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Type Species |
(TRV) |
Virions are tubular particles with no envelope. They are of two predominant lengths, (L) 180-215 nm and (S) ranging from 46 to 115 nm, depending on the isolate (Fig. 1). Many strains produce in addition small amounts of shorter particles. The particle diameter is 21.3-23.1 nm by electron microscopy or 20.5-22.5 nm by X-ray diffraction, and there is a central canal 4-5 nm in diameter. Virions have helical symmetry with a pitch of 2.5 nm; the number of subunits per turn has been variously estimated as 25 or 32.
Physicochemical and Physical Properties
Virion Mr is 48-50 
106 (L particles) and 11-29 106 (S Particles). Buoyant density in CsCl is 1.306-1.324 g/cm3. S20,W is 286-306S (L particles) and 155-245S (S particles). Virions are stable over a wide range of pH and ionic conditions and are resistant to many organic solvents, but are sensitive to treatment with EDTA.
The genome consists of two molecules of linear positive sense ssRNA; RNA-1 is about 6.8 kb and RNA-2 ranges from 1.8 kb to about 4.5 kb in size (varying in different isolates). The 5
-terminus is capped with the structure m7G5ppp5Ap. There is no genome-linked protein or poly(A) tract.
Virions contain a single structural protein (Mr 
22-24 103). RNA-1 of Tobacco rattle virus (TRV) codes for four nonstructural proteins: a 134 103 protein terminated by an opal stop codon and a 194 103 protein produced by readthrough of this stop codon, both of which are probably involved in RNA replication; a 29 103 protein (P1a), probably involved in intercellular transport of the virus; and a 16 103 protein (P1b) of unknown function. The sizes of the analogous proteins in Pea early-browning virus (PEBV) are 141 103, 201 103, 30 103 and 12 103, respectively. In addition to the virion structural protein, RNA-2 codes for two nonstructural proteins, P2b and P2c, one or both of which is required for vector transmission. The size of P2b ranges from 27 to 37 103 in different isolates, and that of P2c from 18 to 33 103. The genes for P2b and P2c are missing from some laboratory strains that have been maintained by mechanical transmission. RNA-2 of some tobravirus isolates contains an additional small ORF between the coat protein (CP) and P2b genes, which codes for a potential 9 103 protein.
Virions contain no lipids.
Virions contain no carbohydrates.
Genome Organization and Replication
RNA-1 is capable of independent replication and systemic spread in plants. The 134-141 103 and 194-201 103 replication proteins are translated directly from it, whereas P1a and P1b are translated from sgRNA species 1a and 1b, respectively. RNA-2 does not itself have messenger activity; the CP is translated from sgRNA-2a. The means by which the other RNA-2 encoded proteins are expressed is unknown. There is sequence homology between RNA-1 and RNA-2 at both ends, but the extent of the homology varies between strains. In some strains, the homologous region at the 3-end is large enough to include some or all of the P1a and P1b genes of RNA-1, but it is not known whether these genes are expressed from RNA-2. Accumulation of virus particles is sensitive to cycloheximide but not to chloramphenicol, suggesting that cytoplasmic ribosomes are involved in viral protein synthesis. Virions accumulate in the cytoplasm. L particles of Pepper ringspot virus (PepRSV) become radially arranged around mitochondria, which are often distorted, and in cells infected with some TRV isolates, ‘X-bodies’ largely composed of abnormal mitochondria and containing small aggregates of virus particles may be produced (Fig. 2).
Viruses are moderately immunogenic. There is little or no serological relationship between members of the genus, and considerable antigenic heterogeneity among different isolates of the same virus.
The host ranges are wide, including members of more than 50 monocotyledonous and dicotyledonous plant families. The natural vectors are nematodes in the genera Trichodorus and Paratrichodorus (Trichodoridae), different species being specific for particular virus strains. Adults and juveniles can transmit, but virus is probably not retained through the moult. Virus can be retained for many months by non-feeding nematodes. Virus particles become attached to the esophageal wall of the nematodes and are thought to be egested with saliva into root cells when the nematodes feed. There is no evidence for multiplication of virus in the vector and it is probably not transmitted through nematode eggs. The viruses are transmitted through seed of many host species. TRV occurs in Europe (including Russia), Japan, New Zealand and North America; PEBV occurs in Europe and North Africa, and PepRSV occurs in South America. TRV causes diseases in a wide variety of crop plants as well as weeds and other wild plants, including spraing (corky ringspot) and stem mottle in potato, rattle in tobacco, streaky mottle in narcissus and tulip, ringspot in aster, notched leaf in gladiolus, malaria in hyacinth and yellow blotch in sugar beet. PEBV is the cause of diseases in several legumes, including broad bean yellow band, distorting mosaic of bean and pea early-browning. PepRSV causes diseases in artichoke, pepper and tomato.
Most tissues of systemically invaded plants can become infected, but in many species virus remains localized at the initial infection site. In some virus-host combinations, notably TRV in some potato cultivars, limited systemic invasion occurs, and virus may not be passed on to all the vegetative progeny of infected mother plants.
Normal particle-producing isolates (called M-type) are readily transmitted by inoculation with sap and by nematodes. Other isolates (called NM-type) have only RNA-1, do not produce particles, are transmitted with difficulty by inoculation with sap, and are probably not transmitted by nematodes. NM-type isolates are obtained from M-type isolates by using inocula containing only L particles, and are also found in naturally infected plants. They often cause more necrosis in plants than do their parent M-type cultures.
List of Species Demarcation Criteria in the Genus
The criteria demarcating species in the genus are:
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RNA-1-specific cDNA probes of > 1 bk fail to hybridize with RNA-1 of heterologous species, |
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Intergenic pseudo-recombinant isolates cannot be made, |
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Host range differ in specific hosts (e.g., legumes), |
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RNA-2 sequences and serological relationships are of limited value. |
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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Pea early-browning virus (120) |
[X14006, X15883, X51828, X78455] |
(PEBV) |
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Pepper ringspot virus (347) |
[L23972, X03241] |
(PepRSV) |
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Tobacco rattle virus (12, 346) |
[X06172, D00155, X03955, J04347, X03685, X03686, Z36974] |
(TRV) |
Tentative Species in the Genus
None reported.
Phylogenetic Relationships within the Genus
Not available.
The 134-141 103 and 194-201 103 nonstructural proteins contain conserved sequence motifs common to RNA-dependent RNA polymerases of many viruses, and are most closely related to the analogous proteins of Tobacco mosaic virus. The 29-30 103 protein encoded by RNA-1 also shares sequence similarities with the analogous 30 103 protein of Tobacco mosaic virus and, to a lesser extent, with non-structural proteins of some other plant viruses. The structural proteins share sequence motifs with those of other plant viruses with rod-shaped particles.
Tobra: sigla from tobacco rattle virus.
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