Properties of a filamentous virus isolated from grapevines affected by corky bark

Metaviromic Characterization of Betaflexivirus Populations Associated with a Vitis cultivar Collection in South Africa

South Africa is associated with a centuries-old viticultural industry, accompanied by a diverse range of wine and table grape cultivars and an extensive history of pervasive introductions of vine material and associated viruses. The Vitis D2 collection in Stellenbosch represents the most comprehensive collection of Vitis species, hybrids, and cultivars in South Africa. We collected leaf petiole material from 229 accessions from this collection. Our metaviromic analyses revealed a total of 406 complete/near complete genomes of various betaflexiviruses. Among these, we identified the presence of grapevine rupestris stem pitting-associated virus and grapevine viruses A, B, E, F, H (GVH), I (GVI), and M (GVM). Notably, this study marks the first report of GVH, GVI, and GVM in South Africa, which were confirmed via RT-PCR. This research significantly contributes to our understanding of viral diversity and introductions in South African viticulture and emphasizes the need for vigilant monitoring and management of viral infections. Our findings lay the groundwork for strategies that mitigate the impact of viruses on South Africa's wine industry, which generates an annual revenue of approximately 500 million USD.

Biology and Ultrastructural Characterization of Grapevine Badnavirus 1 and Grapevine Virus G

The biological characteristics of grapevine viruses, such as their transmission and host range, are important for the adoption of successful prophylaxis strategies. The aim of this study was to investigate the traits of two newly described grapevine viruses widely distributed in Croatia, grapevine badnavirus 1 (GBV-1) and grapevine virus G (GVG). The vine mealybug (Planoccocus ficus) proved to be a vector of GBV-1 and GVG capable of vine-to-vine transmission with overall experimental transmission rates of 61% and 14.6%, respectively. Transmission was also demonstrated by grafting, with an overall transmission rate of 53.8% for GBV-1 and 100% for GVG, as well as by green grafting using the T-budding technique. Symptoms of GBV-1 and GVG were not observed on the woody cylinders of the indicators LN 33, Kober 5BB, 110 Richter and cvs. Chardonnay and Cabernet Sauvignon. Seed transmission and mechanical transmission were not confirmed. Electron microscopy revealed accumulation of GBV-1 particles and viroplasms in the cytoplasm, but no alternations of the cell structure. Infection with GVG revealed the proliferation of tonoplast-associated vesicles inside phloem cells and cell wall thickening.

Incidence and Genetic Diversity of Grapevine Virus G in Croatian Vineyards

Grapevine virus G (GVG) is a recently discovered vitivirus infecting grapevines. Historically, viruses in the genus Vitivirus have been associated with the grapevine rugose wood disease. Based on new and previously reported GVG isolates, primers and probes were developed for real-time RT-PCR. The developed assay successfully detected the virus in infected plants during dormancy and the growing season. A field study of 4327 grapevines from Croatian continental and coastal wine-growing regions confirmed the presence of GVG in 456 (~10.5%) grapevines from three collection plantations and 77 commercial vineyards, with infection rates ranging from 2% to 100%. Interestingly, the virus was confirmed only in vines considered to be Croatian autochthonous cultivars, but not in introduced cultivars. A 564-nucleotide long portion of the coat protein gene from previously known and newly characterized GVG isolates had nucleotide and amino acid identities ranging from 89% to 100% and from 96.8% to 100%, respectively. Phylogenetic analysis revealed five distinct groups, with isolates originating from the same site being close to each other, indicating possible local infection. The information presented in this manuscript sets the stage for future studies to better understand the ecology and epidemiology of GVG and the possible need for inclusion in certification schemes.

Virome of Grapevine Germplasm from the Anapa Ampelographic Collection (Russia)

Grapevine germplasm collections are unique repositories of grape cultivars; therefore, it is necessary to minimize their infection with pathogens, including viruses, and develop various programs to maintain them in a virus-free state. In our study, we examined the virome of the largest Russian grapevine germplasm collection, the Anapa Ampelographic Collection, using high-throughput sequencing of total RNAs. As a result of bioinformatics analysis and validation of its results by reverse transcription PCR (RT-PCR) and quantitative RT-PCR (RT-qPCR), we identified 20 viruses and 3 viroids in 47 libraries. All samples were infected with 2 to 12 viruses and viroids, including those that cause economically significant diseases: leafroll, fleck, and rugose wood complex. For the first time in Russia, we detected Grapevine virus B (GVB), Grapevine virus F (GVF), Grapevine asteroid mosaic-associated virus (GAMaV), Grapevine Red Globe virus (GRGV), Grapevine satellite virus (GV-Sat), Grapevine virga-like virus (GVLV), Grapevine-associated jivivirus 1 (GaJV-1) and Vitis cryptic virus (VCV). A new putative representative of the genus Umbravirus with the provisional name Grapevine umbra-like virus (GULV) was also identified in Russian grape samples.

Detection, Transmission, and Characterization of Grapevine Virus H in Croatia

A survey of recently discovered vitiviruses was performed on 113 Croatian autochthonous grapevine cultivars from the national collection “Jazbina” using one-step RT-PCR. The presence of grapevine virus H (GVH) was confirmed in nine (7.9%) cultivars and grapevine virus G in eight (7.1%), while the presence of grapevine viruses I and J were not detected. GVH was transmitted by the vine mealybug (Planococcus ficus) from a source plant to grapevine seedlings with a 10.5% transmission rate using a combination of 10 first and second instars per plant with 48 and 72 h of acquisition and inoculation access period, respectively. Transmission correlated with the presence of grapevine leafroll-associated virus 3 (GLRaV-3) in the GVH-source plant and recipient seedlings. No alternative GVH host was identified. A comparison of 356 nt fragments of the RdRP and CP coding regions showed nucleotide identity between the Croatian GVH isolates in the range of 95.5–99.2% and 97.5–99.4% and amino acid identity between 95.8 and 100% and between 98.3 and 100%, respectively. Comparison with foreign isolates revealed nucleotide sequence similarity in the RdRP and CP between 94 and 100% and between 97.7–100%, respectively. To the best of our knowledge, this is the first report of GVH in Croatia and the first identification of the vine mealybug as a vector of GVH.

Status of the current vitivirus taxonomy

Since the establishment of the genus Vitivirus, several additional viruses have been sequenced and proposed to represent new species of this genus. Currently, the International Committee on Taxonomy of Viruses recognizes 15 vitivirus species. The report of new vitiviruses that fail to completely adhere to the species demarcation criteria, the incorporation of non-vitivirus grapevine viruses in the unofficial "naming system", and the existence of non-grapevine vitiviruses lead to inconsistencies in classification. In this report, we give a brief overview of vitiviruses and use currently available information to clarify the present status of the vitivirus taxonomy.

Development of RT-PCR assays for the detection and the resultant phylogenetic analysis of four grapevine vitiviruses based on the coat protein sequences

Four sets of primers were designed based on the alignment of the complete coat protein (CP) gene sequences of several isolates of four different vitiviruses, i.e. grapevine virus B (GVB), GVD, GVE and GVF, and tested for their efficiency in RT-PCR assays to detect vitiviruses infections in grapevine. The resultant RT-PCR amplicons were sequenced and analyzed for their genetic variability and phylogenetic studies. The results of the RT-PCR assays showed that these primers were highly efficient in detecting different vitivirus isolates in grapevine material originating from ten different Mediterranean countries. In particular, 76 out of 218 tested samples (ca. 35%) were infected with at least one vitivirus. GVE was the most detected (14.7%), followed by GVF (11.5%), GVB (6.9%), and GVD (2.8%). Nucleotide (nt) sequence analysis of the CP genes from this study and Genbank showed that the sequence identity matrixes among isolates of GVB and GVE were the most variable, with nt identity ranging from 77% to 100%, whereas isolates of GVD and GVF showed more conserved nt identities ranging between 82% to 100% and 86.4% to 99.8%, respectively. The phylogenetic trees constructed based on the CP sequences distinguished two main groups of isolates for each vitivirus species, except for the GVD isolates, which did not show any particular subdivision. In general, the distributions of the isolates in the phylogenetic tree were associated with their geographical origin, thus suggesting limited movement of grapevine materials between the different countries. This study reported for the first time: (i) the development of primers based on the complete CP gene sequences for RT-PCR assays for the universal detection of vitivirus species, (ii) the high genetic variability among Mediterranean isolates of vitiviruses and (iii) the presence of GVD in Jordanian vines, of GVE in grapevines from Hungary, Italy, Jordan, Malta and Palestine, and of GVF in grapevines from Afghanistan, Bulgaria, China, France, Hungary, Italy, Jordan, Lebanon and Malta.

Construction of Full-length Infectious cDNA Clones of Apple chlorotic leaf spot virus and Their Agroinoculation to Woody Plants by a Novel Method of Vacuum Infiltration

Construction and agroinoculation of full-length infectious cDNA clones of plant RNA viruses have been used in plant virology to prove Koch's postulates and for development of viruses as vectors for expressing foreign genes in plants. Four full-length cDNA clones (pIF3-12, pIF3-14, pIF3-15, and pIF3-19) of Apple chlorotic leaf spot virus (ACLSV) isolate 38/85 were produced. Two of the four full-length cDNA clones (pIF3-15 and pIF3-19) proved to be infectious on Nicotiana occidentalis 37B test plants by agroinoculation and were then mechanically transmissible to healthy N. occidentalis 37B. The genomic cDNAs of ACLSV pIF3-15 and pIF3-19 shared nucleotide identity of 77.5%, demonstrating mixed infections of multiple strains of ACLSV in the source tree of isolate 38/85. The two full-length cDNA clones were agroinoculated to apple seedlings by a newly developed vacuum infiltration method. The success rate of agroinoculation was greater than 78%, defined as the number of PCR positive seedlings to the number of apple seedlings that survived. ACLSV was transmissible from agroinoculated seedlings by cleft grafting. The results of this study will be useful for construction of infectious cDNA clones of plant viruses from full-length PCR fragments and agroinoculating woody host plants using the vacuum infiltration method outlined here.

Infection and Colonization of Nicotiana benthamiana by Grapevine leafroll-associated virus 3

Grapevine leafroll disease is an increasing problem in all grape-growing regions of the world. The most widespread agent of the disease, Grapevine leafroll-associated virus 3 (GLRaV-3), has never been shown to infect species outside of the genus Vitis. Virus transmission to several plant species used as model systems was tested using the vine mealybug, Planococcus ficus. We show that GLRaV-3 is able to infect Nicotiana benthamiana. Working with GLRaV-3 infected N. benthamiana revealed distinct advantages in comparison with its natural host Vitis vinifera, yielding both higher viral protein and virion concentrations in western blot and transmission electron microscopy observations, respectively. Immunogold labelling of thin sections through N. benthamiana petioles revealed filamentous particles in the phloem cells of GLRaV-3 positive plants. Comparison of assembled whole genomes from GLRaV-3 infected V. vinifera vs. N. benthamiana revealed substitutions in the 5' UTR. These results open new avenues and opportunities for GLRaV-3 research.

First molecular characterization of grapevine virus B (GVB) in Portuguese grapevine cultivars and improvement of the RT-PCR detection assay

This work describes the first molecular characterization of grapevine virus B (GVB) in Portuguese grapevine cultivars. During a routine screening of 44 accessions in the National Collection of Grapevine Varieties (CAN PRT051), 17 were found infected with GVB in DAS-ELISA assays with commercial antibodies. However, only six of the corresponding isolates were successfully amplified using primer pairs described in the literature. The sequence variants (ORF4-3'UTR, 1147 nt) retrieved from these isolates segregated into two phylogenetic groups, which included sequences from complete genomes available in GenBank. The highly discrepant results obtained using serological and RT-PCR-based diagnostic tools led to the design of a primer pair for detection of GVB, which allowed the amplification of a 606-bp GVB-specific fragment from all DAS-ELISA-positive isolates and also revealed the existence of false negatives in the serological testing.

Brief report of the construction of infectious DNA clones of South African genetic variants of grapevine virus A and grapevine virus B

Background

Recent research results strongly suggest that certain genetic variants of grapevine virus A (GVA) and grapevine virus B (GVB), two members of the Vitivirus genus of the family Betaflexiviridae, are the cause of Shiraz disease and corky bark disease of grapevines in South Africa, respectively. To investigate this hypothesis, work was undertaken to construct DNA clones of these viruses.

Findings and conclusions

Biologically viable and stable DNA clones of genetic variants of GVA and GVB B from South Africa were constructed. The clones share 76.3, 73.2 and 85.2, 77.6 % nt sequence similarity with corresponding clones constructed in Italy and Israel. The results suggest that a derivative of a mini binary vector pCB302 is superior to pCAMBIA1305.1 for the construction of infectious and stable DNA clones of vitiviruses. Successful construction of such DNA clones of GVA and GVB reported in this study is a clear step towards fulfilling Koch’s 3rd postulate in investigating the aetiology of Shiraz disease and corky bark disease.

Multiplex RT-PCR method for the simultaneous detection of nine grapevine viruses

Viral diseases are a serious pathological problem for grapevines, and in recent years the need for increasingly specific and rapid diagnostic methods for the selection of propagation materials has grown. Arabis mosaic virus, Grapevine fanleaf virus, Grapevine virus A, Grapevine virus B, Grapevine rupestris stem pitting-associated virus, Grapevine fleck virus, and Grapevine leafroll-associated viruses 1, 2, and 3 are nine of the most widespread viruses that naturally infect grapevines. A multiplex RT-PCR was developed for simultaneous detection of these nine grapevine viruses, in combination with a plant RNA internal control used as an indicator of the effectiveness of the reaction. One to ten fragments specific for the viruses and an internal control were simultaneously amplified from infected samples and identified by their specific molecular sizes in agarose gel. The protocol reported is an update of previously published protocols for RNA extraction and multiplex diagnosis of viruses. After several years of use and hundreds of samples tested, and following validation in several laboratories, this multiplex RT-PCR provides a reliable and rapid method for detecting grapevine viruses from a large number of samples.

Transmission of six ampeloviruses and two vitiviruses to grapevine by Phenacoccus aceris

Grapevine leafroll disease is caused by grapevine leafroll-associated viruses (GLRaVs). These viruses are common in vineyards worldwide and often associated with vitiviruses that are involved in the rugose wood complex of grapevine. Ten mealybug species are known as vectors of one or several of these grapevine viruses, including the apple mealybug Phenacoccus aceris which is widespread in Holarctic regions and able to transmit Grapevine leafroll-associated virus-1 and -3 (GLRaV-1 and -3). Our aim was to characterize the transmission features of leafroll viruses by Phenacoccus aceris in order to better understand the contribution of this mealybug to leafroll epidemics. Results showed that Phenacoccus aceris is able to transmit GLRaV-1, -3, -4, -5, -6, and -9 to grapevine but not GLRaV-7. This is the first report of GLRaV-6 transmission by a mealybug. Also, for the first time it was shown that Phenacoccus aceris could vector vitiviruses Grapevine virus A (GVA) and Grapevine virus B (GVB). First instar nymphs were the most efficient stage in transmitting GLRaV-1, -3, and GVA. This research sheds light on the transmission biology of grapevine viruses by Phenacoccus aceris and represents a step forward to leafroll disease management.

Detection and characterisation of two novel vitiviruses infecting Actinidia

Two co-infecting novel vitiviruses from Actinidia chinensis were identified from mechanically inoculated Nicotiana occidentalis. Both virus genomes were sequenced and share 64% nucleotide identity. Their overall structure is typical of vitiviruses, with five open reading frames (ORFs) and a polyadenylated 3' end. Open reading frame 4 (ORF4) encodes the coat protein, the most conserved gene of the vitiviruses, in which they share 75% amino acid identity, 61-68% with grapevine virus B, 55-59% with grapevine virus A, and 37-42% with grapevine virus E. Based on the molecular criteria for species demarcation in the family Betaflexiviridae, these are two novel viruses, tentatively named Actinidia virus A and Actinidia virus B.

The ORF3-encoded proteins of vitiviruses GVA and GVB induce tubule-like and punctate structures during virus infection and localize to the plasmodesmata

The genomic RNA of vitiviruses contains 5 open reading frames (ORF). ORF3 encodes a protein to which the function of a movement protein (MP) was assigned, based on sequence homology with other viral proteins. The aim of the research described in this paper was to gain further insight in distribution profile of the ORF3 product encoded by the vitiviruses Grapevine virus A (GVA) and Grapevine virus B (GVB). Expression of the GVA MP-GFP fusion protein via the virus genome in Nicotiana benthamiana leaves resulted in the formation of irregular spots and fibrous network structures on the outermost periphery of epidermal cells. Expression of GVA MP-GFP and GVB MP-GFP was involved in the formation of the tubule-like and punctate structures on the periphery of N. benthamiana and Vitis vinifera protoplasts. Co-expression of the GVA MP-GFP and GVA MP-RFP in protoplasts resulted in co-localization of these proteins into the same punctate structures, indicating that the MP is not accumulated randomly onto the cell surface, but targeted to particular sites at the cell periphery, where punctate and tubule-like structures are likely formed. With the use of cytoskeleton and secretory pathway inhibitors, we showed that the cytoskeletal elements are not likely to be involved in targeting of the MP-GFP to the punctate cellular structures. In addition to MP, a functional coat protein was found to be essential for virus spread within inoculated leaves.

Divergent molecular variants of Grapevine virus B (GVB) from corky bark (CB)-affected and CB-negative LN33 hybrid grapevines

Analysis of two Grapevine virus B (GVB)-infected LN33 hybrid grapevines revealed that a plant exhibiting clear symptoms of corky bark (CB) disease was infected with two molecular variants of the virus, whereas a plant exhibiting no disease symptoms was infected with only one variant. Sequence results indicated that the single variant in the CB-negative grapevine was also one of the two present in the CB-affected hybrid. Plant extracts from these two grapevines were used to successfully transmit the virus to N. benthamiana. After further cloning and sequencing, two clearly divergent variants were identified. Comparative molecular analysis of the variants, named here GVB 953-1 and GVB-H1, respectively, transmitted from CB-affected and consistently CB-negative plants, revealed short genomic regions, most of them highly divergent, that encoded amino acid sequences, containing significant amino acid substitutions altering the net charges of their respective proteins. Interestingly, a comparison of these variants to genome sequence data of GVB variants GVB Italy and GVB 94/971 available from the GenBank, revealed that these significant amino acid substitutions were the same for, and unique to, the variant pairs GVB 953-1/GVB Italy and GVB-H1/GVB 94/971. This despite the variants of each pair being otherwise clearly different at nucleotide and amino acid levels. In addition, both sets of variants differed substantially in their respective 3'-non-translated (3'NTR) regions. The relevance of these findings is discussed.

Sequencing and assembly of a full-length infectious clone of grapevine virus B and its infectivity on herbaceous plants

Grapevine virus B (GVB) has been found associated with corky bark-diseased vines. Although the sequence of a 7.6-kb cDNA clone from a GVB isolate from Italy has been described, striking differences in sequences between GVB isolates prompted us to construct an additional full-length GVB clone from the isolate 94/971 and to determine its complete sequence. The cDNA of GVB 94/971 shared a nucleotide sequence identity of only 77% with the GVB isolate from Italy. The cDNA of GVB 94/971 was infectious on Nicotiana plants as demonstrated by symptoms and by means of Northern blot, Western blot and electron microscopic analyses.

Family Flexiviridae: a case study in virion and genome plasticity

The plant virus family Flexiviridae includes the definitive genera Potexvirus, Mandarivirus, Allexivirus, Carlavirus, Foveavirus, Capillovirus, Vitivirus, Trichovirus, the putative genus Citrivirus, and some unassigned species. Its establishment was based on similarities in virion morphology, common features in genome type and organization, and strong phylogenetic relationships between replicational and structural proteins. In this review, we provide a brief account of the main biological and molecular properties of the members of the family, with special emphasis on the relationships within and among the genera. In phylogenetic analyses the potexvirus-like replicases were more closely related to tymoviruses than to carlaviruses. We postulate a common evolutionary ancestor for the family Tymoviridae and the two distinct evolutionary clusters of the Flexiviridae, i.e., a plant virus with a polyadenylated genome, filamentous virions, and a triple gene block of movement proteins. Subsequent recombination and gene loss would then have generated a very diverse group of plant and fungal viruses.

Simultaneous detection of nine grapevine viruses by multiplex reverse transcription-polymerase chain reaction with coamplification of a plant RNA as internal control

ABSTRACT A multiplex reverse transcription-polymerase chain reaction (mRT-PCR) was developed for simultaneous detection of nine grapevine viruses: Arabis mosaic virus, Grapevine fanleaf virus, Grapevine virus A, Grapevine virus B, Rupestris stem pitting-associated virus, Grapevine fleck virus, Grapevine leafroll-associated virus-1, -2, and -3, in combination with a plant RNA internal control used as an indicator of the effectiveness of RNA extraction and RT-PCR. Primers were designed from conserved regions of each virus and their specificity was confirmed by sequencing PCR products. Two plant total RNA extraction methods (silica capture and modified RNeasy method) and two RT-PCR systems (onestep and two-step) were evaluated to develop a reliable protocol for mRT-PCR. One to nine fragments specific for the viruses were simultaneously amplified from infected samples and identified by their specific molecular sizes in agarose gel electrophoresis. In the two-step mRT-PCR, the detection limits were 10(-3) or 10(-4) extract dilutions, depending on the virus. Leaves, phloem from dormant cuttings, and in vitro plantlets from 103 naturally infected and healthy grapevines were analyzed. The mRT-PCR provided a reliable and rapid method for detecting grapevine viruses from a large number of samples.

Extensive variation of sequence within isolates of Grapevine virus B+

Four regions covering 1247 nucleotides of the RNA genome of 20 isolates of a Vitivirus, Grapevine virus B (GVB), from three countries were analyzed. All the regions in these isolates varied in sequence as compared to the published GVB sequence. Of these, the intergenic region varied the most, with 73.2% nucleotide sequence homology, while ORF4 encoding coat protein varied the least when compared both at nucleotide sequence (80.3% homology) and at amino acid sequence levels (90.6% homology). The variations were scattered along each region length and were higher at the nucleotide level than at the amino acid level, but none resulted in a frame shift or stop codon. These results indicate that GVB may exist as a heterogeneous population, possibly resulting from mixing different strains by grafting practices or by RNA-RNA recombination in the grapevine, the only known natural host of this virus. Although it has been reported that GVB is associated with corky bark disease, no corky bark symptoms were observed in any of the GVB positive grapevine sample collected from Australia.

Isolation of recombinant antibodies (scFvs) to grapevine virus B

A panel of 15 recombinant single chain antibodies (scFv) specific to grapevine virus B (GVB) were recovered from a human combinatorial scFv antibody library using the phage display technique against purified virus particles. Two selected scFv-encoding genes were expressed in recombinant Escherichia coli cells as dimeric antibodies. Successful detection of GVB in tissues of herbaceous hosts and grapevine was obtained in a direct binding assay using dimeric scFvs. This reagent was also shown to substitute efficiently for a GVB polyclonal serum in standard DAS-ELISA test used routinely for diagnosis.

Detection and coat protein gene characterization of an isolate of Grapevine virus B from corky bark-affected grapevines in Southern Brazil

An isolate of Grapevine virus B (GVB), obtained by indexing Vitis labrusca and V. vinifera grapevines on the indicator LN33, was transmitted mechanically to several Nicotiana species. The virus was partially purified from N. cavicola and the coat protein estimated at 23 kDa by SDS-PAGE. In negatively stained leaf extracts of experimentally inoculated N. cavicola and N. occidentalis, flexuous particles with cross banding were observed, predominantly measuring 750-770 x 12 nm, with a modal length of 760 nm. Decoration indicated a clear, positive reaction against AS-GVB. In DAS-ELISA, GVB was detected in N. cavicola and grapevine extracts, and Western blots showed homologous and cross reaction of GVB and GVA antisera with GVB coat protein. Using specific primers for GVB, a fragment of 594 bp, comprising the coat protein gene coding for 197 amino acids, was amplified by RT-PCR with viral RNA extracted from GVB-infected N. occidentalis. The nucleotide and the deduced amino acid sequences of the coat protein gene showed high identities with Italian and Japanese isolates of GVB.

Improved RNA extraction and one-tube RT-PCR assay for simultaneous detection of control plant RNA plus several viruses in plant extracts

A procedure was developed for simultaneous detection of plant RNA viruses and of plant RNA, as a control. RT-PCR amplification with primers designed for the detection of the plant mRNAs encoding malate dehydrogenase (MDH) and the large subunit of ribulose bisphosphate carboxylase oxygenase (RubiscoL) was used for the development of a plant extraction procedure that consistently yields extracts that can be amplified. The control amplification was used successfully on extracts from cane, leaf and/or bud tissues from grapevine, apple, raspberry, strawberry, peach, apricot, plum and wheat. Multiplex RT-PCR conditions were established for the simultaneous detection in grapevine extracts of either arabis mosaic virus, rupestris stem pitting associated virus and malate dehydrogenase mRNA, or grapevine virus A, grapevine virus B, grapevine leafroll associated virus-3, and RubiscoL mRNA.

Impacts of Molecular Diagnostic Technologies on Plant Disease Management

Detection and diagnosis of plant viruses has included serological laboratory tests since the 1960s. Relatively little work was done on serological detection of plant pathogenic bacteria and fungi prior to the development of ELISA and monoclonal antibody technologies. Most applications for laboratory-based tests were directed at virus detection with relatively little emphasis on fungal and bacterial pathogens, though there was some good work done with other groups of plant pathogens. With the advent of molecular biology and the ability to compare regions of genomic DNA representing conserved sequences, the development of laboratory tests increased at an amazing rate for all groups of plant pathogens. Comparison of ITS regions of bacteria, fungi, and nematodes has proven useful for taxonomic purposes. Sequencing of conserved genes has been used to develop PCR-based detection with varying levels of specificity for viruses, fungi, and bacteria. Combinations of ELISA and PCR technologies are used to improve sensitivity of detection and to avoid problems with inhibitors or PCR often found in plants. The application of these technologies in plant pathology has greatly improved our ability to detect plant pathogens and is increasing our understanding of, their ecology and epidemiology.

Characterization of Grapevine Rugose Wood Disease Sources from Italy

A collection of 27 sources of grapevine rugose wood (RW) disease from a viticultural region in northern Italy was analyzed by graft-inoculating vines of three selective Vitis indicators (V. rupestris cv. St. George, V. berlandieri × V. riparia cv. Kober 5BB, and hybrid cv. LN 33). On the basis of stem reactivity, different groups were identified among the selected RW inoculum sources: nine isolates induced pitting only on cv. St. George, whereas four induced grooving only on cv. Kober 5BB. These two groups were classified as isolates of rupestris stem pitting and Kober stem grooving. Three of the remaining isolates induced wood abnormalities on cvs. LN 33 and Kober 5BB, seven induced wood abnormalities on cvs. St. George and Kober 5BB, and four induced symptoms on all three indicators. These groups may represent RW sources with various disease combinations. RW-affected grapevine clones used as inoculum sources also were tested for virus infections by enzyme-linked immunosorbent assay (ELISA). ELISA revealed the presence of grapevine fleck virus, grapevine leafroll-associated closterovirus 1 and 3, and grapevine trichovirus A. These viruses infected most of the selected RW sources. However, eight of the latter were ELISA-negative. The findings are discussed, and the biological and etiological complexity of the RW phenomena in grapevine is confirmed.

Serological detection of grapevine virus a using antiserum to a nonstructural protein, the putative movement protein

ABSTRACT Grapevine virus A (GVA) is implicated in the etiology of the rugose wood disease. The coat protein (CP) and the putative movement protein (MP) genes of GVA were cloned and expressed in Escherichia coli and used to produce antisera. Both the CP and the MP were detected with their corresponding antisera in GVA-infected Nicotiana benthamiana. The MP was first detected at an early stage of the infection, 6 to 12 h after inoculation, and the CP was detected 2 to 3 days after inoculation. The CP and MP were detected by immunoblot analysis in rugose wood-affected grapevines. The MP could be detected in GVA-infected grapevines that tested negative for CP, both with CP antiserum and with a commercially available enzyme-linked immunosorbent assay kit. The study shows that detection of the nonstructural MP may be an effective means for serological detection of GVA infection in grapevines.

Nucleotide sequence of the 3' terminal region of the RNA of two filamentous grapevine viruses

The 3' terminal region of grapevine virus A (GVA) and grapevine virus B (GVB), encompassing 1883 and 2136 nucleotides, respectively, was sequenced by the deoxynucleotide chain termination method. Three putative open reading frames (ORF) were identified in both genomic viral RNAs, denoted 1 to 3 in the 5' to 3' direction. ORF 1 encoded a polypeptide with estimated M(r) of 31 kDa (GVA) and 36.5 kDa (GVB), possessing the G/D motif of the "30 K superfamily" movement proteins, and showing good alignments with putative movement proteins of trichoviruses and capilloviruses. ORF 2 was identified as the coat protein (CP) cistron, coding for polypeptides with an estimated M(r) of 21.5 kDa (GVA) and 21.6 kDa (GVB). These CPs showed substantial sequence homology with one another and with CPs of tricho- and capilloviruses, but not of closteroviruses. ORF 3 potentially coded for two small polypeptides with estimated M(r) of 10 kDa (GVA) and 14 kDa (GVB). The ORF 3 product of GVB (14 K), but not that of GVA, shared some homology with the 3' terminal polypeptides of different plant viruses that exhibit the "zinc finger domain" of proteins with nucleic acid-binding properties. GVA and GVB have many properties in common with trichoviruses but possess an extra open reading frame (ORF 3). Whether this finding may have a bearing on the classification of these viruses is unclear. However, until the taxonomic significance of this difference in genome structure is established, it seems plausible to include GVA and GVB as tentative species in the Trichovirus genus.

Sensitive detection of grapevine virus A, B, or leafroll-associated III from viruliferous mealybugs and infected tissue by cDNA amplification

DNA primers specific for grapevine virus A (GVA), grapevine virus B (GVB) or grapevine leafroll-associated virus III (GLRaV-III) were constructed based on the nucleotide sequence of a segment of each viral genome. DNA primers were utilized for cDNA synthesis and polymerase chain reaction (PCR) amplification of a 430 bp fragment from extracts of GVA-infected grapevine tissue or viruliferous mealybugs and 450 bp and 340 bp DNA fragments from extracts of GVB and GLRaV-III-infected grapevine tissues, respectively. The amplified DNA fragment of each virus was identified by Southern hybridization analysis with a cRNA probe of cloned viral genome. Reverse transcription (RT)-PCR, immunocapture (IC)-RT-PCR and/or multiplex (M)-RT-PCR assays were developed for the detection of GVA, GVB, and/or GLRaV-III in extracts of infected grapevine leaves, dormant cuttings and/or in viruliferous mealybugs. Viral specific DNA was absent from amplified extracts of uninfected grapevine tissue or nonviruliferious mealybugs. IC-RT-PCR was easier to perform than RT-PCR for the detection of GVA from viruliferous mealybugs. M-RT-PCR was easier and faster than IC-RT-PCR for the detection of GLRaV-III from infected grapevine tissue and it allows the sensitive detection of GVB, for which a high titer antiserum is not yet available.