Taxonomic Structure of the Family
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Family |
Luteoviridae |
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Genus |
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Genus |
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Genus |
Virions are 25 to 30 nm in diameter, hexagonal in outline and have no envelope (Fig. 1). They exhibit icosahedral symmetry (T = 3). Particles are composed of two proteins and a core of genomic ssRNA. A small protein (VPg) has been reported to be covalently linked to the 5-end of the genomic RNA of two poleroviruses.
Physicochemical and Physical Properties
For luteoviruses and poleroviruses, virion Mr is about 6 106; buoyant density in CsCl is 1.40 g/cm3; S20w is 106-127S. For enamoviruses, the Mr is about 5.6 106 (B component) and 4.5 106 (T component); buoyant density in CsCl solutions is 1.42 g/cm3 (B component); S20w is 107-122S (B component) and 91-106S (T component). Virions are moderately stable and are insensitive to treatment with chloroform or non-ionic detergents, but are disrupted by prolonged treatment with high concentrations of salts. Luteovirus and polerovirus particles are insensitive to freezing.
Virions contain a single molecule of infectious, linear, positive-sense ssRNA. The genome size is fairly uniform: 5677 nts for Barley yellow dwarf virus-PAV (BYDV-PAV), 5882 nts for Potato leafroll virus (PLRV), 5722 nts for Beet mild yellowing virus (BMYV), 5641 nts for Beet western yellows virus (BWYV), 5600 nts for Cereal yellow dwarf virus-RPV (CYDV-RPV) (partial sequence), 5669 nts for Cucurbit aphid-borne yellows virus (CABYV) and 5705 nts for Pea enation mosaic virus-1 (PEMV-1). The RNAs do not have a 3-terminal poly(A) tract. A VPg is linked to the genome RNA of the poleroviruses, PLRV and CYDV-RPV, presumably at the 5-end, and a protein is associated with virion RNA of PEMV-1.
The five or six proteins encoded by genome RNA have Mr of between 4 and 84 103 (Table 1). Only the capsid protein (CP) gene has been unequivocally assigned to an ORF; it is encoded by ORF3, which is followed in frame by ORF5.
Virion structural proteins are CP and a “readthrough” protein, which is a fusion of the products of the CP gene and the contiguous ORF5. The readthrough protein may be associated with aphid transmission or virus particle stability. The product of ORF4 has been shown to be required for long distance movement of some luteoviruses and poleroviruses. ORF4 is absent from enamovirus RNA. The region containing ORF6 (only present in luteovirus RNA), but not the ORF6 translation product, acts as a translational enhancer of the expression of BYDV-PAV RNA.
None reported.
None reported.
Genome Organization and Replication
The genomes contain 5 or 6 ORFs (Fig. 2). The genera can be distinguished on the basis of the arrangements and sizes of the ORFs. ORF1 of the luteoviruses is not homologous to the corresponding ORF of the poleroviruses. Also, ORF0 is present only in the genomes of poleroviruses and enamoviruses, ORF4 is present only in the genomes of luteoviruses and poleroviruses, and ORF6 is present only in the genomes of luteoviruses. ORF0 overlaps ORF1 (poleroviruses and enamoviruses), which overlaps ORF2. ORF4 is contained completely within ORF3 (luteoviruses and poleroviruses). Finally, ORF5 is positioned directly downstream of, and contiguous with, ORF3.
The differences among luteoviruses, poleroviruses and enamoviruses are principally in the 5-end of the genome, although luteovirus genomes contain an additional ORF (ORF6) at the 3-end. ORFs 0, 1 and 2 are probably translated from the genomic RNA. ORF2 is translated by frameshift from ORF1 and thus shares an amino terminus with the product of ORF1. ORFs 3, 4 (in luteoviruses and poleroviruses) and 5 are expressed from a sgRNA. ORF5 is probably translated via a readthrough following translation of ORF3. In luteoviruses, ORF6 seems to be expressed from a separate sgRNA, but no protein product has been detected. There are no data on post-translational modification.
Luteovirus and polerovirus are strongly immunogenic. Species within a genus are more closely related serologically than are species in different genera. Serological relationships may be detected when comparing disrupted virus particles that are not detectable when intact virions are tested.
Virions produced in plants infected with PEMV-1 together with PEMV-2 (an umbravirus) are moderately antigenic. In gel diffusion assays, aphid-transmissible isolates display an antigenic determinant that is absent from aphid-non-transmissible isolates. No serological relationships have been reported between enamoviruses and either luteoviruses or poleroviruses.
Several members of the family Luteoviridae have host ranges largely restricted to one plant family. For example, BYDV and CYDV infect many grasses, Bean leafroll virus (BLRV) infects mainly legumes, and Carrot red leaf virus (CtRLV) infects mainly plants in the family Umbelliferae. Other members of the family Luteoviridae infect plants in several or many different families. For example, BWYV infects more than 150 species of plants in over 20 families.
Members of the family Luteoviridae have been reported worldwide including from temperate, sub-tropical, and tropical regions. Some of the viruses are found worldwide, such as BYDV, BWYV and PLRV. Others have more restricted distributions, such as TNDV, which has been reported only from Japan, and Groundnut rosette assistor virus (GRAV), which has been reported from south Saharan countries in Africa.
Transmission is in a circulative, non-propagative manner by specific aphid vectors. Virus is acquired by phloem feeding, enters the hemocoel of the aphid via the hindgut (BYDV-PAV) or posterior midgut (PLRV), circulates in hemolymph and enters the accessory salivary gland. Inoculation probably results from transport of virus into the salivary duct and introduction of saliva into the plant during feeding. PEMV-1 is readily transmitted mechanically, a property dependent on its multiplication in cells co-infected with PEMV-2 (an umbravirus), but aphid transmissibility can be lost after several mechanical passages.
Luteovirus and polerovirus particles are largely confined to phloem cells; PEMV-1, with PEMV-2, is found in phloem and mesophyll tissue. Virus particles are found in both the nuclei and cytoplasm of infected cells. Luteoviruses and poleroviruses often cause phloem necrosis that spreads from inoculated sieve elements and causes symptoms by inhibiting translocation, slowing plant growth and inducing the loss of chlorophyll. The genome of PEMV-1 is capable of autonomous replication in protoplasts, but is dependent on PEMV-2 to support systemic invasion which induces mosaic and enation symptoms.
List of Species Demarcation Criteria in the Family
Criteria used to demarcate species of the family Luteoviridae include:
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Differences in breadth and specificity of host range; | |
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Failure of cross-protection in either one-way or two-way relationships; | |
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Differences in serological specificity with discriminatory polyclonal or monoclonal antibodies; | |
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Differences in amino acid sequences of any gene product of greater than 10%. |
The nucleotide sequence of Soybean dwarf virus (SbDV) lacks ORF0, like those of luteoviruses, and the predicted amino acid sequences of its replication proteins are similar to those of the luteoviruses. However, SbDV structural proteins are more closely related to those of the poleroviruses. SbDV may represent a hybrid of sequences from these two genera. Hence, SbDV is not assigned to a genus until data for other viruses make it clear if SbDV is representative of a new genus, or is an unusual member of one of the existing genera.
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