Pelargonium zonate spot virus

D. Gallitelli
Istituto di Patologia Vegetale, Università di Bari, 70126 Bari, Italy

A. Quacquarelli
Istituto di Patologia Vegetale, Università di Bari, 70126 Bari, Italy

G. P. Martelli
Istituto di Patologia Vegetale, Università di Bari, 70126 Bari, Italy

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

Diseases described by Martelli & Cirulli (1969) and by Quacquarelli & Gallitelli (1979); virus characterized by Gallitelli (1982).

A virus with RNA-containing quasi-isometric particles 25-35 nm in diameter which sediment as three components. Readily sap-transmissible to a moderately wide range of herbaceous hosts. The virus is seed-borne and pollen-borne in Nicotiana glutinosa but its vector is unknown. Reported only from Southern Italy.

Main Diseases

Causes concentric chrome-yellow bands in the leaves of Pelargonium zonale (Quacquarelli & Gallitelli, 1979; Fig.2) and unfruitfulness and stunting of tomato (Lycopersicon esculentum) accompanied by yellowish rings and line patterns (Fig.1) and malformation of the leaves (Martelli & Cirulli, 1969; Gallitelli, 1982).

Geographical Distribution

Found only in Southern Italy.

Host Range and Symptomatology

Besides pelargonium and tomato the virus has been isolated from naturally infected Chrysanthemum segetum plants with yellow mottling of the leaves and from artichoke with foliar mosaic and malformations (G. L. Rana, unpublished data). Hosts include species in seven dicotyledonous families (Martelli & Cirulli, 1969; Quacquarelli & Gallitelli, 1979; Gallitelli, 1982).

Diagnostic species

Nicotiana glutinosa. Local chlorotic rings 3-4 days after inoculation, followed by systemic mosaic with marginal necrosis and rolling of the leaves.

N. tabacum (tobacco) cv. Xanthi. Local chlorotic rings and systemic mottling.

Cucurbita pepo (zucchini squash). Chlorotic/necrotic local lesions, systemic apical necrosis and death.

Phaseolus vulgaris (French bean). cv. La Victoire. Chlorotic/necrotic local lesions and top necrosis. Systemic mottling may develop on trifoliolate leaves.

Propagation species

N. glutinosa is a good source of virus for purification and is suitable for maintaining cultures.

Assay species

Cucumis sativus (cucumber) is a satisfactory local lesion host.

Strains

Minor variants may exist. Virus isolates from pelargonium and tomato differ in their effects on some herbaceous hosts.

Transmission by Vectors

No vector known. Aphis gossypii and Myzus persicae did not transmit the virus (Quacquarelli & Gallitelli, 1979).

Transmission through Seed

Transmitted to c. 5% of N. glutinosa seed; the virus is also present in the pollen.

Serology

The virus is weakly immunogenic; a rabbit injected with 2 mg purified nucleoprotein in four injections gave an antiserum with a titre of 1/256 in gel-diffusion tests. Before injection into a rabbit virus preparations must be fixed, for example with formaldehyde. In gel-diffusion tests a single precipitin band is formed.

Relationships

The particles resemble those of ilarviruses and similar viruses. However, particle preparations did not react with antisera to five members of the ilarvirus group (apple mosaic, prune dwarf, prunus necrotic ringspot, tobacco streak and Tulare apple mosaic), or to raspberry bushy dwarf virus, alfalfa mosaic virus or any of 26 other viruses with isometric particles.

Stability in Sap

In sap of N. glutinosa infectivity is lost after dilution to 10^-2, heating for 10 min at 40°C or storing for 7 h at 25°C.

Purification

The virus was purified from expressed sap of Nicotiana glutinosa or Cucumis sativus by the method described by Lister & Saksena (1976): adjustment of slurry to pH 4.8, precipitation of virus with polyethylene glycol M. Wt 6000, differential centrifugation and sucrose density gradient centrifugation (Gallitelli, 1982). Fulton’s (1968) procedure (clarification with hydrated calcium phosphate) was also useful but higher amounts of contaminating host components were retained (Quacquarelli & Gallitelli, 1979; Gallitelli, 1982). Average yield of virus is 10-12 mg/100 g infected material.

Properties of Particles

In sucrose density gradients and analytical ultracentrifugation (Fig.3) purified virus preparations form three nucleoprotein components (TV, MV and BV) sedimenting at different rates. Component BV is probably an aggregate of TV (Quacquarelli & Gallitelli, 1979; Gallitelli, 1982). Distribution of infectivity in fractionated virus preparations is bimodal corresponding to where mixtures of TV + MV and MV + BV components occur. At equilibrium in CsCl or Cs₂SO₄ (Fig.4) virus preparations band as a single component. The virus particles are stabilized by protein-RNA linkages (Gallitelli, 1982).

Sedimentation coefficients, s_20,w (svedbergs): 80 (TV), 90 (MV), 118 (BV).

A₂₆₀/A₂₈₀: 1.64.

Buoyant density of formaldehyde-stabilized virus in Cs₂SO₄ at 25°C (g/cm³): 1.286; in CsCl virus particles aggregate and band at a density of 1.346 g/cm³.

Thermal denaturation mid-point of nucleoproteins (Quacquarelli et al., 1977): 63°C.

Particle Structure

Particles are quasi-isometric with diameter ranging from 25 to 35 nm with a modal value of 29 nm (Fig.5). They are stable in 2% uranyl acetate but not in neutral potassium phosphotungstate unless they are stabilized with formaldehyde (Gallitelli, 1982).

Particle Composition

Nucleic acid: single-stranded RNA constituting about 18% of the particle weight (estimated from the A₂₆₀/A₂₈₀ ratio). In polyacrylamide gel electrophoresis under non-denaturing conditions RNA migrates as two species with estimated M. Wt of 0.95 x 10⁶ (RNA-2) and 1.25 x 10⁶ (RNA-1); both species are needed for infectivity (Quacquarelli & Gallitelli, 1979; Gallitelli, 1982). Addition of coat protein does not enhance the intrinsic infectivity of mixtures of the two RNA species.

Protein: When virus preparations were analysed in 15% polyacrylamide slab-gels in the discontinuous buffer system of Laemmli (1970) a single protein species was observed with M. Wt of c. 23,000. A second band, corresponding to a M. Wt of 44,000, was observed when urea and 2-mercaptoethanol were omitted from the dissociation medium and is probably a dimer of the 23K protein.

Relations with Cells and Tissues

Virus particles are present in leaf hair cells, epidermis, foliar parenchyma and, occasionally, in differentiating and mature sieve tubes of artificially infected French bean plants (Castellano & Martelli, 1981). Virus particles occur both in the cytoplasm and in the nuclei of infected cells. Intranuclear particles may be associated with the nucleolus (Fig.6) whereas the cytoplasmic particles occur randomly scattered, or in large clusters, or in single rows within tubular structures (Fig.7). Vesicles containing fibrillar material resembling nucleic acid are present in the cytoplasm, in a perinuclear position or inside the nucleus. Chloroplasts are damaged and cell wall modifications (thickenings and/or outgrowths) are commonly seen.

Notes

The field syndromes were readily reproduced in pelargonium and tomato by inoculation with infective sap or with purified virus (Quacquarelli & Gallitelli, 1979; Gallitelli, 1982). The tomato disease is thought to be the same as that recorded years ago in Southern Italy by Martelli & Cirulli (1969) and then attributed to infection with tobacco streak virus. Pelargonium zonate spot virus shows similarities to ilarviruses in particle size, morphology, instability, coat protein M. Wt, sedimentation coefficients of TV and MV nucleoprotein components and transmission through seed and pollen. However, it differs significantly from members of this group in the number of centrifugal components, in having only two RNA species, in the lack of effect of coat protein on the infectivity of RNA preparations, and in the type of ultrastructural modification. Besides these characters, the symptoms shown by pelargonium and tomato plants affected by pelargonium zonate spot virus are very distinctive. None of the other viruses infecting the same species in nature is known to induce comparable syndromes. In southern Italy, alfalfa mosaic and tobacco rattle viruses may induce yellow rings and line patterns in the leaves of tomato, but with a much brighter hue.

References

1. Castellano & Martelli, Phytopath. Medit. 20: 64, 1981.
2. Fulton, Phytopathology 58: 631, 1968.
3. Gallitelli, Ann. appl. Biol. 100: 457, 1982.
4. Laemmli, Nature, Lond. 227: 680, 1970.
5. Lister & Saksena, Virology 70: 440, 1976.
6. Martelli & Cirulli, Phytopath. Medit. 8: 154, 1969.
7. Quacquarelli & Gallitelli, Phytopath. Medit. 18: 61, 1979.
8. Quacquarelli, Piazzolla, Avgelis & Gallitelli, J. gen Virol. 35: 25, 1977.

Figure 1

Malformations, yellow rings and lines in a leaflet of a naturally infected tomato plant.

Figure 2

Malformations, puckering and zonate yellow bands in a leaf of a naturally infected Pelargonium zonale plant.

Figure 3

Schlieren diagram of partially purified virus preparation after centrifuging for 19 min at 32,000 rev/min. TV, MV and BV are virus-specific components.

Figure 4

Schlieren diagram of an unfractionated purified virus preparation at equilibrium after centrifugation at 44,000 rev/min for 20 h at 25°C in a Cs₂SO₄ solution of initial density of 1.285 g/cm³. All particles have the same buoyant density (1.286 g/cm³). Meniscus is at left.

Figure 5

Virus particles in 2% uranyl acetate. Bar represents 100 nm.

Figure 6

Virus particles (V) next to and within a nucleolus (Nu) in an infected cell. Bar represents 200 nm.

Figure 7

Tubular structures containing virus particles in the cytoplasm of an infected cell. Bar represents 200 nm.