189
August 1978
Family: Virgaviridae
Genus: Unassigned Virgaviridae
Species: Nicotiana velutina mosaic virus
Acronym: NVMV


Nicotiana velutina mosaic virus

J. W. Randles
Department of Plant Pathology, Waite Agricultural Research Institute, The University of Adelaide, Glen Osmond, South Australia, 5064

Contents

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

Introduction

Described by Randles, Harrison & Roberts (1976).

An RNA-containing virus with straight rod-shaped particles 18 nm in diameter, and a large range of lengths, the commonest being 125-150 nm. Transmissible through seed, and by mechanical inoculation of sap to plants in three families. Has no known economic importance.

Main Diseases

Isolated from a Nicotiana velutina plant showing bright yellow mosaic.

Geographical Distribution

Only known occurrence is adjacent to a dam 8 km from Lake Frome Station Homestead, in the semi-arid region of South Australia.

Host Range and Symptomatology

Fairly readily transmissible by inoculation of sap. Infects species in the Solanaceae (including many Nicotiana species) and Chenopodiaceae; Gomphrena globosa (Amaranthaceae) is the only known host outside these families. Symptom intensity and speed of development vary with light and temperature conditions, and are favoured by continuous light of approximately 5000 lux at 25°C.

Diagnostic species

Nicotiana glutinosa. Inoculated leaves are usually symptomless but may develop chlorotic rings. Systemic vein chlorosis and mosaic persist throughout the life of the plant (Fig.2).

Nicotiana tabacum (tobacco) cv. Xanthi-nc. Inoculated leaves develop delicate necrotic rings and lines. A systemic mild mosaic is produced.

Nicotiana debneyi. Large spreading chlorotic blotches develop in inoculated leaves, sometimes accompanied by delicate necrotic rings. The first leaf with systemic symptoms becomes yellow, often with necrosis. Leaves produced later show a mild mosaic, rugosity, and narrowing of the lamina (Fig.5).

Chenopodium amaranticolor and C. quinoa. Chlorotic local lesions, becoming necrotic. Not infected systemically (Fig.4).

Beta macrocarpa. Chlorotic local lesions and systemic mosaic (Fig.3).

Gomphrena globosa. Enlarging local necrotic ringspots. Systemic chlorosis with crinkling and epinasty of leaves (Fig.1).

Tetragonia expansa is not a host.

Propagation species

Nicotiana glutinosa plants and seed are suitable for maintaining inoculum; N. debneyi is favoured as a source for virus purification.

Assay species

Chenopodium amaranticolor and C. quinoa are useful local lesion hosts.

Strains

None found.

Transmission by Vectors

None found.

Transmission through Seed

Frequently transmitted through the seed of Nicotiana glutinosa (up to 72%), N. clevelandii, N. debneyi and N. rustica.

Serology

The virus is moderately to poorly immunogenic. Tube precipitin tests, and double diffusion in gels containing agarose at 0.5% and below are satisfactory. Observation of antigen-antibody agglutination by electron microscopy is favoured for studying cross-reactions because specific attachment of antibody to antigen can be observed, sensitivity is high, and requirements for reactants are small.

Relationships

Host range and structure of the virus suggest relatedness to the rod-shaped fungus-transmitted viruses such as potato mop-top virus, which are now regarded as tentative members of the tobamovirus group. However, the size of the coat protein subunits, and the frequent seed transmission suggest that it may also be allied to barley stripe mosaic virus.

No serological relationship has been found between Nicotiana velutina mosaic virus and potato mop-top, soil-borne wheat mosaic, tobacco mosaic, beet necrotic yellow vein, barley stripe mosaic or tobacco rattle viruses.

Stability in Sap

Sap of infected N. glutinosa or N. debneyi loses infectivity when stored from 1 to 4 days at room temperature. Infectivity declines in sap heated at 50°C for 10 min, and is lost after 10 min at between 60° and 70°. The dilution end-point of infective N. glutinosa sap, diluted in water and tested on N. glutinosa, is between 10-1 and 10-2.

Infectivity is enhanced and greatly stabilized by adding reducing agents (for example 0.002 M dithiothreitol or 0.05% thioglycerol), but not chelating agents (for example 0.01 M sodium diethyl dithiocarbamate), to preparations in 0.05 M Tris-HCl buffer, pH 8.1.

Purification

The purification method employs a high pH step to dissociate virus particles from rapidly sedimenting material. Clarification is achieved by sedimentation through 55% sucrose.

The following modification (D. Cartwright, unpublished data) of the original method of Randles et al. (1976) yields up to 20 mg of virus per kg of N. debneyi leaf. Blend tissue with 3 vol. 0.067 M phosphate buffer (pH 5.6; containing 0.01 M sodium diethyl dithiocarbamate and 0.05% thioglycerol), strain through muslin and centrifuge at 12,000 g for 20 min. Resuspend the pellet in 0.05 M glycine buffer (pH 9.0; containing 0.05% thioglycerol), stir 4-6 h. Clarify at 2500 g for 20 min, layer over 55% sucrose containing 0.05% thioglycerol, and centrifuge at 176,000 g for 90 min. Resuspend the pellet in 1-2 ml 0.05% thioglycerol for 16 h. Clarify and centrifuge again through 55% sucrose, resuspend for 2 h and centrifuge through a 10-40% linear sucrose gradient (buffered with 0.05 M glycine, pH 9.0) at 74,000 g for 25 min. Recover the very broad light-scattering zone and sediment the virus at 176,000 g for 40 min. Work at 4°C.

Properties of Particles

Ultraviolet absorption spectrum shows a minimum at 248-250 nm, maximum at 262-264 nm.

A260/A280: 1.15-1.22.

Particle Structure

Particles are straight rods, 18-19 nm in diameter (Fig.6). In partially purified preparations most particles are 100-175 nm long. However, the length distribution is broad with the frequency of particles in the smaller size classes increasing during purification to give a peak in the 125-150 nm class. This suggests that particles are either fragile, or undergo progressive dissociation of end-to-end aggregates during purification. Particles have helical symmetry, with a pitch of 2.86 ± 0.03 nm when stained in ammonium molybdate or uranyl formate (Fig.7).

Particle Composition

Nucleic acid: Nucleic acids extracted from infected leaf show a zone of infectivity between 15 S and 33 S when sedimented in sucrose density gradients. Infectivity of these extracts is lost upon treatment with ribonuclease A (1 µg/ml). Nucleic acid extracts from purified virus show a 2.3 x 106 M. Wt component and up to 8 smaller minor species when fractionated on polyacrylamide gels (D. Cartwright, unpublished data).

Protein: Subunits have M. Wt of 2.14 x 104.

Other components: None known.

Relations with Cells and Tissues

Particles were not seen in locally or systemically infected tissue, and no intracellular changes have been observed.

Notes

Nicotiana velutina mosaic virus resembles the seedborne rod-shaped barley stripe mosaic virus (Atabekov & Novikov, 1971), and also the fungus-transmitted tobamoviruses, such as potato mop-top virus (Harrison, 1974), soil-borne wheat mosaic virus (Brakke, 1971), beet necrotic yellow vein virus (Tamada, 1975; Putz, 1977), and possibly peanut clump virus (Thouvenel, Dollet & Fauquet, 1976). However, it is distinguishable from these by its host range, efficient seed transmissibility in some Nicotiana species, particle instability and serological uniqueness. A useful test which distinguishes it from all the above viruses is its ability to infect Nicotiana species and Gomphrena globosa, but not Tetragonia expansa.

References

  1. Atabekov & Novikov, CMI/AAB Descriptions of Plant Viruses 68, 4 pp., 1971.
  2. Brakke, CMI/AAB Descriptions of Plant Viruses 77, 4 pp., 1971.
  3. Harrison, CMI/AAB Descriptions of Plant Viruses 138, 4 pp., 1974.
  4. Putz, J. gen. Virol. 35: 397, 1977.
  5. Randles, Harrison & Roberts, Ann. appl. Biol. 84: 193, 1976.
  6. Tamada, CMI/AAB Descriptions of Plant Viruses 144, 4 pp., 1975.
  7. Thouvenel, Dollet & Fauquet, Ann. appl. Biol. 84: 311, 1976.

Acknowledgements

Figs 1, 3, 6, and 7 by courtesy of Scottish Horticultural Research Institute.


Figure 1

Systemically infected Gomphrena globosa.

Figure 2

Systemic mosaic and distortion in Nicotiana glutinosa leaves.

Figure 3

Systemically infected Beta macrocarpa leaf.

Figure 4

Chlorotic local lesions in Chenopodium quinoa.

Figure 5

Systemic chlorosis, interveinal necrosis and leaf distortion of Nicotiana debneyi.

Figure 6

Virus particles from a purified preparation, stained with 2% ammonium molybdate. Bar represents 200 nm.

Figure 7

Helical structure of virus particles; stained with uranyl formate. Bar represents 100 nm.