Genetic diversity and tissue and host specificity of Grapevine vein clearing virus

Grapevine vein clearing virus Is Prevalent and Genetically Variable in Grape Aphid (Aphis illinoisensis Shimer) Populations

Grapevine vein clearing virus (GVCV) causes severe stunting and death of cultivated grapevines and is prevalent in native Vitis spp. and Ampelopsis cordata in the Midwest region of the United States. GVCV can be transmitted from wild A. cordata to Vitis spp. by grape aphid (Aphis illinoisensis) under greenhouse conditions, but its prevalence, genetic composition, and genome number in native grape aphids are unknown. In this study, we collected grape aphids from native Vitaceae across the state of Missouri in 2018 and 2019, and conducted diagnostic, genetic, and quantitative analyses. GVCV was detected in 91 of the 105 randomly sampled communities on 71 Vitaceae plants (87%). It was present in 211 of 525 single grape aphids (40%). Diverse GVCV variants from aphids were present on both GVCV-negative and GVCV-positive plants. Identical GVCV variants were found in grape aphids sampled from wild and cultivated Vitaceae, indicating that viruliferous aphids likely migrate and disperse GVCV variants among wild and cultivated Vitaceae. In addition, we found that the number of GVCV genomes varies largely in the stylet and body of individual aphids. Our study provides a snapshot of GVCV epidemics and genetic structure in its mobile vector and sessile hosts. This presents a good model for studying the epidemiology, ecology, and evolution of a plant virus.

North American Grape 'Norton' is Resistant to Grapevine Vein Clearing Virus

Grapevines (Vitis spp.) host viruses belonging to 17 families. Virus-associated diseases are a constant challenge to grape production. Genetic resources for breeding virus-resistant grape cultivars are scarce. 'Norton' is a hybrid grape of North American Vitis aestivalis and is resistant to powdery mildew and downy mildew. In this study, we assessed resistance of 'Norton' to grapevine vein clearing virus (GVCV), which is prevalent in native, wild Vitaceae and in vineyards in the Midwest region of the U.S. We did not detect GVCV in 'Norton' as either the scion or the rootstock up to 3 years after it was grafted with a GVCV-infected 'Chardonel' grapevine. Upon sequencing of small RNAs, we were able to assemble the GVCV genome from virus small RNAs in GVCV-infected 'Chardonel' scion or rootstock, but not from grafted 'Norton' scion and rootstock. This study unveils a new trait of 'Norton' that can be used in breeding GVCV-resistant grape cultivars, and to investigate genetic mechanisms of 'Norton' resistance to GVCV.

A Natural Reservoir and Transmission Vector of Grapevine Vein Clearing Virus

Grapevine vein clearing virus (GVCV) is associated with a vein-clearing and vine-decline disease. In this study, we surveyed wild Ampelopsis cordata from the Vitaceae family and found that 31% (35 of 113) of native A. cordata plants are infected with GVCV. The full-length genome sequence of one GVCV isolate from A. cordata shared 99.8% identical nucleotides with an isolate from a nearby cultivated 'Chardonel' grapevine, suggesting the occurrence of an insect vector. To identify a vector, we collected Aphis illinoisensis (common name: grape aphids) from wild A. cordata plants and detected GVCV in the aphid populations. We found that A. illinoisensis is capable of transmitting GVCV from infected A. cordata to Chardonel grapevines in the greenhouse. Upon transmission, GVCV caused severe symptoms on the infected Chardonel 45 days post transmission. We conclude that wild GVCV isolates from A. cordata are capable of inducing a severe disease on cultivated grapevines once they spread from native A. cordata to vineyards via grape aphids. The discovery of a natural reservoir and an insect vector of GVCV provides timely knowledge for disease management in vineyards and critical clues on viral evolution and epidemiology.

Viral small RNAs reveal the genomic variations of three grapevine vein clearing virus quasispecies populations

Viral small RNAs (vsRNAs) include viral small interfering RNAs (vsiRNAs) that are initiators and products of RNA silencing, and small RNAs that are derived from viral RNAs with function still unknown. Sequencing of vsRNAs allows assembling of viral genomes and revelation of viral population variations at genomic levels. Grapevine vein clearing virus (GVCV) is a new member of the family Caulimoviridae whose DNA genome is replicated by reverse transcription of pre-genomic RNA molecules. In this short report, three genomic sequences of GVCV were assembled from vsRNAs that were isolated and sequenced from three individual grapevines in commercial vineyards and compared to the GVCV-CHA reference genome. Profiles of single nucleotide polymorphism among three viral populations indicated a closer relatedness between two populations in different grape cultivars at the same location than those in the same grape cultivar at different locations, suggesting the spread of GVCV populations among vineyards of close proximity. Classic types of vsiRNAs (21-nt, 22-nt, and 24-nt) were found in the three GVCV vsiRNA populations, but these did not produce alignment hotspots on the GVCV-CHA reference genome. The number of 36-nt reads is the highest among vsRNAs, the role of these vsRNAs remains unclear. The analysis of vsRNAs provides a first holistic picture of genomic variations among GVCV viral quasispecies populations that help monitor epidemics and evolution of GVCV populations, an emerging virus that is becoming a threat to grape production in the Midwest region of the USA.

Genetic and Phenotypic Characterization of Grapevine vein clearing virus from Wild Vitis rupestris

Grapevine vein clearing virus (GVCV), a new member of the genus Badnavirus in the family Caulimoviridae, is associated with a vein clearing and vine decline disease that severely affects grape production and berry quality in commercial vineyards in the Midwest region of the United States. In this paper, the genetic and phenotypic characteristics of GVCV-VRU1 and GVCV-VRU2, two isolates from wild Vitis rupestris grapevines in their native habitat, are described. The GVCV-VRU1 genome is 7,755 bp long while the GVCV-VRU2 genome consists of 7,725 bp, both of which are different from the genome of the GVCV-CHA isolate (7,753 bp), which was originally discovered in the grape cultivar 'Chardonel'. The nucleotide sequence identity among GVCV-VRU1, GVCV-VRU2, and GVCV-CHA ranges from 91.6 to 93.4%, and open reading frame (ORF) II is the most divergent ORF with only 83.3 to 88.5% identity. Sequence analysis of the ORF II indicated that GVCV isolates genetically similar to GVCV-VRU1 and GVCV-VRU2 also are present in commercial vineyards. Symptoms of GVCV-VRU1- or GVCV-VRU2-infected wild V. rupestris grapevine appeared initially as translucent vein clearing on young leaves and progressed to vein necrosis on mature leaves. Inoculation of GVCV-VRU1 or GVCV-VRU2 by grafting onto grape cultivar Chardonel resulted in mild mottle and leaf distortion. The natural range of wild V. rupestris grapevines overlaps with commercial vineyards in the Midwestern United States. Therefore, the discovery of GVCV isolates in wild V. rupestris grapevines has important implications for epidemics and management of the GVCV-associated disease.

Characterization of the promoter of Grapevine vein clearing virus

Grapevine vein clearing virus (GVCV) is a recently discovered DNA virus in grapevine that is closely associated with the grapevine vein clearing syndrome observed in vineyards in Missouri and surrounding states. The genome sequence of GVCV indicates that it belongs to the genus Badnavirus in the family Caulimoviridae. To identify the GVCV promoter, we cloned portions of the GVCV large intergenic region in front of a GFP gene present in an Agrobacterium tumefaciens binary vector. GFP expression was assessed by ELISA 3 days after agroinfiltration of Nicotiana benthamiana leaves. We found that the GVCV DNA segment between nts 7332 and 7672 directed expression of GFP and this expression was stronger than expression using the Cauliflower mosaic virus 35S promoter. It was revealed by 5' and 3' RACE that transcription was initiated predominantly at nt 7571 and terminated at nt 7676.