Immuno-electron microscopical detection of tomato spotted wilt virus and its nucleocapsids in crude plant extracts

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.

Comparison of transcriptomes of an orthotospovirus vector and non-vector thrips species

Thrips transmit one of the most devastating plant viruses worldwide–tomato spotted wilt tospovirus (TSWV). Tomato spotted wilt tospovirus is a type species in the genus Orthotospovirus and family Tospoviridae. Although there are more than 7,000 thrips species, only nine thrips species are known to transmit TSWV. In this study, we investigated the molecular factors that could affect thrips ability to transmit TSWV. We assembled transcriptomes of a vector, Frankliniella fusca [Hinds], and a non-vector, Frankliniella tritici [Fitch], and performed qualitative comparisons of contigs associated with virus reception, virus infection, and innate immunity. Annotations of F. fusca and F. tritici contigs revealed slight differences across biological process and molecular functional groups. Comparison of virus cell surface receptors revealed that homologs of integrin were present in both species. However, homologs of another receptor, heperan sulfate, were present in F. fusca alone. Contigs associated with virus replication were identified in both species, but a contig involved in inhibition of virus replication (radical s-adenosylmethionine) was only present in the non-vector, F. tritici. Additionally, some differences in immune signaling pathways were identified between vector and non-vector thrips. Detailed investigations are necessary to functionally characterize these differences between vector and non-vector thrips and assess their relevance in orthotospovirus transmission.

The glycoprotein gene of Chrysanthemum stem necrosis virus and Zucchini lethal chlorosis virus and molecular relationship with other tospoviruses

Two tospoviruses, Chrysanthemum stem necrosis virus (CSNV) and Zucchini lethal chlorosis virus (ZLCV), cause economical losses in several ornamental and vegetable crops in Brazil. The nucleocapsid gene and movement protein sequences had already been reported for both viruses, though the glycoprotein precursor gene sequence was not available. In this study, cDNA fragments (ca. 4 kb) of the M RNA 3' portion of CSNV (isolate Chry-1) and ZLCV (isolate 1038), including the complete glycoprotein precursor gene, partial NSm gene, and the entire intergenic and 3' untranslated regions, were cloned and sequenced. The sequences were assembled with the corresponding 5' region sequence (NSm gene and 5'UTR) of the same isolates to build up the complete sequence of the M RNA segment of both species. The M RNA of CSNV was 4,828 nucleotide-long, while of ZLCV 4,836 nucleotides. Both M RNA molecules comprised two ORFs in an ambisense arrangement. The vcRNA coded for the viral glycoprotein (Gn/Gc) precursor gene of CSNV and ZLCV (both with 127.5 kDa). Comparison of deduced amino acids of the CSNV and ZLCV glycoprotein precursor genes with those of other tospoviruses showed the highest identity with that of Tomato spotted wilt virus (86%) and with that of CSNV (82%), respectively. However, the nucleotide sequence of the intergenic and 3' untranslated regions of CSNV and ZLCV shared lower identities with other tospoviruses. The glycoprotein precursor gene is thought to be a good candidate as additional classification parameter for Tospovirus taxonomy. The presence of the RGD motif in both Gc proteins indicated that they are typical American tospoviruses, which was confirmed by phylogenetic analysis. The membrane topology of both glycoproteins is discussed.

Sequence analysis of the glycoproteins of Tomato chlorotic spot virus and Groundnut ringspot virus and comparison with other tospoviruses

The tospoviruses Tomato chlorotic spot virus (TCSV) and Groundnut ringspot virus (GRSV) cause high economic losses in several vegetable crops in Brazil. The glycoprotein precursor coding sequence was still not available for these two viruses. In this study, the 3' 4 kb M RNA of TCSV and GRSV genome was cloned and sequenced. The sequences were compiled with the available 5' region sequence (NSM gene and 5' UTR) of the same isolates. The M RNA of TCSV was deduced as formed by 4,882 nucleotides, while of GRSV by 4,855 nucleotides. Both M RNA comprised two ORFs in an ambisense arrangement. The vcRNA ORF coded for viral glycoprotein (G1/G2) precursor of TCSV (128.46 kDa) and for glycoprotein precursor of GRSV (128.16 kDa). Comparison of the TCSV and GRSV glycoprotein precursor proteins with those of other tospoviruses showed the highest identity with Tomato spotted wilt virus (81 and 79%, respectively). The amino acid sequence comparison of glycoprotein precursor between TCSV and GRSV revealed a high identity of 92%. However, the nucleotide sequence of the M RNA intergenie region showed only 78%. Phylogenetic analysis was done based on glycoprotein precursor and on M RNA intergenic region of tospoviruses and parameters on tospovirus taxonomic classification were discussed.

RT-PCR and Dot Blot hybridization methods for a universal detection of tospoviruses

Transcriptase reverse - polymerase chain reaction (RT-PCR) and dot blot hybridization with digoxigenin-labeled probes were applied for the universal detection of Tospovirus species. The virus species tested were Tomato spotted wilt virus, Tomato chlorotic spot virus, Groundnut ringspot virus, Chrysanthemum stem necrosis virus, Impatiens necrotic spot virus, Zucchini lethal chlorosis virus, Iris yellow spot virus. Primers for PCR amplification were designed to match conserved regions of the tospovirus genome. RT-PCR using distinct primer combinations was unable to simultaneously amplify all tospovirus species and consistently failed to detect ZLCV and IYSV in total RNA extracts. However, all tospovirus species were detected by RT-PCR when viral RNA was used as template. RNA-specific PCR products were used as probes for dot hybridization. This assay with a M probe (directed to the G1/G2 gene) detected at low stringency conditions all Tospovirus species, except IYSV. At low stringency conditions, the L non-radioactive probe detected the seven Tospovirus species in a single assay. This method for broad spectrum detection can be potentially employed in quarantine services for indexing in vitro germplasm.

Tomato spotted wilt virus-positive steps towards negative success

Abstract Taxonomy: Tomato spotted wilt virus (TSWV) is the type member of the plant-infecting Tospovirus genus in the family Bunyaviridae, a large group of predominantly vertebrate- and insect-infecting RNA viruses. Physical properties: Virions are 80-120-nm pleomorphic particles with surface projections composed of two viral glycoproteins, G1 and G2 (Fig. 1). Virion composition is 5% nucleic acid, 70% protein, 5% carbohydrate and 20% lipid. The genome consists of three negative or ambisense ssRNAs designated S (2.9 kb), M (4.8 kb) and L (8.9 kb), with partially complementary terminal sequences that allow the RNA to adopt a pseudocircular or panhandle conformation. Each genomic RNA is encapsidated by multiple copies of the viral nucleocapsid (N) protein to form ribonucleoprotein structures also known as nucleocapsids. The nucleocapsids are enclosed in a host-derived membrane bilayer along with an estimated 10-20 copies of the L protein, the putative RNA-dependent RNA polymerase. Hosts: Over 800 plant species, both dicots and monocots, in more than 80 plant families are susceptible to TSWV (Goldbach and Peters, 1994). The Solanaceae and Compositae families contain the largest numbers of susceptible plant species (Prins and Kormelink, 1998). TSWV also replicates in its insect vector, thrips (Thysanoptera: Thripidae) (Ullman et al., 1993; Wijkamp et al., 1993). Useful web site: http://www4.ncbi.nlm.nih.gov/ICTVdb/ICTVdB/11050003.htm.