-cap (m7GpppG) and a 3-terminal tRNA-like structure (Fig. 2). The RNA molecules of Sorghum chlorotic spot virus (SgCSV) are not polyadenylated.
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Type Species |
(SBWMV) |
Virions are non-enveloped hollow rods, which have helical symmetry. Virions are about 20 nm in diameter, with predominant lengths of 140-160 nm and 260-300 nm. The length distribution of the Soil-borne wheat mosaic virus (SBWMV) short particles is broad, 80-160 nm, due to the presence of deletion mutants in some cultures (Fig. 1).
Physicochemical and Physical Properties
Virions sediment as two (or three) components; for SBWMV the S20w values are 220-230S (long particles) and 170-225S (short particles), and 126-177S (deletion mutants). SBWMV loses infectivity in extracts of wheat kept at 60-65°C for 10 minutes.
The genome is bipartite, linear, positive-sense ssRNA. RNA-1 is about 6-7 kb and RNA-2 about 3.5-3.6 kb. The complete nucleotide sequence has been determined for SBWMV and the RNA molecules have a 5-cap (m7GpppG) and a 3-terminal tRNA-like structure (Fig. 2). The RNA molecules of Sorghum chlorotic spot virus (SgCSV) are not polyadenylated.
The capsid comprises multiple copies of a single polypeptide of approximately Mr 19-20.5 
103.
None reported.
None reported.
Genome Organization and Replication
Only the genome of SBWMV has been completely sequenced. RNA-1 and RNA-2 are both required for infectivity. RNA-1 (7099 nts) encodes a protein with an Mr 150 103, a readthrough product with Mr 209 103 and a protein with Mr 37 103 (Fig. 2). The protein Mr 150 103 contains an NTP-binding helicase motif and the readthrough protein an RNA polymerase motif, indicating that these proteins are involved in replication. The protein Mr 37 103 is thought to be involved in virus movement as it shares partial sequence similarity to the movement proteins of dianthoviruses. RNA-2 (3593 nts) encodes the capsid protein (CP) (Mr 19 103), the sequence of which terminates in a UGA codon that can be suppressed to give a readthrough product of Mr 84 103. A polypeptide with Mr 25 103 is initiated from a CUG codon upstream of the CP AUG. An ORF towards the 3-end of the RNA-2 encodes a protein with Mr 19 103 that contains 9 cysteine residues. Products corresponding to the protein Mr 37 103 and the cysteine-rich protein with Mr 19 103 were not found in in vitro transcription/translation experiments, and these proteins are thought to be expressed from sgRNAs. Spontaneous deletions in the CP readthrough domain occur on successive passage by manual inoculation, and in field isolates in older infected plants.
The capsid proteins of Oat golden stripe virus (OGSV) and isolates of SBWMV share 96.7-99.1% nucleotide sequence identity. In Northern blots, cDNA probes prepared to SBWMV RNA-1 and RNA-2 hybridized to the respective OGSV RNAs. Whereas cDNA probes prepared to SBWMV RNAs did not hybridize to SrCSV RNA in Northern blots or vice versa.
Virions are immunogenic. SBWMV is serologically related to SrCSV and OGSV, and a monoclonal antibody raised to SBWMV reacted strongly with OGSV.
The natural host range is narrow. SBWMV is mainly confined to species within the Graminae, it induces green or yellow mosaic and stunting in winter wheat (Triticum aestivum) causing up to 80% yield loss in severely infected crops but it is not readily mechanically transmissible. OGSV infects oats (Avena sativa) but failed to infect wheat when plants were grown in viruliferous soil. SrCSV infects Sorghum bicolor, and experimentally it can be mechanically transmitted to a range of species including Chenopodium quinoa, C. amaranticolor, Nicotiana clevelandii, Arachis hypogaea, Zea mays and T. aestivum.
The viruses are soil-borne, and SBWMV is transmitted by the plasmodiophorid fungus Polymyxa graminis. Virions are thought to be carried internally within motile fungal zoospores. Soil containing the fungal resting spores remains infective for many years.
SBWMV is found in many countries in temperate regions but the others have more limited distribution.
Virions are found scattered, or in aggregates and inclusion bodies in the cytoplasm and vacuole. Inclusion bodies can be crystalline inclusions or comprise loose clusters of virus particles in association with masses of microtubules. Amorphous inclusion bodies can be seen in tissue sections by light microscopy.
List of Species Demarcation Criteria in the Genus
Not applicable.
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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Soil-borne wheat mosaic virus (77) |
[L07937,L07938] |
(SBWMV) |
|
(Oat golden stripe virus) |
Tentative Species in the Genus
|
Hypochoeris mosaic virus (223) |
(HyMV) |
|
Rice stripe necrosis virus |
(RSNV) |
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Soil-borne rye mosaic virus |
(SBRMV) |
|
Sorghum chlorotic spot virus |
(SrCSV) |
Phylogenetic Relationships within the Genus
Not available.
Furoviruses have similar particle morphology to members of the genera Pecluvirus, Pomovirus, Benyvirus, Tobamovirus and Hordeivirus. The furoviruses differ in that they do not possess a triple block of genes encoding movement proteins. The amino acid sequences of the putative replicase proteins form a cluster with those of the pomo-, peclu-, hordei-, tobamo- and tobraviruses. SBWMV has distant serological relationships with potato mop-top virus and tobacco mosaic virus.
Furo: siglum from fungus-borne, rod-shaped virus.
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