-ends; there is no information about the structures at their 5-ends.
|
Type Species |
(CMoV) |
Umbraviruses do not form conventional virus particles, and the three genomes whose sequences are known lack plausible ORFs for capsid proteins (CPs). Umbraviruses rely on the CP of a helper virus, characteristically from the family Luteoviridae, for encapsidation and for transmission by the vector of the helper virus. However, in single infections of umbraviruses, the infectivity in buffer extracts of leaves is surprisingly stable, though very sensitive to treatment with ganic solvents, suggesting that the infective RNA is protected in lipid-containing structures. In Carrot mottle virus (CMoV), enveloped structures about 52 nm in diameter occur in the vacuoles of infected cells and in partially purified preparations, and these structures may be involved in virus replication and/or serve to protect the RNA (Fig. 1). Similar structures occur in plants infected with Bean yellow vein-banding virus (BYVBV), Groundnut rosette virus (GRV) and Lettuce speckles mottle virus (LSMV), but no information is available for other umbraviruses.
Physicochemical and Physical Properties
Infectivity in leaf extracts is stable for several hours at room temperature or several days at 5°C, but is abolished by treatment with organic solvents. Partially purified preparations of CMoV consist predominantly of cell membranes but contain infective components which, because they have a sedimentation coefficient of about 270S and a buoyant density of about 1.15 g/cm3 in CsCl, are probably the 52 nm-diameter enveloped structures observed in these preparations.
Nucleic acid preparations made by extracting leaves with phenol are often much more infective than buffer extracts. The infective agent in these preparations is a ssRNA, but the preparations also contain abundant dsRNA. The genome consists of one linear segment of positive-sense ssRNA. Nucleotide sequences have been determined for only three umbraviruses, Carrot mottle mimic virus (CMoMV) (4201 nts), GRV (4019 nts) and Pea enation mosaic virus-2 (PEMV-2) (4253 nts). These three genomes are probably not polyadenylated at their 3-ends; there is no information about the structures at their 5-ends.
No structural proteins are reported. The nucleotide sequences of CMoMV, GRV and PEMV-2 lack plausible ORFs for CPs but possess ORFs for four potential non-structural protein products.
Although no conventional virus particles are formed, the sensitivity to organic solvents, and low buoyant density, of the infective components in partially purified preparations suggests that this infectivity is associated with lipid, probably of plant origin. The infective components probably correspond to the enveloped structures seen in sections of infected leaves.
None reported.
Genome Organization and Replication
Figure 2 shows the genome organization of GRV; those of CMoMV and PEMV-2 are very similar. For each RNA there is at the 5-end a very short non-coding region preceding ORF1, which encodes a putative product of Mr 
31-37 
103. ORF2, which slightly overlaps the end of ORF1, could encode a product of Mr 64-65 103 but lacks an AUG initiation codon near its 5-end. However, immediately before the stop codon of ORF1 there is a 7-nt sequence that is associated with frameshifting in several plant and animal viruses, and it seems probable that ORF1 and ORF2 are translated as a single polypeptide of Mr 94-98 103 by a mechanism involving a-1 frameshift. The predicted product contains, in the ORF2 region, sequence motifs characteristic of viral RNA-dependent RNA polymerases (RdRp). A short untranslated region separates ORF2 from ORFs 3 and 4, which overlap each other almost completely in different reading frames and each yield a putative product of Mr 26-29 103. The ORF4 product contains sequences characteristic of plant virus cell-to-cell movement proteins (MPs). The ORF3 products of CMoMV, GRV and PEMV-2 have 42-50% similarity to each other but no significant similarity to any other viral or non-viral proteins. The sequence of the ORF3 product therefore gives no clue to its possible function.
Umbravirus-infected leaf tissue contains abundant dsRNA which is not itself infective but becomes so when heat-denatured. Two dsRNA species are common to all umbraviruses: dsRNA-1 (about 4.2-4.8 kbp) and dsRNA-2 (about 1.1-1.5 kbp). cDNA copies of dsRNA-1 hybridize with dsRNA-2 and these molecules are thought to represent double-stranded forms of, respectively, genomic and subgenomic ssRNA species. It seems probable that ORFs 3 and 4 are expressed from sgRNA. There is evidence for the presence in GRV-infected plants of two less-than-full-length RNA species of very similar size, close to that expected for such sgRNAs, and corresponding to that of dsRNA-2. The dsRNA-2 of CMoMV has been shown to include the sequences of ORFs 3 and 4, and the 3 untranslated region.
Some umbraviruses possess one or more additional dsRNA species, associated in at least one instance (GRV) with the presence of a satellite RNA. PEMV-2 too has a satellite RNA, and each of these satellites can be supported by the helper virus of the other.
None reported.
Individual umbraviruses are confined in nature to one or a few host plant species. Their experimental host ranges are broader but still restricted. The symptoms induced in infected plants are mottles or mosaics. Symptoms of GRV are greatly influenced by the associated satellite RNA.
Umbraviruses are transmissible, sometimes with difficulty, by mechanical inoculation. However, in nature each is dependent on a specific helper virus for transmission in a persistent (circulative, non-propagative) manner by aphids. All helper viruses that have been characterized are members of the family Luteoviridae. The mechanism of this dependence is encapsidation of the dependent virus RNA in the CP of the helper. In GRV the satellite RNA plays an essential role in mediating this luteovirus-dependent aphid transmission. There is no evidence for multiplication of umbraviruses in the insect vector. Seed transmission has not been reported.
CMoV and/or CMoMV, and PEMV-2, apparently occur worldwide wherever their crop hosts are grown; other umbraviruses have a restricted distribution. Several umbraviruses, notably GRV, occur only in Africa.
Although all umbraviruses depend on a helper virus for transmission by vector insects, several of them are equally or more important than their helpers in the causation of disease symptoms. The umbravirus of greatest economic importance is GRV, which causes the most devastating virus disease of groundnut (peanut) in Africa. However, in this case it is a GRV-dependent satellite RNA that is the actual cause of the symptoms.
In most instances umbraviruses have not been shown to contribute functions essential for the biological success of their associated helper viruses. However, a notable exception is PEMV-2, which is essential for the systemic spread of PEMV-1 in plants, and even allows it, unlike typical members of the family Luteoviridae, to spread out of the phloem into mesophyll tissue and thereby to become transmissible by manual inoculation. Another member of the Luteoviridae, Beet western yellows virus, has been reported to show limited manual transmissibility when in the presence of Lettuce speckles mottle virus a member of the genus Umbravirus.
Umbraviruses, even in the absence of their helper viruses, exhibit rapid systemic spread in plants. They infect cells throughout the leaf, though presumably the aphid-transmissible particles are restricted to the same tissues (in most instances the phloem) as the luteoviruses that provide their CP. In mesophyll cells infected with CMoV there is extensive development of cell wall outgrowths sheatng elongated plasmodesmatal tubules.
List of Species Demarcation Criteria in the Genus
The criteria demarcating species in the genus are:
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Natural host range, |
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dsRNA band pattern (bearing in mind that some bands may represent satellite RNA species), |
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Nucleotide sequence identity less than 55%, and |
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Little or no hybridization with cDNA probes representing most parts of the genome. |
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:
|
Bean yellow vein-banding virus |
(BYVBV) | |
|
Carrot mottle virus (137) |
(CMoV) | |
|
Carrot mottle mimic virus |
[U57305] |
(CMoMV) |
|
Groundnut rosette virus (355) |
[Z69910; satellite RNA: Z29702-Z29711] |
(GRV) |
|
Lettuce speckles mottle virus |
(LSMV) | |
|
Pea enation mosaic virus-2 (257) |
[U03563; satellite RNA: U03564] |
(PEMV-2) |
|
Tobacco mottle virus |
(TMoV) |
Tentative Species in the Genus
|
Sunflower crinkle virus (Sunflower rugose mosaic virus) |
(SuCV) |
|
Sunflower yellow blotch virus (Sunflower yellow ringspot virus) |
(SuYBV) |
|
Tobacco bushy top virus |
(TBTV) |
|
Tobacco yellow vein virus |
(TYVV) |
Amino acid sequence comparisons show that the putative RdRp encoded by the genomic RNA molecules of CMoMV, GRV and PEMV-2 belong to the so-called supergroup 2 of RNA polymerases, as do those of viruses in the genera Carmovirus, Dianthovirus, Luteovirus, Machlomovirus, Necrovirus, and Tombusvirus. Since these enzymes are the only universally conserved proteins of positive-strand RNA viruses, the genus Umbravirus might be considered to be in or close to the family Tombusviridae.
Umbra: From Latin, a shadow. In English, a shadow, an uninvited guest that comes with an invited one.
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