Search

BACKGROUND: Grapevine red blotch virus (GRBV) is the causal agent of grapevine red blotch disease and is known to delay grape ripening. However, grape cell‐wall modifications during GRBV infection are largely unknown, even though the cell wall plays a large role in pathogenicity, viral interactions with host plants, and phenolic extractability during winemaking. Understanding the impact of GRBV infection on cell‐wall metabolism is important for the development of potential mitigations strategies. In this study, high‐throughput transcriptome sequencing was conducted on Vitis vinifera L. 'Merlot' grapes during ripening. The cell‐wall composition, phenolic content, and phenolic extractability at two different commercial harvest points were also determined. RESULTS: Log fold changes indicated a strong induction in diseased grapes at harvest of several transcripts involved in cell‐wall solubilization and degradation. However, these observations did not translate to changes in cell‐wall composition at either harvest point in diseased grapes, potentially suggesting post‐transcriptional regulation. Moderate induction of pectin methylesterase inhibitor transcripts and transcripts associated with pathogenesis‐related proteins coincided with increases in pectin and soluble proteins in cell walls of diseased grapes at harvest. Both pectin and pathogenesis‐related proteins are known to retain phenolic compounds during winemaking. CONCLUSION: Our study corroborates this finding when the percentage extractability of flavonols in wines was significantly lower when made from GRBV‐infected fruit. These results suggest GRBV alters the grape cell walls, consequently decreasing phenolic extraction during winemaking. © 2023 Society of Chemical Industry. [ABSTRACT FROM AUTHOR]

Grapevine red blotch virus (GRBV) is a recently identified virus. Previous research indicates primarily a substantial impact on berry ripening in all varieties studied. The current study analyzed grapes' primary and secondary metabolism across grapevine genotypes and seasons to reveal both conserved and variable impacts to GRBV infection. Vitis vinifera cv. Cabernet Sauvignon (CS) grapevines grafted on two different rootstocks (110R and 420A) were analyzed in 2016 and 2017. Metabolite profiling revealed a considerable impact on amino acid and malate acid levels, volatile aroma compounds derived from the lipoxygenase pathway, and anthocyanins synthesized in the phenylpropanoid pathway. Conserved transcriptional responses to GRBV showed induction of auxin-mediated pathways and photosynthesis with inhibition of transcription and translation processes mainly at harvest. There was an induction of plant-pathogen interactions at pre-veraison, for all genotypes and seasons, except for CS 110R in 2017. Lastly, differential co-expression analysis revealed a transcriptional shift from metabolic synthesis and energy metabolism to transcription and translation processes associated with a virus-induced gene silencing transcript. This plant-derived defense response transcript was only significantly upregulated at veraison for all genotypes and seasons, suggesting a phenological association with disease expression and plant immune responses. [ABSTRACT FROM AUTHOR]

The three-cornered alfalfa hopper, Spissistilus festinus (Say) was shown to transmit Grapevine red blotch virus (GRBV) in a greenhouse study. GRBV is the causal agent of Grapevine Red Blotch Disease, which reduces the quality of wine produced from infected grapes. Due to the general lack of prior concern regarding S. festinus on grapevines, the biology of this species in vineyards has been largely unknown. A 2-yr study with weekly sampling was conducted in a Californian vineyard to increase the knowledge of S. festinus seasonal dynamics and distribution. The overwintering S. festinus adults were first captured in the vineyard before bud break. Detection of late-instar S. festinus nymphs, the first in-field adult generation, and grape anthesis occurred concurrently in 2016 and 2017. Two in-field S. festinus generations were documented by peaks in sweep net sampling of vineyard groundcover in 2016, whereas only one generation was observed in 2017. There appears to be an inverse relationship between the number of S. festinus adults sampled on ground cover and the number of girdles in the grapevine canopy. Spissistilus festinus exhibited an aggregated distribution in the vineyard and a significant edge effect. Results from this study will contribute to the development of sampling and management guidelines and determine timing of control measures to reduce populations of S. festinus within vineyards to minimize the virus spread. [ABSTRACT FROM AUTHOR]

Grapevine leafroll-associated virus 3 (GLRaV-3) is the most widely prevalent and economically important of the complex of RNA viruses associated with grapevine leafroll disease (GLD). Phylogenetic studies have grouped GLRaV-3 isolates into nine different monophyletic groups and four supergroups, making GLRaV-3 a genetically highly diverse virus species. In addition, new divergent variants have been discovered recently around the world. Accurate identification of the virus is an essential component in the management and control of GLRaV-3; however, the diversity of GLRaV-3, coupled with the limited sequence information, have complicated the development of a reliable detection assay. In this study, GLRaV-3 sequence data available in GenBank and those generated at Foundation Plant Services, University of California-Davis, was used to develop a new RT-qPCR assay with the capacity to detect all known GLRaV-3 variants. The new assay, referred to as FPST, was challenged against samples that included plants infected with different GLRaV-3 variants and originating from 46 countries. The FPST assay detected all known GLRaV-3 variants, including the highly divergent variants, by amplifying a small highly conserved region in the 3’ untranslated terminal region (UTR) of the virus genome. The reliability of the new RT-qPCR assay was confirmed by an enzyme linked immunosorbent assay (ELISA) that can detect all known GLRaV-3 variants characterized to date. Additionally, three new GLRaV-3 divergent variants, represented by four isolates, were identified using a hierarchical testing process involving the FPST assay, GLRaV-3 variant-specific assays and high-throughput sequencing analysis. These variants were distantly related to groups I, II, III, V, VI, VII and IX, but much similar to GLRaV-3 variants with no assigned group; thus, they may represent new clades. Finally, based on the phylogenetic analysis, a new GLRaV-3 subclade is proposed and named as group X. [ABSTRACT FROM AUTHOR]

Grapevine red blotch virus (GRBV) currently poses a serious threat to the wine industry. The three-cornered alfalfa hopper (3CAH), Spissistilus festinus (Say) (Hemiptera: Membracidae), was recently shown in a greenhouse study to be a vector of GRBV in grapes. Feeding and reproductive status of 10 weed and 10 cover crop species commonly found in grape vineyards were evaluated in no-choice tests. Species confirmed as reproductive hosts were subsequently evaluated in choice tests to determine S. festinus oviposition preference. Spanish clover, dandelion, birdsfoot trefoil, common groundsel, field bindweed, magnus peas, bell beans, blando brome, purple vetch, black medick, subterranean clover, crimson clover, and woollypod vetch were all found to be reproductive hosts. The plant species that exhibited the greatest nymph emergence in each of the three groups tested for preference were all in the family Fabaceae. Knowledge of plant species present in vineyards that serve as alternative hosts for S. festinus can contribute to developing management strategies to mitigate virus spread. [ABSTRACT FROM AUTHOR]

Grapevine red blotch-associated virus (GRBaV) is a single-stranded DNA vims, proposed to be a member of the family Geminiviridae, and is associated with grapevines showing red blotch symptoms in North America. The existence of the vims was reported in 2012, and subsequently detected in grapevines in major grape production regions. We investigated if a vector exists that can transmit GRBaV in vineyards. Phylogenetic analysis of the predicted amino acid sequence of coat protein (CP) of GRBaV with the CP of 23 geminivimses representing all seven genera of the family Geminiviridae revealed that GRBaV-CP was most similar to that of Tomato pseudo-curly top virus, a geminivirus known to be transmitted by a treehopper (Membracidae), a family that is closely related to leafhoppers (Cicadellidae). To identify vectors of GRBaV. hemipteran species within and nearby wine grape vineyards where virus spread was suspected were collected and transmission assays were conducted. Among the species tested, the three-cornered alfalfa hopper Spissistilus festinus (Hemiptera: Membracidae) was able to both acquire the virus from a grapevine infected with GRBaV and transmit the virus to healthy grapevines in the laboratory. In commercial vineyards, lateral shoots of grapevines girdled due to feeding injury by the adult three-cornered alfalfa hopper also tested positive for the virus using digital PCR. These findings represent an important step in understanding the biology of GRBaV and develop management guidelines. [ABSTRACT FROM AUTHOR]

Grapevine red blotch-associated virus(GRBaV), which causes grapevine red blotch disease, is an emerging problem for grapevine production in the United States. However, little is known about how viruses, such as GRBaV, affect host physiology even though it is crucial to understanding host-pathogen interactions, symptom development, and potential effects on other pathogens and insect pests including potential vectors. Thus, foliar levels of amino acids, sugars, phenolics and terpenoids were examined in healthy or GRBaV-infected ‘Cabernet Franc’ (CF) or ‘Cabernet Sauvignon’ (CS) grapevines both before and after development of red blotch symptoms, in July and September of 2014, respectively. Particular amino acids were increased both before and after symptom development in both cultivars, with some of these amino acids having previously defined associations with host defences. Fructose and glucose were increased in GRBaV-infected CF at both sampling times. However, for CS only, glucose was increased in infected grapevines and only when pre-symptomatic. Phenolic levels were greater in GRBaV-infected CF and CS after symptom expression. Terpenoids were greater in infected CF in July, with no other apparent differences. Taken together, these results demonstrate the effects of GRBaV infection on host physiology, with shifts potentially associated with symptom development and changes in resistance to other organisms. [ABSTRACT FROM PUBLISHER]

Grapevine red blotch-associated vims (GRBaV) is a newly identified vims of grapevines and a putative member of a new genus within the family Geminiviridae. This vims is associated with red blotch disease that was first reported in California in 2008. It affects the profitability of vineyards by substantially reducing fruit quality and ripening. In red-berried grapevine cultivars, foliar disease symptoms consist of red blotches early in the season that can expand and coalesce across most of the leaf blade later in the season. In white-berried grapevine cultivars, foliar disease symptoms are less conspicuous and generally involve irregular chlorotic areas that may become necrotic late in the season. Determining the GRBaV genome sequence yielded critical information for the design of primers for polymerase chain reaction-based diagnostics. To date. GRBaV has been reported in the major grape-growing areas in North America and two distinct phylogenetic clades have been described. Spread of GRBaV is suspected in certain vineyards but a vector of epidemiological significance has yet to be identified. Future research will need to focus on virus spread, the production of clean planting stocks, and the development of management options that are effective, economical, and environmentally friendly. [ABSTRACT FROM AUTHOR]

Copyright of Canadian Journal of Plant Pathology is the property of Taylor & Francis Ltd and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Abstract: Impact of drip and overhead sprinkler irrigation on the persistence of attenuated Escherichia coli O157:H7 in the lettuce phyllosphere was investigated using a split-plot design in four field trials conducted in the Salinas Valley, California, between summer 2007 and fall 2009. Rifampicin-resistant attenuated E. coli O157:H7 ATCC 700728 (BLS1) was inoculated onto the soil beds after seeding with a backpack sprayer or onto 2- or 4-week-old lettuce plant foliage with a spray bottle at a level of 7 log CFU ml−1. When E. coli O157:H7 was inoculated onto 2-week-old plants, the organism was recovered by enrichment in 1 of 120 or 0 of 240 plants at 21 or 28 days post-inoculation, respectively. For the four trials where inoculum was applied to 4-week-old plants, the population size of E. coli O157:H7 declined rapidly and by day 7, counts were near or below the limit of detection (10 cells per plant) for 82% or more of the samples. However, in 3 out 4 field trials E. coli O157:H7 was still detected in lettuce plants by enrichment 4-weeks post-inoculation. Neither drip nor overhead sprinkler irrigation consistently influenced the survival of E. coli O157:H7 on lettuce. [Copyright &y& Elsevier]

Grapevine red blotch virus (GRBV) has become widespread in the United States since its identification in 2012. GRBV is the causative agent of grapevine red blotch disease (GRBD), which has caused detrimental economic impacts to the grape and wine industry. Understanding viral function, plant–pathogen interactions, and the effects of GRBV on grapevine performance remains essential to developing potential mitigation strategies. This comprehensive review examines the current body of knowledge regarding GRBV, to highlight gaps in the knowledge and potential mitigation strategies for grape growers and winemakers. [ABSTRACT FROM AUTHOR]

A novel cucumber mosaic virus inducible viral amplicon (CMViva) expression system has been developed that allows for tightly regulated chemically inducible expression of heterologous genes in plant hosts. Transient production of recombinant α1-antitrypsin (rAAT), a human blood protein, was demonstrated in Nicotiana benthamiana leaves. The highest production levels were obtained by co-infiltrating leaves with Agrobacterium tumefaciens cells containing CMViva carrying the AAT gene and A. tumefaciens cells carrying a binary vector constitutively expressing the gene silencing suppressor p19. Accumulation of up to thirty-fold more rAAT was observed in leaves (24 mg per 100 g leaf tissue) when compared with the expression levels observed using the cauliflower mosaic virus (CaMV) 35S promoter. Significantly, 70% of the rAAT produced using the CMViva expression system was found to be biologically active, a 170-fold increase in functional protein compared with the CaMV 35S expression system. [ABSTRACT FROM AUTHOR]

RNA silencing has been shown to be an important mechanism for conferring resistance in transgenic, virus-resistant plants. We used this approach to evaluate resistance in Nicotiana benthamiana plants transformed with chimeric coding and noncoding sequences from Citrus tristeza virus (CTV). Several independent transgenic plant lines were generated. using two constructs (pCTV1 and pCTV2) designed to produce self-complementary transcripts. The pCTV contained cDNA Sequences from the CTV capsid protein (CP), p20, and 3′ untranslated region (UTR); and pCTV2 contained CP, p23, and 3′ UTR sequences. Heterologous recombinant Potato virus X (PVX) containing either homologous or heterologous CTV sequences was used to challenge plants and resistance was evaluated phenotypically and validated with reverse-transcriptase polymerase chain reaction and northern hybridization analysis. Transgenic plants (T1 generation) for each construct showed resistance to recombinant PVX constructs used for challenge experiments when PVX contained p20 or UTR (for CTV1 plants), or p23 or UTR (for CTV2 plants). However, no resistance was seen when plants were challenged with PVX containing the CTV CP. T2 generation plants also showed resistance even when challenged with PVX containing the cognate CTV sequences obtained from heterologous CTV isolates. The presence of transgene-specific small interfering RNAs in the resistant CTV1 and CTV2 plants indicated that resistance was mediated by Post-transcriptional gene silencing. [ABSTRACT FROM AUTHOR]

Feeding by the three-cornered alfalfa hopper, Spissistilus festinus (Say) (Hemiptera: Membracidae) results in girdling of grapevine petioles and shoots. Its significance as an economic pest of grape has increased since it was shown to transmit Grapevine red blotch virus (GRBV) in a greenhouse study. However, the status of grapevines as a reproductive host for S. festinus remained undetermined. Adult S. festinus were caged onto three regions of the grapevines: apical shoot, green shoot, and dormant cane. Their ability to reproduce was determined by weekly destructive sampling for 7 wk. Successful oviposition and nymphal emergence were observed on apical and green shoots, but not on dormant canes. However, insect development beyond the second nymphal instar did not occur. Knowledge of S. festinus reproduction on grapevines will be an important consideration in designing management guidelines to minimize the spread of GRBV in vineyards. [ABSTRACT FROM AUTHOR]

Grapevine red blotch-associated virus (GRBaV) is a recently discovered virus of concern to wine grape production in North America. While the vector of this virus is unknown, other elements of virus epidemiology are essential to develop guidelines for the management of the virus as well as to assist in the search for its vector. The objective of this study was to evaluate vegetation within and surrounding GRBaV-infected vineyards to identify potential virus reservoirs that may serve as sources of inoculum. In this study, 13 plant species were sampled throughout the year and were tested for the presence of GRBaV. Of the 13 species tested, two species, Rubus armeniacus and wild grapes (Vitis californica x V. vinifera), tested positive by quantitative PCR. Of these two species, only wild grapes were determined to be a true host. This study documents the first time GRBaV has been confirmed in an alternative host or in a species outside of a commercial vineyard and suggests that a mechanism exists by which GRBaV moves between plant species that is not human-mediated. The precise role that wild grapes play in the epidemiology of GRBaV remains unknown. [ABSTRACT FROM AUTHOR]

Abstract: Lettuce infectious yellows virus (LIYV) encodes a 26 kDa protein (P26) previously shown to associate with plasmalemma deposits (PLDs), unique LIYV-induced cytopathologies located at the plasmalemma over plasmodesmata pit fields in companion cells and phloem parenchyma. To further characterize the relationship of P26 and PLDs, we assessed localization and cytopathology induction of P26 expressed from either LIYV or a heterologous Tobacco mosaic virus (TMV) vector using green fluorescent protein (GFP) fusions, immunofluorescence microscopy, biochemical fractionation, and transmission electron microscopy (TEM). TEM analyses demonstrated that P26 not only associated with, but induced formation of PLDs in the absence of other LIYV proteins. Interestingly, PLDs induced by P26-expressing TMV were no longer confined to phloem cells. Putative P26 orthologs from two other members of the genus Crinivirus which do not induce conspicuous PLDs exhibited fractionation properties similar to LIYV P26 but were not associated with any PLD-like cytopathology. [Copyright &y& Elsevier]

• Grapevine red blotch virus reduced °Brix and increased berry weight at harvest. • Primary and secondary metabolites were affected in fruit skin/flesh and seeds. • Different responses were observed for amino acid contents in both tissues. • Glucose, fructose and sucrose had variable responses in grape seeds. • Prominent phenolic compounds were reduced in skin/pulp tissues of diseased vines. The grapevine red blotch disease (GRBD) was first noticed in 2008, impacting grape ripening. In general, GRBD reduces grape and wine quality resulting in significant economic losses. The purpose of the present study was to evaluate the effect of GRBD on agronomical parameters of 'Cabernet Sauvignon' vines at harvest. Using a metabolomics approach, the influence on primary and secondary metabolite profiling in skin + pulp/flesh and seeds were also determined. GRBD influenced °Brix and berry weight, as well as primary and secondary metabolites in both tissues. 1D 1H NMR was effective in quantifying the main primary and secondary metabolites affected by GRBD. RP-HPLC was similarly able to quantify the main phenolics affected. Multivariate analysis showed the influence of the virus on grape metabolites using both tools in two berry tissues. The effectiveness of both tools to describe sample variability was compared and the most affected metabolites in each tissue could be identified. [ABSTRACT FROM AUTHOR]

In spring 2013, 5-year-old nectarine (Prunus persica) trees, grafted on peach rootstock Nemaguard, were found stunted in a propagation block in California. These trees had been propagated from budwood of three nectarine cultivars imported from France and cleared through the post-entry quarantine procedure. Examination of the canopy failed to reveal any obvious symptoms. However, examination of the trunks, after snipping the bark, revealed extensive pitting on the woody cylinder. To investigate the etiological agent, double-stranded RNA was extracted from bark scrapings from the scion and rootstock portions, and a cDNA library was prepared and sequenced using the Illumina platform. BLAST analysis of the contigs generated by the de novo assembly of sequence reads indicated the presence of a novel luteovirus. Complete sequence of the viral genome was determined by sequencing of three overlapping cDNA clones generated by reverse transcription-polymerase chain reaction (RT-PCR) and by rapid amplification of the 5'- and 3'-termini. The virus genome was comprised of 4,991 nucleotides with a gene organization similar to members of the genus Luteovirus (family Luteoviridae). The presence of the virus, tentatively named Nectarine stem pitting-associated virus, was confirmed in symptomatic trees by RT-PCR. Discovery of a new virus in nectarine trees after postentry quarantine indicates the importance of including (i) metagenomic analysis by next-generation sequencing approach as an essential tool to assess the plant health status, and (ii) examination of the woody cylinders as part of the indexing process. [ABSTRACT FROM AUTHOR]

In the Napa Valley of California, vineyards of 'Cabernet Franc' (CF) clone 214, 'Cabernet Sauvignon' clone 337, and 'Zinfandel' clone 1A (Z1A) with grapevines exhibiting foliar symptoms of red blotches, marginal reddening, and red veins that were accompanied by reduced sugar accumulation in fruit at harvest were initially suspected to be infected with leafroll-associated viruses. However, reverse-transcription polymerase chain reaction (PCR) tests were negative for all known leafroll-associated viruses, with the exception of Grapevine leafrollassociated virus 2 in Z1A. Metagenomic analysis of cDNA libraries obtained from double-stranded RNA enriched nucleic acid (NA) preparations from bark scrapings of dormant canes on an Illumina platform revealed sequences having a distant relationship with members of the family Geminiviridae. Sequencing of products obtained by PCR assays using overlapping primers and rolling circle amplification (RCA) confirmed the presence of a single circular genome of 3,206 nucleotides which was nearly identical to the genome of a recently reported Grapevine cabernet franc-associated virus found in declining grapevines in New York. We propose to call this virus "Grapevine red blotch-associated virus" (GRBaV) to describe its association with grapevine red blotch disease. Primers specific to GRBaV amplified a product of expected size (557 bp) from NA preparations obtained from petioles of several diseased source vines. Chip bud inoculations successfully transmitted GRBaV to test plants of CF, as confirmed by PCR analysis. This is the first report of a DNA virus associated with red blotch disease of grapevines in California. [ABSTRACT FROM AUTHOR]

Spread and in-field spatial patterns of vines infected with grapevine red blotch virus (GRBV) were documented in Oregon vineyards using field sampling, molecular diagnostics, and spatial analysis. Grapevine petiole tissue collected from 2013 to 2016 was tested using quantitative polymerase chain reaction for GRBV. At Jacksonville in southern Oregon, 3.1% of vines were infected with GRBV in 2014, and GRBV incidence reached 58.5% of study vines by 2016. GRBV-infected plants and GRBV-uninfected plants were spatially aggregated at this site in 2015, and infected plants were spatially associated between years 2015 and 2016. In a southern Oregon vineyard near Talent, 10.4% of vines were infected with GRBV in 2014, and infection increased annually to 21.5% in 2016. At Talent, distribution of the infected vines was spatially associated across all years. GRBV infection was highest at Yamhill, in the Willamette Valley, where 31.7% of the tested vines had GRBV infection in 2014. By 2016, 59.2% of the vines tested positive for GRBV. Areas of aggregation increased and were spatially associated across all years. From 2013 to 2015, GRBV was not detected at Milton-Freewater in eastern Oregon. Spatial patterns of GRBV infection support evidence of spread by a mobile insect vector. GRBV is a significant threat to Oregon wine grape production because of its drastic year-over-year spread in affected vineyards.

Management of geminiviruses is a worldwide challenge because of the widespread distribution of economically important diseases caused by these viruses. Regardless of the type of agriculture, management is most effective with an integrated pest management (IPM) approach that involves measures before, during, and after the growing season. This includes starting with resistant cultivars and virus- and vector-free transplants and propagative plants. For high value vegetables, protected culture (e.g., greenhouses and screenhouses) allows for effective management but is limited owing to high cost. Protection of young plants in open fields is provided by row covers, but other measures are typically required. Measures that are used for crops in open fields include roguing infected plants and insect vector management. Application of insecticide to manage vectors (whiteflies and leafhoppers) is the most widely used measure but can cause undesirable environmental and human health issues. For annual crops, these measures can be more effective when combined with host-free periods of two to three months. Finally, given the great diversity of the viruses, their insect vectors, and the crops affected, IPM approaches need to be based on the biology and ecology of the virus and vector and the crop production system. Here, we present the general measures that can be used in an IPM program for geminivirus diseases, specific case studies, and future challenges.

Grapevine red blotch-associated virus (GRBaV) is a single-stranded DNA virus, proposed to be a member of the family Geminiviridae, and is associated with grapevines showing red blotch symptoms in North America. The existence of the virus was reported in 2012, and subsequently detected in grapevines in major grape production regions. We investigated if a vector exists that can transmit GRBaV in vineyards. Phylogenetic analysis of the predicted amino acid sequence of coat protein (CP) of GRBaV with the CP of 23 geminiviruses representing all seven genera of the family Geminiviridae revealed that GRBaV-CP was most similar to that of Tomato pseudo-curly top virus, a geminivirus known to be transmitted by a treehopper (Membracidae), a family that is closely related to leafhoppers (Cicadellidae). To identify vectors of GRBaV, hemipteran species within and nearby wine grape vineyards where virus spread was suspected were collected and transmission assays were conducted. Among the species tested, the three-cornered alfalfa hopper Spissistilus festinus (Hemiptera: Membracidae) was able to both acquire the virus from a grapevine infected with GRBaV and transmit the virus to healthy grapevines in the laboratory. In commercial vineyards, lateral shoots of grapevines girdled due to feeding injury by the adult three-cornered alfalfa hopper also tested positive for the virus using digital PCR. These findings represent an important step in understanding the biology of GRBaV and develop management guidelines.

We have identified the genome of a novel viral satellite in deep sequence analysis of double-stranded RNA from grapevine. The genome was 1,060 bases in length, and encoded two open reading frames. Neither frame was related to any known plant virus gene. But translation of the longer frame showed a protein sequence similar to those of other plant virus satellites. Other than in commonalities they shared in this gene sequence, members of that group were extensively divergent. The reading frame in this gene from the novel satellite could be translationally coupled to an adjacent reading frame in the -1 register, through overlapping start/stop codons. These overlapping AUGA start/stop codons were adjacent to a sequence that could be folded into a pseudoknot structure. Field surveys with PCR probes specific for the novel satellite revealed its presence in 3% of the grapevines (n = 346) sampled.

A novel virus-like sequence from grapevine was identified by Illumina sequencing. The complete genome is 7,551 nucleotides in length, with polyadenylation at the 3' end. Translation of the sequence revealed five open reading frames (ORFs). The genomic organization was most similar to those of vitiviruses. The polymerase (ORF1) and coat protein (ORF4) genes shared 31 to 49% nucleotide and 40 to 70% amino acid sequence identities, respectively, with other grapevine vitiviruses. The virus was tentatively named grapevine virus F (GVF).

The development of genetically engineered resistance to plant viruses is a result of efforts to understand the plant-virus interactions involved in "crossprotection," a phenomenon observed with several plant virus diseases. Historically, expression of the coat protein gene of Tobacco mosaic virus in transgenic tobacco (Nicotiana tabacum) plants is the first example of transgene-mediated resistance to a plant virus. Subsequently, virus-derived sequences of several plant viruses were shown to confer virus resistance in experimental and/or natural hosts. For plant RNA viruses, virus complementary DNA sequences shown to confer resistance include wild-type genes, mutated genes that produced truncated protein products, and nontranslatable sense or antisense transcripts to various regions of the virus genome. Resistance also has been demonstrated for some viruses by mutant trans-dominant gene products, derived from the movement protein and replication-associated protein genes. In addition to virus-derived sequences, gene sequences of plant origin have also been used for transgenic resistance, and such resistance can be virus-specific, for instance, R genes isolated from resistant plant genotypes, or nonspecific, for example, ribosome inactivating proteins and proteinase inhibitors. Plantibodies and 2-5A synthetase, a class of proteins of mammalian origin, have also been useful in engineering plant virus resistance. In the case of transgenic resistance mediated by viral coat protein, the mechanism of resistance was suggested to operate during the early events of virus infection. However, transgene-mediated RNA silencing and generation of small interfering RNAs appears to be the primary mechanism that confers resistance to plant viruses. Despite the advantages of transgene-mediated resistance, current interest in the development and use of transgenic virus resistant plants is low in most parts of the world. However, because of its real potential, we believe that this technology will have more widespread and renewed interest in the near future.