![]() 393 October 2002 | Family: Potyviridae Genus: Tritimovirus Species: Wheat streak mosaic virus Acronym: WSMV |
This is a revised version of DPV 48 |
Roy French
USDA, ARS, Department of Plant Pathology, University of Nebraska, Lincoln, USA
Drake C. Stenger
USDA, ARS, Department of Plant Pathology, University of Nebraska, Lincoln, USA
Introduction Main Diseases Geographical Distribution Host Range and Symptomatology Strains Transmission by Vectors Transmission through Seed Transmission by Grafting Transmission by Dodder Serology Nucleic Acid Hybridization Relationships Stability in Sap | Purification Properties of Particles Particle Structure Particle Composition Properties of Infective Nucleic Acid Molecular Structure Genome Properties Satellites Relations with Cells and Tissues Ecology and Control Notes References Acknowledgements Figures |
Described by McKinney (1937).
A mite-transmitted virus with filamentous particles (about 15 x 700 nm) with a monopartite, single-stranded RNA genome. The virus is readily transmitted by mechanical inoculation of plant sap to many species in the Poaceae but does not infect any dicotyledonous plants.
Infects most (see STRAINS) varieties of wheat (Triticum aestivum), oats (Avena sativa), barley (Hordeum vulgare), and rye (Secale cerale), and certain inbred lines or varieties of maize (Zea mays), sorghum (Sorghum vulgare), and millets (Panicum, Setaria, and Echinochloa spp.); all develop yellow-green mosaic. Infects many wild grass species in the genera Aeiglops, Agropyron, Bouteloua, Bromus, Cenchrus, Digitaria, Echinochloa, Elymus, Eragrostis, Haynaldia, Hordeum, Lolium, Panicum, Phalaris, Poa, Orizopsis, Setaria, and Stipa. Does not infect Agropyron repens, Bromus inermis, Hordeum jubatum, Oryza sativa (rice) or Saccharum officinarum (sugar cane).
No dicotyledonous hosts have been reported (Brakke, 1971; Kahn & Dickerson, 1957; Seifers, et al., 1996; Sill & Agusioba, 1953; Sill & Conin, 1953).
In the field, virus symptoms first appear as temperatures become warmer in mid to late spring. Infected plants exhibit mosaic symptoms with tillers much less erect than those of uninfected plants. Symptoms persist until plant maturity.
Causes systemic mosaic symptoms, usually with stunting in nearly all varieties of wheat (Fig.2), barley, and oats, and some maize varieties, at both cool and warm temperatures. Some isolates give sporadic necrotic lesions on some sorghum (Sorghum bicolor) varieties.
Wheat. Essentially any winter wheat variety is suitable. Historically the variety Michigan Amber has been used because it resists mildew in typical greenhouse conditions.
No satisfactory local lesion assay host has been reported. Systemic assays of diluted sap may be made on wheat seedlings. Vector transmission tests also are best done with wheat, which is also a good host of the mite.
Transmitted by an eriophyid mite, Aceria tosichella (Fig.3). Earlier literature refers to this species (common name: wheat curl mite) as either A. tulipae or Eriophyes tulipae. Both adult and nymphal stages of the mite can transmit virus, but only nymphs may acquire virus. Mites tend not to settle down and feed upon transfer to new leaves, which makes timing studies difficult. The virus acquisition access feeding time is 15 min or more and the virus persists for at least 9 days in mites kept on virus-immune plants. There is no evidence either way regarding the presence of a latent period following virus acquisition. Virus inoculation access feeding time is several hours although an access feeding time of 15 min resulted in 2.5% transmission. The virus is not vertically transmitted to progeny through the egg. Mite transmission can serve as a bottleneck to reduce genetic variation in the virus (Amrine & Stansy, 1994; Hall et al., 2001; Orlob, 1966; Slykhuis, 1955; Staples & Allington, 1956).
Paliwal and Slykhuis (1967) detected WSMV by serology and by infectivity assays in homogenates of A. tosichella reared on WSMV-infected plants but not in homogenates of two non-vector mite species reared on similar plants. These observations led to speculation that back flow of gut contents during feeding or, excretion of intact virus, were potential routes for virus transmission by A. tosichella. Electron microscopy of thin sections of viruliferous A. tosichella detected aggregates of virus particles in the posterior midgut and hindgut that persisted for at least 5 days after feeding (Paliwal & Slykhuis, 1967). In later electron microscope studies, Paliwal (1980) reported the detection of WSMV particles in the haemocoel and salivary glands, but not in the hindgut, of viruliferous A. tosichella. When Abacarus hysterix, a non-vector of WSMV, reared on WSMV-infected plants, was examined by electron microscopy, small numbers of degraded WSMV particles were detected only in the gut. Although such studies may suggest that virus transmission by A. tosichella may be circulative, further evidence for this is lacking.
A slight modification of the method described by Brakke (1971) gives reliable yields. Harvest the youngest 2 or 3 leaves of systemically infected wheat seedlings about 10 days after inoculation and triturate in 2 volumes (w/v) of 0.01 M K2HPO4. Adjust the pH to 6.1 with 1 M acetic acid and centrifuge for 30 min at 12,000 g. Remove and adjust the supernatant fluid to pH 8.0 and add 1/10 vol 0.1 M sodium citrate, pH 8.0 and bring to 1% Triton X-100. Layer the mixture over 4 ml cushion of 20% (w/v) sucrose in 0.01 M sodium citrate, pH 8.0 and centrifuge for 2 hr at 85,000 g. Resuspend the pellets in 0.01 M sodium citrate, pH 8.0 and clarify by centrifuging for 5 min at 3,000 g. Preparations can be purified further by centrifugation in sucrose (10-40%) density gradients (Brakke et al., 1990).
Alternative methods for purifying virus from frozen material (Slykhuis & Bell, 1965) and from infected maize leaves (Uyemoto & Ferguson, 1980) have been described.
Sedimentation coefficient as determined in sucrose density gradients is about 165S. Buoyant density in CsCl is 1.30-1.32 g.cm-3.
A260/280: 1.37; A230/260: 2.7; A260/320: 16 (Brakke & Van Pelt, 1968).
Nucleic acid: Virus particles contain a single species of ssRNA. The complete nucleotide sequence (9,339 to 9,384 nt) of five isolates have been determined and deposited in GenBank as Accession Nos. AF057533, AF285169, AF285170, AF454454, AF454455 (Stenger et al., 1998; Choi et al., 2001; Rabenstein et al., 2002).
Protein: In polyacrylamide gel electrophoresis, virus particle preparations typically contain a major protein of Mr 45,000-47,000. Electrophoretic mobility is anomalous because coat protein size estimates vary with gel concentration and do not correspond to the molecular mass calculated from its predicted amino acid sequence. Most preparations contain varying amounts of smaller (Mr 31,000-40,000), partially degraded forms of the protein (Brakke et al., 1990). By analogy to other potyviruses, virions presumably contain a VPg protein linked to the 5'-end of the genomic RNA, but this has not been determined experimentally.
The RNA molecule encodes a single ORF of about 350 kDa (Fig.5) that is processed into mature proteins analogous to those of other potyviruses. P1 (41.2 kDa) has proteinase activity and probably cleaves at a GLRWY/G motif; HC-Pro (44.9 kDa) also is a proteinase; P3 (32.6 kDa) is of unknown function; CI (75.6 kDa) is the cylindrical inclusion body protein and has motifs shared by the DEAD/H class of RNA helicases; 6K1 and 6K2 are two short proteins of unknown function flanking CI; NIa (49.9 kDa) is the viral proteinase and probably also encodes a genome-linked VPg sequences; NIb (58.5 kDa) is the RNA-dependent RNA polymerase and is followed by the carboxy-terminal coat protein (40.9 kDa) (Stenger et al., 1998; Choi et al., 2000a; 2002).
Infectious cDNA clones have been produced for the virus. Foreign gene sequences have been inserted into these and expressed in wheat, barley, maize and oat (Choi et al., 1999; 2000b).
There are several viruses with similar morphology that infect wheat. However, of these, Agropyron mosaic virus infects Agropyron repens and Hordeum mosaic virus infects Hordeum jubatum, species that are immune to WSMV. Another virus, Wheat spindle streak mosaic virus is difficult to transmit mechanically and mosaic symptoms due to this virus fade as temperatures increase above 15°C, unlike those of WSMV. Brome streak mosaic virus can be distinguished by serologically from WSMV. The other tritimovirus, Oat necrotic mottle virus, does not infect wheat. Reverse transcription-polymerase chain reaction based assays are convenient for diagnosing WSMV (Hall et al., 2001).
In mixed infections with Maize chlorotic mottle virus titres of both viruses are increased several-fold (Scheets, 1998).
A severely stunted infected wheat plant with oblique growth habit of tillers.
Wheat leaves showing a range of chlorotic symptoms; bottom leaf is healthy.
Scanning electron micrograph of the wheat curl mite vector, Aceria tosichella. Bar represents 10 µm.
Transmission electron micrograph of virus particles stained with uranyl acetate. Bar represents 100 nm.
Genome organization; box represents the polyprotein open reading frame with vertical lines demarcating sites of cleavage by viral proteinases. Predicted mature proteins are labelled by analogy to other potyviruses: P1 - P1 protein, HC-Pro - helper component and proteinase (note that it is not known whether HC-Pro of WSMV is involved in transmission of the virus by mites), P3 - P3 protein, 6K1 and 6K2 - 6 kDa proteins, CI - cytoplasmic inclusion protein, NIa - comprised of a 5' genome-linked protein (VPg) and proteinase (Pro) domains, NIb - replicase, and CP - coat protein.