361
September 1998
Family: Bromoviridae
Genus: Alfamovirus
Species: Cassava Ivorian bacilliform virus
Acronym: CsIBV


Cassava Ivorian bacilliform virus

D. Fargette
Laboratoire de Phytovirologie des Regions Chaudes, CIRAD, ORSTOM, BP5035, 34032 Montpellier, France

B. D. Harrison
Scottish Crop Research Institute, Invergowrie, Dundee DD2 5DA, UK

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 Aiton et al. (1988) and Fargette et al. (1991).

A virus with bacilliform particles c. 18 nm in diameter and predominant lengths of 42, 49 and 76 nm. Transmissible by inoculation with sap and having affinities to the genus Alfamovirus. Reported from cassava in Côte d’Ivoire.

Main Diseases

Found naturally in cassava (Manihot esculenta: Euphorbiaceae) plants co-infected with African cassava mosaic begomovirus. Its effects on begomovirus-free cassava are unclear and it may be of only trivial economic importance. Probably maintained in cassava by vegetative propagation of infected plants.

Geographical Distribution

The type isolate was originally obtained from a cassava plant collected at Touresso, Côte d'Ivoire. However, the occurrence of CIBV at several other sites in north western Côte d'Ivoire suggests that it is more than a casual pathogen of cassava.

Host Range and Symptomatology

Only known natural host is cassava but the virus was transmitted experimentally to 12 species in five other families. All were infected systemically but several remained symptomless. Nineteen other species in eight plant families were not infected.

Diagnostic species

Chenopodium amaranticolor. Inoculated leaves symptomless. Slight distortion and stunting of systemically infected tip leaves.

C. murale. Necrotic lesions developed in inoculated leaves after 2-3 days (Fig.1). Systemic necrosis (Fig.2) appeared in tip leaves about 5 days after inoculation.

C. quinoa. Faint chlorotic lesions, sometimes with some necrosis, in inoculated leaves (Fig.3). Systemic shoot necrosis developed after 5-7 days (Fig.4), eventually spreading to kill some plants.

Tetragonia expansa. Necrotic local lesions and symptomless systemic infection.

Nicotiana benthamiana, N. clevelandii and Phaseolus vulgaris become infected systemically but develop few or no symptoms.

Propagation species

C. quinoa is suitable for maintaining isolates and is a good source of virus for purification.

Assay species

C. murale is a useful local lesion assay host.

Strains

Isolates differ in virulence. Isolate F143 caused symptomless systemic infection instead of systemic necrosis in C. quinoa and chlorotic instead of necrotic local lesions in C. murale.

Transmission by Vectors

No vector is known. CIBV was not transmitted by Myzus persicae after an acquisition access period of <2 h.

Transmission through Seed

No information.

Serology

Poorly immunogenic. Antiserum from a rabbit given nine intramuscular injections had a titre of only 1/64 in double diffusion tests in agarose gel. In DAS-ELISA, purified virus particles were detected at concentrations down to about 6 ng/ml. With crude sap of systemically infected C. quinoa, the detection end point was about 10-4. In immunosorbent electron microscopy (ISEM), the number of virus particles on grids was more than 50-fold greater than on uncoated grids.

Relationships

In ISEM no increase in particle numbers was observed when grids were coated with antiserum to alfalfa mosaic, ourmia melon, prune dwarf, prunus necrotic ringspot or olive latent 2 viruses. Little if any reaction occurred when CIBV antibodies were used in DAS-ELISA in attempts to detect alfalfa mosaic virus. CIBV appears to be a distinct virus which nevertheless has strong affinities with alfalfa mosaic virus.

Stability in Sap

C. quinoa leaf extracts in 50 mM borate buffer, pH 8.5, were infective after dilution to 10-2 but not at 10-3, and after heating for 10 min at 50°C but not at 55°C. In N. benthamiana, CIBV concentration seemed unaffected by co-infection with African cassava mosaic begomovirus.

Purification

Extract systemically infected C. quinoa leaves, harvested 5-7 days after plants are inoculated, in 5 mM borate buffer (4 ml/g leaf) at pH 8.5, using a Waring blendor. Re-extract fibre. Emulsify combined extracts with chloroform, separate aqueous phase by centrifugation (10,000 g, 10 min) and add polyethylene glycol (M. Wt 6000) and sodium chloride to 60 g/l and 0.2 M, respectively. Collect precipitate by centrifugation (10,000 g, 10 min), resuspend in 0.1 M phosphate buffer, pH 5.8, containing 4 mM EDTA (1 ml buffer/5 g initial tissue), and further purify and concentrate virus particles by one cycle of differential centrifugation (10,000 g, 10 min; 280,000 g, 45 min), followed by resuspension, sucrose density gradient centrifugation (250,000 g, for 60 min in Beckman SW50.1 rotor), and then ultracentrifugation of the virus-containing fractions. Resuspend the final pellets in 1 mM Tris-HCl buffer, pH 7.0, containing 2 mM EDTA.

Properties of Particles

Particles are bacilliform, 18 nm wide and about 42, 49 or 76 nm long (Fig.5). Three corresponding sedimenting components are resolved by sucrose density gradient centrifugation.

A260/A280 = 1.7 ± 0.05 (suggestive of about 25% nucleic acid).

A258(max)/A240(min)= 1.30 ± 0.05.

Particle Structure

CIBV particles stained with uranyl acetate (Fig.6) have a structure resembling that of particles of alfalfa mosaic virus, in which the stain penetrates ‘eyes’ at regular intervals along the length of appropriately orientated particles (see Johnson & Argos, 1985). In that virus, the tubular part of the particle is made up of interlocking hexagonal arrays of protein subunits, with holes penetrated by stain at the 6-fold vertices. This structure repeats at 8 nm intervals; the rounded ends of the particles are thought to consist of the two halves of an icosahedron. The appearance of CIBV particles, notably the occurrence of ‘eyes’ and the end-on view, is compatible with a similar model. Assuming a longitudinal repeat of the structure of CIBV particles every 8 nm, particle lengths of 42, 50 and 74 nm are possible, close to the observed values of 42, 49 and 76 nm.

Particle Composition

Nucleic acid: Three RNA components were detected by electrophoresis in agarose gel. Assuming they are single-stranded, they have Mr (x 10-6) of 0.9, 1.0 and 1.3.

Protein: One particle protein of Mr c. 22,000.

Relations with Cells and Tissues

No information.

Ecology and Control

Vegetative planting material should not be moved from Côte d'Ivoire to other countries unless it is apparently free of CIBV after suitable tests, e.g. mechanical inoculation of leaf extracts to C. murale, DAS-ELISA with CIBV antibodies (Fargette et al., 1991; Frison & Feliu, 1991).

Notes

Except where indicated, the above data were obtained by Fargette et al. (1991).

CIBV resembles alfalfa mosaic virus (Hull, 1969; Jaspars & Bos, 1980) in the shape, diameter and, probably, structure of its particles. However, only three predominant lengths of particle and only three RNA species were found, whereas alfalfa mosaic virus has four of each. CIBV resembles alfalfa mosaic virus in infecting species in the Fabaceae and Solanaceae, but its host range is more limited: of the 19 species not infected by CIBV, 13 (six plant families) are hosts of alfalfa mosaic virus. Also, the concentration of CIBV in sap is 10- to 100-fold less than that of alfalfa mosaic virus, no relationship between the two viruses could be detected by ISEM and only a very distant, and dubious, relationship could be found by DAS-ELISA. Evidently CIBV is a distinct virus. It is considered to be a tentative member of the genus Alfamovirus.

References

  1. Aiton, Lennon, Roberts & Harrison, Abstr. 5th Int. Congr. Pl. Path. Kyoto, p.43, 1988.
  2. Fargette, Roberts & Harrison, Ann. appl. Biol. 119: 303, 1991.
  3. Frison & Feliu (eds), FAO/IBPGR Technical Guidelines for the Safe Movement of Cassava Germplasm, 47 pp, FAO/IBPGR, Rome, 1991.
  4. Hull, Adv. Virus Res. 15: 335, 1969.
  5. Jaspars & Bos, CMI/AAB Descr. Pl. Viruses 229, 7 pp., 1980.
  6. Johnson & Argos, In The Plant Viruses. 1. Polyhedral Virions with Tripartite Genomes, p. 19, ed. R. I. B. Francki, Plenum Press, New York,1985.


Figure 1

Necrotic lesions in inoculated leaf of C. murale.

Figure 2

Systemic necrosis in C. murale.

Figure 3

Necrotic spots in inoculated leaf of C. quinoa.

Figure 4

Systemic stunt and necrosis in C. quinoa.

Figure 5

Purified particles of CIBV (18 nm wide) negatively stained with ammonium molybdate.

Figure 6

Purified particles of CIBV negatively stained with uranyl acetate.