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Type Species |
(ACLSV) |
Virions are very flexuous filaments, 640-760 12 nm in size, helically constructed with a pitch of 3.3-3.5 nm, and about 10 subunits per turn of the helix. Virions may show cross-banding, criss-cross or rope-like features according to the negative contrast material used (Fig. 1).
Physicochemical and Physical Properties
Virions sediment as single or as two very close bands with an S20w of about 100S. Virions of Apple chlorotic leaf spot virus (ACLSV) are sensitive to ribonucleases. Virions of all species resist moderately high temperatures (thermal inactivation is around 55-60°C) and are moderately resistant to organic solvents.
Virions contain a single molecule of linear, positive sense, ssRNA about 7.5 kb in size, with a polyadenylated 3-terminus, accounting for approximately 5% of the particle weight. Indirect evidence suggests that the genomic RNA of ACLSV is capped at its 5-end. The complete nucleotide sequences are available for three isolates of ACLSV and for Potato virus T (PVT). Grapevine berry inner necrosis virus (GINV) was sequenced in part. ACLSV isolates show a high variability in their nucleotide sequence with an overall homology between 76 and 82%. The capsid protein (CP) is the most conserved (87-93% identity), whilst the putative movement protein (MP) is the most divergent (77-85% identity).
Virions of all species are composed of a single polypeptide (Mr 22-27 103). Non-structural proteins of ACLSV and PVT are: (1) a protein of about 180-220 103 containing RNA-dependent RNA polymerase, helicase and methyltransferase signature sequences typical of replication-associated proteins of the “alpha-like” supergroup of ssRNA viruses; (2) a polypeptide of 40-50 103 with weak homologies to some plant virus movement proteins (MP).
None reported.
None reported.
Genome Organization and Replication
The genome of all species contains three slightly overlapping ORFs (Fig. 2). The large 5 ORF is directly expressed from genomic RNA, whereas the two smaller downstream ORFs that code, respectively, for the putative MP and CP, are expressed via sgRNAs. ACLSV-infected tissues contain six dsRNA species of approximately 7.5, 6.4, 5.4, 2.2, 1.1, and 1.0 kbp. The 7.5 kbp species represents the double-stranded form of the full-length genome, whereas the 2.2 and the 1.1 kbp species are the double-stranded forms of sgRNAs coding for the putative MP and the CP, respectively. The most abundant dsRNA species, the function of which are unknown, are 5 co-terminal with genomic RNA, and have a size of 6.4 and 5.4 kbp, respectively. Replication is presumed to be cytoplasmic and to involve the product of ORF1.
Virions are moderate to poor antigens. None of the known species is serologically related to any of the others.
The natural host range of individual species is relatively narrow (ACLSV), or restricted to a single host (PVT, GINV). The experimental host range is somewhat wider, but still limited to a few herbaceous species. In the natural hosts, infections induce little or no symptoms (PVT, ACLSV in certain hosts), or mottling, rings, line patterns, and fruit injuries (i.e., pseudosharka) (ACLSV), or mottling with stunting and internal necrosis of shoots and berries (GINV).
The viruses are readily transmitted by mechanical inoculation, by grafting (ACLSV, GINV) and through propagating material. PVT is seed-transmitted in several hosts, including Solanum spp. No vector of any of the species is known, but GINV is reported to spread naturally in the field.
Geographical distribution varies from wide to restricted, according to the virus species. ACLSV is ubiquitous, whereas PVT is reported only from the Andean region of South America, and GINV from Japan.
Infected cells are damaged to varying extents. Virions are found in phloem and parenchyma cells of leaves and roots and accumulate in the cytoplasm, sometimes in the nucleus, in bundles or paracrystalline aggregates. No inclusion bodies are formed.
List of Species Demarcation Criteria in the Genus
The criteria demarcating species in the genus are:
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Natural and experimental host range, | |
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Serological specificity (all known species are serologically unrelated), | |
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Amino acid sequences of CPs differing by more than 15%. |
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:
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Apple chlorotic leaf spot virus (30) |
[M58152, D14996, X99752] |
(ACLSV) |
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Grapevine berry inner necrosis virus |
[D88448] |
(GINV) |
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Potato virus T (187) |
[D10172] |
(PVT) |
Tentative Species in the Genus
None reported.
Phylogenetic Relationships within the Genus
Not available.
Virion morphology resembles that of members of the family Closteroviridae and of the genera Vitivirus and Capillovirus. The ORF1-encoded polypeptide (putative polymerase) contains signature sequences homologous to those found in other members of the “alpha-like” supergroup of ssRNA viruses, especially those of the genera Carlavirus, Capillovirus, Vitivirus, Potexvirus, and Tymovirus. The ORF2-encoded polypeptide (putative MP) has weak homology with MPs of other plant viruses, the closest relative being the 36 103 protein of Apple stem grooving capillovirus. Capsid proteins of all species share distinct homology with that of capillo- and vitiviruses but not with CPs of closteroviruses.
Tricho: from Greek “thrix”, hair.
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