335
September 1988
Family: Alphaflexiviridae
Genus: Potexvirus
Species: Dioscorea latent virus
Acronym: DLV


Dioscorea latent virus

Sue Phillips
AFRC Institute of Horticultural Research, Littlehampton, Sussex, England, BN17 6LP

A. A. Brunt
AFRC Institute of Horticultural Research, Littlehampton, Sussex, England, BN17 6LP

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

First reported by Lawson et al. (1973), partially characterised by Waterworth et al. (1974), and more fully described by Phillips et al. (1986).

A virus with filamentous particles about 445 x 13 nm, known to occur naturally only in Dioscorea composita and D. floribunda in Central America. It has no known vector but is readily transmissible by inoculation of sap to species in six families.

Main Diseases

The virus alone causes no obvious symptoms in Dioscorea composita and D. floribunda, its only known natural hosts, but it often occurs together with dioscorea green banding virus which induces mosaic and green-banding leaf symptoms (Lawson et al., 1973; Hearon et al., 1978).

Geographical Distribution

Reported only from Puerto Rico, but probably occurs in other parts of Central America where its two natural hosts are cultivated.

Host Range and Symptomatology

The virus is transmissible by mechanical inoculation of sap to D. composita and D. floribunda and to 13 of 34 other species in five of 12 families. It induces symptomless infection in inoculated leaves only of most experimental hosts, but symptomless systemic infection also in Nicotiana benthamiana and N. megalosiphon (Hearon et al., 1978; Phillips et al., 1986).

Diagnostic species

Philodendron selloum. Faint yellowing and mottling of inoculated leaves 3 wk after inoculation. No systemic infection.

Dioscorea composita and D. floribunda. Symptomless infection in inoculated and systemically infected leaves.

Nicotiana benthamiana and N. megalosiphon. Symptomless infection in inoculated and uninoculated leaves.

Propagation species

Nicotiana megalosiphon. Infected plants, although symptomless, are good sources of virus for purification.

Assay species

No satisfactory local lesion assay host is available. Nicotiana megalosiphon, if tested subsequently for the presence of virus by serological and/or electron microscopical procedures, can be used indirectly for assays.

Strains

None reported.

Transmission by Vectors

Not transmitted by Aphis gossypii or Myzus persicae in either the persistent or the non-persistent manner (Lawson et al., 1973; Waterworth et al., 1974; Phillips et al., 1986).

Transmission through Seed

Not seed-borne in Nicotiana megalosiphon (Phillips et al., 1986).

Serology

The virus is a good immunogen; antisera with titres of 1/4096 to 1/16,000 in precipitin tube tests have been obtained from rabbits injected intramuscularly on each of 2 to 4 occasions with c. 1 mg virus emulsified with Freund’s complete adjuvant (Waterworth et al., 1974; Phillips et al., 1986).

Relationships

The virus has properties typical of members of the potexvirus group. It shows a distant serological relationship to commelina X and lily X viruses, but is serologically unrelated to cactus X, clover yellow mosaic, cymbidium mosaic, hydrangea ringspot, narcissus mosaic, nerine X (Agapanthus strain), papaya mosaic (Ullucus strain), pepino mosaic, potato X, potato aucuba mosaic, viola mottle or white clover mosaic viruses (Waterworth et al., 1974; Phillips et al., 1986).

Stability in Sap

Sap from systemically infected Nicotiana megalosiphon leaves remains infective after heating for 10 min at 75-80 but not 85°C, after at least 12 months at 22°C and after dilution to 10-6 but not 10-7 (Phillips et al., 1986).

Purification

Although virus can be recovered from infected Dioscorea floribunda and D. composita leaves, purification is hindered by the presence of mucilaginous substances (Waterworth et al., 1974). Nicotiana megalosiphon is a better source of virus for purification, yielding up to 30 mg virus/kg leaf tissue using the following method (Phillips et al., 1986). Grind leaves (1 g/3 ml) in 0.07 M neutral phosphate buffer containing 1 ml/l 2-mercaptoacetic acid, express the extract through cheesecloth and, with stirring, slowly add Triton X-100 (50 ml/l). After 30 min, subject the mixture to two cycles of differential centrifugation (20 min at 12,000 g; 75 min at 65,000 g). Purify the virus further by rate zonal sucrose density gradient centrifugation (2.5 h at 85,000 g in 100-400 g/l sucrose density- gradient columns).

Properties of Particles

Sedimentation coefficient (s20,W): 111 S. Aggregated particles, probably end-to-end dimers, sediment at 131 S (Phillips et al., 1986).

Buoyant density in caesium chloride: 1.33 g cm-3.

Particle Structure

The virus has slightly flexuous filamentous particles with a modal length of 445 nm and a diameter of 13 nm when mounted in sodium phosphotungstate (Lawson et al., 1973; Waterworth et al., 1974; Phillips et al., 1986).

Particle Composition

Nucleic acid: RNA, single-stranded, c. 5% of the particle weight. One species, of M. Wt 2.3 x 106 (estimated under denaturing conditions by electrophoresis in 1.5% agarose gels).

Protein: One species of polypeptide, M. Wt 24,900; a second component of M. Wt 23,100 present in some preparations is probably a degradation product of the former (Phillips et al., 1986).

Relations with Cells and Tissues

Infected Dioscorea floribunda plants contain amorphous, vacuolated, cytoplasmic inclusions, and banded inclusions consisting of closely-packed virus particles (Hearon, 1974; Hearon et al., 1978).

Notes

Although the particles of dioscorea latent virus are shorter than those of most other potexviruses, their morphology, physico-chemical properties, in vivo occurrence and distant serological relationship to lily X and commelina X viruses indicate that the virus is a distinct but definite member of the potexvirus group.

Dioscorea composita and D. floribunda produce tubers that are major sources of diosgenin, a sapogenin used in the commercial production of cortisone and progesterone. These two species, indigenous to Mexico and some South American countries, are the only known natural hosts of dioscorea latent virus; it is especially interesting that the virus has not been detected in edible yam species such as D. alata, D. esculenta and D. bulbifera.

References

  1. Hearon, Proc. Am. Phytopath. Soc. 1: 151, 1974.
  2. Hearon, Corbett, Lawson, Gillaspie & Waterworth, Phytopathology 68: 1137, 1978.
  3. Lawson, Hearon, Smith & Kahn, Phytopathology 63: 1435, 1973.
  4. Phillips, Piggott & Brunt, Ann. appl. Biol. 109: 137, 1986.
  5. Waterworth, Lawson & Kahn, J. Agric. Univ. P. Rico 58: 351, 1974.