An assemblage of closteroviruses infects Hawaiian ti (Cordyline fruticosa L.)

Molecular characterization of Cordyline virus 1 isolates infecting yam (Dioscorea spp)

Cordyline virus 1 (CoV1) is a velarivirus that has so far only been reported in ornamental Ti plants (Cordyline fruticosa). Using high-throughput sequencing, we identified CoV1 infection in yam accessions from Vanuatu. Using a specific RT-PCR assay, we found that CoV1 is also present and highly prevalent in Dioscorea alata, D. cayenensis, and D. trifida in Guadeloupe. Phylogenetic analysis showed that CoV1 isolates infecting yam in Guadeloupe display a low level of molecular diversity. These data provide insights into the transmission of CoV1 in yam in Guadeloupe.

Prevalence of Yellow Leaf Disease (YLD) and its Associated Areca Palm Velarivirus 1 (APV1) in Betel Palm (Areca catechu) Plantations in Hainan, China

Yellow leaf disease (YLD) is an economically important disease affecting betel palm in several countries, the cause of which remains unclear despite associations with putative agents, including phytoplasmas. In this study, we screened the potential casual agents associated with YLD in Hainan, China using next-generation sequencing and revealed the association of areca palm velarivirus 1 (APV1) with the YLD-affected palm. The complete genome of the APV1-WNY isolate was determined to be 17,546 nucleotides in length, approximately 1.5 kb longer than the previously reported APV1_HN genome. Transmission electron microscopy showed that APV1 particles are flexuous and filamentous, a typical morphology of species in the Closteroviridae family. Comparison of symptomatic and symptomless tree populations showed a strong association between APV1 and YLD. APV1 was detected in Pseudococcus sp. mealybugs sampled from YLD-affected trees in many locations, suggesting that mealybugs are a potential transmission vector for APV1. Although further studies are needed to confirm a causal relationship, these results provide timely information for the prevention and management of YLD associated with APV1.

Mixed infecting of grapevine with viruses in the commercial vineyards of the Crimean Peninsula

Viral infections cause the reduction of yields and efficiency of vine cultivation in all viticultural regions of the world. Simultaneous infecting with several viruses, the so-called mixed infection, can significantly increase the severity of symptoms. The article presents the results of study of mixed infecting of grapevine with viruses in the vineyards of the Crimean Peninsula during 2011-2020. More than 800 grapevine samples of 153 varieties selected in 23 vineyards of the Crimea were analyzed over the entire period of study. Presence of phytopathogenic viruses in the samples was determined by RT-PCR using virus specific primers. The share of 25.5 % of all the samples tested gave positive result for one or more of the following viruses: GFLV, GLRaV-1, GLRV-2, GLRaV-3, GFkV, GVA, GRSPaV, ArMV, GFLV. During the process of studyno vines affected by GVB virus were identified. Single-agent infection was found in 20% of the examined vines. The amount of vines affected by mixed infection was 5.48 % of all tested. In a mixed viral infection more common combinations are: GRSPaV/GFkV; GVA/GRSPaV; GVA/GLRaV-1/GRSPaV; GVA/GLRaV-3/GRSPaV. Vines were infected the most with GRSPaV and GFkV viruses, which is visually manifested in the form of mosaic leaf variegation, leaf chlorosis and the appearance of necrotic spots.

Characterization of Ti Ringspot-Associated Virus, a Novel Emaravirus Associated with an Emerging Ringspot Disease of Cordyline fruticosa

Ti ringspot is an emerging foliar disease of the ti plant (Cordyline fruticosa) in Hawaii that is quickly spreading throughout the islands. Symptoms include small chlorotic ringspots on leaves that often coalesce to form larger lesions. Although several virus species have been discovered in symptomatic plants, none have been associated with these symptoms. Here, we report and characterize a novel virus closely associated with ti ringspot symptoms in Hawaii. The presence of double membrane bodies approximately 85 nm in diameter in symptomatic cells and sequence analyses of five genomic RNA segments obtained by high-throughput sequencing indicate that this virus is most closely related to members of the plant virus genus Emaravirus. Phylogenetic and sequence homology analyses place this virus on a distinct clade within the Emaravirus genus along with High Plains wheat mosaic emaravirus, blue palo verde broom virus, and Raspberry leaf blotch emaravirus. Sequence identity values with taxonomically relevant proteins indicate that this represents a new virus species, which we are tentatively naming ti ringspot-associated virus (TiRSaV). TiRSaV-specific reverse transcription PCR assays detected the virus in several experimental herbaceous host species following mechanical inoculation. TiRSaV was also detected in eriophyid mites collected from symptomatic ti plants, which may represent a putative arthropod vector of the virus.

Strawberry Mottle Virus (Family Secoviridae, Order Picornavirales) Encodes a Novel Glutamic Protease To Process the RNA2 Polyprotein at Two Cleavage Sites

Strawberry mottle virus (SMoV) belongs to the family Secoviridae (order Picornavirales) and has a bipartite genome with each RNA encoding one polyprotein. All characterized secovirids encode a single protease related to the picornavirus 3C protease. The SMoV 3C-like protease was previously shown to cut the RNA2 polyprotein (P2) at a single site between the predicted movement protein and coat protein (CP) domains. However, the SMoV P2 polyprotein includes an extended C-terminal region with a coding capacity of up to 70 kDa downstream of the presumed CP domain, an unusual characteristic for this family. In this study, we identified a novel cleavage event at a P↓AFP sequence immediately downstream of the CP domain. Following deletion of the PAFP sequence, the polyprotein was processed at or near a related PKFP sequence 40 kDa further downstream, defining two protein domains in the C-terminal region of the P2 polyprotein. Both processing events were dependent on a novel protease domain located between the two cleavage sites. Mutagenesis of amino acids that are conserved among isolates of SMoV and of the related Black raspberry necrosis virus did not identify essential cysteine, serine, or histidine residues, suggesting that the RNA2-encoded SMoV protease is not related to serine or cysteine proteases of other picorna-like viruses. Rather, two highly conserved glutamic acid residues spaced by 82 residues were found to be strictly required for protease activity. We conclude that the processing of SMoV polyproteins requires two viral proteases, the RNA1-encoded 3C-like protease and a novel glutamic protease encoded by RNA2.IMPORTANCE Many viruses encode proteases to release mature proteins and intermediate polyproteins from viral polyproteins. Polyprotein processing allows regulation of the accumulation and activity of viral proteins. Many viral proteases also cleave host factors to facilitate virus infection. Thus, viral proteases are key virulence factors. To date, viruses with a positive-strand RNA genome are only known to encode cysteine or serine proteases, most of which are related to the cellular papain, trypsin, or chymotrypsin proteases. Here, we characterize the first glutamic protease encoded by a plant virus or by a positive-strand RNA virus. The novel glutamic protease is unique to a few members of the family Secoviridae, suggesting that it is a recent acquisition in the evolution of this family. The protease does not resemble known cellular proteases. Rather, it is predicted to share structural similarities with a family of fungal and bacterial glutamic proteases that adopt a lectin fold.

Biotechnology Applications of Grapevine Viruses

Plant virus genomes are engineered as vectors for functional genomics and production of foreign proteins. The application of plant virus vectors is of potential interest to the worldwide, multibillion dollar, grape and wine industries. These applications include grapevine functional genomics, pathogen control, and production of beneficial proteins such as vaccines and enzymes. However, grapevine virus biology exerts certain limitations on the utility of the virus-derived gene expression and RNA interference vectors. As is typical for viruses infecting woody plants, several grapevine viruses exhibit prolonged infection cycles and relatively low overall accumulation levels, mainly because of their phloem-specific pattern of systemic infection. Here we consider the biotechnology potential of grapevine virus vectors with a special emphasis on members of the families Closteroviridae and Betaflexiviridae.

Genomic detection and characterization of a Korean isolate of Little cherry virus 1 sampled from a peach tree

A peach tree (Prunus persica) showing yellowing and mild mottle symptoms was analyzed using high-throughput RNA sequencing to determine the causal agent. A total of nine contigs similar to Little cherry virus 1 (LChV-1) were produced, and all the contigs showed nucleotide sequence identity (lower than 83 %) and query coverage (higher than 73 %) with LChV-1. The symptomatic peach sample was confirmed to be infected with LChV-1-like virus as a result of reverse transcription-polymerase chain reaction using primers designed based on sequences of the contigs. Occurrence of diseases caused by LChV-1 in Prunus species has been reported. Complete 16,931-nt genome of the peach virus composed of eight open reading frames was determined, and conserved domains including viral methyltransferase, viral helicase 1, RNA-dependent RNA polymerase (RdRp), heat shock protein 70 homologue (HSP70h), HSP90h and closterovirus coat protein (CP) were identified. Phylogenetic trees based on amino acid sequence alignments between the peach virus and members in the family Closteroviridae showed that the virus was most similar to LChV-1. Pairwise comparisons based on amino acid sequence alignments of three genes (RdRp, HSP70h and CP) between the peach virus and LChV-1 isolates showed the highest amino acid sequence identities, with 84.32 % for RdRp, 85.48 % for HSP70h and 80.45 % for CP. These results indicate that this is the first report for the presence of LChV-1 in South Korea and may be one of the first reports of natural infection of peach by LChV-1. Although it is not clear if LChV-1 YD isolate was responsible for specific symptoms observed, detection and characterization of the peach tree-infecting LChV-1 in South Korea would be useful in terms of the epidemiology of LChV-1.

Differentiation and distribution of cordyline viruses 1-4 in Hawaiian ti plants (Cordyline fruticosa L.)

Common green ti plants (Cordyline fruticosa L.) in Hawaii can be infected by four recently characterized closteroviruses that are tentative members of the proposed genus Velarivirus. In this study, a reverse-transcription polymerase chain reaction (RT-PCR) assay developed to detect and distinguish Cordyline virus 1 (CoV-1), CoV-2, CoV-3, and CoV-4 was used to determine: (i) the distribution of these viruses in Hawaii; and (ii) if they are involved in the etiology of ti ringspot disease. One hundred and thirty-seven common green ti plants with and without ti ringspot symptoms were sampled from 43 sites on five of the Hawaiian Islands and underwent the RT-PCR assay. Eleven ornamental ti varieties were also sampled and assayed. Based on this survey, it appears none of the CoVs are involved in the etiology of ti ringspot. The observation of a non-uniform geographic distribution of the CoVs in common green ti, combined with the presence of CoVs in seed-derived ornamental varieties, suggests active vector transmission. Eight herbarium specimens collected between 1903 and 2003 from plants on the island of Oahu also underwent the RT-PCR assay. Amplifiable RNA was isolated from accessions collected in 1985 or later, however only the 2003 accession was found to harbor CoVs.

Molecular characterization of closteroviruses infecting Cordyline fruticosa L. in Hawaii

In Hawaii, common green ti plants (Cordyline fruticosa L.) have been shown to harbor Cordyline virus 1 (CoV-1) which, along with Little cherry virus 1 (LChV-1), and Grapevine leafroll-associated virus 7 (GLRaV-7), form a distinct clade within the family Closteroviridae. Preliminary work has indicated that, aside from CoV-1, three additional closteroviruses may infect common green ti plants in Hawaii. In this study, pyrosequencing was used to characterize the genomes of closteroviruses infecting a single common green ti plant. The sequence data confirmed the presence of CoV-1 as well as three additional closteroviruses. Although all four viruses had the same general genome organization, the sequence divergence between the RNA-dependent RNA polymerase, heat shock protein 70 homolog, and coat protein ranged from 22 to 61%, indicating these represent four distinct closterovirus species. The names CoV-2, CoV-3, and CoV-4 are proposed for the three new viruses. Phylogenetic analyses placed CoV-2, CoV-3, and CoV-4 in the same clade as CoV-1, LChV-1, and GLRaV-7.

Genomic and biological analysis of Grapevine leafroll-associated virus 7 reveals a possible new genus within the family Closteroviridae

Deep sequencing analysis of an asymptomatic grapevine revealed a virome containing five RNA viruses and a viroid. Of these, Grapevine leafroll-associated virus 7 (GLRaV-7), an unassigned closterovirus, was by far the most prominently represented sequence in the analysis. Graft-inoculation of the infection to another grape variety confirmed the lack of the leafroll disease symptoms, even though GLRaV-7 could be detected in the inoculated indicator plants. A 16,496 nucleotide-long genomic sequence of this virus was determined from the deep sequencing data. Its genome architecture and the sequences encoding its nine predicted proteins were compared with those of other closteroviruses. The comparison revealed that two other viruses, Little cherry virus-1 and Cordyline virus-1 formed a well supported phylogenetic cluster with GLRaV-7.

Molecular characterization and taxonomy of grapevine leafroll-associated virus 7

The complete nucleotide sequence of an Albanian isolate of grapevine leafroll-associated virus 7 (GLRaV-7-Alb) was determined. The viral genome consists of 16,404 nucleotides and has nine open reading frames (ORFs) that potentially encode proteins, most of which are typical for members of the family Closteroviridae. Only the 25-kDa (ORF8) and 27-kDa (ORF9) proteins had no apparent similarity to other viral proteins in the sequence databases. The genome structure of GLRaV-7-Alb closely resembles that of little cherry virus 1 and cordyline virus 1. In phylogenetic trees constructed with HSP70h sequences, these three viruses cluster together in a clade next to that comprising members of the genus Crinivirus, to which they are more closely related than to the clostero- and ampeloviruses. The molecular properties of these three viruses differ sufficiently from those of members of the three extant genera of the family Closteroviridae to warrant their classification in a novel genus.