11
June 1970
Family: Tymoviridae
Genus: Tymovirus
Species: Cacao yellow mosaic virus
Acronym: CYMV


Cacao yellow mosaic virus

A. A. Brunt
Glasshouse Crops Research Institute, Littlehampton, Sussex, England

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 Brunt et al. (1965).

A virus infecting cacao in Sierra Leone, readily transmitted by inoculation of sap to a moderately wide range of hosts, and having isometric particles c. 28 nm in diameter. Although no vector has yet been found, the virus is serologically distantly related to the beetle-transmitted viruses turnip yellow mosaic and wild cucumber mosaic.

Main Diseases

Naturally infected West African Amelonado cacao leaves produce large circular chlorotic blotches which often coalesce to give a complete chlorosis or yellow mosaic (Fig.1). Infected trees are not severely debilitated, but the actual effect of infection on growth and yield has not been determined. The virus occurs also, but is virtually symptomless, in Cola nitida and Culcasia scandens (Blencowe et al., 1963).

Geographical Distribution

Recorded only from Sierra Leone (Blencowe et al., 1963).

Host Range and Symptomatology

Natural host range is narrow but the virus is transmitted readily by mechanical inoculation of sap to members of 9 of the 21 dicotyledonous families tested.

Diagnostic species

Theobroma cacao (cacao). Seedlings grown from inoculated Amelonado cacao beans produce conspicuous chlorotic leaf mottling symptoms, usually within 20-30 days of inoculation (Fig.2).

Chenopodium amaranticolor. Numerous circular necrotic local lesions c. 1-2 mm in diameter within 5 days (Fig.3), soon followed by systemic chlorotic and necrotic flecking (Fig.4).

Chenopodium quinoa. Numerous circular chlorotic local lesions 1-2 mm in diameter within 5 days, soon becoming necrotic. Systemic symptoms usually a mosaic.

Tetragonia expansa. Circular chlorotic local lesions 8-10 days after inoculation.

Propagation species

Cacao and Vinca rosea; Chenopodium quinoa, Nicotiana clevelandii and Nicandra physalodes are also useful sources of virus for purification.

Assay species

Chenopodium amaranticolor and C. quinoa.

Strains

None distinguished - single isolate only studied.

Transmission by Vectors

Not soil-borne or transmitted by mealybugs (Brunt et al., 1965), but beetles not yet tested as vectors.

Transmission through Seed

Not seed-borne in cacao.

Transmission by Dodder

Not tested.

Serology

The virus is strongly immunogenic, antisera with titres of at least 1/8192 being prepared without difficulty. The virus and its homologous antiserum combine well both in conventional precipitin tube tests, producing typical granular (‘somatic’) precipitates, and in agar gel-diffusion tests, producing a single curved line of precipitate adjacent to the antigen depot.

Relationships

The virus reacts with antisera to wild cucumber mosaic virus but not with those to turnip yellow mosaic virus, even though the last two viruses are themselves distantly related (MacLeod & Markham, 1963; Brunt et al., 1965). Cross-absorption tests, however, indicate that wild cucumber mosaic virus has some antigenic groups common to both turnip yellow mosaic and cacao yellow mosaic viruses (Brunt et al., 1965).

No serological relationship has been detected between cacao yellow mosaic virus and the following beetle-borne viruses: cowpea mosaic, Andean potato latent, dulcamara mottle, ononis yellow mosaic, bean pod mottle, bean (southern) mosaic, cocksfoot mottle, radish mosaic, squash mosaic, turnip crinkle, turnip rosette and Echtes Ackerbohnemosaik viruses (Brunt et al., 1965; Gibbs et al., 1966; Brunt, unpublished).

Stability in Sap

In cacao leaf extracts, the thermal inactivation point (10 min) is 60-65°C, dilution end-point about 20-3-20-4, and infectivity is retained for 16-32 days at 25-30°C or over 100 days at 0-4°C. Lyophilized sap stored in vacuo remains infective for over 5 years without appreciable loss of infectivity.

Purification

The virus is easily purified from cacao and other hosts. Infected cacao leaves are homogenized in an extractant (wt./vol. = 1/20) at pH 7.0 containing 0.05 M phosphate buffer and either 0.005 M diethyldithiocarbamate or 0.01 M thioglycollate. After clarifying the extract by low speed centrifugation (8000 g for 20 min), virus is precipitated by 50% saturation with ammonium sulphate (but not ethanol or acetone), or by ultracentrifugation (75,000 g for 90-120 min is adequate). After resuspension and dialysis against 0.005 M phosphate buffer, virus may be further concentrated by ultracentrifugation. Higher yields of ‘top’ component are obtained by precipitation with ammonium sulphate.

The virus may also be purified from Nicotiana clevelandii, Chenopodium amaranticolor and C. quinoa by one or more cycles of differential centrifugation after treating the extract with chloroform (25% v/v) or n-butanol (to 8.5% v/v) to aid clarification.

Purified preparations are readily fractionated into their two components by centrifugation in sucrose density-gradient columns.

Properties of Particles

The particles are all the same size, but sediment as two components (Fig.6), empty protein shells without nucleic acid (‘top’ component), and infective nucleoprotein (‘bottom’ component).

Sedimentation coefficients (s20, w) at infinite dilution: 49 and 108 S.

A260/A280 (‘bottom’ component): 1.42; maximum absorbance at 263-265 nm is presumably caused by relatively large amounts of cytidylic acid and small amounts of guanylic acid.

Electrophoretic mobilities: two components, -11.8 x 10-5 and -11.3 x 10-5 cm2 sec-1 volt-1 in 0.06 M phosphate buffer pH 7.0. These two components apparently do not correspond to the 49 and 108 S particles (Brunt et al., 1965).

Particle Structure

Particles are isometric c. 28 nm in diameter (Fig.5) with icosahedral symmetry, and are composed of 180 structural subunits arranged in pentamer-hexamer clusters (20 sixes and 12 fives) to produce 32 morphological units. ‘Top’ component particles are penetrated by 2% potassium phosphotungstate but ‘bottom’ component particles are not.

Particle Composition

RNA: Probably single-stranded. Molar percentages of nucleotides: G 16; A 22; C 33; U 29. RNA is estimated to be c. 38% of particle weight; sedimentation coefficient and molecular weight not determined.

Protein: 62% of particle weight; molecular weight and amino acid composition unknown.

Relations with Cells and Tissues

No intracellular inclusions detected by light microscopy.

Notes

Particles of cacao yellow mosaic, turnip yellow mosaic and wild cucumber mosaic viruses are similar in size, shape, external morphology and sedimentation velocity, and contain similar amounts of nucleic acid of similar composition. Although beetles have not yet been tested for their ability to transmit cacao yellow mosaic virus, its properties and its distant serological relationship to the other two viruses indicate that it should be grouped with beetle-transmitted viruses.

References

  1. Blencowe, Brunt, Kenten & Lovi, Trop. Agric. Trin. 40: 233, 1963.
  2. Brunt, Kenten, Gibbs & Nixon, J. gen. Microbiol. 38: 81, 1965.
  3. Gibbs, Hecht-Poinar, Woods & McKee, J. gen. Microbiol. 44: 177, 1966.
  4. MacLeod & Markham, Virology 19: 190, 1963.


Figure 1

Symptoms in a naturally infected West African Amelonado cacao leaf.

Figure 2

Systemically infected Amelonado leaf from a seedling grown from an inoculated cacao bean.

Figure 3

Circular necrotic lesions in an inoculated Chenopodium amaranticolor leaf.

Figure 4

Systemically infected C. amaranticolor leaf.

Figure 5

Purified virus preparation mounted in 2% neutral potassium phosphotungstate. Bar represents 100 nm. (Micrograph courtesy R. D. Woods.)

Figure 6

Schlieren diagram produced by purified virus in the analytical ultracentrifuge showing (left) 49 S and (right) 108 S components. (Photo courtesy R. D. Woods.)