Genome sequences and structures of two biologically distinct strains of Grapevine leafroll-associated virus 2 and sequence analysis

A Chronological Study on Grapevine Leafroll-Associated Virus 2 in Australia

Grapevine leafroll disease affects the health status of grapevines worldwide. Most studies in Australia have focused on grapevine leafroll-associated viruses 1 and 3, while little attention has been given to other leafroll virus types, in particular, grapevine leafroll-associated virus 2 (GLRaV-2). A chronological record of the temporal occurrence of GLRaV-2 in Australia since 2001 is reported. From a total of 11,257 samples, 313 tested positive, with an overall incidence of 2.7%. This virus has been detected in 18 grapevine varieties and Vitis rootstocks in different regions of Australia. Most varieties were symptomless on their own roots, while Chardonnay showed a decline in virus-sensitive rootstocks. An isolate of GLRaV-2, on own-rooted Vitis vinifera cv. Grenache, clone SA137, was associated with severe leafroll symptoms after veraison with abnormal leaf necrosis. The metagenomic sequencing results of the virus in two plants of this variety confirmed the presence of GLRaV-2, as well as two inert viruses, grapevine rupestris stem pitting-associated virus (GRSPaV) and grapevine rupestris vein feathering virus (GRVFV). No other leafroll-associated viruses were detected. Among the viroids, hop stunt viroid and grapevine yellow speckle viroid 1 were detected. Of the six phylogenetic groups identified in GLRaV-2, we report the presence of four groups in Australia. Three of these groups were detected in two plants of cv. Grenache, without finding any recombination event. The hypersensitive reaction of certain American hybrid rootstocks to GLRaV-2 is discussed. Due to the association of GLRaV-2 with graft incompatibility and vine decline, the risk from this virus in regions where hybrid Vitis rootstocks are used cannot be overlooked.

Discovery of a Closterovirus Infecting Jujube Plants Grown at Aksu Area in Xinjiang of China

Chinese jujube (Ziziphus jujuba Mill.) is a widely grown fruit crop at Aksu in Xinjiang Uygur Autonomous Region of China. Viral disease-like symptoms are common on jujube plants. Here, for the first time, we report a virus tentatively named persimmon ampelovirus jujube isolate (PAmpV-Ju) infecting jujube plants. The virus was identified using high-throughput sequencing from a jujube plant (ID: AKS15) and molecularly related to viruses in the family Closteroviridae. The genomic sequences of two PAmpV-Ju variants named AKS15-20 and AKS15-17 were determined by RT-PCR amplifications. The genome structure of PAmpV-Ju was identical to that of a recently reported persimmon ampelovirus (PAmpV) and consisted of seven open reading frames. The genomes of AKS15-20 and AKS15-17 shared 83.7% nt identity with each other, and the highest nt sequence identity of 79% with two variants of PAmpV. The incidence of PAmpV-Ju on Aksu jujube plants was evaluated by RT-PCR assays. The phylogenetic analysis of amplified partial sequences coding for polymerase, HSP70h, and CP revealed two phylogenetic clades represented by AKS15-20 and AKS15-17. Our study provides important evidence for understanding viruses infecting jujube plants and establishing efficient measures to prevent virus spread.

High-Throughput Sequencing of Small RNAs for the Sanitary Certification of Viruses in Grapevine

Biological indexing is the method generally recognized for the certification of propagative grapevines in many countries, and it is mandatory in the European Union. It consists of the evaluation of the plant material after grafting on indicators that are inspected for symptom development. This is a lengthy process that requires well-trained workers, testing field, etc. Alternative diagnostic methods such as serology and RT-qPCR have been discarded for certification because of their intrinsic drawbacks. In turn, high-throughput sequencing (HTS) of plant RNA has been proposed as a plausible alternative to bioassay, but before it is accepted, different aspects of this process must be evaluated. We have compared the HTS of small RNAs with bioassays and other diagnostic methods from a set of 40 grapevine plants submitted for certification. The results allowed the authors the identification of numerous grapevine viruses in the samples, as well as different variants. Besides, relationships between symptom expression and viromes were investigated, in particular leafroll-associated viruses. We compared HTS results using analytical and bioinformatics approaches in order to define minimum acceptable quality standards for certification schemes, resulting in a pipeline proposal. Finally, the comparison between HTS and bioassay resulted favorable for the former in terms of reliability, cost, and timing.

Probing into the Effects of Grapevine Leafroll-Associated Viruses on the Physiology, Fruit Quality and Gene Expression of Grapes

Grapevine leafroll is one of the most widespread and highly destructive grapevine diseases that is responsible for great economic losses to the grape and wine industries throughout the world. Six distinct viruses have been implicated in this disease complex. They belong to three genera, all in the family Closteroviridae. For the sake of convenience, these viruses are named as grapevine leafroll-associated viruses (GLRaV-1, -2, -3, -4, -7, and -13). However, their etiological role in the disease has yet to be established. Furthermore, how infections with each GLRaV induce the characteristic disease symptoms remains unresolved. Here, we first provide a brief overview on each of these GLRaVs with a focus on genome structure, expression strategies and gene functions, where available. We then provide a review on the effects of GLRaV infection on the physiology, fruit quality, fruit chemical composition, and gene expression of grapevine based on the limited information so far reported in the literature. We outline key methodologies that have been used to study how GLRaV infections alter gene expression in the grapevine host at the transcriptomic level. Finally, we present a working model as an initial attempt to explain how infections with GLRaVs lead to the characteristic symptoms of grapevine leafroll disease: leaf discoloration and downward rolling. It is our hope that this review will serve as a starting point for grapevine virology and the related research community to tackle this vastly important and yet virtually uncharted territory in virus-host interactions involving woody and perennial fruit crops.

Next-Generation Sequencing Combined With Conventional Sanger Sequencing Reveals High Molecular Diversity in Actinidia Virus 1 Populations From Kiwifruit Grown in China

Kiwifruit (Actinidia spp.) is native to China. Viral disease–like symptoms are common on kiwifruit plants. In this study, six libraries prepared from total RNA of leaf samples from 69 kiwifruit plants were subjected to next-generation sequencing (NGS). Actinidia virus 1 (AcV-1), a tentative species in the family Closteroviridae, was discovered in the six libraries. Two full-length and two near-full genome sequences of AcV-1 variants were determined by Sanger sequencing. The genome structure of these Chinese AcV-1 variants was identical to that of isolate K75 and consisted of 12 open reading frames (ORFs). Analyses of these sequences together with the NGS-derived contig sequences revealed high molecular diversity in AcV-1 populations, with the highest sequence variation occurring at ORF1a, ORF2, and ORF3, and the available variants clustered into three phylogenetic clades. For the first time, our study revealed different domain compositions in the viral ORF1a and molecular recombination events among AcV-1 variants. Specific reverse transcriptase–polymerase chain reaction assays disclosed the presence of AcV-1 in plants of four kiwifruit species and unknown Actinidia spp. in seven provinces and one city.

Expression of the p24 silencing suppressor of Grapevine leafroll-associated virus 2 from Potato virus X or Barley stripe mosaic virus vector elicits hypersensitive responses in Nicotiana benthamiana

The 24-kDa protein (p24) encoded by Grapevine leafroll-associated virus 2 (GLRaV-2) is an RNA-silencing suppressor (RSS), but its effect on active viral infection is unclear. Using a Potato virus X (PVX)-based expression system, we demonstrated that p24 elicits lethal systemic necrosis in Nicotiana benthamiana, sharing typical characteristics of the hypersensitive response (HR), and that NbRAR1 (a cytoplasmic Zn2+-binding protein) is involved in the PVX-p24-mediated systemic necrosis. Moreover, expression of p24 from Barley stripe mosaic virus (BSMV) vector triggered local necrosis in infiltrated patches of N. benthamiana, likely inhibiting viral systemic spread. By deletion analysis, we demonstrated that amino acids (aa) 1 to 180, which are located in the region (aa 1-188) previously shown to be necessary for p24's RSS activity, is sufficient for p24 to elicit systemic necrosis in the context of PVX infection. Using substitution mutants, we revealed that silencing-suppression-defective mutants R2A and W54A induce only a mild necrotic response; two mutants without self-interaction ability previously shown to lose or retain weak suppression function also displayed decreased pathogenicity: W149A without RSS activity elicited a mild necrotic response, whereas V162H/L169H/L170H which retains weak RSS activity was able to induce systemic necrosis, but with a 1- to 2-day delay. Taken together, p24 plays an important role in GLRaV-2 pathogenesis, triggering HR-like necrosis in N. benthamiana plants when expressed from PVX or BSMV vector; both the silencing suppression and self-interaction are crucial for p24's pathogenicity activity, and the region of p24 for determining systemic necrosis is mapped to aa 1-180.

Characterization of silencing suppressor p24 of Grapevine leafroll-associated virus 2

Grapevine leafroll-associated virus 2 (GLRaV-2) p24 has been reported to be an RNA silencing suppressor (RSS). However, the mechanisms underlying p24's suppression of RNA silencing are unknown. Using Agrobacterium infiltration-mediated RNA silencing assays, we showed that GLRaV-2 p24 is a strong RSS triggered by positive-sense green fluorescent protein (GFP) RNA, and that silencing suppression by p24 effectively blocks the accumulation of small interfering RNAs. Deletion analyses showed that the region of amino acids 1-188, which contains all predicted α-helices and β-strands, is required for the RSS activity of p24. Hydrophobic residues I35/F38/V85/V89/W149 and V162/L169/L170, previously shown to be critical for p24 self-interaction, are also crucial for silencing suppression, and western blotting results suggested that a lack of self-interaction ability results in decreased p24 accumulation in plants. The mutants showed greatly weakened or a lack of RSS activity. Substitution with two basic residues at positions 2 or 86, putatively involved in RNA binding, totally abolished the RSS activity of p24, suggesting that p24 uses an RNA-binding strategy to suppress RNA silencing. Our results also showed that W54 in the WG/GW-like motif (W54/G55) is crucial for the RSS activity of p24, whereas p24 does not physically interact with AGO1 of Nicotiana benthamiana. Furthermore, p24 did not promote AGO1 degradation, but significantly up-regulated AGO1 mRNA expression, and this effect was correlated with the RSS activity of p24, indicating that p24 may interfere with microRNA-directed processes. The presented results contribute to our understanding of viral suppression of RNA silencing and the molecular mechanisms underlying GLRaV-2 infection.

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.

Critical regions and residues for self-interaction of grapevine leafroll-associated virus 2 protein p24

The 24-kDa protein (p24) encoded by grapevine leafroll-associated virus 2 (GLRaV-2) is an RNA-silencing suppressor. In this work, a yeast two-hybrid system (YTHS) and bimolecular fluorescence complementation analyses showed that GLRaV-2 p24 can interact with itself, and that this interaction occurs in the cytoplasm of Nicotiana benthamiana cells. To identify the functional region(s) and crucial amino acid residues required for p24 self-interaction, various truncated and substitution mutants were generated. YTHS assay showed that in both homologous pairing and pairing with the wild-type p24, the functional regions mapped to aa 10-180 or 1-170 which contain, respectively, all seven α-helices or the first six α-helices and the N-terminal end (aa 1-9) of the protein. When only the full-length p24 was an interaction partner, the functional region of aa 1-170 could be further mapped to aa 1-140 which contains four α-helices plus most of the fifth α-helix. Further analysis with substitution mutants demonstrated that hydrophobic residues I35/F38/V85/V89/W149 and V162/L169/L170, which may, respectively, mediate the inter-domain interaction of the same p24 monomer and the tail-to-tail association between two p24 counterparts, are crucial for homotypic p24-p24 interaction. In addition, substitution of two basic residues-R2 or R86-of p24, which may play important functional roles in RNA binding, did not seem to affect self-interaction of the mutants in yeast but had obvious effects in plant cells. Taken together, our results demonstrate the functional regions and crucial amino acids for p24 self-interaction.

Phylogenomic analysis reveals deep divergence and recombination in an economically important grapevine virus

The evolutionary history of the exclusively grapevine (Vitis spp.) infecting, grapevine leafroll-associated virus 3 (GLRaV-3) has not been studied extensively, partly due to limited available sequence data. In this study we trace the evolutionary history of GLRaV-3, focussing on isolate GH24, a newly discovered variant. GH24 was discovered through the use of next-generation sequencing (NGS) and the whole genome sequence determined and validated with Sanger sequencing. We assembled an alignment of all 13 available whole genomes of GLRaV-3 isolates and all other publicly available GLRaV-3 sequence data. Using multiple recombination detection methods we identified a clear signal for recombination in one whole genome sequence and further evidence for recombination in two more, including GH24. We inferred phylogenetic trees and networks and estimated the ages of common ancestors of GLRaV-3 clades by means of relaxed clock models calibrated with asynchronous sampling dates. Our results generally confirm previously identified variant groups as well as two new groups (VII and VIII). Higher order groups were defined as supergroups designated A to D. Supergroup A includes variant groups I-V and supergroup B group VI and its related unclassified isolates. Supergroups C and D are less well known, including the newly identified groups VII (including isolate GH24) and VIII respectively. The inferred node ages suggest that the origins of the major groups of GLRaV-3, including isolate GH24, may have occurred prior to worldwide cultivation of grapevines, whilst the current diversity represents closely related isolates that diverged from common ancestors within the last century.

Genome organization, serology and phylogeny of Grapevine leafroll-associated viruses 4 and 6: taxonomic implications

Complete nucleotide sequences of the type isolate of Grapevine leafroll-associated virus 4 (GLRaV-4) and of an isolate of GLRaV-6 from cv 'Estellat' (GLRaV-6Est) were generated and compared mutually and with related viruses. The genome organization of both viruses resembled that of members of Subgroup I in the genus Ampelovirus (fam. Closteroviridae). The availability of these sequences, along with previously existing data on related GLRaVs, allowed critical review of the taxonomy and nomenclature of these viruses. In phylogenetic analyses, GLRaV-4 and -6Est consistently grouped with GLRaV-5, -9, and -Pr forming a poorly resolved sub-cluster ("GLRaV-4 group") within the genus Ampelovirus. In-depth study showed that genetic distances between these viruses do not exceed the intra-species diversity observed in other closteroviruses. In Western blots, partially purified preparations of GLRaVs -4, -5, -6 and -9 reacted only with homologous monoclonal antibodies, but were all recognized by polyclonal antisera to GLRaV-5 and GLRaV-9. Serological relatedness among these viruses was further confirmed in DAS-ELISA. In immuno-electron microscopy, GLRaV-6 particles appeared uniformly decorated with homologous monoclonal antibodies, whereas GLRaV-2, used as a control, showed "bipolar" morphology of the virion. Results of this study challenge taxonomy and nomenclature of several GLRaVs suggesting that they are divergent isolates of Grapevine leafroll-associated virus 4 and not, as has been assumed, distinct species (definitive and/or putative) in the genus Ampelovirus.

Vitis californica and Vitis californica × Vitis vinifera Hybrids are Hosts for Grapevine leafroll-associated virus-2 and -3 and Grapevine virus A and B

Vitis and non-Vitis spp. surrounding nine Napa Valley vineyards were surveyed for Grapevine leafroll-associated virus (GLRaV)-1 to -5 and -9, Grapevine virus A (GVA), Grapevine virus B (GVB), and Grapevine virus D (GVD). Vitis spp. from three riparian areas not adjacent to vineyards were also included. DNA fingerprinting and probability analyses indicated that the Vitis samples consisted primarily of Vitis californica followed by V. californica × V. vinifera hybrids. Single and mixed infections of GLRaV-2, -3, GVA, or GVB were detected by conventional or quantitative reverse-transcription polymerase chain reaction in 6 of the 66 V. californica and 11 of the 19 V. californica × V. vinifera hybrids. GLRaV-1, -4, -5, -9, and GVD were not detected. Phylogenetic analysis of GLRaV-2 and -3 partial coat protein gene nucleotide sequences indicated that the isolates from V. californica and V. californica × V. vinifera hybrids were closely related to isolates from V. vinifera.

Biological, molecular, and serological studies of a novel strain of grapevine leafroll-associated virus 2

In California, a novel closterovirus was detected in "Redglobe" grapevine, associated with graft incompatibility and given a trivial name "Grapevine rootstock stem lesion associated virus (GRSLaV)." The biological properties of the putative virus were ascertained when asymptomatic yet infected Redglobe scion buds were graft-inoculated onto test plants of Cabernet Sauvignon propagated on 18 different rootstocks. It proved lethal on test plants growing on rootstocks 1616C, 5BB, 5C, 3309C, and 1103 P, whereas latent infections occurred on the remaining scion-rootstock combinations. In contrast, GLRaV-2 type (type strain) produced only typical leafroll symptoms. In a different experiment, GLRaV-2 type was successfully sap-transmitted to N. benthamiana, whereas sap transmission of GRSLaV was unsuccessful. Double-stranded RNA was extracted from infected Redglobe grapevines, cloned, sequenced, and determined a genome length of 16,527 nucleotides. Computer-assisted analysis of open-reading frames (ORFs) revealed a genome organization typical of monopartite viruses in the genus Closterovirus with nine ORFs (range 71-79% identity) with GLRaV-2 type, the closest similar virus species within the family Closteroviridae. Also the 3'-UTR of GRSLaV consisted of 223 nucleotides with an extended oligo(A) tract similar to that of GLRaV-2 type, Beet yellow stunt virus, and Beet yellows virus. Recombinant GRSLaV coat protein was expressed in E. coli, purified, and immunized a rabbit to produce polyclonal antiserum. Serological data matched the molecular data, whereby exposed plant tissue extracts of grapevines infected by both viruses (GRSLaV and GLRaV-2) reacted positively with homologous and heterologous viral antisera but not with healthy grapevine extracts in ELISA and Western blot tests. Based on the comparative sequence data and shared antigens, GRSLaV is now considered a strain of GLRaV-2 and redesignated as Grapevine leafroll associated virus-2 Redglobe (GLRaV-2RG). Primers specific for GLRaV-2RG were developed, which did not amplify GLRaV-2 type strain. When both sets of specific primers were used in assays of different grapevine collections, the incidence of the respective viruses varied considerably, e.g., 1.7 and 13.5%, respectively, for GLRaV-2RG and GLRaV-2 type.

Genetic variability of natural populations of Grapevine leafroll-associated virus 2 in Pacific Northwest vineyards

Genetic variability of field populations of Grapevine leafroll-associated virus 2 (GLRaV-2) in Pacific Northwest (PNW) vineyards was characterized by sequencing the entire coat protein (CP) and a portion of the heat-shock protein-70 homolog (HSP70h) genes. Phylogenetic analysis of CP and HSP70h nucleotide sequences obtained in this study and corresponding sequences from GenBank revealed segregation of GLRaV-2 isolates into six lineages with virus isolates from PNW distributed in 'PN', 'H4', and 'RG' lineages. An estimation of the ratio of nonsynonymous substitutions per nonsynonymous site to synonymous substitutions per synonymous site indicated that different selection pressures may be acting on the two genomic regions encoding proteins with distinct functions. Multiple alignments of CP amino acid sequences showed lineage-specific differences. Enzyme-linked immunosorbent assay results indicated that GLRaV-2-specific antibodies from a commercial source are unable to reliably detect GLRaV-2 isolates in the RG lineage, thereby limiting antibody-based diagnosis of all GLRaV-2 isolates currently found in PNW vineyards. A protocol based on reverse-transcription polymerase chain reaction and restriction fragment length polymorphism analysis was developed for differentiating GLRaV-2 isolates belonging to the three lineages present in the region. The taxonomic status of GLRaV-2 is discussed in light of the current knowledge of global genetic diversity of the virus.

Survey for the Three Major Leafroll Disease-Associated Viruses in Finger Lakes Vineyards in New York

Vineyards in the Finger Lakes region in New York were surveyed for the three major viruses associated with leafroll disease, i.e., Grapevine leafroll-associated virus 1 (GLRaV-1), Grapevine leafroll-associated virus 2 (GLRaV-2), and Grapevine leafroll-associated virus 3 (GLRaV-3). Target viruses were detected in nearly two-thirds (68%, 65 of 95) of the vineyard blocks surveyed by enzyme-linked immunosorbent assay. Single infections by GLRaV-1, GLRaV-2, and GLRaV-3 occurred in 10% (113 of 1,124), 3% (36 of 1,124), and 15% (173 of 1,124) of the samples tested, respectively, whereas mixed infections affected 3.6% (40 of 1,124) of them, essentially with GLRaV-1 and GLRaV-3 (2.5%, 28 of 1,124). Presence of the target viruses was confirmed in selected samples by reverse transcription-polymerase chain reaction and sequencing. Comparative analysis indicated moderate to high nucleotide sequence identities in the second diverged copy of the GLRaV-1 coat protein gene (81.0 to 86.7%), GLRaV-2 coat protein gene (87.6 to 99.2%), and GLRaV-3 heat shock protein 70 homologue gene (91.5 to 98.3%) of New York isolates with corresponding virus reference strains. The prevalence of the three major leafroll disease-associated viruses in Finger Lakes vineyards results likely from poor sanitary status of planting materials, stressing the need to reinstate a certification program in New York.

Tandem leader proteases of Grapevine leafroll-associated virus-2: host-specific functions in the infection cycle

Several viruses in the genus Closterovirus including Grapevine leafroll-associated virus-2 (GLRaV-2), encode a tandem of papain-like leader proteases (L1 and L2) whose functional profiles remained largely uncharacterized. We generated a series of the full-length, reporter-tagged, clones of GLRaV-2 and demonstrated that they are systemically infectious upon agroinfection of an experimental host plant Nicotiana benthamiana. These clones and corresponding minireplicon derivatives were used to address L1 and L2 functions in GLRaV-2 infection cycle. It was found that the deletion of genome region encoding the entire L1-L2 tandem resulted in a ~100-fold reduction in minireplicon RNA accumulation. Five-fold reduction in RNA level was observed upon deletion of L1 coding region. In contrast, deletion of L2 coding region did not affect RNA accumulation. It was also found that the autocatalytic cleavage by L2 but not by L1 is essential for genome replication. Analysis of the corresponding mutants in the context of N. benthamiana infection launched by the full-length GLRaV-2 clone revealed that L1 or its coding region is essential for virus ability to establish infection, while L2 plays an accessory role in the viral systemic transport. Strikingly, when tagged minireplicon variants were used for the leaf agroinfiltration of the GLRaV-2 natural host, Vitis vinifera, deletion of either L1 or L2 resulted in a dramatic reduction of minireplicon ability to establish infection attesting to a host-specific requirement for tandem proteases in the virus infection cycle.

Resistance to Grapevine leafroll associated virus-2 is conferred by post-transcriptional gene silencing in transgenic Nicotiana benthamiana

Grapevine leafroll-associated virus-2 (GLRaV-2) is an important component of the leafroll disease complex in grapevine. We have previously sequenced the GLRaV-2 genome and identified the coat protein (CP) gene. The objective of this study is to test the concept of pathogen-derived resistance against a closterovirus associated with grapevine leafroll disease. Because GLRaV-2 is capable of infecting Nicotiana benthamiana, we decided to test the concept on this herbaceous host. Thirty-seven T(0) transgenic N. benthamiana plants expressing the GLRaV-2 CP gene were regenerated following Agrobacterium-mediated transformation. Disease resistance was evaluated in greenhouse-grown T(1) and T(2) plants by mechanical inoculation with GLRaV-2. Although all the inoculated non-transgenic plants showed symptoms 2-4 weeks post inoculation, various numbers of transgenic plants (16-100%) in 14 of 20 T(1) lines tested were not infected. In these resistant plants, GLRaV-2 was not detectable by enzyme linked immunosorbent assay. Although virus resistance was confirmed in T(2) progenies, the percentage of resistant plants was generally lower (0-63%) than that of the corresponding T(1) lines (0-100%). Northern blot and nuclear run-off results showed that virus resistance in the transgenic plants was consistently associated with the low level of transgene RNA transcript suggesting a post-transcriptional gene silencing. The success of pathogen-derived resistance to GLRaV-2 in transgenic N. benthamiana plants represents the first step towards eventual control of the leafroll disease in grapevines using this strategy.

Evolutionary relationships of virus species belonging to a distinct lineage within the Ampelovirus genus

A study of the evolutionary relationships of GLRaV-4,-5,-6 and -9, and two new Ampelovirus isolates (GLRaV-Pr and -De) related to grapevine leafroll disease was conducted based on molecular variability, positive selection analysis and maximum likelihood phylogenetic reconstructions. Sequences corresponding to the N-terminal HSP70h and full CP encoding genes were determined for these viruses and datasets including homologous genomic regions from different members of the Closteroviridae were analyzed. GLRaV-Pr and -De were further characterised as distinct from the other closely related species after determination of a large genomic region (4319-4358 nts). ML phylogenetic topologies for both genes established the closer phylogenetic relationships of GLRaV-4,-5,-6,-9,-Pr and -De in regard to the other ampeloviruses, revealing very low inter-species evolutionary distances for this multitudinous lineage. The HSP70h segment phylogeny and bootstrap analysis enabled the identification of species within this lineage and provides a useful taxonomic tool for the rapid demarcation of these viruses. Estimations of d(N)/d(S) using the CP and HSP70h datasets revealed that, within the Closteroviridae, these viruses are subjected to the strongest constraints against amino acid substitutions. These estimations demonstrated a distinct evolutionary trait for this lineage probably related to its particular ecological niche that involves successful adaptation to the host, transmission through vegetative propagation and lack of vectors with high transmission efficiency.

Identification and characterization of Raspberry mottle virus, a novel member of the Closteroviridae

Raspberry mosaic is one of the most important viral diseases of raspberry. Four virus and virus-like agents, two of which are poorly characterized, have been implicated in the disease complex based on symptom development in Rubus indicators. Three novel viruses were identified in a red raspberry plant that caused typical raspberry mosaic symptoms when grafted onto indicators. This communication focuses on one of these viruses, Raspberry mottle virus (RMoV), a new member of the family Closteroviridae. The complete nucleotide sequence of RMoV has been determined and exceeds 17 kilobases encoding 10 genes. The genome organization of RMoV is similar to that of Beet yellows virus, the type member of the Closterovirus genus, and phylogenetic analysis using the polymerase conserved motifs and the heat shock protein 70 homolog revealed a close relationship of RMoV with Strawberry chlorotic fleck associated virus and Citrus tristeza virus, which suggests the possibility of an aphid vector. The virus was detected in symptomatic raspberry plants in production areas in mixed infections with several other viruses, indicating that RMoV may impact raspberry production.

A sensitive one-step real-time RT-PCR method for detecting Grapevine leafroll-associated virus 2 variants in grapevine

Grapevine leafroll syndrome is caused by a complex of up to nine different Grapevine leafroll-associated viruses (GLRaV-1-9) with GLRaV-2 being reported as one of the most variable species of this group. Many methods, including indexing, serological and molecular procedures, have been developed for the detection of GLRaV-2. However, due to the low concentration of the virus in plants and the high variability of GLRaV-2, a method with improved sensitivity and with the capacity to detect of all known variants is required. Such improvement is essential for grapevine rootstocks, as these are suspected to harbour frequent GLRaV-2 infections difficult to detect, thus contributing to the spread of the leafroll disease. The development of new universal primers is described using a target sequence located in the 3' end of the virus genome. These primers were combined with a one-step SYBR Green real-time RT-PCR assay to achieve quantitative detection. All 43 GLRaV-2 isolates tested in this study were identified readily and reproducibly, regardless of their geographical origin or variety of grapevine. Using the procedure developed in this study, the sensitivity was increased 125 times compared to a conventional single-tube RT-PCR. This real-time method opens new perspectives for the sanitary selection of grapevine and in leafroll 2 disease monitoring.

Molecular analysis of double-stranded RNAs reveals complex infection of grapevines with multiple viruses

The table grape variety "Waltham Cross" was infected with Leafroll and Shiraz Disease. To reveal specific viruses that are associated with the diseased plants, we used an RT-PCR-based strategy to determine partial genome sequences of these viruses. Upon cloning and sequencing of the RT-PCR products, we detected seven groups of viral variants that are related to four species of the Closteroviridae: Grapevine leafroll-associated virus 1, 2, 3, and 5, in addition to Rupestris stem pitting-associated virus. The population composition of GLRaV-2 and GLRaV-3-like viruses is complex and consists of two or three distinct groups of viral variants. Based on the consensus sequence of several GLRaV-2 strains, we designed a pair of broad-spectrum primers (GLR2-4 and GLR2-5) and used them to detect a range of GLRaV-2 variants from "Waltham Cross". Moreover, we identified a novel group of viral variants from the diseased grapevines, which possess a stretch of 19 nucleotides inserted in the 3' non-coding region as compared to strain "PN" and "93/955" for which the complete genomes have been sequenced. In contrast, the population composition of GLRaV-1 and GLRaV-5-like virus seems to be more uniform and each consists of a single viral variant. Furthermore, the central 5.7kb genomic region encompassing ORF1b-ORF4 of the GLRaV-1 isolate detected in "Waltham Cross" was sequenced. The new isolate is designated GLRaV-1 "WC", which differs from GLRaV-1 "Type" by 16% in nucleotide sequence. The taxonomic standing of the GLRaV-5-like and GLRaV-3-like viruses detected in "Waltham Cross" is discussed.

Strawberry chlorotic fleck: identification and characterization of a novel Closterovirus associated with the disease

Chlorotic fleck, a strawberry disease caused by a graft and aphid transmissible agent, was identified more than 45 years ago in Louisiana. Since its discovery there has been no additional information on the agent that causes the disease. The mode of transmission implies that a virus is the causal agent of chlorotic fleck. We identified four closteroviruses in the single chlorotic fleck infected strawberry clone known to exist in the United States. Sequence analysis indicated that two of the viruses are novel and one of them is closely related to members of the Closterovirus genus, the aphid-transmitted viruses in the family Closteroviridae, a feature that is in accordance with the aphid transmissibility of the chlorotic fleck agent. The genome of the novel Closterovirus, designated as Strawberry chlorotic fleck associated virus exceeds 17 kilobases and encodes 10 open reading frames, including the signature closterovirus genes as well as a gene without obvious homologs in the family. RNA folding predicted a pseudoknot structure near the 3' terminus of the virus that may be involved in template recognition by the viral polymerase. Phylogenetic analysis indicates that Strawberry chlorotic fleck associated virus is most closely related to Citrus tristeza virus among sequenced members of the family. Detection protocols have been developed and the virus was detected in several strawberry plants from production fields.