148
October 1975
Family: Tombusviridae
Genus: Carmovirus
Species: Saguaro cactus virus
Acronym: SgCV


Saguaro cactus virus

M. R. Nelson
University of Arizona, Tucson, Arizona 85721, USA

M. A. Yoshimura
University of Arizona, Tucson, Arizona 85721, USA

J. H. Tremaine
Agriculture Canada Research Station, 6660 NW Marine Drive, Vancouver, B.C., Canada

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 Milbrath & Nelson (1972).

An RNA-containing virus with isometric particles found in the giant saguaro cactus (Carnegia gigantea) in south-eastern Arizona. Infects species in several dicotyledonous plant families. It is readily transmitted mechanically but has no known vector.

Main Diseases

Found in nature only in saguaro cactus (Carnegia gigantea); it causes no readily discernible symptom in naturally or experimentally infected saguaro plants.

Geographical Distribution

South-eastern Arizona, near cultivated areas (Milbrath et al., 1973).

Host Range and Symptomatology

Infects several species in the families Cactaceae, Chenopodiaceae, Portulacaceae, Amaranthaceae, Nyctaginaceae, Aizoaceae (M. A. Yoshimura & M. R. Nelson, unpublished data).

Diagnostic species

Carnegia gigantea (saguaro). Symptomless systemic infection (Fig.1).

Chenopodium capitatum. Systemic vein-clearing, with eventual systemic necrosis (Fig.2).

C. amaranticolor. Necrotic red-bordered local lesions (Fig.3); no systemic infection.

C. quinoa. Chlorotic local lesions; no systemic infection.

Gomphrena globosa. Chlorotic local lesions; systemic spots.

Propagation species

C. capitatum or C. murale.

Assay species

C. amaranticolor or C. quinoa.

Strains

No major strains found, but isolates differ in the proportions of the electrophoretic components.

Transmission by Vectors

None known. Field observations suggest a slow rate of spread.

Transmission through Seed

No virus was detected in any of 60 progeny seedlings from an infected saguaro cactus plant (Milbrath, 1971).

Serology

A good immunogen; antibody titres of 1/1024 are readily obtained. The gel double diffusion test gives excellent results; two precipitin bands may be produced with low dilutions of antiserum, presumably corresponding to virus particles and soluble antigen.

Relationships

In the sizes of its RNA and protein components it resembles tomato bushy stunt and turnip crinkle viruses but has a lower sedimentation coefficient. No serological relationship was detected to these viruses or to any of thirteen other isometric viruses, including nine known to have single sedimenting components (Milbrath & Nelson, 1972; Nelson & Tremaine, 1975). An isometric virus described by Casper et al. (1970) from Opuntia tuna was found to have no serological relationship to the saguaro virus (M. R. Nelson & R. Casper, unpublished data).

Stability in Sap

In Chenopodium capitatum sap, the saguaro virus is completely inactivated after 10 min at temperatures between 55° and 65°C depending on the dilution of the sap. When assayed using Chenopodium quinoa, dilutions of sap produced the maximum number of lesions at 10-3 with a 95% drop at 10-4. A 10-1 dilution of sap retained infectivity after 34 days.

Purification

Nelson & Tremaine, 1975: Extract each 100 g of leaves in 100 ml 0.2 M sodium acetate buffer, pH 5.0. Squeeze through cheesecloth, adjust pH to 5.0 and centrifuge at low speed, discarding the pellets. Concentrate the virus from the supernatant fluid by two cycles of differential centrifugation, resuspending virus pellets in 0.1 M sodium acetate buffer, pH 5.0. Use of pH 7.0 phosphate or Tris buffers should be avoided because it results in precipitation of the virus on storage at 5°C.

Properties of Particles

Sedimentation coefficient s20, w: about 118 S. No accessory viral components are found by analytical centrifugation in 0.1 M phosphate or acetate buffers (Fig.4). However, in NaCl solutions greater than 1 M, an additional component of about 80 S occurs (Fig.5). When dialysed to remove salt only the 118 S particles remain and they are still infectious. When the two components are removed separately from salt gradients (CsCl or sucrose + 1 M NaCl) only the bottom component is infectious. When the virus is fixed in formaldehyde prior to salt treatment, only the 118 S component is observed (M. A. Yoshimura & M. R. Nelson, unpublished data).

Molecular weight: 7.9 x 106.

Diffusion coefficient (D20, w x 10-7 cm2/sec): 1.22.

Partial specific volume: 0.702 cm3/g.

Isoelectric point: about pH 3.5, 5.0 and 5.5 for the three electrophoretic components.

Electrophoretic mobility: In sucrose density gradient and polyacrylamide gel electrophoresis there are three electrophoretic components, all infectious, but of unknown origin (Fig.6). Electrophoretic mobilities (x 10-5 cm2 sec-1 volt-1): -5.6, 0 and 1.9 in 0.02 ionic strength acetate buffer, pH 5.0. Similar electrophoretic components were observed in two isolates but their relative proportions differed.

Absorbance at 260 nm (1 mg/ml, 1 cm light path): 6.

A260/A280: 1.4.

Buoyant density in CsCl: 1.33.

Particle Structure

Isometric particles, 32 ± 1 nm in diameter (Fig.7). None of the particles are penetrated by negative stain. Particles of 80 S formed in 1 M NaCl are 37 nm in diameter.

Particle Composition

Nucleic acid: Single-stranded RNA comprising 17% of particle weight. M. Wt 1.4 x 10-6. Molar percentage of nucleotides G29; A24; C21; U27 (Nelson & Tremaine, 1975).

Protein: 83% of particle weight. Major component (90-95 %) of M. Wt 38,900, probably 180 sub-units per particle; minor component (5-10%) of M. Wt 90,200; a third component (0-4%) of M. Wt 29,400. M. Wts were determined by polyacrylamide gel electrophoresis. Amino acid composition of total protein was obtained by Nelson & Tremaine (1975).

Relations with Cells and Tissues

In saguaro, the virus occurs in high concentrations in buds, flowers and fruit. Isolation from vegetative tissue is difficult.

Notes

Saguaro virus particles degrade in the presence of RNase at pH 7.0 but less readily at pH 5.0 (M. A. Yoshimura & M. R. Nelson, unpublished data). The virus is dissociated into protein and RNA components by low concentrations of sodium dodecyl sulphate (Nelson & Tremaine, 1975). Carnation mottle virus has been successfully inoculated to saguaro seedlings experimentally (M. A. Yoshimura & M. R. Nelson, unpublished data) but no other virus has yet been found to occur naturally.

References

  1. Milbrath, Dissertation, Univ. Arizona, 1971.
  2. Milbrath & Nelson, Phytopathology 62: 739, 1972.
  3. Milbrath, Nelson & Wheeler, Phytopathology 63: 1133, 1973.
  4. Nelson & Tremaine, Virology 65: 309-319, 1975.
  5. Casper, Lesemann & Bartels, Pl. Dis. Reptr 54: 851, 1970.


Figure 1

Infected but symptomless saguaro plant.

Figure 2

(Left) systemically infected Chenopodium capitatum plant; (right) healthy plant.

Figure 3

Lesions in inoculated leaf of C. amaranticolor.

Figure 4

Schlieren diagram of moving boundary analytical ultracentrifugation of virus preparation in 0.1 M phosphate buffer, pH 5.

Figure 5

As Fig.4, but preparation in 0.1 M phosphate buffer, pH 5, containing 1 M NaCl.

Figure 6

Electrophoresis in sucrose gradient, showing three components (after 4 h; in 0.03 ionic strength phosphate, pH 6.0).

Figure 7

Electron micrograph of virus particles prepared with 1% uranyl acetate at pH 4. Bar represents 100 nm.