Tomato spotted wilt virus particle morphogenesis in plant cells

Autophagy plays an antiviral defence role against tomato spotted wilt orthotospovirus and is counteracted by viral effector NSs

Autophagy, an intracellular degradation process, has emerged as a crucial innate immune response against various plant pathogens, including viruses. Tomato spotted wilt orthotospovirus (TSWV) is a highly destructive plant pathogen that infects over 1000 plant species and poses a significant threat to global food security. However, the role of autophagy in defence against the TSWV pathogen, and whether the virus counteracts this defence, remains unknown. In this study, we report that autophagy plays an important role in antiviral defence against TSWV infection; however, this autophagy-mediated defence is counteracted by the viral effector NSs. Transcriptome profiling revealed the up-regulation of autophagy-related genes (ATGs) upon TSWV infection. Blocking autophagy induction by chemical treatment or knockout/down of ATG5/ATG7 significantly enhanced TSWV accumulation. Notably, the TSWV nucleocapsid (N) protein, a major component of the viral replication unit, strongly induced autophagy. However, the TSWV nonstructural protein NSs was able to effectively suppress N-induced autophagy in a dose-dependent manner. Further investigation revealed that NSs inhibited ATG6-mediated autophagy induction. These findings provide new insights into the defence role of autophagy against TSWV, a representative segmented negative-strand RNA virus, as well as the tospoviral pathogen counterdefence mechanism.

Using Raman spectroscopy for early detection of resistance-breaking strains of tomato spotted wilt orthotospovirus in tomatoes

Tomato spotted wilt (TSW) disease caused by tomato spotted wilt orthotospovirus (TSWV, Orthotospovirus tomatomaculae) poses a significant threat to specialty and staple crops worldwide by causing over a billion dollars in crop losses annually. Current strategies for TSWV diagnosis heavily rely on nucleic acid or protein-based techniques which require significant technical expertise, and are invasive, time-consuming, and expensive, thereby catalyzing the search for better alternatives. In this study, we explored the potential of Raman spectroscopy (RS) in early detection of TSW in a non-invasive and non-destructive manner. Specifically, we investigated whether RS could be used to detect strain specific TSW symptoms associated with four TSWV strains infecting three differentially resistant tomato cultivars. In the acquired spectra, we observed notable reductions in the intensity of vibrational peaks associated with carotenoids. Using high-performance liquid chromatography (HPLC), we confirmed that TSWV caused a substantial decrease in the concentration of lutein that was detected by RS. Finally, we demonstrated that Partial Least Squares-Discriminant Analysis (PLS-DA) could be used to differentiate strain-specific TSW symptoms across all tested cultivars. These results demonstrate that RS can be a promising solution for early diagnosis of TSW, enabling timely disease intervention and thereby mitigating crop losses inflicted by TSWV.

Imaging Bunyavirus Infections by Transmission Electron Microscopy: Conventional Sample Preparation vs High-Pressure Freezing and Freeze-Substitution

Transmission electron microscopy significantly contributed to unveil the course of virus entry, replication, morphogenesis, and egress. For these studies, the most widely used approach is imaging ultrathin sections of virus-infected cells embedded in a plastic resin that is transparent to electrons. Before infiltration in a resin, cells must be processed to stabilize their components under the observation conditions in an electron microscope, such as high vacuum and irradiation with electrons. For conventional sample preparation, chemical fixation and dehydration are followed by infiltration in the resin and polymerization to produce a hard block that can be sectioned with an ultramicrotome. Another method that provides a superior preservation of cell components is high-pressure freezing (HPF) followed by freeze substitution (FS) before resin infiltration and polymerization. This chapter describes both procedures with cells infected with Bunyamwera virus (BUNV), a well characterized member of the Bunyavirales, and compares the morphological details of different viral structures imaged in the two types of samples. Advantages, disadvantages, and applications of conventional processing and HPF/FS are also presented and discussed.

Pathological Features and Genetic Polymorphism Analysis of Tomato Spotted Wilt Virus in Infected Tomato Fruit

An in-house tomato inbred line, YNAU335, was planted in a greenhouse in spring from 2014 to 2017, and showed immunity to tomato spotted wilt virus (TSWV). YNAU335 was infected with TSWV in the spring from 2018 to 2020, and disease was observed on the leaves, sepals, and fruits. In 2021 and 2022, YNAU335 was planted in spring in the same greenhouse, which was suspected of being infected with TSWV, and visible disease symptoms were observed on the fruits. Transmission electron microscopy, deep sequencing of small RNAs, and molecular mutation diagnosis were used to analyze the pathological features and genetic polymorphism of TSWV infecting tomato fruit. Typical TSWV virions were observed in the infected fruits, but not leaves from YNAU335 grown between 2021 and 2022, and cross-infection was very rarely observed. The number of mitochondria and chloroplasts increased, but the damage to the mitochondria was greater than that seen in the chloroplasts. Small RNA deep sequencing revealed the presence of multiple viral species in TSWV-infected and non-infected tomato samples grown between 2014-2022. Many virus species, including TSWV, which accounted for the largest proportion, were detected in the TSWV-infected tomato leaves and fruit. However, a variety of viruses other than TSWV were also detected in the non-infected tissues. The amino acids of TSWV nucleocapsid proteins (NPs) and movement proteins (MPs) from diseased fruits of YNAU335 picked in 2021-2022 were found to be very diverse. Compared with previously identified NPs and MPs from TSWV isolates, those found in this study could be divided into three types: non-resistance-breaking, resistance-breaking, and other isolates. The number of positive clones and a comparison with previously identified amino acid mutations suggested that mutation F at AA118 of the MP (GenBank OL310707) is likely the key to breaking the resistance to TSWV, and this mutation developed only in the infected fruit of YNAU335 grown in 2021 and 2022.

Interspecies/Intergroup Complementation of Orthotospovirus Replication and Movement through Reverse Genetics Systems

Orthotospoviruses, the plant-infecting bunyaviruses, cause serious diseases in agronomic crops and pose major threats to global food security. The family of Tospoviridae contains more than 30 members that are classified into two geographic groups, American-type and Euro/Asian-type orthotospovirus. However, the genetic interaction between different species and the possibility, during mixed infections, for transcomplementation of gene functions by orthotospoviruses from different geographic groups remains underexplored. In this study, minireplicon-based reverse genetics (RG) systems have been established for Impatiens necrotic spot virus (INSV) (an American-type orthotospovirus) and for Calla lily chlorotic spot virus and Tomato zonate spot virus (CCSV and TZSV) (two representative Euro/Asian orthotospoviruses). Together with the earlier established RG system for Tomato spotted wilt virus (TSWV), a type species of the Orthotospovirus American-clade, viral replicase/movement proteins were exchanged and analyzed on interspecies transcomplementation. Whereas the homologous RNA-dependent RNA polymerase (RdRp) and nucleocapsid (N) protein supported the replication of orthotospoviruses from both geographic groups, heterologous combinations of RdRp from one group and N from the other group were unable to support the replication of viruses from both groups. Furthermore, the NSm movement protein (MP), from both geographic groups of orthotospoviruses, was able to transcomplement heterologous orthotospoviruses or a positive-strand Cucumber mosaic virus (CMV) in their movement, albeit with varying efficiency. MP from Rice stripe tenuivirus (RSV), a plant-infecting bunyavirus that is distinct from orthotospoviruses, or MP from CMV also moves orthotospoviruses. Our findings gain insights into the genetic interaction/reassortant potentials for the segmented plant orthotospoviruses. IMPORTANCE Orthotospoviruses are agriculturally important negative-strand RNA viruses and cause severe yield-losses on many crops worldwide. Whereas the emergence of new animal-infecting bunyaviruses is frequently associated with genetic reassortants, this issue remains underexposed with the plant-infecting orthotospovirus. With the development of reverse genetics systems for orthotospoviruses from different geographic regions, the interspecies/intergroup replication/movement complementation between American- and Euro/Asian-type orthotospoviruses were investigated. Genomic RNAs from American orthotospoviruses can be replicated by the RdRp and N from those of Euro/Asia-group orthotospoviruses, and vice versa. However, their genomic RNAs cannot be replicated by a heterologous combination of RdRp from one geographic group and N from another geographic group. Cell-to-cell movement of viral entity is supported by NSm from both geographic groups, with highest efficiency by NSm from viruses belonging to the same group. Our findings provide important insights into the genetic interaction and exchange ability of viral gene functions between different species of orthotospovirus.

Three-dimensional analysis of membrane structures associated with tomato spotted wilt virus infection

To study viral infection, the direct structural visualization of the viral life cycle consisting of virus attachment, entry, replication, assembly and transport is essential. Although conventional electron microscopy (EM) has been extremely helpful in the investigation of virus-host cell interactions, three-dimensional (3D) EM not only provides important information at the nanometer resolution, but can also create 3D maps of large volumes, even entire virus-infected cells. Here, we determined the ultrastructural details of tomato spotted wilt virus (TSWV)-infected plant cells using focused ion beam scanning EM (FIB-SEM). The viral morphogenesis and dynamic transformation of paired parallel membranes (PPMs) were analyzed. The endoplasmic reticulum (ER) membrane network consisting of tubules and sheets was related to viral intracellular trafficking and virion storage. Abundant lipid-like bodies, clustering mitochondria, cell membrane tubules, and myelin-like bodies were likely associated with viral infection. Additionally, connecting structures between neighboring cells were found only in infected plant tissues and showed the characteristics of tubular structure. These novel connections that formed continuously in the cell wall or were wrapped by the cell membranes of neighboring cells appeared frequently in the large-scale 3D model, suggesting additional strategies for viral trafficking that were difficult to distinguish using conventional EM.

Functional analysis of the nonstructural protein NSs of tomato zonate spot virus

Tomato zonate spot virus (TZSV), a member of the genus orthotospovirus, causes severe damage to vegetables and ornamental crops in southwest China. The NSs protein is an RNA silencing suppressor in various orthotospovirus like TZSV, but its mechanism and role in virus infection are poorly understood. Here, we observed that an NSs-GFP fusion protein was transiently expressed on the plasma membrane and Golgi bodies in Nicotiana benthamiana plants. The TZSV NSs gene was silenced and infiltrated into N. benthamiana and N. tabacum cv. K326. RT-qPCR and Indirect enzyme-linked immunosorbent assay (ID-ELISA) showed that the transcription and the protein expression of the NSs gene were inhibited by more than 90.00%, and the symptoms on silenced plants were alleviated. We also found that the expression of the Zingipain-2-like gene significantly decreased when the NSs gene was silenced, resulting in co-localization of the NSs-GFP and the Zingipain-2-like-mCherry fusion protein. The findings of this study provide new insights into the mechanism of silencing suppression by NSs, as well as its effect on systemic virus infection, and also support the theory of disease resistance breeding and control and prevention of TZSV in the field.

An Isoform of the Eukaryotic Translation Elongation Factor 1A (eEF1a) Acts as a Pro-Viral Factor Required for Tomato Spotted Wilt Virus Disease in Nicotiana benthamiana

The tripartite genome of the negative-stranded RNA virus Tomato spotted wilt orthotospovirus (TSWV) is assembled, together with two viral proteins, the nucleocapsid protein and the RNA-dependent RNA polymerase, into infectious ribonucleoprotein complexes (RNPs). These two viral proteins are, together, essential for viral replication and transcription, yet our knowledge on the host factors supporting these two processes remains limited. To fill this knowledge gap, the protein composition of viral RNPs collected from TSWV-infected Nicotiana benthamiana plants, and of those collected from a reconstituted TSWV replicon system in the yeast Saccharomyces cerevisiae, was analysed. RNPs obtained from infected plant material were enriched for plant proteins implicated in (i) sugar and phosphate transport and (ii) responses to cellular stress. In contrast, the yeast-derived viral RNPs primarily contained proteins implicated in RNA processing and ribosome biogenesis. The latter suggests that, in yeast, the translational machinery is recruited to these viral RNPs. To examine whether one of these cellular proteins is important for a TSWV infection, the corresponding N. benthamiana genes were targeted for virus-induced gene silencing, and these plants were subsequently challenged with TSWV. This approach revealed four host factors that are important for systemic spread of TSWV and disease symptom development.

Ultrastructural Features of Membranous Replication Organelles Induced by Positive-Stranded RNA Viruses

All intracellular pathogens critically depend on host cell organelles and metabolites for successful infection and replication. One hallmark of positive-strand RNA viruses is to induce alterations of the (endo)membrane system in order to shield their double-stranded RNA replication intermediates from detection by the host cell’s surveillance systems. This spatial seclusion also allows for accruing host and viral factors and building blocks required for efficient replication of the genome and prevents access of antiviral effectors. Even though the principle is iterated by almost all positive-strand RNA viruses infecting plants and animals, the specific structure and the organellar source of membranes differs. Here, we discuss the characteristic ultrastructural features of the virus-induced membranous replication organelles in plant and animal cells and the scientific progress gained by advanced microscopy methods.

Antiviral RISC mainly targets viral mRNA but not genomic RNA of tospovirus

Antiviral RNA silencing/interference (RNAi) of negative-strand (-) RNA plant viruses (NSVs) has been studied less than for single-stranded, positive-sense (+)RNA plant viruses. From the latter, genomic and subgenomic mRNA molecules are targeted by RNAi. However, genomic RNA strands from plant NSVs are generally wrapped tightly within viral nucleocapsid (N) protein to form ribonucleoproteins (RNPs), the core unit for viral replication, transcription and movement. In this study, the targeting of the NSV tospoviral genomic RNA and mRNA molecules by antiviral RNA-induced silencing complexes (RISC) was investigated, in vitro and in planta. RISC fractions isolated from tospovirus-infected N. benthamiana plants specifically cleaved naked, purified tospoviral genomic RNAs in vitro, but not genomic RNAs complexed with viral N protein. In planta RISC complexes, activated by a tobacco rattle virus (TRV) carrying tospovirus NSs or Gn gene fragments, mainly targeted the corresponding viral mRNAs and hardly genomic (viral and viral-complementary strands) RNA assembled into RNPs. In contrast, for the (+)ssRNA cucumber mosaic virus (CMV), RISC complexes, activated by TRV carrying CMV 2a or 2b gene fragments, targeted CMV genomic RNA. Altogether, the results indicated that antiviral RNAi primarily targets tospoviral mRNAs whilst their genomic RNA is well protected in RNPs against RISC-mediated cleavage. Considering the important role of RNPs in the replication cycle of all NSVs, the findings made in this study are likely applicable to all viruses belonging to this group.

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.

Improved quantification of immune-gold labeling and its use to compare the distribution of cellular factors among sub-chloroplast compartments

Quantification of immuno-gold labeling can provide valuable information on the quantity and localization of a target within a region of interest (ROI). Background subtraction usually requires preparation of material with a deliberately reduced amount of target component often by gene knockout/knockdown. This paper reports a modified method without the need for gene knockout/knockdown, by using a region outside the ROI as a background and non-immune serum to verify the reliability of the data. An optimized parameter for use in image processing was also developed to improve semi-automatic segmentation of gold particles, by using the standard deviation of pixel intensity together with default parameters (size and intensity) to improve specificity. The modified methods were used to quantify the gold labeling of various components within chloroplasts and their 3 sub-organelle compartments (thylakoid, stroma and starch). Rubisco, actin, myosin, β-tubulin, Endoplasmic reticulum-retention signal HDEL, Sterol methyltransferase 1, and double stranded RNA were all effectively and consistently quantified at the level of the different sub-chloroplast compartments. The approach should be applicable more widely for high resolution labelling of samples in which a background requiring gene knockout/knockdown is not a realistic option.

Plant negative-stranded RNA virus biology and host interactions revitalized by reverse genetics

Our understanding of the biology and pathogenesis of plant negative-stranded RNA viruses (NSVs) has lagged behind those made with positive-stranded RNA and DNA virus counterparts. This tardiness is mainly due to the lack of reverse genetics tools for NSV genome engineering for many years. The eventual establishment and application of recombinant systems with diverse plant NSVs has provided renewed momentum for investigations of these important viral pathogens. In this review, we summarize the recent advances in plant NSV reverse genetics systems, highlighting the general principles and the uniqueness of each system and emphasizing important considerations for strategy designing. We also provide a brief overview of the insights about NSV morphogenesis, movement, and virus-host interactions gained from reverse genetics-enabled studies.

Crystal structure of tomato spotted wilt virus GN reveals a dimer complex formation and evolutionary link to animal-infecting viruses

Tospoviridae is a family of enveloped RNA plant viruses that infect many field crops, inflicting a heavy global economic burden. These tripartite, single-stranded, negative-sense RNA viruses are transmitted from plant to plant by thrips as the insect vector. The medium (M) segment of the viral genome encodes two envelope glycoproteins, GN and GC, which together form the envelope spikes. GC is considered the virus fusogen, while the accompanying GN protein serves as an attachment protein that binds to a yet unknown receptor, mediating the virus acquisition by the thrips carrier. Here we present the crystal structure of glycoprotein N (GN) from the tomato spotted wilt virus (TSWV), a representative member of the Tospoviridae family. The structure suggests that GN is organized as dimers on TSWV's outer shell. Our structural data also suggest that this dimerization is required for maintaining GN structural integrity. Although the structure of the TSWV GN is different from other bunyavirus GN proteins, they all share similar domain connectivity that resembles glycoproteins from unrelated animal-infecting viruses, suggesting a common ancestor for these accompanying proteins.

Rescue of tomato spotted wilt virus entirely from complementary DNA clones

Negative-stranded/ambisense RNA viruses (NSVs) include not only dangerous pathogens of medical importance but also serious plant pathogens of agronomic importance. Tomato spotted wilt virus (TSWV) is one of the most important plant NSVs, infecting more than 1,000 plant species, and poses major threats to global food security. The segmented negative-stranded/ambisense RNA genomes of TSWV, however, have been a major obstacle to molecular genetic manipulation. In this study, we report the complete recovery of infectious TSWV entirely from complementary DNA (cDNA) clones. First, a replication- and transcription-competent minigenome replication system was established based on 35S-driven constructs of the S(-)-genomic (g) or S(+)-antigenomic (ag) RNA template, flanked by the 5' hammerhead and 3' ribozyme sequence of hepatitis delta virus, a nucleocapsid (N) protein gene and codon-optimized viral RNA-dependent RNA polymerase (RdRp) gene. Next, a movement-competent minigenome replication system was developed based on M(-)-gRNA, which was able to complement cell-to-cell and systemic movement of reconstituted ribonucleoprotein complexes (RNPs) of S RNA replicon. Finally, infectious TSWV and derivatives carrying eGFP reporters were rescued in planta via simultaneous expression of full-length cDNA constructs coding for S(+)-agRNA, M(-)-gRNA, and L(+)-agRNA in which the glycoprotein gene sequence of M(-)-gRNA was optimized. Viral rescue occurred with the addition of various RNAi suppressors including P19, HcPro, and γb, but TSWV NSs interfered with the rescue of genomic RNA. This reverse genetics system for TSWV now allows detailed molecular genetic analysis of all aspects of viral infection cycle and pathogenicity.

Changes in the GN/GCof the M segment show positive selection and recombination of one aggressive isolate and two mild isolates of tomato spotted wilt virus

We isolated and compared three tomato spotted wilt virus (TSWV) isolates from lettuce (TSWV-Let), pepper (TSWV-Pep), and tomato (TSWV-Tom) from central Mexico to determine their ability to infect a set of eighteen differential plant species from seven families. TWSV-Let was an aggressive isolate with the ability to infect up to 52% of the differential plants, including maize, under greenhouse conditions. The nucleotide (nt) sequences of the three isolates are more than 90% similar in the M and S RNA segments. In the M segment of the TSWV-Let isolate, we detected nt changes in their intergenic region (IGR) and, in the Gc gene, a region containing a recombination site, as well as a synapomorphy associated with one of three sites under positive selection with a change in one aa residue (a cysteine-to-valine mutation). We speculate on the association of these features in the Gc gene with host selection, adaptation, aggressiveness, and ability to infect maize plants.

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.

Identification and Characterization of a Novel Emaravirus Associated With Jujube (Ziziphus jujuba Mill.) Yellow Mottle Disease

A previously unreported disease affecting jujube (Ziziphus jujuba Mill.) trees was observed in China (Liaoning province) in 2015 and named jujube yellow mottle disease (JYMD), due to prevalent symptoms on the leaves. Diseased plants produced also malformed and discolored fruits. In an attempt to identify the possible causal agent of JYMD, high-throughput sequencing of small RNA libraries was performed and a novel virus, tentatively named jujube yellow mottle-associated virus (JYMaV), was identified and characterized. Six genomic RNA segments of JYMaV were completely sequenced. Each one contains a single open reading frame in the viral complementary strand and two untranslated regions with complementary 5' and 3' terminal ends, thus showing typical features of other negative-stranded RNA viruses. RNA1 (7.1 kb), RNA2 (2.2 kb) and RNA3 (1.2 kb) encode putative proteins that, based on their conserved motifs, have been identified as the RNA dependent RNA polymerase, the glycoprotein and the nucleocapsid protein, respectively. These proteins share significant sequence identity (52.1-70.4%) with proteins encoded by raspberry leaf blotch virus (RLBV). RNA4 (1.5 kb) and RNA5 (1.2 kb) code for two putative 30 K movement proteins also related to the homologous RLBV protein. The functional role of the protein encoded by JYMaV RNA6 remains unknown. These data together with the phylogenetic relationships of JYMaV with other recognized emaraviruses support the proposal that JYMaV is the representative member of a novel species in the genus Emaravirus. In agreement with this proposal, virus-like particles and double-membrane-bound bodies, similar to those previously reported for other emaraviruses, were observed by transmission electron microscopy in extracts and tissues from symptomatic leaves, respectively. A specific RT-PCR-based detection method has been developed and used in a preliminary field survey that provided results strongly supporting the close association of JYMaV with the novel disease.

Endoplasmic reticulum remodeling induced by Wheat yellow mosaic virus infection studied by transmission electron microscopy

Plant virus was a kind of organism lived depending on infecting viable host cell and propagated their posterity by replicating its hereditary nucleotide, transcripting into protein, assembling protein and nucleotide into virion (Ortín and Parra, 2006; Sanfaçon, 2005). Viral infection usually induces remodeling of host cell, especially endoplasmic reticulum (ER) for generating membrane packed viral factory. During the infection of Bymovirus, a kind of membranous body (MB) was generated in host cells, which is thought as an ER aggregate. In present study we performed a study on Wheat yellow mosaic virus (WYMV) induced MB by several transmission electron microscopy (TEM) based methods, including cytological observation, component analysis by immuno-gold labeling and structural analysis by electron tomography (ET). WYMV infection induced at least two morphologies of MB, including the lamella dominated morphology (lamella-MB) looked like sprawling cirrus, and the tubule dominated morphology (tubule-MB) looked like latticed network. MB was verified composing of ER as revealed by immuno-gold labeling by antibody against endoplasmic reticulum (ER) retention signal as well as by detailed observation of MB construction modules as double layer membrane. By immuno-gold labeling, both two MB morphologies (lamella-MB and tubule-MB) had same components in viral derived protein and membrane origination (from ER). Structural analysis by ET reconstruction revealed the organization of ER in MB. Lamella-MB was composed of cesER like structures arranged irregularly whereas tubule-MB was composed of tubER like structures arranged regularly. This study provided insights into the structural details in how Bymovirus utilizing host membrane system.

Genome packaging of the Bunyavirales

The order Bunyavirales comprises nine families of enveloped, negative-strand RNA viruses. Depending on the family and genus, bunyaviruses (i.e. now referring to all members of the Bunyavirales) contain genomes consisting of two to six segments. Each genome segment is encapsidated by multiple copies of the nucleocapsid (N) protein and one or a few molecules of the viral polymerase, forming so-called ribonucleoproteins (RNPs). Incorporation of RNPs into virions is mediated by the interaction of N with the cytoplasmic tails of the structural glycoproteins. Although some selectivity exists in the packaging of RNPs into virions, which seems to be driven by the 5' and 3'-untranslated regions of the genomic RNA segments, evidence is accumulating that bunyavirus genome packaging is a stochastic process.

Characterization of the genome of a phylogenetically distinct tospovirus and its interactions with the local lesion-induced host Chenopodium quinoa by whole-transcriptome analyses

Chenopodium quinoa is a natural local lesion host of numerous plant viruses, including tospoviruses (family Bunyaviridae). Groundnut chlorotic fan-spot tospovirus (GCFSV) has been shown to consistently induce local lesions on the leaves of C. quinoa 4 days post-inoculation (dpi). To reveal the whole genome of GCFSV and its interactions with C. quinoa, RNA-seq was performed to determine the transcriptome profiles of C. quinoa leaves. The high-throughput reads from infected C. quinoa leaves were used to identify the whole genome sequence of GCFSV and its single nucleotide polymorphisms. Our results indicated that GCFSV is a phylogenetically distinct tospovirus. Moreover, 27,170 coding and 29,563 non-coding sequences of C. quinoa were identified through de novo assembly, mixing reads from mock and infected samples. Several key genes involved in the modulation of hypersensitive response (HR) were identified. The expression levels of 4,893 deduced complete genes annotated using the Arabidopsis genome indicated that several HR-related orthologues of pathogenesis-related proteins, transcription factors, mitogen-activated protein kinases, and defense proteins were significantly expressed in leaves that formed local lesions. Here, we also provide new insights into the replication progression of a tospovirus and the molecular regulation of the C. quinoa response to virus infection.

Intracellular Localization, Interactions and Functions of Capsicum Chlorosis Virus Proteins

Tospoviruses are among the most devastating viruses of horticultural and field crops. Capsicum chlorosis virus (CaCV) has emerged as an important pathogen of capsicum and tomato in Australia and South-east Asia. Present knowledge about CaCV protein functions in host cells is lacking. We determined intracellular localization and interactions of CaCV proteins by live plant cell imaging to gain insight into the associations of viral proteins during infection. Proteins were transiently expressed as fusions to autofluorescent proteins in leaf epidermal cells of Nicotiana benthamiana and capsicum. All viral proteins localized at least partially in the cell periphery suggestive of cytoplasmic replication and assembly of CaCV. Nucleocapsid (N) and non-structural movement (NSm) proteins localized exclusively in the cell periphery, while non-structural suppressor of silencing (NSs) protein and Gc and Gn glycoproteins accumulated in both the cell periphery and the nucleus. Nuclear localization of CaCV Gn and NSs is unique among tospoviruses. We validated nuclear localization of NSs by immunofluorescence in protoplasts. Bimolecular fluorescence complementation showed self-interactions of CaCV N, NSs and NSm, and heterotypic interactions of N with NSs and Gn. All interactions occurred in the cytoplasm, except NSs self-interaction was exclusively nuclear. Interactions of a tospoviral NSs protein with itself and with N had not been reported previously. Functionally, CaCV NSs showed strong local and systemic RNA silencing suppressor activity and appears to delay short-distance spread of silencing signal. Cell-to-cell movement activity of NSm was demonstrated by trans-complementation of a movement-defective tobamovirus replicon. CaCV NSm localized at plasmodesmata and its transient expression led to the formation of tubular structures that protruded from protoplasts. The D155 residue in the 30K-like movement protein-specific LxD/N50-70G motif of NSm was critical for plasmodesmata localization and movement activity. Compared to other tospoviruses, CaCV proteins have both conserved and unique properties in terms of in planta localization, interactions and protein functions which will effect viral multiplication and movement in host plants.

Host-specific accumulation and temperature effects on the generation of dimeric viral RNA species derived from the S-RNA of members of the Tospovirus genus

Polygonum ringspot virus (PolRSV) is a recently characterized Tospovirus reported in Italy. Northern blot analyses of PolRSV infections in Nicotiana benthamiana and tomato plants showed that a viral RNA species with nearly twice the length of the Small genomic RNA (S-RNA) accumulated abundantly in the former host, but was not detected in the latter. Additional assays confirmed that biogenesis of this novel RNA species was common to all PolRSV isolates tested and also to an isolate of Tomato spotted wilt virus (TSWV). Given its size, we hypothesized that the novel RNA species was a dimer molecule and we confirmed this hypothesis by RNA sequencing (RNAseq) analysis and reverse transcription (RT)-PCR of putative predicted dimer junction sites in RNA extracts of N. benthamiana challenged with PolRSV isolates Plg6 and Plg13/2. We also confirmed that these molecules are derived from head-to-tail dimers and often contain deletions at their junction sites. We named these novel molecules imperfect dimer RNAs (IMPD-RNAs). PolRSV IMPD-RNAs systemic accumulation in a range of host plants was restricted to N. benthamiana and Nicotiana occidentalis. Notably, IMPD-RNAs accumulation was modulated by temperature and their generation was restricted to late stages of systemic infection (12 days post-inoculation) in N. benthamiana. Differently from all other PolRSV isolates used in this study, Plg13/2 generated more IMPD-RNAs coupled with low amounts of genomic S-RNA and maintained them even at 18 °C, besides having lost the ability to infect tomato plants. This is the first characterization of S-RNA dimers for Tospovirus, and of occurrence of dimers of genomic segments at the whole organism level for Bunyaviridae.

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.

Resistance to Tospoviruses in Vegetable Crops: Epidemiological and Molecular Aspects

During the past three decades, the economic impact of tospoviruses has increased, causing high yield losses in a variety of crops and ornamentals. Owing to the difficulty in combating thrips vectors with insecticides, the best way to limit/prevent tospovirus-induced diseases involves a management strategy that includes virus resistance. This review briefly presents current tospovirus taxonomy, diversity, molecular biology, and cytopathology as an introduction to a more extensive description of the two main resistance genes employed against tospoviruses: the Sw5 gene in tomato and the Tsw in pepper. Natural and experimental resistance-breaking (RB) isolates allowed the identification of the viral avirulence protein triggering each of the two resistance gene products; epidemiology of RB isolates is discussed to reinforce the need for allelic variants and the need to search for new/alternative resistance genes. Ongoing efforts for alternative resistance strategies are described not only for Tomato spotted wilt virus (TSWV) in pepper and tomato but also for other vegetable crops heavily impacted by tospoviruses.

Cell Walls and the Convergent Evolution of the Viral Envelope

Why some viruses are enveloped while others lack an outer lipid bilayer is a major question in viral evolution but one that has received relatively little attention. The viral envelope serves several functions, including protecting the RNA or DNA molecule(s), evading recognition by the immune system, and facilitating virus entry. Despite these commonalities, viral envelopes come in a wide variety of shapes and configurations. The evolution of the viral envelope is made more puzzling by the fact that nonenveloped viruses are able to infect a diverse range of hosts across the tree of life. We reviewed the entry, transmission, and exit pathways of all (101) viral families on the 2013 International Committee on Taxonomy of Viruses (ICTV) list. By doing this, we revealed a strong association between the lack of a viral envelope and the presence of a cell wall in the hosts these viruses infect. We were able to propose a new hypothesis for the existence of enveloped and nonenveloped viruses, in which the latter represent an adaptation to cells surrounded by a cell wall, while the former are an adaptation to animal cells where cell walls are absent. In particular, cell walls inhibit viral entry and exit, as well as viral transport within an organism, all of which are critical waypoints for successful infection and spread. Finally, we discuss how this new model for the origin of the viral envelope impacts our overall understanding of virus evolution.

Salivary gland morphology, tissue tropism and the progression of tospovirus infection in Frankliniella occidentalis

Tomato spotted wilt virus (TSWV) is transmitted by thrips in a propagative manner; however, progression of virus infection in the insect is not fully understood. The goal of this work was to study the morphology and infection of thrips salivary glands. The primary salivary glands (PSG) are complex, with three distinct regions that may have unique functions. Analysis of TSWV progression in thrips revealed the presence of viral proteins in the foregut, midgut, ligaments, tubular salivary glands (TSG), and efferent duct and filament structures connecting the TSG and PSG of first and second instar larvae. The primary site of virus infection shifted from the midgut and TSG in the larvae to the PSG in adults, suggesting that tissue tropism changes with insect development. TSG infection was detected in advance of PSG infection. These findings support the hypothesis that the TSG are involved in trafficking of TSWV to the PSG.

Effect of Spinosad Resistance on Transmission of Tomato Spotted Wilt Virus by the Western Flower Thrips (Thysanoptera: Thripidae)

Tomato spotted wilt virus (TSWV) is transmitted by Frankliniella occidentalis (Pergande) in a persistent-propagative manner. We previously observed significant results in terms of feeding behavior of spinosad-susceptible (Ivf03) and -resistant (Spin-R) strains of F. occidentalis using electrical penetration graph. TSWV transmission by the two strains was compared in the present study. The results showed that the titer of TSWV-N RNA (a part of S RNA of TSWV and encoding the nucleocapsid protein) in Ivf03 and Spin-R strains was not significantly different after a 48-h inoculation access period. The TSWV transmission rate did not significantly differ between the two strains and was 51.0% for Ivf03 and 44.4% for Spin-R. The virus transmission rate was significantly higher for males than females of both strains. The virus transmission rate for males and females of Ivf03 was 68.1 and 33.8%, respectively; however, in case of Spin-R, it was 60 and 28.8% for males and females, respectively. Additionally, number of probes and duration of probes were generally greater for viruliferous females of Ivf03 than for viruliferous females of Spin-R but the total number and duration of noningestion probes did not significantly differ between males of the two strains. The latter finding behavior may help explain the similar transmission rates for the susceptible and resistant strains.

Clustering and cellular distribution characteristics of virus particles of Tomato spotted wilt virus and Tomato zonate spot virus in different plant hosts

BACKGROUND

Tomato spotted wilt virus (TSWV) and Tomato zonate spot virus (TZSV) are the two dominant species of thrip-transmitted tospoviruses, cause significant losses in crop yield in Yunnan and its neighboring provinces in China. TSWV and TZSV belong to different serogroup of tospoviruses but induce similar symptoms in the same host plant species, which makes diagnostic difficult. We used different electron microscopy preparing methods to investigate clustering and cellular distribution of TSWV and TZSV in the host plant species.

RESULTS

Negative staining of samples infected with TSWV and TZSV revealed that particles usually clustered in the vesicles, including single particle (SP), double particles clustering (DPC), triple particles clustering (TPC). In the immunogold labeling negative staining against proteins of TZSV, the antibodies against Gn protein were stained more strongly than the N protein. Ultrathin section and high pressure freeze (HPF)-electron microscopy preparations revealed that TSWV particles were distributed in the cisternae of endoplasmic reticulum (ER), filamentous inclusions (FI) and Golgi bodies in the mesophyll cells. The TSWV particles clustered as multiple particles clustering (MPC) and distributed in globular viroplasm or cisternae of ER in the top leaf cell. TZSV particles were distributed more abundantly in the swollen membrane of ER in the mesophyll cell than those in the phloem parenchyma cells and were not observed in the top leaf cell. However, TZSV virions were mainly present as single particle in the cytoplasm, with few clustering as MPC.

CONCLUSION

In this study, we identified TSWV and TZSV particles had the distinct cellular distribution patterns in the cytoplasm from different tissues and host plants. This is the first report of specific clustering characteristics of tospoviruses particles as well as the cellular distribution of TSWV particles in the FI and globular viroplasm where as TZSV particles inside the membrane of ER. These results indicated that tospoviruses particles possessed specific and similar clustering in the saps of diseased plants. Furthermore, the results of this study will also provide a basis for further study on the tospoviruses assembling, maturation and movement.

Pigeonpea sterility mosaic virus: a legume-infecting Emaravirus from South Asia

Pigeonpea sterility mosaic virus (PPSMV), a species of the genus Emaravirus, is the causal agent of sterility mosaic disease (SMD) of pigeonpea [Cajanus cajan (L.) Millsp]. This disease, dubbed the 'green plague', as the infected plants remain in the vegetative state without flower production, has been reported from India and a few other South-East Asian countries. SMD is estimated to result in an annual yield loss of over US$300 million in India alone. The aetiology of SMD, which remained a mystery for over 70 years, was resolved with the discovery of PPSMV in 2000 and its complete genome sequence in 2014.

AETIOLOGY AND VIRUS TRANSMISSION

SMD is characterized by stunted and bushy plants, leaves of reduced size with chlorotic rings or mosaic symptoms, and partial or complete cessation of flower production (i.e. sterility). The causal agent of the disease is PPSMV, a virus with a segmented, negative-sense, single-stranded RNA genome, transmitted in a semi-persistent manner by an eriophyid mite Aceria cajani Channabassavanna (Acari: Arthropoda). Both the virus and vector are highly specific to pigeonpea and a few of its wild relatives, such as C. scarabaeoides and C. cajanifolius. Under experimental conditions, PPSMV was transmitted to Nicotiana benthamiana by sap inoculation using fresh extract of SMD-infected leaves (but not to pigeonpea); however, purified nucleoprotein preparations are not infectious. The virus was also transmitted to French bean (Phaseolus vulgaris L.) using viruliferous eriophyid mites. PPSMV is not seed transmitted in pigeonpea or other hosts known to be infected by this virus. On the basis of the differential host reactions in different geographical locations, the occurrence of diverse PPSMV strains was suspected.

HOST RANGE AND EPIDEMIOLOGY

PPSMV can infect several genotypes of cultivated and wild relatives of pigeonpea. Experimental hosts include N. benthamiana, N. clevelandii, P. vulgaris and Chrozophora rottleri. However, pigeonpea alone and a few wild species of Cajanus were found to support the vector A. cajani. SMD is endemic in most of the pigeonpea-growing regions of India, but the incidence varies widely between regions and years. In nature, A. cajani populations were almost exclusively observed on SMD-infected pigeonpea, but not on healthy plants, indicating a strong communalistic relationship between the virus-infected plants and the vector. The epidemiology of SMD involves the virus, mite vector, cultivar and environmental conditions. Infected perennial and volunteer plants serve as a source for both the virus and its vector mites, and play an important role in the disease cycle.

GENOME ORGANIZATION, GENE FUNCTION AND TAXONOMY

The PPSMV genome contains five segments of single-stranded RNA that are predicted to encode proteins in negative sense. The ribonucleoprotein complex is encased in quasi-spherical, membrane-bound virus particles of 100-150 nm. The largest segment, RNA-1, is 7022 nucleotides in length and codes for RNA-dependent RNA polymerase (2295 amino acids); RNA-2, with a sequence length of 2223 nucleotides, codes for glycoproteins (649 amino acids); RNA-3, with a sequence length of 1442 nucleotides, codes for nucleocapsid protein (309 amino acids); RNA-4, with a sequence length of 1563 nucleotides, codes for a putative movement protein p4 (362 amino acids); and RNA-5, with a sequence length of 1689 nucleotides, codes for p5 (474 amino acids), a protein with unknown function. PPSMV was recently classified as a species in the genus Emaravirus, a genus whose members show features resembling those of members of the genera Tospovirus (Family: Bunyaviridae) and Tenuivirus, both of which comprise single-stranded RNA viruses that encode proteins by an ambisense strategy.

SMD CONTROL

The disease is mainly controlled using SMD-resistant cultivars. However, the occurrence of distinct strains/isolates of PPSMV in different locations makes it difficult to incorporate broad-spectrum resistance. Studies on the inheritance of SMD resistance in different cultivars against different isolates of PPSMV indicate that the resistance is mostly governed by recessive genes, although there are contrasting interpretations of the data. Genetic engineering through RNA-interference (RNAi) and resistant gene-based strategies are some of the potential approaches for the transgenic control of SMD. Seed treatment or soil and foliar application of a number of organophosphorus-based insecticides or acaricides, which are recommended for the management of the vector mites, are seldom practised because of prohibitive costs and also their risks to human health and the environment.

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.

Nucleocapsid protein from fig mosaic virus forms cytoplasmic agglomerates that are hauled by endoplasmic reticulum streaming

Although many studies have demonstrated intracellular movement of viral proteins or viral replication complexes, little is known about the mechanisms of their motility. In this study, we analyzed the localization and motility of the nucleocapsid protein (NP) of Fig mosaic virus (FMV), a negative-strand RNA virus belonging to the recently established genus Emaravirus. Electron microscopy of FMV-infected cells using immunogold labeling showed that NPs formed cytoplasmic agglomerates that were predominantly enveloped by the endoplasmic reticulum (ER) membrane, while nonenveloped NP agglomerates also localized along the ER. Likewise, transiently expressed NPs formed agglomerates, designated NP bodies (NBs), in close proximity to the ER, as was the case in FMV-infected cells. Subcellular fractionation and electron microscopic analyses of NP-expressing cells revealed that NBs localized in the cytoplasm. Furthermore, we found that NBs moved rapidly with the streaming of the ER in an actomyosin-dependent manner. Brefeldin A treatment at a high concentration to disturb the ER network configuration induced aberrant accumulation of NBs in the perinuclear region, indicating that the ER network configuration is related to NB localization. Dominant negative inhibition of the class XI myosins, XI-1, XI-2, and XI-K, affected both ER streaming and NB movement in a similar pattern. Taken together, these results showed that NBs localize in the cytoplasm but in close proximity to the ER membrane to form enveloped particles and that this causes passive movements of cytoplasmic NBs by ER streaming.

IMPORTANCE

Intracellular trafficking is a primary and essential step for the cell-to-cell movement of viruses. To date, many studies have demonstrated the rapid intracellular movement of viral factors but have failed to provide evidence for the mechanism or biological significance of this motility. Here, we observed that agglomerates of nucleocapsid protein (NP) moved rapidly throughout the cell, and we performed live imaging and ultrastructural analysis to identify the mechanism of motility. We provide evidence that cytoplasmic protein agglomerates were passively dragged by actomyosin-mediated streaming of the endoplasmic reticulum (ER) in plant cells. In virus-infected cells, NP agglomerates were surrounded by the ER membranes, indicating that NP agglomerates form the basis of enveloped virus particles in close proximity to the ER. Our work provides a sophisticated model of macromolecular trafficking in plant cells and improves our understanding of the formation of enveloped particles of negative-strand RNA viruses.

The Tomato spotted wilt virus cell-to-cell movement protein (NSM ) triggers a hypersensitive response in Sw-5-containing resistant tomato lines and in Nicotiana benthamiana transformed with the functional Sw-5b resistance gene copy

Although the Sw-5 gene cluster has been cloned, and Sw-5b has been identified as the functional gene copy that confers resistance to Tomato spotted wilt virus (TSWV), its avirulence (Avr) determinant has not been identified to date. Nicotiana tabacum 'SR1' plants transformed with a copy of the Sw-5b gene are immune without producing a clear visual response on challenge with TSWV, whereas it is shown here that N. benthamiana transformed with Sw-5b gives a rapid and conspicuous hypersensitive response (HR). Using these plants, from all structural and non-structural TSWV proteins tested, the TSWV cell-to-cell movement protein (NSM ) was confirmed as the Avr determinant using a Potato virus X (PVX) replicon or a non-replicative pEAQ-HT expression vector system. HR was induced in Sw-5b-transgenic N. benthamiana as well as in resistant near-isogenic tomato lines after agroinfiltration with a functional cell-to-cell movement protein (NSM ) from a resistance-inducing (RI) TSWV strain (BR-01), but not with NSM from a Sw-5 resistance-breaking (RB) strain (GRAU). This is the first biological demonstration that Sw-5-mediated resistance is triggered by the TSWV NSM cell-to-cell movement protein.

Rice stripe tenuivirus NSvc2 glycoproteins targeted to the golgi body by the N-terminal transmembrane domain and adjacent cytosolic 24 amino acids via the COP I- and COP II-dependent secretion pathway

The NSvc2 glycoproteins encoded by Rice stripe tenuivirus (RSV) share many characteristics common to the glycoproteins found among Bunyaviridae. Within this viral family, glycoproteins targeting to the Golgi apparatus play a pivotal role in the maturation of the enveloped spherical particles. RSV particles, however, adopt a long filamentous morphology. Recently, RSV NSvc2 glycoproteins were shown to localize exclusively to the ER in Sf9 insect cells. Here, we demonstrate that the amino-terminal NSvc2 (NSvc2-N) targets to the Golgi apparatus in Nicotiana benthamiana cells, whereas the carboxyl-terminal NSvc2 (NSvc2-C) accumulates in the endoplasmic reticulum (ER). Upon coexpression, NSvc2-N redirects NSvc2-C from the ER to the Golgi bodies. The NSvc2 glycoproteins move together with the Golgi stacks along the ER/actin network. The targeting of the NSvc2 glycoproteins to the Golgi bodies was strictly dependent on functional anterograde traffic out of the ER to the Golgi bodies or on a retrograde transport route from the Golgi apparatus. The analysis of truncated and chimeric NSvc2 proteins demonstrates that the Golgi targeting signal comprises amino acids 269 to 315 of NSvc2-N, encompassing the transmembrane domain and 24 adjacent amino acids in the cytosolic tail. Our findings demonstrate for the first time that the glycoproteins from an unenveloped Tenuivirus could target Golgi bodies in plant cells.

IMPORTANCE

NSvc2 glycoprotein encoded by unenveloped Rice stripe tenuivirus (RSV) share many characteristics in common with glycoprotein found among Bunyaviridae in which all members have membrane-enveloped sphere particle. Recently, RSV NSvc2 glycoproteins were shown to localize exclusively to the ER in Sf9 insect cells. In this study, we demonstrated that the RSV glycoproteins could target Golgi bodies in plant cells. The targeting of NSvc2 glycoproteins to the Golgi bodies was dependent on active COP II or COP I. The Golgi targeting signal was mapped to the 23-amino-acid transmembrane domain and the adjacent 24 amino acids of the cytosolic tail of the NSvc2-N. In light of the evidence from viruses in Bunyaviridae that targeting Golgi bodies is important for the viral particle assembly and vector transmission, we propose that targeting of RSV glycoproteins into Golgi bodies in plant cells represents a physiologically relevant mechanism in the maturation of RSV particle complex for insect vector transmission.

The cytosolic nucleoprotein of the plant-infecting bunyavirus tomato spotted wilt recruits endoplasmic reticulum-resident proteins to endoplasmic reticulum export sites

In contrast with animal-infecting viruses, few known plant viruses contain a lipid envelope, and the processes leading to their membrane envelopment remain largely unknown. Plant viruses with lipid envelopes include viruses of the Bunyaviridae, which obtain their envelope from the Golgi complex. The envelopment process is predominantly dictated by two viral glycoproteins (Gn and Gc) and the viral nucleoprotein (N). During maturation of the plant-infecting bunyavirus Tomato spotted wilt, Gc localizes at endoplasmic reticulum (ER) membranes and becomes ER export competent only upon coexpression with Gn. In the presence of cytosolic N, Gc remains arrested in the ER but changes its distribution from reticular into punctate spots. Here, we show that these areas correspond to ER export sites (ERESs), distinct ER domains where glycoprotein cargo concentrates prior to coat protein II vesicle-mediated transport to the Golgi. Gc concentration at ERES is mediated by an interaction between its cytoplasmic tail (CT) and N. Interestingly, an ER-resident calnexin provided with Gc-CT was similarly recruited to ERES when coexpressed with N. Furthermore, disruption of actin filaments caused the appearance of a larger amount of smaller ERES loaded with N-Gc complexes, suggesting that glycoprotein cargo concentration acts as a trigger for de novo synthesis of ERES.

Epidemiology, molecular virology and diagnostics of Schmallenberg virus, an emerging orthobunyavirus in Europe

After the unexpected emergence of Bluetongue virus serotype 8 (BTV-8) in northern Europe in 2006, another arbovirus, Schmallenberg virus (SBV), emerged in Europe in 2011 causing a new economically important disease in ruminants. The virus, belonging to the Orthobunyavirus genus in the Bunyaviridae family, was first detected in Germany, in The Netherlands and in Belgium in 2011 and soon after in the United Kingdom, France, Italy, Luxembourg, Spain, Denmark and Switzerland. This review describes the current knowledge on the emergence, epidemiology, clinical signs, molecular virology and diagnosis of SBV infection.

Molecular characterization of the full-length L and M RNAs of Tomato yellow ring virus, a member of the genus Tospovirus

Tomato yellow ring virus (TYRV), first isolated from tomato in Iran, was classified as a non-approved species of the genus Tospovirus based on the characterization of its genomic S RNA. In the current study, the complete sequences of the genomic L and M RNAs of TYRV were determined and analyzed. The L RNA has 8,877 nucleotides (nt) and codes in the viral complementary (vc) strand for the putative RNA-dependent RNA polymerase (RdRp) of 2,873 amino acids (aa) (331 kDa). The RdRp of TYRV shares the highest aa sequence identity (88.7 %) with that of Iris yellow spot virus (IYSV), and contains conserved motifs shared with those of the animal-infecting bunyaviruses. The M RNA contains 4,786 nt and codes in ambisense arrangement for the NSm protein of 308 aa (34.5 kDa) in viral sense, and the Gn/Gc glycoprotein precursor (GP) of 1,310 aa (128 kDa) in vc-sense. Phylogenetic analyses indicated that TYRV is closely clustered with IYSV and Polygonum ringspot virus (PolRSV). The NSm and GP of TYRV share the highest aa sequence identity with those of IYSV and PolRSV (89.9 and 80.2-86.5 %, respectively). Moreover, the GPs of TYRV, IYSV, and PolRSV share highly similar characteristics, among which an identical deduced N-terminal protease cleavage site that is distinct from all tospoviral GPs analyzed thus far. Taken together, the elucidation of the complete genome sequence and biological features of TYRV support a close ancestral relationship with IYSV and PolRSV.

Emaravirus: a novel genus of multipartite, negative strand RNA plant viruses

Ringspot symptoms in European mountain ash (Sorbus aucuparia L.), fig mosaic, rose rosette, raspberry leaf blotch, pigeonpea sterility mosaic (Cajanus cajan) and High Plains disease of maize and wheat were found to be associated with viruses that share several characteristics. They all have single-stranded multipartite RNA genomes of negative orientation. In some cases, double membrane-bound virus-like particles of 80 to 200 nm in diameter were found in infected tissue. Furthermore, at least five of these viruses were shown to be vectored by eriophyid mites. Sequences of European mountain ash ringspot-associated virus (EMARaV), Fig mosaic virus (FMV), rose rosette virus (RRV), raspberry leaf blotch virus (RLBV), pigeonpea sterility mosaic virus and High Plains virus strongly support their potential phylogenetic relationship. Therefore, after characterization of EMARaV, the novel genus Emaravirus was established, and FMV was the second virus species assigned to this genus. The recently sequenced RRV and RLBV are supposed to be additional members of this new group of plant RNA viruses.

Hijack it, change it: how do plant viruses utilize the host secretory pathway for efficient viral replication and spread?

The secretory pathway of eukaryotic cells has an elaborated set of endomembrane compartments involved in the synthesis, modification, and sorting of proteins and lipids. The secretory pathway in plant cells shares many features with that in other eukaryotic cells but also has distinct characteristics important for fundamental cell and developmental processes and for proper immune responses. Recently, there has been evidence that the remodeling of this pathway, and often the formation of viral-induced organelles, play an important role in viral replication and spread. The modification of the host secretory pathway seems to be a common feature among most single-stranded positive ss(+)RNA and even some DNA viruses. In this review, we will present the recent advances in the understanding of the organization and dynamics of the plant secretory pathway and the molecular regulation of membrane trafficking in the pathway. We will also discuss how different plant viruses may interact with the host secretory pathway for their efficient replication and spread, with a focus on tobacco mosaic virus and turnip mosaic virus.

Intracellular transport of plant viruses: finding the door out of the cell

Plant viruses are a class of plant pathogens that specialize in movement from cell to cell. As part of their arsenal for infection of plants, every virus encodes a movement protein (MP), a protein dedicated to enlarging the pore size of plasmodesmata (PD) and actively transporting the viral nucleic acid into the adjacent cell. As our knowledge of intercellular transport has increased, it has become apparent that viruses must also use an active mechanism to target the virus from their site of replication within the cell to the PD. Just as viruses are too large to fit through an unmodified plasmodesma, they are also too large to be freely diffused through the cytoplasm of the cell. Evidence has accumulated now for the involvement of other categories of viral proteins in intracellular movement in addition to the MP, including viral proteins originally associated with replication or gene expression. In this review, we will discuss the strategies that viruses use for intracellular movement from the replication site to the PD, in particular focusing on the role of host membranes for intracellular transport and the coordinated interactions between virus proteins within cells that are necessary for successful virus spread.

Recent advances in the molecular and cellular biology of bunyaviruses

The family Bunyaviridae of segmented, negative-stranded RNA viruses includes over 350 members that infect a bewildering variety of animals and plants. Many of these bunyaviruses are the causative agents of serious disease in their respective hosts, and are classified as emerging viruses because of their increased incidence in new populations and geographical locations throughout the world. Emerging bunyaviruses, such as Crimean-Congo hemorrhagic fever virus, tomato spotted wilt virus and Rift Valley fever virus, are currently attracting great interest due to migration of their arthropod vectors, a situation possibly linked to climate change. These and other examples of continued emergence suggest that bunyaviruses will probably continue to pose a sustained global threat to agricultural productivity, animal welfare and human health. The threat of emergence is particularly acute in light of the lack of effective preventative or therapeutic treatments for any of these viruses, making their study an important priority. This review presents recent advances in the understanding of the bunyavirus life cycle, including aspects of their molecular, cellular and structural biology. Whilst special emphasis is placed upon the emerging bunyaviruses, we also describe the extensive body of work involving model bunyaviruses, which have been the subject of major contributions to our overall understanding of this important group of viruses.

Development of a lentiviral vector system to study the role of the Andes virus glycoproteins

To infect target cells, enveloped viruses use their virion surface proteins to direct cell attachment and subsequent entry via virus-cell membrane fusion. How hantaviruses enter cells has been largely unexplored. To study early steps of Andes virus (ANDV) cell infection, a lentiviral vector system was developed based on a Simian immunodeficiency virus (SIV) vector pseudotyped with the ANDV-Gn/Gc envelope glycoproteins. The incorporation of Gn and Gc onto SIV-derived vector particles was assessed using newly generated monoclonal antibodies against ANDV glycoproteins. In addition, sera of ANDV infected humans were able to block cell entry of the SIV vector pseudotyped with ANDV glycoproteins, suggesting that their antigenic conformation is similar to that in the native virus. The use of such SIV vector pseudotyped with ANDV-Gn/Gc glycoproteins should facilitate studies on ANDV cell entry. Along this line, it was found that depletion of cholesterol from target cells strongly diminished cell infection, indicating a possible role of lipid rafts in ANDV cell entry. The Gn/Gc pseudotyped SIV vector has several advantages, notably high titer vector production and easy quantification of cell infection by monitoring GFP reporter gene expression by flow cytometry. Such pseudotyped SIV vectors can be used to identify functional domains in the Gn/Gc glycoproteins and to screen for potential hantavirus cell entry inhibitors.

Requirements for ER-arrest and sequential exit to the golgi of Tomato spotted wilt virus glycoproteins

The envelope glycoproteins Gn and Gc are major determinants in the assembly of Tomato spotted wilt virus (TSWV) particles at the Golgi complex. In this article, the ER-arrest of singly expressed Gc and the transport of both glycoproteins to the Golgi upon coexpression have been analyzed.While preliminary results suggest that the arrest of Gc at the ER (endoplasmic reticulum) did not appear to result from improper folding, transient expression of chimeric Gc, in which the transmembrane domain (TMD) and/or cytoplasmic tail (CT) were swapped for those from Gn, showed that the TMD of Gn was sufficient to allow ER exit and transport to the Golgi. Expression of both glycoproteins in the presence of overexpressed Sar1p specific guanosine nucleotide exchange factor Sec12p, resulted in ER-retention demonstrating that the viral glycoproteins are transported to the Golgi in a COPII (coat protein II)-dependent manner. Inhibition of ER Golgi transport by brefeldin A (BFA) had a similar effect on the localization of Gn. However, inhibition of ER (endoplasmic reticulum) to Golgi transport of coexpressed Gc and Gn by overexpression of Sec12p or by BFA revealed distinct localization patterns, i.e. diffuse ER localization versus concentration at specific spots.

Tomato spotted wilt virus nucleocapsid protein interacts with both viral glycoproteins Gn and Gc in planta

Recently, the Tomato Spotted Wilt Virus (TSWV) Gn and Gc glycoproteins were shown to induce the formation of (pseudo-) circular and pleomorphic membrane structures upon transient expression in plant cells. Furthermore, when singly expressed, Gc retains in the ER, while Gn is able to further migrate to the Golgi. Upon co-expression, Gn rescues Gc and co-migrates to the Golgi complex. Here, we have studied the behavior of the glycoproteins in the presence of the viral nucleocapsid (N) protein and in vivo analyzed the occurrence of protein-protein interactions by fluorescence life time imaging microscopy (FLIM). The analysis demonstrated that N co-localizes and interacts with both glycoproteins, with a preference for Gn. Additionally, it is shown that N causes a dramatic change in the distribution of Gc within the ER, from reticular to punctate spots. The observations are discussed in the context of the virus particle formation during the infection process.

Tomato spotted wilt virus glycoproteins induce the formation of endoplasmic reticulum- and Golgi-derived pleomorphic membrane structures in plant cells

Tomato spotted wilt virus (TSWV) particles are spherical and enveloped, an uncommon feature among plant infecting viruses. Previous studies have shown that virus particle formation involves the enwrapment of ribonucleoproteins with viral glycoprotein containing Golgi stacks. In this study, the localization and behaviour of the viral glycoproteins Gn and Gc were analysed, upon transient expression in plant protoplasts. When separately expressed, Gc was solely observed in the endoplasmic reticulum (ER), whereas Gn was found both within the ER and Golgi membranes. Upon co-expression, both glycoproteins were found at ER-export sites and ultimately at the Golgi complex, confirming the ability of Gn to rescue Gc from the ER, possibly due to heterodimerization. Interestingly, both Gc and Gn were shown to induce the deformation of ER and Golgi membranes, respectively, also observed upon co-expression of the two glycoproteins. The behaviour of both glycoproteins within the plant cell and the phenomenon of membrane deformation are discussed in light of the natural process of viral infection.

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.

The cytoplasmic domain of tomato spotted wilt virus Gn glycoprotein is required for Golgi localisation and interaction with Gc

Envelopment of tomato spotted wilt virus nucleocapsids occurs at the Golgi stacks of infected cells. This is also the place where the two membrane glycoproteins Gn and Gc accumulate upon coexpression. The required Golgi retention signal has previously been demonstrated to reside within Gn. Using a series of truncated Gn proteins, the Golgi retention signal was mapped to a stretch of 10 amino acids on this protein's cytoplasmic tail, 20 residues downstream the transmembrane domain. Studies on the intracellular distribution of chimeric Gc proteins in which the cytoplasmic tail and/or transmembrane domain were exchanged by those from Gn, demonstrated the additional requirement of the Gn transmembrane domain for Golgi targeting. Truncated Gn constructs lacking the C-terminal 20 amino acids but still localising to the Golgi were no longer able to redirect Gc, suggesting the requirement of this domain for interaction with Gc.