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

Transmission blockage of an orthotospovirus using synthetic peptides

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

Soybean vein necrosis virus: an emerging virus in North America

Few diseases have emerged in such a short period of time as soybean vein necrosis. The disease is present in all major producing areas in North America, affecting one of the major row field instead of row crops for the United States. Because of the significance of soybean in the agricultural economy and the widespread presence of the disease, the causal agent, soybean vein necrosis virus has been studied by several research groups. Research in the past 10 years has focused on virus epidemiology, management, and effects on yield and seed quality. This communication provides a review of the current knowledge on the virus and the disease.

Alstroemeria yellow spot virus (AYSV): a new orthotospovirus species within a growing Eurasian clade

An orthotospovirus distinct from all other orthotospoviruses was isolated from naturally infected alstroemeria plants. Disease symptoms caused by this virus mainly consisted of yellow spots on the leaves based on which the name alstroemeria yellow spot virus (AYSV) was coined. A host range analysis was performed and a polyclonal antiserum was produced against purified AYSV ribonucleoproteins which only reacted with the homologous antigen and not with any other (established or tentative) orthotospovirus from a selection of American and Asian species. Upon thrips transmission assays the virus was successfully transmitted by a population of Thrips tabaci. The entire nucleotide sequence of the M and S RNA segments was elucidated by a conventional cloning and sequencing strategy, and contained 4797 respectively 2734 nucleotides (nt). Simultaneously, a next generation sequencing (NGS) approach (RNAseq) was employed and generated contigs covering the entire viral tripartite RNA genome. In addition to the M and S RNA nucleotide sequences, the L RNA (8865 nt) was obtained. The nucleocapsid (N) gene encoded by the S RNA of this virus consisted of 819 nucleotides with a deduced N protein of 272 amino acids and by comparative sequence alignments to other established orthotospovirus species showed highest homology (69.5% identity) to the N protein of polygonum ringspot virus. The data altogether support the proposal of AYSV as a new orthotospovirus species within a growing clade of orthotospoviruses that seem to share the Middle East basin as a region of origin.

Characterization of the first complete genome sequence of an Impatiens necrotic spot orthotospovirus isolate from the United States and worldwide phylogenetic analyses of INSV isolates

OBJECTIVE

Impatiens necrotic spot orthotospovirus (INSV) can impact economically important ornamental plants and vegetables worldwide. Characterization studies on INSV are limited. For most INSV isolates, there are no complete genome sequences available. This lack of genomic information has a negative impact on the understanding of the INSV genetic diversity and evolution. Here we report the first complete nucleotide sequence of a US INSV isolate.

RESULTS

INSV-UP01 was isolated from an impatiens in Pennsylvania, US. RT-PCR was used to clone its full-length genome and Vector NTI to assemble overlapping sequences. Phylogenetic trees were constructed by using MEGA7 software to show the phylogenetic relationships with other available INSV sequences worldwide. This US isolate has genome and biological features classical of INSV species and clusters in the Western Hemisphere clade, but its origin appears to be recent. Furthermore, INSV-UP01 might have been involved in a recombination event with an Italian isolate belonging to the Asian clade. Our analyses support that INSV isolates infect a broad plant-host range they group by geographic origin and not by host, and are subjected to frequent recombination events. These results justify the need to generate and analyze complete genome sequences of orthotospoviruses in general and INSV in particular.

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.

Full-length M and L RNA sequences of tospovirus isolate 2009-GZT, which causes necrotic ringspot on tomato in China

The virus isolate 2009-GZT, collected from tomato in Guizhou province of China, was identified as a new member of the genus Tospovirus based on its S RNA sequence. Because its provisional name, "tomato necrotic spot virus" (TNSV), was identical to an already existing member of the genus Ilarvirus, 2009-GZT was renamed "tomato necrotic spot-associated virus" (TNSaV). In this study, the full-length sequences of the genomic M and L RNAs of TNSaV were determined and analyzed. The M RNA has 4,773 nucleotides (nt), encoding the NSm protein of 309 aa (34.4 kDa) in the viral (v) strand and the glycoprotein precursor (Gn/Gc) of 1123 aa (128 kDa) in the viral complementary (vc) strand. The NSm and Gn/Gc of TNSaV share the highest aa sequence identity (86.2 % and 86.9 %, respectively) with those of tomato zonate spot virus. The L RNA contains 8,908 nt and codes for the putative RNA-dependent RNA polymerase (RdRp) of 2885 aa (332 kDa) in the vc strand. The RdRp of TNSaV shares the highest aa sequence identity (85.2 %) with that of calla lily chlorotic spot virus (CCSV). Serological assays showed that TNSaV cross-reacts with rabbit antisera against watermelon silver mottle virus (WSMoV) NP and CCSV NP, indicating that TNSaV is a member of the WSMoV serogroup.

Complete Genome Sequence of Mulberry Vein Banding Associated Virus, a New Tospovirus Infecting Mulberry

Mulberry vein banding associated virus (MVBaV) that infects mulberry plants with typical vein banding symptoms had been identified as a tentative species of the genus Tospovirus based on the homology of N gene sequence to those of tospoviruses. In this study, the complete sequence of the tripartite RNA genome of MVBaV was determined and analyzed. The L RNA has 8905 nucleotides (nt) and encodes the putative RNA-dependent RNA polymerase (RdRp) of 2877 aa amino acids (aa) in the viral complementary (vc) strand. The RdRp of MVBaV shares the highest aa sequence identity (85.9%) with that of Watermelon silver mottle virus (WSMoV), and contains conserved motifs shared with those of the species of the genus Tospovirus. The M RNA contains 4731 nt and codes in ambisense arrangement for the NSm protein of 309 aa in the sense strand and the Gn/Gc glycoprotein precursor (GP) of 1,124 aa in the vc strand. The NSm and GP of MVBaV share the highest aa sequence identities with those of Capsicum chlorosis virus (CaCV) and Groundnut bud necrosis virus (GBNV) (83.2% and 84.3%, respectively). The S RNA is 3294 nt in length and contains two open reading frames (ORFs) in an ambisense coding strategy, encoding a 439-aa non-structural protein (NSs) and the 277-aa nucleocapsid protein (N), respectively. The NSs and N also share the highest aa sequence identity (71.1% and 74.4%, respectively) with those of CaCV. Phylogenetic analysis of the RdRp, NSm, GP, NSs, and N proteins showed that MVBaV is most closely related to CaCV and GBNV and that these proteins cluster with those of the WSMoV serogroup, and that MVBaV seems to be a species bridging the two subgroups within the WSMoV serogroup of tospoviruses in evolutionary aspect, suggesting that MVBaV represents a distinct tospovirus. Analysis of S RNA sequence uncovered the highly conserved 5’-/3’-ends and the coding regions, and the variable region of IGR with divergent patterns among MVBaV isolates.

The complete nucleotide sequence of chrysanthemum stem necrosis virus

The complete genome sequence of chrysanthemum stem necrosis virus (CSNV) was determined using Roche 454 next-generation sequencing. CSNV is a tentative member of the genus Tospovirus within the family Bunyaviridae, whose members are arthropod-borne. This is the first report of the entire RNA genome sequence of a CSNV isolate. The large RNA of CSNV is 8955 nucleotides (nt) in size and contains a single open reading frame of 8625 nt in the antisense arrangement, coding for the putative RNA-dependent RNA polymerase (L protein) of 2874 aa with a predicted Mr of 331 kDa. Two untranslated regions of 397 and 33 nt are present at the 5' and 3' termini, respectively. The medium (M) and small (S) RNAs are 4830 and 2947 nt in size, respectively, and show 99 % identity to the corresponding genomic segments of previously partially characterized CSNV genomes. Protein sequences for the precursor of the Gn/Gc proteins, N and NSs, are identical in length in all of the analysed CSNV isolates.

Broad-spectrum transgenic resistance against distinct tospovirus species at the genus level

Thrips-borne tospoviruses cause severe damage to crops worldwide. In this investigation, tobacco lines transgenic for individual WLm constructs containing the conserved motifs of the L RNA-encoded RNA-dependent RNA polymerase (L) gene of Watermelon silver mottle virus (WSMoV) were generated by Agrobacterium-mediated transformation. The WLm constructs included: (i) translatable WLm in a sense orientation; (ii) untranslatable WLmt with two stop codons; (iii) untranslatable WLmts with stop codons and a frame-shift; (iv) untranslatable antisense WLmA; and (v) WLmhp with an untranslatable inverted repeat of WLm containing the tospoviral S RNA 3'-terminal consensus sequence (5'-ATTGCTCT-3') and an NcoI site as a linker to generate a double-stranded hairpin transcript. A total of 46.7-70.0% transgenic tobacco lines derived from individual constructs showed resistance to the homologous WSMoV; 35.7-100% plants of these different WSMoV-resistant lines exhibited broad-spectrum resistance against four other serologically unrelated tospoviruses Tomato spotted wilt virus, Groundnut yellow spot virus, Impatiens necrotic spot virus and Groundnut chlorotic fan-spot virus. The selected transgenic tobacco lines also exhibited broad-spectrum resistance against five additional tospoviruses from WSMoV and Iris yellow spot virus clades, but not against RNA viruses from other genera. Northern analyses indicated that the broad-spectrum resistance is mediated by RNA silencing. To validate the L conserved region resistance in vegetable crops, the constructs were also used to generate transgenic tomato lines, which also showed effective resistance against WSMoV and other tospoviruses. Thus, our approach of using the conserved motifs of tospoviral L gene as a transgene generates broad-spectrum resistance against tospoviruses at the genus level.

Tospovirus ambisense genomic RNA segments use almost complete repertoire of stable tetraloops in the intergenic region

The intergenic regions of the ambisense RNA segments of viruses from the Tospovirus genus form large extended RNA structures that regulate virus replication. Using comparative structure analysis, we show the presence of conserved alternative conformations at the apical parts of these structures. In one conformation, a branched Y-shape, the 5'-proximal hairpin arms are mostly capped by exceptionally stable tetraloop motifs. The tetraloop hairpins are folded in both virus and virus-complementary sense RNAs, and different tetraloops can functionally replace each other. Folding simulations show that the branched Y-shape structures can undergo a conformational transition to alternative extended rod-like conformations. Functional importance of both alternatives is supported by nucleotide covariations. The balanced equilibrium between alternative structures is evidenced by native gel electrophoresis of mutant RNA transcripts with shifted equilibria. The tetraloops play a role in the stability and dynamics of structures but may also be recognized by proteins involved in translation and/or replication.