A silencing suppressor protein (NSs) of a tospovirus enhances baculovirus replication in permissive and semipermissive insect cell lines

WIV, a protein domain found in a wide number of arthropod viruses, which probably facilitates infection

The most powerful approach to detect distant homologues of a protein is based on structure prediction and comparison. Yet this approach is still inapplicable to many viral proteins. Therefore, we applied a powerful sequence-based procedure to identify distant homologues of viral proteins. It relies on three principles: (1) traces of sequence similarity can persist beyond the significance cutoff of homology detection programmes; (2) candidate homologues can be identified among proteins with weak sequence similarity to the query by using 'contextual' information, e.g. taxonomy or type of host infected; (3) these candidate homologues can be validated using highly sensitive profile-profile comparison. As a test case, this approach was applied to a protein without known homologues, encoded by ORF4 of Lake Sinai viruses (which infect bees). We discovered that the ORF4 protein contains a domain that has homologues in proteins from >20 taxa of viruses infecting arthropods. We called this domain 'widespread, intriguing, versatile' (WIV), because it is found in proteins with a wide variety of functions and within varied domain contexts. For example, WIV is found in the NSs protein of tospoviruses, a global threat to food security, which infect plants as well as their arthropod vectors; in the RNA2 ORF1-encoded protein of chronic bee paralysis virus, a widespread virus of bees; and in various proteins of cypoviruses, which infect the silkworm Bombyx mori. Structural modelling with AlphaFold indicated that the WIV domain has a previously unknown fold, and bibliographical evidence suggests that it facilitates infection of arthropods.

Knockout of Dicer-2 in the Sf9 cell line enhances the replication of Spodoptera frugiperda rhabdovirus and conditionally increases baculovirus replication

The Sf9 cell line, originally isolated from the ovarian tissue of Spodoptera frugiperda larvae, is widely used in academia and industry for the baculovirus-mediated production of recombinant proteins and virus-like particles. RNA interference (RNAi) is a conserved antiviral pathway present in eukaryotic organisms and is the primary antiviral defence mechanism in insects. Recent evidence has implicated RNAi as an antiviral response to baculovirus infection in Sf9 cells. To test this hypothesis, CRISPR/Cas9 technology was used to disable the RNAi pathway in Sf9 cells by knocking out Dicer-2, the protein responsible for cleaving viral double-stranded RNA precursors into short interfering RNAs. Infection of Dicer-2 knockout Sf9 cells with either the wild-type baculovirus Autographa californica nucleopolyhedrovirus (AcMNPV), recombinant AcMNPV (rAcMNPV) expressing β-galactosidase (β-gal), or rAcMNPV expressing a wasp venom protein (Vn50) at a multiplicity of infection (m.o.i.) of 1 resulted in a modest increase in virus replication compared to control Sf9 cells under adherent culture conditions. In contrast, Dicer-2 knockout Sf9 monolayer or suspension cultures infected by the rAcMNPV expressing β-gal at higher m.o.i.s (3.5 and 20) did not exhibit increases in either viral DNA replication or β-gal production. Intriguingly, during long-term passaging in suspension, Dicer-2 knockout Sf9 cultures underwent transient crashes in cell proliferation and viability. It was discovered that these periods of low growth and viability coincided with a dramatic increase in the RNA levels of S. frugiperda rhabdovirus, a recently identified adventitious virus that persistently infects the Sf9 cell line, suggesting a role for Dicer-2 in managing chronic viral infections in this industrially relevant insect cell line.

Analysis of luciferase dsRNA production during baculovirus infection of Hi5 cells: RNA hairpins expressed by very late promoters do not trigger gene silencing

The baculovirus expression vector system (BEVS) has become an important platform for the expression of recombinant proteins and is especially useful for the production of large protein complexes such as virus-like particles (VLPs). An important application for VLPs is their use as vehicles for targeted delivery of drugs or toxins which requires the development of methods for efficient loading with the intended cargo. Our research intends to employ the BEVS for the production of VLPs for the delivery of insecticidal dsRNA molecules to targeted insect pests (as "dsRNA-VLPs"). A convenient strategy would be the co-expression of long dsRNAs with viral capsid proteins and their simultaneous encapsulation during VLP assembly but the capacity of the BEVS for the production of long dsRNA has not been assessed so far. In this study, the efficiency of production of long RNA hairpins targeting the luciferase gene ("dsLuc") by the polyhedrin promoter during baculovirus infection was evaluated. However, RNAi reporter assays could not detect significant amounts of dsLuc in Hi5 cells infected with recombinant baculovirus, even in the presence of co-expressed dsRNA-binding protein B2-GFP or the employment of the MS2-MCP system. Nevertheless, dot blot analyses using anti-dsRNA antibody revealed that baculovirus-mediated expression of B2-GFP resulted in significant increases in dsRNA levels in infected cells that may correspond to hybridized complementary viral transcripts. Using B2-GFP as a genetically encoded sensor, dsRNA foci were detected in the nuclei that partially co-localized with DAPI staining, consistent with their localization at the virogenic stroma. Co-localization experiments with the baculovirus proteins vp39, Ac93, ODV-E25 and gp64 indicated limited overlap between B2-GFP and the ring zone compartment where assembly of nucleocapsids and virions occurs. Stability experiments showed that exogenous dsRNA is resistant to degradation in extracts of non-infected and infected Hi5 cells and it is proposed that strong unwinding activity at the virogenic stroma in the infected nuclei may neutralize the annealing of complementary RNA strands and block the production of long dsRNAs. Because the strong stability of exogenous dsRNA, transfection can be explored as an alternative method for delivery of cargo for dsRNA-VLPs during their assembly in baculovirus-infected Hi5 cells.

Tagitinin A from Tithonia diversifolia provides resistance to tomato spotted wilt orthotospovirus by inducing systemic resistance

Tomato spotted wilt orthotospovirus (TSWV) causes devastating losses to agronomic and ornamental crops worldwide. Currently, there is no effective strategy to control this disease. Use of biotic inducers to enhance plant resistance to viruses maybe an effective approach. Our previous study indicated that Tagitinin A (Tag A) has a high curative and protective effect against TSWV. However, the underlying molecular mechanism of Tag A-mediated antiviral activity remains unknown. In this study, Tag A reduced the expression of the NSs, NSm genes was very low in untreated leaves following TSWV infection. In addition, the expression of all TSWV genes in the inoculated and systemic leaves was inhibited in the protective assay, and with an inhibition rate of more than 85% in systemic leaves. Tag A increased phenylalanine ammonia-lyase (PAL) activity in the curative and protective assays. The concentrations of jasmonic acid (JA) and jasmonic acid -isoleucine (JA-Ile) and the expression of its key gene NtCOI1 in Tag A-treated and systemic leaves of treated plants were significantly higher than those of the control plant. Furthermore, Tag A-induced resistance to TSWV could be eliminated by VIGS-mediated silencing of the NtCOI1 gene. These indicated that Tag A acts against TSWV by activating the JA defense signaling pathway.

A Conserved Helix in the C-Terminal Region of Watermelon Silver Mottle Virus Nonstructural Protein S Is Imperative For Protein Stability Affecting Self-Interaction, RNA Silencing Suppression, and Pathogenicity

In orthotospovirus, the nonstructural protein S (NSs) is the RNA-silencing suppressor (RSS) and pathogenicity determinant. Here, we demonstrate that a putative α-helix, designated H8, spanning amino acids 338 to 369 of the C-terminal region of the NSs protein, is crucial for self-interaction of watermelon silver mottle virus NSs protein and that the H8 affects RSS function. Co-immunoprecipitation, yeast two-hybrid, and bimolecular fluorescence complementation analyses revealed that the triple point mutation (TPM) of H8 amino acids Y338A, H350A, and F353A resulted in NSs protein self-interaction dysfunction. Transient expression of H8-deleted (ΔH8) and TPM NSs proteins in Nicotiana benthamiana plants by agroinfitration indicated that these proteins have weaker RSS activity and are far less stable than wild-type (WT) NSs. However, an electrophoretic mobility assay revealed that small interfering RNA (siRNA) binding ability of TPM NSs protein is not compromised. The pathogenicity assay of WT NSs protein expressed by the attenuated turnip mosaic virus vector restored severe symptoms in recombinant-infected N. benthamiana plants but not for ΔH8 or TPM proteins. Taken together, we conclude that the H8 helix in the C-terminal region of NSs protein is crucial for stabilizing NSs protein through self-interaction to maintain normal functions of RSS and pathogenicity, but not for NSs-siRNA binding activity.

Inhibition of dicer activity in lepidopteran and dipteran cells by baculovirus-mediated expression of Flock House virus B2

Prior studies have suggested that insect DNA viruses are negatively affected by dicer-2-mediated RNA interference (RNAi). To examine this further, we utilized an in vitro assay to measure dicer activity in lepidopteran and dipteran cells, combined with baculoviruses expressing the RNAi suppressor B2 from Flock House virus or Aedes aegypti dicer-2 (Aedicer-2) using a constitutive heat shock promoter. Addition of cell lysates containing baculovirus-expressed B2 to lysates from dipteran (S2, Aag2) or lepidopteran (Sf9) cells inhibited endogenous dicer activity in a dose-dependent manner, while expression of Aedicer-2 restored siRNA production in Ae. albopictus C6/36 cells, which are dicer-2 defective. However, B2 expression from the constitutive heat shock promoter had no impact on baculovirus replication or virulence in cell lines or larvae that were either highly permissive (Trichoplusia ni) or less susceptible (Spodoptera frugiperda) to infection. We determined that this constitutive level of B2 expression had little to no ability to suppress dicer activity in cell lysates, but higher expression of B2, following heat shock treatment, inhibited dicer activity in all cells tested. Thus, we cannot rule out the possibility that optimized expression of B2 or other RNAi suppressors may increase baculovirus replication and expression of heterologous proteins by baculoviruses.

Paving the Way to Tospovirus Infection: Multilined Interplays with Plant Innate Immunity

Tospoviruses are among the most important plant pathogens and cause serious crop losses worldwide. Tospoviruses have evolved to smartly utilize the host cellular machinery to accomplish their life cycle. Plants mount two layers of defense to combat their invasion. The first one involves the activation of an antiviral RNA interference (RNAi) defense response. However, tospoviruses encode an RNA silencing suppressor that enables them to counteract antiviral RNAi. To further combat viral invasion, plants also employ intracellular innate immune receptors (e.g., Sw-5b and Tsw) to recognize different viral effectors (e.g., NSm and NSs). This leads to the triggering of a much more robust defense against tospoviruses called effector-triggered immunity (ETI). Tospoviruses have further evolved their effectors and can break Sw-5b-/Tsw-mediated resistance. The arms race between tospoviruses and both layers of innate immunity drives the coevolution of host defense and viral genes involved in counter defense. In this review, a state-of-the-art overview is presented on the tospoviral life cycle and the multilined interplays between tospoviruses and the distinct layers of defense.

Diterpenoid compounds from Wedelia trilobata induce resistance to Tomato spotted wilt virus via the JA signal pathway in tobacco plants

Tomato spotted wilt virus (TSWV) causes major losses of many crops worldwide. Several strategies have been attempted to control disease caused by TSWV. However, many challenges for the effective control of this disease remain. A promising approach is the use of abiotic or biotic inducers to enhance plant resistance to pathogens. We screened a diterpenoid compound from Wedelia trilobata, 3α-Angeloyloxy-9β-hydroxy-ent-kaur-16-en-19-oic acid (AHK), which had higher curative and protective effects against TSWV than the ningnanmycin control. The rapid initiation of the expression of all the TSWV genes was delayed by more than 1d in the curative assay, and the expression of the NSs, NSm and RdRp genes was inhibited. In addition, the replication of all TSWV genes in systemic leaves was inhibited in the protective assay, with an inhibition rate of more than 90%. The concentrations of jasmonic acid (JA) and jasmonic acid isoleucine (JA-ILE) in the AHK-treated and systemic leaves of the treated plants were significantly higher than those observed in the control. The results suggested that AHK can induce systemic resistance in treated plants. The transcription of the NtCOI1 gene, a key gene in the JA pathway, was significantly higher in both the inoculated and systemic leaves of the AHK-treated plants compared to the control. The AHK-induced resistance to TSWV in Nicotiana benthamiana could be eliminated by VIGS-mediated silencing of the NtCOI1 gene. These results indicated that AHK can activate the JA pathway and induce systemic resistance to TSWV infection.

Discovery of Negative-Sense RNA Viruses in Trees Infected with Apple Rubbery Wood Disease by Next-Generation Sequencing

Apple rubbery wood is a disease of apple found around the world, often associated with Apple flat limb disease, and regulated in many countries. Despite its long history in apple cultivation, the disease's causal agent has remained elusive. In this study, next-generation sequencing (NGS) was used to identify and characterize several related novel viral agents from apple rubbery wood-infected plants, which have been named Apple rubbery wood virus (ARWV) 1 and 2. Additional specimens with apple rubbery wood disease tested positive by polymerase chain reaction with primers designed to ARWV 1 and 2 genomic RNA segments. In an NGS-based screening of over 100 Malus and 100 Prunus specimens from a collection of virus-infected trees, only one Malus specimen was found to be infected with ARWV not known to be infected with the disease, which strongly suggests that ARWV is not commonly found in Malus spp. or other fruit trees. The two viruses are most closely related to members of the order Bunyavirales. Three RNA segments (large, medium, and small) were characterized and the viruses likely represent a new genus under the family Phenuiviridae, with a suggested name of Rubodvirus (Rubbery wood virus).

RNA Interference in Insect Vectors for Plant Viruses

Insects and other arthropods are the most important vectors of plant pathogens. The majority of plant pathogens are disseminated by arthropod vectors such as aphids, beetles, leafhoppers, planthoppers, thrips and whiteflies. Transmission of plant pathogens and the challenges in managing insect vectors due to insecticide resistance are factors that contribute to major food losses in agriculture. RNA interference (RNAi) was recently suggested as a promising strategy for controlling insect pests, including those that serve as important vectors for plant pathogens. The last decade has witnessed a dramatic increase in the functional analysis of insect genes, especially those whose silencing results in mortality or interference with pathogen transmission. The identification of such candidates poses a major challenge for increasing the role of RNAi in pest control. Another challenge is to understand the RNAi machinery in insect cells and whether components that were identified in other organisms are also present in insect. This review will focus on summarizing success cases in which RNAi was used for silencing genes in insect vector for plant pathogens, and will be particularly helpful for vector biologists.

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 RNA Silencing Suppression: The Enigma of Bunyavirus NSs Proteins

The Bunyaviridae is a family of arboviruses including both plant- and vertebrate-infecting representatives. The Tospovirus genus accommodates plant-infecting bunyaviruses, which not only replicate in their plant host, but also in their insect thrips vector during persistent propagative transmission. For this reason, they are generally assumed to encounter antiviral RNA silencing in plants and insects. Here we present an overview on how tospovirus nonstructural NSs protein counteracts antiviral RNA silencing in plants and what is known so far in insects. Like tospoviruses, members of the related vertebrate-infecting bunyaviruses classified in the genera Orthobunyavirus, Hantavirus and Phlebovirus also code for a NSs protein. However, for none of them RNA silencing suppressor activity has been unambiguously demonstrated in neither vertebrate host nor arthropod vector. The second part of this review will briefly describe the role of these NSs proteins in modulation of innate immune responses in mammals and elaborate on a hypothetical scenario to explain if and how NSs proteins from vertebrate-infecting bunyaviruses affect RNA silencing. If so, why this discovery has been hampered so far.

Small interfering RNA pathway modulates persistent infection of a plant virus in its insect vector

Plant reoviruses, rhabdoviruses, tospoviruses, and tenuiviruses are transmitted by insect vectors in a persistent-propagative manner. How such persistent infection of plant viruses in insect vectors is established and maintained remains poorly understood. In this study, we used rice gall dwarf virus (RGDV), a plant reovirus, and its main vector leafhopper Recilia dorsalis as a virus-insect system to determine how the small interference (siRNA) pathway modulates persistent infection of a plant virus in its insect vector. We showed that a conserved siRNA antiviral response was triggered by the persistent replication of RGDV in cultured leafhopper cells and in intact insects, by appearance of virus-specific siRNAs, primarily 21-nt long, and the increased expression of siRNA pathway core components Dicer-2 and Argonaute-2. Silencing of Dicer-2 using RNA interference strongly suppressed production of virus-specific siRNAs, promoted viral accumulation, and caused cytopathological changes in vitro and in vivo. When the viral accumulation level rose above a certain threshold of viral genome copy (1.32 × 10(14) copies/μg insect RNA), the infection of the leafhopper by RGDV was lethal rather than persistent. Taken together, our results revealed a new finding that the siRNA pathway in insect vector can modulate persistent infection of plant viruses.

Small Interfering RNA Pathway Modulates Initial Viral Infection in Midgut Epithelium of Insect after Ingestion of Virus

Numerous viruses are transmitted in a persistent manner by insect vectors. Persistent viruses establish their initial infection in the midgut epithelium, from where they disseminate to the midgut visceral muscles. Although propagation of viruses in insect vectors can be controlled by the small interfering RNA (siRNA) antiviral pathway, whether the siRNA pathway can control viral dissemination from the midgut epithelium is unknown. Infection by a rice virus (Southern rice black streaked dwarf virus [SRBSDV]) of its incompetent vector (the small brown planthopper [SBPH]) is restricted to the midgut epithelium. Here, we show that the siRNA pathway is triggered by SRBSDV infection in continuously cultured cells derived from the SBPH and in the midgut of the intact insect. Knockdown of the expression of the core component Dicer-2 of the siRNA pathway by RNA interference strongly increased the ability of SRBSDV to propagate in continuously cultured SBPH cells and in the midgut epithelium, allowing viral titers in the midgut epithelium to reach the threshold (1.99 × 10(9) copies of the SRBSDV P10 gene/μg of midgut RNA) needed for viral dissemination into the SBPH midgut muscles. Our results thus represent the first elucidation of the threshold for viral dissemination from the insect midgut epithelium. Silencing of Dicer-2 further facilitated the transmission of SRBSDV into rice plants by SBPHs. Taken together, our results reveal the new finding that the siRNA pathway can control the initial infection of the insect midgut epithelium by a virus, which finally affects the competence of the virus's vector.

IMPORTANCE

Many viral pathogens that cause significant global health and agricultural problems are transmitted via insect vectors. The first bottleneck in viral infection, the midgut epithelium, is a principal determinant of the ability of an insect species to transmit a virus. Southern rice black streaked dwarf virus (SRBSDV) is restricted exclusively to the midgut epithelium of an incompetent vector, the small brown planthopper (SBPH). Here, we show that silencing of the core component Dicer-2 of the small interfering RNA (siRNA) pathway increases viral titers in the midgut epithelium past the threshold (1.99 × 10(9) copies of the SRBSDV P10 gene/μg of midgut RNA) for viral dissemination into the midgut muscles and then into the salivary glands, allowing the SBPH to become a competent vector of SRBSDV. This result is the first evidence that the siRNA antiviral pathway has a direct role in the control of viral dissemination from the midgut epithelium and that it affects the competence of the virus's vector.

The silencing suppressor (NSs) protein of the plant virus Tomato spotted wilt virus enhances heterologous protein expression and baculovirus pathogenicity in cells and lepidopteran insects

In this work, we showed that cell death induced by a recombinant (vAcNSs) Autographa californica multiple nucleopolyhedrovirus (AcMNPV) expressing the silencing suppressor (NSs) protein of Tomato spotted wilt virus (TSWV) was enhanced on permissive and semipermissive cell lines. The expression of a heterologous gene (firefly luciferase) during co-infection of insect cells with vAcNSs and a second recombinant baculovirus (vAgppolhfluc) was shown to increase when compared to single vAgppolhfluc infections. Furthermore, the vAcNSs mean time-to-death values were significantly lower than those for wild-type AcMNPV on larvae of Spodoptera frugiperda and Anticarsia gemmatalis. These results showed that the TSWV-NSs protein could efficiently increase heterologous protein expression in insect cells as well as baculovirus pathogenicity and virulence, probably by suppressing the gene-silencing machinery in insects.

Functional analysis of RNAi suppressor P19 on improving baculovirus yield and transgene expression in Sf9 cells

OBJECTIVE

To investigate whether RNA interference suppressor P19 derived from tombusvirus can enhance baculovirus yield and transgene expression.

RESULTS

A number of recombinant baculoviruses with P19 under the control of white spot syndrome virus immediate-early promoter, baculovirus early-to-late promoter, or P10 late promoter were constructed The budded virus titer and the expression levels of eGFP and luciferase were determined. P19 was clearly functional in Sf9 cells and could enhance baculovirus yield, eGFP and luciferase expression levels up to 6.8-, 1.8-, and 2.1-fold respectively, at 72 h post infection.

CONCLUSION

P19 enhanced baculovirus production and transgene expression, and thus has potential applications in baculovirus-based gene therapy and vaccine studies.

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.

A recombinant Anticarsia gemmatalis MNPV harboring chiA and v-cath genes from Choristoneura fumiferana defective NPV induce host liquefaction and increased insecticidal activity

One of the interesting features of Anticarsia gemmatalis multiple nucleopolyhedrovirus isolate 2D (AgMNPV-2D) genome is the absence of chitinase (chiA) and cathepsin (v-cath) genes. This characteristic may be responsible for the lack of liquefaction and melanization in A. gemmatalis larvae killed by AgMNPV-2D infection. This study aimed to test the hypothesis that CHIA and V-CATH proteins from Choristonera fumiferana DEF multiple nucleopolyhedrovirus (CfDEFNPV) are able to liquefy and melanize the cuticle of A. gemmatalis larvae infected by a recombinant AgMNPV containing chiA and v-cath genes inserted in its genome. A fragment from the CfDefNPV genome containing chiA and v-cath genes was inserted into the genome of AgMNPV-2D. The recombinant virus (vAgp2100Cf.chiA/v-cath) was purified and used to infect insect cells and larvae. Transcripts of v-cath and chiA genes were detected along the infection of insect cells by qRT-PCR, from early to late phases of infection. The analysis of A. gemmatalis larvae killed by vAgp2100Cf.chiA/v-cath infection confirmed the hypothesis proposed. The vAgp2100Cf.chiA/v-cath showed higher insecticidal activity against third instar A. gemmatalis larvae when compared to AgMNPV-2D. The mean time to death was also lower for the vAgp2100Cf.chiA/v-cath when compared to AgMNPV-2D at 10 days post infection. Occlusion body production was higher in A. gemmatalis larvae infected with vAgp2100Cf.chiA/v-cath when compared to AgMNPV-2D. Enzyme assays showed higher chitinase and cysteine protease activities in insect cells and insects infected with vAgp2100Cf.chiA/v-cath when compared to AgMNPV-2D. The introduction of chiA and v-cath genes into the genome of AgMNPV improves its insecticidal activity against A. gemmatalis larvae and this recombinant virus could be used as an alternative to the wild type virus to control this important insect pest.

Complementation between two tospoviruses facilitates the systemic movement of a plant virus silencing suppressor in an otherwise restrictive host

BACKGROUND

New viruses pathogenic to plants continue to emerge due to mutation, recombination, or reassortment among genomic segments among individual viruses. Tospoviruses cause significant economic damage to a wide range of crops in many parts of the world. The genetic or molecular basis of the continued emergence of new tospoviruses and new hosts is not well understood though it is generally accepted that reassortment and/or genetic complementation among the three genomic segments of individual viruses could be contributing to this variability since plants infected with more than one tospovirus are not uncommon in nature.

METHODOLOGY/PRINCIPAL FINDINGS

Two distinct and economically important tospoviruses, Iris yellow spot virus (IYSV) and Tomato spotted wilt virus (TSWV), were investigated for inter-virus interactions at the molecular level in dually-infected plants. Datura (Datura stramonium) is a permissive host for TSWV, while it restricts the movement of IYSV to inoculated leaves. In plants infected with both viruses, however, TSWV facilitated the selective movement of the viral gene silencing suppressor (NSs) gene of IYSV to the younger, uninoculated leaves. The small RNA expression profiles of IYSV and TSWV in single- and dually-infected datura plants showed that systemic leaves of dually-infected plants had reduced levels of TSWV N gene-specific small interfering RNAs (siRNAs). No TSWV NSs-specific siRNAs were detected either in the inoculated or systemic leaves of dually-infected datura plants indicating a more efficient suppression of host silencing machinery in the presence of NSs from both viruses as compared to the presence of only TSWV NSs.

CONCLUSION/SIGNIFICANCE

Our study identifies a new role for the viral gene silencing suppressor in potentially modulating the biology and host range of viruses and underscores the importance of virally-coded suppressors of gene silencing in virus infection of plants. This is the first experimental evidence of functional complementation between two distinct tospoviruses in the Bunyaviridae family.

Sequence determination and analysis of the NSs genes of two tospoviruses

The tospoviruses groundnut ringspot virus (GRSV) and zucchini lethal chlorosis virus (ZLCV) cause severe losses in many crops, especially in solanaceous and cucurbit species. In this study, the non-structural NSs gene and the 5'UTRs of these two biologically distinct tospoviruses were cloned and sequenced. The NSs sequence of GRSV and ZLCV were both 1,404 nucleotides long. Pairwise comparison showed that the NSs amino acid sequence of GRSV shared 69.6% identity with that of ZLCV and 75.9% identity with that of TSWV, while the NSs sequence of ZLCV and TSWV shared 67.9% identity. Phylogenetic analysis based on NSs sequences confirmed that these viruses cluster in the American clade.