

A peptide that binds the pea aphid gut impedes entry of Pea enation mosaic virus into the aphid hemocoel

Development of ways to block virus transmission by aphids could lead to novel and broad-spectrum means of controlling plant viruses. Viruses in the Luteoviridae enhanced are obligately transmitted by aphids in a persistent manner that requires virion accumulation in the aphid hemocoel. To enter the hemocoel, the virion must bind and traverse the aphid gut epithelium. By screening a phage display library, we identified a 12-residue gut binding peptide (GBP3.1) that binds to the midgut and hindgut of the pea aphid Acyrthosiphon pisum. Binding was confirmed by labeling the aphid gut with a GBP3.1-green fluorescent protein fusion. GBP3.1 reduced uptake of Pea enation mosaic virus (Luteoviridae) from the pea aphid gut into the hemocoel. GBP3.1 also bound to the gut epithelia of the green peach aphid and the soybean aphid. These results suggest a novel strategy for inhibiting plant virus transmission by at least three major aphid pest species.

Variation in Tomato spotted wilt virus titer in Frankliniella occidentalis and its association with frequency of transmission

Tomato spotted wilt virus (TSWV) is transmitted in a persistent propagative manner by Frankliniella occidentalis, the western flower thrips. While it is well established that vector competence depends on TSWV acquisition by young larvae and virus replication within the insect, the biological factors associated with frequency of transmission have not been well characterized. We hypothesized that the number of transmission events by a single adult thrips is determined, in part, by the amount of virus harbored (titer) by the insect. Transmission time-course experiments were conducted using a leaf disk assay to determine the efficiency and frequency of TSWV transmission following 2-day inoculation access periods (IAPs). Virus titer in individual adult thrips was determined by real-time quantitative reverse transcriptase-PCR (qRT-PCR) at the end of the experiments. On average, 59% of adults transmitted the virus during the first IAP (2 to 3 days post adult-eclosion). Male thrips were more efficient at transmitting TSWV multiple times compared with female thrips of the same cohort. However, females harbored two to three times more copies of TSWV-N RNA per insect, indicating that factors other than absolute virus titer in the insect contribute to a successful transmission event. Examination of virus titer in individual insects at the end of the third IAP (7 days post adult-eclosion) revealed significant and consistent positive associations between frequency of transmission and virus titer. Our data support the hypothesis that a viruliferous thrips is more likely to transmit multiple times if it harbors a high titer of virus. This quantitative relationship provides new insights into the biological parameters that may influence the spread of TSWV by thrips.

Insect vector interactions with persistently transmitted viruses

The majority of described plant viruses are transmitted by insects of the Hemipteroid assemblage that includes aphids, whiteflies, leafhoppers, planthoppers, and thrips. In this review we highlight progress made in research on vector interactions of the more than 200 plant viruses that are transmitted by hemipteroid insects beginning a few hours or days after acquisition and for up to the life of the insect, i.e., in a persistent-circulative or persistent-propagative mode. These plant viruses move through the insect vector, from the gut lumen into the hemolymph or other tissues and finally into the salivary glands, from which these viruses are introduced back into the plant host during insect feeding. The movement and/or replication of the viruses in the insect vectors require specific interactions between virus and vector components. Recent investigations have resulted in a better understanding of the replication sites and tissue tropism of several plant viruses that propagate in insect vectors. Furthermore, virus and insect proteins involved in overcoming transmission barriers in the vector have been identified for some virus-vector combinations.