Voandzeia necrotic mosaic virus
C. Fauquet
Laboratoire de Virologie, Centre ORSTOM d'Adiopodoumé, B.P. V.51, Abidjan, Côte d'lvoire
A. Monsarrat
Laboratoire de Virologie, Centre ORSTOM d'Adiopodoumé, B.P. V.51, Abidjan, Côte d'lvoire
J. C. Thouvenel
Laboratoire de Virologie, Centre ORSTOM d'Adiopodoumé, B.P. V.51, Abidjan, Côte d'lvoire
Contents
Introduction
-
Described by
Fauquet, Monsarrat & Thouvenel (1981) and
Monsarrat, Fauquet & Thouvenel (1984).
-
A virus with RNA-containing isometric particles c. 28 nm in diameter which sediment
as two components. It is found in nature only in Voandzeia subterranea. Its experimental
host range is restricted almost entirely to the Leguminosae. It is transmissible by inoculation
with sap but not through seed. No vector is known. The virus occurs in the Ivory Coast and Upper
Volta.
Main Diseases
Bambarra groundnut (
Voandzeia subterranea) is the only natural host found. The virus
causes stunting, mosaic, necrosis, distortion
(
Fig.1,
Fig.2,
Fig.3) and sometimes the death of the plant
(
Monsarrat et al., 1984).
Geographical Distribution
Found occasionally in fields of Bambarra groundnut in northern Ivory Coast and southern Upper
Volta.
Host Range and Symptomatology
Transmitted from
Voandzeia to
Voandzeia by inoculation with sap: 10-100% of
plants become infected. Best results were obtained by using inocula prepared in 0.1 M borate
buffer, pH 8.8, containing 0.02 M cysteine hydrochloride, 0.05 M sodium bisulphite and 0.5%
bentonite
(
Monsarrat et al., 1984).
Among 87 species tested from the families
Amaranthaceae, Chenopodiaceae, Cruciferae, Cucurbitaceae, Leguminosae, Malvaceae, Passifloraceae,
Scrophulariaceae and Solanaceae, only
Chenopodium amaranticolor, Dolichos lablab, Vigna
sinensis and
V. unguiculata were infected
(
Monsarrat et al., 1984).
-
Diagnostic species
- Chenopodium amaranticolor.
Local necrotic lesions, developing later into red ringspots
with yellow discoloration of the entire leaf.
- Dolichos lablab. Chlorotic local lesions and systemic mosaic.
- Vigna unguiculata subspp. unguiculata and sinensis (cowpea).
Chlorotic
local lesions and systemic mosaic.
- Voandzeia subterranea (Bambarra groundnut). No local symptoms, but the plants are
stunted and fail to develop. Systemically infected leaves show a yellow mosaic
(Fig.2)
with
necrosis along the veins leading to distortion of the leaflets
(Fig.3).
Plants infected young
may be killed.
-
Propagation species
- Voandzeia subterranea
is a good source of virus.
-
Assay species
- Chenopodium amaranticolor
is a useful local lesion host.
Strains
None found.
Transmission by Vectors
No vector reported. The okra leaf beetle,
Podagrica decolorata, and the
groundnut aphid,
Aphis craccivora, did not transmit the virus
(
Monsarrat et al., 1984).
Transmission through Seed
Not seed-borne in
Voandzeia subterranea.Serology
Very immunogenic in rabbits. Antisera with titres of 1/512-1/4096 in gel diffusion tests
have been prepared.
Relationships
Voandzeia necrotic mosaic virus has many properties typical of
tymoviruses
including
particle morphology, sedimentation behaviour, coat protein M. Wt, high thermal inactivation
point, high concentration of particles in host plants, typical effects on chloroplasts and
serological relationship to other members of the group. In agar gel double-diffusion tests,
particles of the virus react with antisera to only four members of the group. The virus is
related closely to
kennedya yellow mosaic virus
(SDI = 1) and distantly to
turnip yellow mosaic virus (SDI = 6),
desmodium yellow mottle virus (SDI = 6) and
okra mosaic virus (SDI = 5)
(
Fauquet et al., 1981).
The interrelationships among these five viruses can be expressed
diagrammatically as a second loop structure similar to that proposed by
Koenig (1976)
for
other members of the group.
Stability in Sap
When assayed in
Chenopodium amaranticolor, the virus, in crude sap of
Voandzeia
subterranea, loses infectivity after dilution 10
-2, after heating for 10 min at
70°C, and after storage for less than 2 days at 20°C or for more than 10 days at
4°C or -20°C. Infectivity survives for several months in frozen or dried leaves.
Purification
(
Monsarrat et al., 1984).
Homogenise infected
Voandzeia leaves in 0.2 M
phosphate buffer at pH 7, containing 0.4% thioglycollic acid (3 ml/g tissue). Clarify by
adding an equal volume of chloroform and concentrate by ultracentrifugation, resuspending the
pellets in 0.01 M phosphate buffer, pH 7. Centrifuge the virus particles through a 20% sucrose
cushion, resuspending the pellet in the same buffer.
The components may be separated in
sucrose density gradients. Yield is about 200 mg virus per kg infected leaves.
Properties of Particles
The particles sediment as two components in sucrose gradients, a fast sedimenting
(B) component consisting of infective nucleoprotein particles, and a slower sedimenting (T)
component consisting
of protein shells. The two components are serologically indistinguishable.
Sedimentation coefficient (s20, w) at infinite dilution: 51 S (T),
113 S (B).
Isoelectric point: pH 4.65 ± 0.10 (B).
A260/A280: 0.64 (T), 1.75 (B).
Amax: 280 nm (T), 260 nm (B). Amin: 250 nm (T),
242 nm (B).
Amax/Amin: 1.91 (T), 1.35 (B).
Buoyant densities in CsCl (g/cm3): 1.29 (T), 1.45 (B).
Particle Structure
Particles are about 28 nm in diameter
(
Fig.4)
and have icosahedral symmetry. T component
particles are penetrated by uranyl acetate (2%), whereas B component particles are not.
Particle Composition
Nucleic acid: Probably RNA, single-stranded, comprising more than 30% of the weight
of the B particles (estimated from the absorption spectrum).
Protein: Polyacrylamide gel electrophoresis of coat protein reveals one polypeptide
of M. Wt c. 20,000 ± 500.
Relations with Cells and Tissues
Crystals or high concentrations of virus-like particles occur in the cytoplasm
(
Fig.5)
and in the vacuoles of cells of infected
Voandzeia subterranea. Peripheral vesicles
bounded by a double membrane occur in the chloroplasts, as shown for other
tymoviruses
(
Fig.6).
Notes
Voandzeia necrotic mosaic virus is the first
tymovirus
to be found naturally infecting
Voandzeia subterranea. Among the four tymoviruses serologically related to it,
okra mosaic virus,
desmodium yellow mottle virus and
kennedya yellow mosaic virus
have appreciably wider host ranges, whereas
turnip yellow mosaic virus
infects only species in the Cruciferae.
Although voandzeia necrotic mosaic virus is closely serologically related to kennedya
yellow mosaic virus, the host ranges of the two viruses are completely different. Also,
the characteristic red local lesions induced by voandzeia necrotic mosaic virus in
Chenopodium amaranticolor are unlike those induced by other tymoviruses.
References
- Fauquet, Monsarrat & Thouvenel, Abstr. 5th Congr. Virology, Strasbourg, France, 1981: 237, 1981.
- Koenig, Virology 72: 1, 1976.
- Monsarrat, Fauquet & Thouvenel, C. r. hebd. Seanc. Acad. Sci., Paris Ser. 3, 299(3): 53, 1984.
Healthy leaves of Voandzeia subterranea.
Systemically infected leaves of Voandzeia subterranea, showing mosaic.
Systemically infected leaves of Voandzeia subterranea, showing necrosis.
B component particles in uranyl acetate. Bar represents 100 nm.
Virus particles aggregated on membranes in the cytoplasm of a V. subterranea
leaf cell. Bar represents 100 nm.
Ultrathin section of infected V. subterranea leaf showing vesicles
at different
stages bounded by double membranes at the periphery of a chloroplast.
Bar represents 500 nm.