12 nm in size, showing distinct cross-banding, helically constructed with a pitch of 3.3-3.5 nm and about 10 subunits per turn of the helix (Fig. 1).
|
Type Species |
(GVA) |
Virions are flexuous filaments 725-825 12 nm in size, showing distinct cross-banding, helically constructed with a pitch of 3.3-3.5 nm and about 10 subunits per turn of the helix (Fig. 1).
Physicochemical and Physical Properties
Virions sediment as a single or two very close bands in sucrose or Cs2SO4 gradients, with S20w of approximately 92S. Virions of Heracleum latent virus (HLV) are sensitive to ribonucleases. Virions of all species resist moderately high temperatures (thermal inactivation is around 60°C) and are moderately resistant to organic solvents.
Virions contain a single molecule of positive sense ssRNA, about 7.6 kb in size, capped at the 5
-terminus and polyadenylated at the 3-terminus. The RNA accounts for approximately 5% of the particle weight. The complete nucleotide sequence of Grapevine virus A (GVA) and Grapevine virus B (GVB) is available. The genomes of Grapevine virus D (GVD) and Heracleum latent virus (HLV) have been sequenced in part.
The viral capsid of all species is composed of a single polypeptide of 18-21.5 103 in size. Non structural proteins are: (1) a 194 103 polypeptide with conserved motifs of replication-related proteins of the “alpha-like” supergroup of positive-strand ssRNA viruses (i.e., methyltransferase, helicase and RNA-dependent RNA polymerase, in that order from the N to the C terminus); (2) a 19-20 103 polypeptide of unknown function with no significant sequence homology with protein sequences from databases; (3) a 31-36.5 103 polypeptide possessing the G/D motif of the “30K superfamily” movement proteins (MP) which, in the case of GVA and GVB, are associated with cell walls and plasmodesmata; (4) a 10-14 103 polypeptide with weak homologies to proteins with RNA-binding properties.
None reported.
None reported.
Genome Organization and Replication
The genome of GVA, GVB and GVD contains five slightly overlapping ORFs (Fig. 2). The 5 region of GVA and GVB initiates with an A/T-rich (60-68%) non coding sequence of 47-86 nts. In addition to the 21.5 103 capsid protein (CP) encoded by ORF4, viral genomes express the aforementioned non-structural proteins. The strategy of expression is based on sgRNA production, as suggested by the analysis of dsRNA patterns from infected tissues. dsRNAs consist of four major bands with a size of 7.6, 6.48, 5.68 and 5.1 kbp for GVA and GVD, and 7.6, 6.25, 5.03 and 1.97 kbp for GVB. Replication occurs in the cytoplasm, possibly in association with membranous vesicles.
Virions are moderate or poor antigens. Most species are very distantly serologically related. Monoclonal antibodies to GVA and GVB have been produced. The relationship between GVA, GVB and GVD is due to a few common internal antigenic determinants (cryptotopes).
The natural host range of individual species is restricted to a single host. Infections induce either no symptoms (HLV) or severe diseases characterized by pitting and grooving of the wood (GVA, GVB, GVD). The experimental host range varies from wide (HLV) to restricted (GVA, GVB, GVC, GVD).
All species are transmitted by mechanical inoculation, those infecting grapevines with greater difficulty. Transmission by grafting and dispersal through propagating material is common with grapevine-infecting species. GVA and GVB are transmitted in a semi-persistent manner by different species of pseudococcid mealybugs of the genera Pseudococcus and Planococcus. GVA is also transmitted by the scale insect Neopulvinaria innumerabilis. HLV is transmitted semi-persistently by aphids, in association with a helper virus.
Geographical distribution varies from very wide (GVA, GVB) to restricted (HLV), according to the virus species.
Infected cells are damaged to a varying extent. All species elicit the formation of vesicular evaginations of the tonoplast containing finely fibrillar material, possibly representig replicative forms of viral RNA. Virions of grapevine-infecting species are strictly phloem-limited, but in herbaceous hosts they invade also parenchyma tissues. Virus particles accumulate in the cytoplasm in bundles or paracrystalline aggregates.
List of Species Demarcation Criteria in the Genus
The criteria demarcating species in the genus are:
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The natural host range, |
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Serological specificity using discriminatory polyclonal and monoclonal antibodies, |
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Epidemiology: individual species or groups of species are transmitted by different types and species of vectors, |
|
Differences in dsRNA pattern, |
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Amino acid sequences of any gene product differing by more than 10%. |
Official virus species names are in italics. Tentative virus species names, alternative names ( ), strains or serotypes are not italicized. Virus names, CMI/AAB description numbers ( ), genome sequence accession numbers [ ], and assigned abbreviations ( ) are:
|
Grapevine virus A |
[X75433] |
(GVA) |
|
Grapevine virus B |
[X75448] |
(GVB) |
|
Grapevine virus D |
[Y07764] |
(GVD) |
|
Heracleum latent virus (228) |
(HLV) |
Tentative Species in the Genus
|
Grapevine virus C |
(GVC) |
Phylogenetic Relationships within the Genus
Not available.
Virions have the same particle morphology as those of viral species of the family Closteroviridae and the genera Trichovirus, and Capillovirus. The replication-associated proteins encoded by ORF1 contain signature sequences homologous to those of other members of the “alpha-like” supergroup of ssRNA viruses, especially those of the genera Trichovirus, Capillovirus, Potexvirus, Carlavirus and Tymovirus. The putative MP has a weak homology with comparable proteins of other plant viruses. The 10-13 103 polypeptide encoded by ORF5 of GVA, GVB, and GVD has weak homologies with the 12-15 103 product of carlaviruses, which has RNA-binding properties. The 13 103 protein of GVB shows similarity to the ORF12 product of Citrus tristeza virus which has also RNA-binding properties. Capsid proteins of GVA, GVB, and GVD share distinct homologies with those of trichoviruses and capilloviruses, and are somewhat more distantly related to CPs of carla- and potexviruses.
Viti: from Vitis, generic name of the grapevine, Vitis vinifera.
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