195
August 1978
Family: Alphaflexiviridae
Genus: Potexvirus
Species: Daphne virus X
Acronym: DVX


Daphne virus X

R. L. Forster
Plant Diseases Division, D.S.I.R., Private Bag, Auckland, New Zealand

K. S. Milne
Dept. Horticulture and Plant Health, Massey University, Palmerston North, New Zealand

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 Forster & Milne (1975, 1978b).

A virus with flexuous filamentous particles c. 500 x 12 nm. No vector is known but the virus is spread by vegetative propagation.

Main Diseases

The virus occurs naturally in Daphne cneorum and D. odora but is not associated with symptoms in either species.

Geographical Distribution

Recorded only from New Zealand.

Host Range and Symptomatology

The virus infected 19 of 34 species in 6 of 11 dicotyledonous families (Amaranthaceae, Chenopodiaceae, Cucurbitaceae, Leguminosae, Scrophulariaceae, Solanaceae). Only 7 species were systemically infected (Chenopodium quinoa, Cucumis melo, Cucumis sativus, Cucurbita maxima, Datura stramonium, Pisum sativum, Nicotiana clevelandii).

Diagnostic species

Cucumis sativus (cucumber). Sunken chlorotic local lesions in cotyledons; faint chlorotic spotting on first systemically infected leaf, later leaves being symptomlessly infected (Fig.1).

Dolichos biflorus. Small necrotic local lesions; no systemic infection.

Gomphrena globosa. Necrotic local lesions, often with red haloes; no systemic infection (Fig.3).

Nicotiana clevelandii. Chlorotic and necrotic local lesions; systemic chlorotic rings or mottle.

Pisum sativum (pea). Necrotic local lesions; systemic chlorotic mottle and sometimes blackening of the stem in the region of inoculated leaves.

Propagation species

The virus is best maintained in Nicotiana clevelandii; inoculated leaves of this host are a good source of virus for purification.

Assay species

Cucumis sativus, Dolichos biflorus and Gomphrena globosa consistently give local lesions.

Strains

None reported.

Transmission by Vectors

The virus was not transmitted from pea to pea by the aphid Myzus persicae.

Transmission through Seed

Not reported.

Serology

The virus is a good immunogen; flocculent precipitates were observed in microprecipitin tests at antiserum dilutions up to 1/2048.

Relationships

Preliminary tests (Milne & Forster, 1976) indicated a distant serological relationship to white clover mosaic virus but later experiments (Forster & Milne, 1978b) have shown no serological relationship to this virus or to clover yellow mosaic, narcissus mosaic or potato X viruses.

Stability in Sap

Infectivity was retained in N. clevelandii sap held at room temperature (c. 20°C) for 5 weeks but not for 6 weeks, in sap diluted to 10-5 but not 10-6, and after heating for 10 min at 80°C but not 85°C.

Purification

Inoculated leaves of Nicotiana clevelandii infected for 7-10 days were homogenised in 0.5 M phosphate buffer, pH 7.1, containing 0.01 M disodium ethylenediamine-tetraacetate (EDTA), 0.01 M sodium diethyldithiocarbamate (DIECA) and 0.3% 2-mercaptoethanol. Extracts were emulsified with chloroform and n-butanol and clarified by low speed centrifugation. The virus was sedimented by high speed centrifugation, resuspended in 0.05 M borate buffer, pH 8.2, containing 0.01 M EDTA and further purified by sucrose-gradient centrifugation.

Properties of Particles

The virus sediments in the analytical ultracentrifuge as a single component with a sedimentation coefficient (s°20,w) of c. 110 S.

A260/A280: 1.22.

Buoyant density: 1.28 g/cm3 in CsCl.

Particle Structure

Particles are flexuous filaments c. 500 x 12 nm (Fig.2). In uranyl acetate many particles show distinct cross-banding.

Particle Composition

Nucleic acid: c. 6% of particle weight (estimated spectrophotometrically).

Protein: In limited tests using polyacrylamide gel electrophoresis with sodium dodecyl sulphate, a single polypeptide was observed with M. Wt c. 23,000.

Relations with Cells and Tissues

In ultrathin sections of systemically infected pea leaflets, virus-like particles were observed in large aggregates (Fig.4) or as unaggregated particles scattered through the cytoplasm.

Notes

In New Zealand, no D. cneorum plants have been found free of daphne virus X out of more than 100 tested. The virus has properties typical of members of the potexvirus group, but can be distinguished from other members of the group by host range, symptomatology and serology. Only two other viruses infecting daphne have flexuous filamentous particles (Forster & Milne, 1975); daphne virus S has particles c. 720 nm long (Forster & Milne, 1978a), and daphne virus Y has particles c. 730 nm long (Forster & Milne, 1976). Unlike daphne virus X, these viruses do not infect Cucumis sativus or Gomphrena globosa. None of the other viruses recorded in daphne give a range of symptoms resembling those of daphne virus X on the species listed under ‘Diagnostic hosts’.

References

  1. Forster & Milne, N. Z. Jl agric. Res. 18: 391, 1975.
  2. Forster & Milne, N. Z. Jl agric. Res. 19: 359, 1976.
  3. Forster & Milne, N. Z. Jl agric. Res. 21: 131, 1978a.
  4. Forster & Milne, N. Z. Jl agric. Res. 21: 137, 1978b.
  5. Milne & Forster, Acta Hort. 59: 95, 1976.


Figure 1

(Left) Local lesions and (right) systemic symptoms in cucumber.

Figure 2

Virus particles from sap of D. cneorum flowers, negatively stained with neutral phosphotungstate. Bar represents 200 nm.

Figure 3

Local lesions in Gomphrena globosa.

Figure 4

Aggregates of virus-like particles in a systemically infected pea leaf cell. Bar represents 300 nm.