Pea enation mosaic virus
R. J. Shepherd
Department of Plant Pathology, University of California, Davis, California, USA
Contents
Introduction
-
Described by
Osborn (1935).
-
Selected synonyms
-
Enation pea mosaic virus
(Stubbs, 1935)
-
Pea virus 1 (Rev. appl. Mycol. 15: 418)
-
Pisum virus 1
(Smith, 1937)
-
An RNA-containing virus with isometric particles about 30 nm in diameter. The virus
has a restricted host range, is easily transmissible by mechanical means, and by aphids
in a persistent manner. It is widely distributed in temperate regions.
Main Diseases
Causes common diseases of broad bean, garden pea and sweet pea. Infected plants
develop mosaic, translucent spots, and enations.
Geographical Distribution
Widely distributed in northern temperate regions.
Host Range and Symptomatology
Host range of the virus is narrow. Though it infects many legumes
(
Hagedorn, Layne & Ruppel, 1964;
McEwan & Schroeder, 1956),
few species in other families are
susceptible. It infects
Nicotiana clevelandii systemically and induces local
lesions in
Chenopodium album,
C. amaranticolor and
C. quinoa.
- Diagnostic species
- Pisum sativum (garden pea), Trifolium incarnatum (crimson clover), and
Vicia faba (broad bean). Systemically infected leaves show conspicuous,
hyaline, translucent spots (‘windows’), mosaic patterns, and, frequently, small
necrotic flecks and streaks
(Fig.1,
Fig.2).
Chronically infected plants generally
develop enations associated with the veins on undersides of leaves
(Hagedorn et al., 1964;
Quantz, 1952)
(Fig.3).
- Propagation species
- Garden peas contain the highest virus concentration 10-12 days after mechanical
inoculation when plants are grown at 18-22°C
(Izadpanah & Shepherd, 1966).
- Assay species
- The chenopodiaceous species that give local lesions can be used for assay, though not
all seed stock of these species gives suitable local lesions.
Strains
Isolates from different geographical areas differ in host range, symptomatology,
and properties
in vitro
(
Ruppel & Hagedorn, 1963;
McWhorter & Cook, 1958).
Strains commonly differ in ease of mechanical transmission. Divergent types
arise when isolates are transmitted solely by aphids or by mechanical inoculation
(
Osborn, 1938).
Transmission by Vectors
Transmissible by 5 of more than 20 species of aphids tested
(
Kennedy, Day & Eastop, 1962),
including
Acyrthosiphon pisum and
Myzus persicae. All
instars transmit, nymphs more efficiently than adults. Nymphs can acquire virus in
15 min, adults in 1-2 h; all exhibit a temperature-dependent latent period of
4-70 h
(
Osborn, 1935;
Sylvester, 1965;
Sylvester & Richardson, 1966).
After
the latent period, insects can inoculate plants in 1-2 min
(
Nault, Gyrisco & Rochow, 1964).
The virus can be retained by moulting individuals up to 30 days but
most cease to transmit within a few days. Not transmitted to progeny. Multiplication
in insects uncertain.
Transmission through Seed
Not known to be seed-borne.
Serology
The virus is not strongly immunogenic in rabbits. It diffuses well through agar
gels and two specific precipitin bands may be obtained. Sap from recently infected
plants contains a rapidly diffusing virus protein which may precipitate in
double-diffusion tests. Chronically infected plants may contain too little virus for
positive reactions in agar gels. Specific precipitates in liquid are of the granular
(somatic) type.
Relationships
No serologically related viruses have been reported.
Stability in Sap
In sap of
Vicia faba the thermal inactivation temperature (10 min) is
about 65°C, dilution end-point 10
-4 and longevity at 20°C 4 days
(
Osborn, 1938).
Purification
Inoculate young pea plants with a highly infectious extract and collect material
10-12 days after inoculation to insure that sufficient virus is present to make
purification practical.
Chronically infected plants contain very little virus. The following method for
purification
(
Izadpanah & Shepherd, 1966;
Shepherd, Wakeman & Ghabrial, 1968)
frequently yields
0.1-0.3 mg virus per g
infected material:
Blend pea tissue in 0.2 M acetate buffer (pH 6.0). To clarify, adjust the
homogenate to pH 5.2 and after 30 min adjust back to pH 6.0 and centrifuge at slow
speed. The partially clarified supernatant is given 2-3 cycles of differential
centrifugation using 0.1 M acetate, pH 6.0, as suspending fluid. The virus may be
further purified by sucrose density-gradient centrifugation or further incubation
of the extracts at pH 5.2 followed by additional differential cycles. Exposure to
strong salt solutions (e.g. 1 M CaCl2) degrades the virus.
Properties of Particles
Purified preparations contain two sedimenting components. It is not clear whether
one or both of these is associated with infectivity
(
Bozarth & Chow, 1966;
Gibbs, Harrison & Woods, 1966,
Izadpanah & Shepherd, 1966).
Sedimentation coefficients (s20,w): c. 100 and
120 S.
Diffusion coefficient (D20,w): c. 1.89 x 10-7
cm2/sec
(Bozarth & Chow, 1966).
Absorbance at 260 nm (1 mg/ml, 1 cm light path): 7.5.
Buoyant density (in CsCl)= 1.42 g/cm3.
Particle Structure
Purified virus preparations appear to contain particles of two types. Though
both types are isometric and about 30 nm in diameter when mounted in neutral
phosphotungstate, one type appears irregular and distorted unless previously
treated with 1% formaldehyde; the other type exhibits a more regular appearance
(
Gibbs et al., 1966).
When shadowed, the two types of particle are not
differentiated and both appear somewhat larger in diameter (36 nm)
(
Izadpanah & Shepherd, 1966).
Fig.4
shows particles mounted in uranyl acetate.
Particle Composition
Contains 2.48% phosphorus.
RNA: Single-stranded; 28% by weight of particle. Molar percentages of
nucleotides G26; A24; C24; U26.
(Shepherd et al., 1968).
Top component
estimated to contain 18% RNA
(Bozarth & Chow, 1966).
Protein: 72% of particle by weight. In the particle there is a single
type of protein which has a relatively large content of basic amino acids. The
protein contains about 199 amino acid residues, and has a molecular weight of
21,800
(Shepherd et al., 1968).
Other components: No polyamine detected.
Relations with Cells and Tissues
In plants, large accumulations of virus occur in cell nuclei and the virus
may multiply there; less virus is found in the cytoplasm and vacuoles. Virus-like
particles have been observed in the cytoplasm of fat body cells of an aphid vector,
Acyrthosiphon pisum
(
Shikata, Maramorosch & Granados, 1966;
Shikata & Maramorosch, 1966).
Notes
The virus is readily distinguished from all others by its characteristic symptoms,
mechanical transmissibility and persistence in its aphid vector.
References
- Bozarth & Chow, Contr. Boyce Thompson Inst. Pl. Res. 23: 301, 1966.
- Gibbs, Harrison & Woods, Virology 29: 348, 1966.
- Hagedorn, Layne & Ruppel, Phytopathology 54: 843, 1964.
- Izadpanah & Shepherd, Virology 28: 463, 1966.
- Kennedy, Day & Eastop, A conspectus of aphids as vectors of plant viruses, London, Commonwealth Institute of Entomology, 1962.
- McEwan & Schroeder, Pl. Dis. Reptr 40: 11, 1956.
- McWhorter & Cook, Pl. Dis. Reptr 42: 51, 1958.
- Nault, Gyrisco & Rochow, Phytopathology 54: 1269, 1964.
- Osborn, Phytopathology 25: 160, 1935.
- Osborn, Phytopathology 28: 923, 1938.
- Quantz, NachrBl. dt. PflSchutzdienst (Braunschw.), Stuttg. 4: 24, 1952.
- Ruppel & Hagedorn, Phytopathology 53: 813, 1963.
- Shepherd, Wakeman & Ghabrial, Virology 35: 255, 1968.
- Shikata, Maramorosch & Granados, Virology 29: 426, 1966.
- Shikata & Maramorosch, Virology 30: 439, 1966.
- Smith, Textbook of Plant Virus Diseases, London, Churchill, 1937.
- Stubbs, Viruses of the garden pea (Pisum sativum), Ph.D. thesis, Univ. Wisc., 1935.
- Sylvester, Virology 25: 62, 1965.
- Sylvester & Richardson, J. econ. Ent. 59: 255, 1966.
(Left and centre) systemically infected plants of Pisum sativum,
(right) healthy plant.
Systemically infected leaflet of Vicia faba.
Enations on leaf of systemically infected Metilotus indica
(courtesy of D. J. Hagedorn).
Virus particles from a purified preparation mounted in uranyl acetate. Bar
represents 100 nm.
