Papaya ringspot virus

D. Purcifull
Departments of Plant Pathology and Agronomy, University of Florida, Gainesville, Florida 32611, USA

J. Edwardson
Departments of Plant Pathology and Agronomy, University of Florida, Gainesville, Florida 32611, USA

E. Hiebert
Departments of Plant Pathology and Agronomy, University of Florida, Gainesville, Florida 32611, USA

D. Gonsalves
Department of Plant Pathology, Cornell University, New York State Agricultural Experiment Station, Geneva, New York 14456, USA

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 Jensen (1949a), Conover (1962, 1964a), De Bokx (1965) and Webb & Scott (1965). A strain of this virus, watermelon mosaic virus 1, was confounded in Description No. 63 with watermelon mosaic virus 2).

Selected synonyms

Papaya (papaw) distortion ringspot virus (Rev. appl. Mycol. 41: 530)

Papaya (papaw) mosaic virus (Rev. appl. Mycol. 39: 147; Rev. appl. Mycol. 43: 1719)

Specific watermelon mosaic virus (Rev. appl. Mycol. 46: 818)

Watermelon mosaic virus 1 (Rev. appl. Mycol. 45: 318)

A virus with RNA-containing, flexuous filamentous particles c. 780 x 12 nm. It induces cylindrical (pinwheel) inclusions and amorphous inclusions in the cytoplasm of host cells. It is transmissible mechanically, and by numerous species of aphid in a non-persistent manner, and has a narrow host range. Most strains belong to one of two major types, which are serologically very closely related. Type P (papaya-infecting) isolates cause an important disease in papaya and also occur in cucurbits. Type W (= watermelon mosaic virus 1) isolates cause important diseases in watermelon and other cucurbits, but do not infect papaya. Widely distributed throughout the world.

Main Diseases

In papaya, type P isolates cause mottling and distortion of leaves, rings and spots on fruit, and streaks on stems and petioles. Plants are stunted and fruit set is reduced (Conover, 1964a; Jensen, 1949a). Cool weather favours the development of severe leaf distortion symptoms (Jensen, 1949a; Conover, 1964a; Lima & Gomes, 1975). In squash, watermelon and other cucurbits, type W isolates cause mottling and distortion of leaves and fruit (Halliwell, Johnson & Cotner, 1979). Cucurbitaceous weeds (e.g. Melothria pendula and Momordica charantia) can serve as natural sources of type W isolates (Adlerz, 1972a, 1972b).

Geographical Distribution

Type P isolates occur in most tropical and subtropical areas where papaya is grown (Purcifull, 1972), including the USA (Conover, 1964a; Jensen, 1949a; Wan & Conover, 1983), South America (Herold & Weibel, 1962; Lima & Gomes, 1975), the Caribbean countries (Adsuar, 1947; Story & Halliwell, 1969), India (Capoor & Varma, 1958), Taiwan (Wang et al., 1978), Africa (Lana, 1980) and Okinawa (Yonaha, 1976). Type W isolates have been reported in cucurbits in many areas, including the USA (Webb, Bohn & Scott, 1965), Mexico (Milne, Grogan & Kimble, 1969), Caribbean countries (Schmelzer, 1969; Quiot, Kaan & Beramis, 1971), Australia (Greber, 1978), Germany (Hein, 1977), France (Lecoq, Lot & Pitrat, 1982), Italy (Ragozzino & Stefanis, 1977), India (Ghosh & Mukhopadhyay, 1979), Middle eastern countries (Ebrahim-Nesbat, 1974; Russo et al., 1979; Makkouk & Lesemann, 1980) and South America (Silva & Costa, 1978). An antigenic variant distinguishable from type P and type W isolates has been discovered in Guadeloupe (L. Quiot-Douine, J. E. Quiot & G. Labonne, unpublished data).

Host Range and Symptomatology

Transmissible by inoculation with sap, but host range is narrow. Fifteen dicotyledonous species in the families Caricaceae, Chenopodiaceae and Cucurbitaceae have been infected experimentally by type P isolates, with papaya (Jensen, 1949a; Conover, 1964a) and cucurbits (Wang et al., 1978) reported as natural hosts. Type W isolates are reported to infect 38 species in 11 genera of Cucurbitaceae, and two species of Chenopodiaceae, with squash, watermelon, cucumber and cantaloupe among the economically important natural hosts. The Guadeloupe strain infects various cucurbits (L. Quiot-Douine, J. E. Quiot & G. Labonne, unpublished data).

Diagnostic species

Carica papaya (papaya). Symptoms induced in papaya by type P isolates are variable and depend on stage of infection, plant vigour, temperature, virus strain and plant size (Conover, 1964a, 1964b). Seedlings show prominent vein clearing and downward cupping of the young leaves in about 1-2 weeks. After several weeks, the leaves become mottled and distorted, the lobes being markedly reduced in size (Fig.1 and Fig.2). Plant- to-plant variation in the expression of distortion symptoms was noted (Conover, 1964a). Spots or streaks with a greasy or water-soaked appearance sometimes occur on the stems (Jensen, 1949a). Fruit symptoms on plants in the field consist of spots or rings (Fig.4). Fruit distortion also sometimes occurs. Type W isolates do not infect papaya (Milne & Grogan, 1969; Purcifull & Hiebert, 1979; Yeh, Gonsalves & Provvidenti, 1984).

Cucurbita pepo (zucchini squash, pumpkin). Some type P isolates induce prominent mosaic and leaf distortion, whereas others induce mild mottle (Gonsalves & Ishii, 1980). Type W isolates typically induce foliar symptoms consisting of mosaic (Fig.8), dark green blisters and distortion. The Guadeloupe strain causes stripe mosaic and distortion.

Cucumis metuliferus (horned cucumber). Systemic mottle or mosaic symptoms are induced by type P and type W isolates in Accession 2459. Plant introduction P1 292190 is resistant to both types (Provvidenti & Robinson, 1977; Provvidenti & Gonsalves, 1982).

Luffa acutangula. Chlorotic mottling or spotting on systemically infected leaves is induced by numerous type W isolates (Webb, 1965; Purcifull & Hiebert, 1979; Russo et al., 1979). A Florida isolate of type P also infects this species (Milne et al., 1969). Many watermelon mosaic virus 2 isolates (Purcifull, Hiebert & Edwardson, 1984b) do not infect L. acutangula, and this species has been used as a means of obtaining type W isolates free from watermelon mosaic virus 2 (Webb, 1965; Greber, 1978; D. Purcifull, unpublished data).

Nicotiana benthamiana. Not susceptible to isolates of type P or type W. However, watermelon mosaic virus 2 invades this host systemically (Christie & Crawford, 1978; Purcifull & Hiebert, 1979; Russo et al., 1979).

Propagation species

Papaya, squash and C. metuliferus are suitable for maintaining cultures of type P isolates and the last-named species is useful as a source of virus for purification (Yeh et al., 1984). Type W isolates are maintained readily in squash or pumpkin, which are useful as hosts in which to propagate the virus for purification.

Assay species

Type P isolates (Fig.3) (Gonsalves & Ishii, 1980; Yeh et al., 1984) and many, but not all, isolates of type W (Russo et al., 1979; Yeh et al., 1984) induce necrotic local lesions in Chenopodium amaranticolor and C. quinoa. C. quinoa is useful as an assay host for detecting nitrous acid-induced mutants (Yeh & Gonsalves, 1984). Pumpkin and squash are useful hosts for vector studies.

Strains

Type P isolates infect papaya whereas type W isolates do not. Isolates of type P and type W are antigenically indistinguishable. A variant found in Guadeloupe is serologically related to, but distinct from, typical P and W isolates (L. Quiot-Douine, D. Purcifull & E. Hiebert, unpublished data). Isolates within type P differ in the severity of the symptoms they induce (Conover, 1964b; Gonsalves & Ishii, 1980). Mild isolates that cross-protect against severe ones in papaya have been detected following nitrous acid treatment of virus (Yeh & Gonsalves, 1984). Such avirulent forms have potential as a means of disease control by cross-protection. A strain that induces wilt in papaya has been noted in Taiwan (Chang, 1979). Some type W isolates induce lesions in C. quinoa or C. amaranticolor whereas others do not (Russo et al., 1979).

Transmission by Vectors

Transmitted in a non-persistent manner by numerous species of aphid. Type P isolates are transmitted by 21 species in 11 genera, including Myzus persicae (Jensen, 1949b; Conover, 1964a; Zettler, Edwardson & Purcifull, 1968) and Aphis gossypii. Type W isolates are transmitted by 24 species in 15 genera, including Myzus persicae, Aulacorthum solani, Aphis craccivora and Macrosiphum euphorbiae (Karl & Schmelzer, 1971). The virus was transmitted by M. persicae following acquisition probes of 15-45 s (Milne & Grogan, 1969), and by M. persicae and Aphis citricola following acquisition probes of 10-60 s and inoculation access periods of 1 h (Adlerz, 1974).

Leafminer flies (Liriomyza sativae) transmitted two type W isolates from squash to squash in greenhouse trials, but transmission frequencies were low (Zitter & Tsai, 1977).

Transmission through Seed

None detected in papaya (Ishii & Holtzmann, 1963; Conover, 1964a; Wang et al., 1978). No transmission of type W isolates was detected in Cucurbita (Camino Lavin, Téliz & Soso Moss, 1974; Hein, 1977).

Serology

The virus is a good immunogen. Liquid immunoprecipitin tests with clarified plant extracts (Webb & Scott, 1965) or with purified virus particles (Milne & Grogan, 1969) have been used for virus detection and for studying relationships. The intact virus particles do not diffuse readily in agar gels but sodium dodecyl sulphate (SDS) (Purcifull & Hiebert, 1979) or pyrrolidine (Shepard, Secor & Purcifull, 1974) can be added to disrupt the particles into diffusible fragments. SDS-immunodiffusion tests have been used to detect isolates of types P and W in cucurbit leaf extracts with antisera to untreated virus (Fig.9) (Purcifull & Hiebert, 1979; Russo et al., 1979) or with antisera to SDS-treated particle protein (Gonsalves & Ishii, 1980). Freeze-drying of leaf extracts is a convenient method for preparing reference antigens for use in SDS-immunodiffusion tests (Purcifull & Hiebert, 1979). Virus detection in papaya by SDS-immunodiffusion is possible (Fig.9) but unreliable (Wan & Conover, 1983) unless 5% ascorbic acid is added to the leaf extracts (S.-D. Yeh & D. Gonsalves, unpublished data). Enzyme-linked immunosorbent assay (ELISA) has been used to detect the virus; much higher A₄₀₅ readings are obtained when extracts from infected papaya or squash leaves are prepared in 0.25 M potassium phosphate + 0.1 M EDTA, pH 7.5, than in extracts prepared in standard phosphate-buffered saline (Gonsalves & Ishii, 1980). Immunoelectron microscopic procedures have been used to study relationships (Makkouk & Lesemann, 1980; Samah, 1982).

Antisera have been prepared to the proteins of cylindrical (pinwheel) inclusions (CI) and amorphous inclusions (AI). The CI proteins are detectable by SDS-immunodiffusion methods (Baum, 1980; Baum & Purcifull, 1981; Yeh, 1984). Antisera prepared to purified CI proteins of type P and type W isolates had titres of 1/8 to 1/16 in SDS-immunodiffusion tests. With indirect ELISA, CI proteins were detected at concentrations as low as 1 .6 ng/ml and at sap dilutions of 3.2 x 10⁵ (Yeh, 1984). AI protein of a type W isolate was detectable by SDS-immunodiffusion tests, by Western blotting tests, and by immunofluorescence in situ (M. De Mejia, E. Hiebert & D. Purcifull, unpublished data).

Relationships

Its particle morphology, aphid transmissibility, serological relationships and ability to induce pinwheel inclusions in host cells place papaya ringspot virus in the potyvirus group (Matthews, 1982; Hollings & Brunt, 1981). Edwardson (1974) assigned both type P and type W isolates to subgroup I because they typically induce pinwheels and scrolls, but not laminated aggregates, in host cells.

Type W isolates of papaya ringspot virus were called watermelon mosaic virus 1 by Webb & Scott (1965); however, the coat proteins of these isolates are serologically very closely related, if not identical, to those of type P isolates (Fig.9) (Purcifull & Hiebert, 1979; Russo et al., 1979; Gonsalves & Ishii, 1980; Barbosa & Paguio, 1982; Yeh et al., 1984) as also are their CI proteins (Baum, 1980; Yeh, 1984). Also, in cross-protection tests, mild type P isolates protect not only against severe type P isolates but also against type W isolates (Yeh & Gonsalves, 1984). Moreover, isolates of type P and type W are controlled by the same resistance gene in Cucumis metuliferus, and genotypes of C. melo, C. sativus, Lagenaria siceraria and Cucurbita ecuadorensis that are resistant to isolates of type W are also resistant to those of type P (Provvidenti & Gonsalves, 1982). The properties of type P and type W isolates, and their biological and cytological similarities, justify their grouping here under one name, papaya ringspot virus, as suggested by Lovisolo (1980).

Webb & Scott (1965) also identified a second type of isolate (which they called watermelon mosaic virus 2) distinguishable from watermelon mosaic virus 1 by serological, cross-protection and host range tests. A contradictory report by Milne & Grogan (1969) led to these two viruses being included together in Description No. 63. However, there is now much evidence that watermelon mosaic virus 2 is serologically very different, not only from type W (= watermelon mosaic virus 1), but also from type P isolates of papaya ringspot virus (Fig.9) (Greber, 1978; Purcifull & Hiebert, 1979; Russo et al., 1979; Gonsalves & Ishii, 1980; Yeh et al., 1984), although distant relationships have been detected (Purcifull & Hiebert, 1979, and unpublished data; Makkouk & Lesemann, 1980; Samah, 1982; Dodds et al., 1984). Also, the CI induced by watermelon mosaic virus 2 are antigenically distinct from those induced by type P and type W isolates of papaya ringspot virus (Baum & Purcifull, 1981; Yeh, 1984). Moreover, no nucleotide sequence homology was detected by molecular hybridisation between cDNA prepared from a type W isolate of papaya ringspot virus and RNA molecules of watermelon mosaic virus 2 (Samah, 1982); nor was any such homology detected between the RNA molecules of lettuce mosaic, potato Y, bean yellow mosaic, soybean mosaic or bean common mosaic viruses (Samah, 1982). Watermelon mosaic 2, is now the subject of a separate, revised Description (No. 293).

Papaya ringspot virus has affinities with some other potyviruses. Pyrrolidine degradation products of its particles reacted with antiserum to the D-protein of tobacco etch virus (Shepard et al., 1974), and the particle proteins and CI proteins of a potyvirus isolated from cucurbits in Morocco (Fischer & Lockhart, 1974) are serologically related to, but distinct from, those of papaya ringspot virus (Baum, 1980; Baum & Purcifull, 1981). The relationship of papaya ringspot virus to certain watermelon mosaic virus isolates from the USA (Anderson, 1954) and South Africa (Van Regenmortel, 1971) is unclear.

Stability in Sap

Infectivity of the virus in papaya sap was lost after 8 h at room temperature, after heating to 54-56°C for 10 min and by dilution to 10^-3 (Conover, 1964a). In squash sap, type W isolates were inactivated by aging for 40-60 days, by heating to 60°C for 10 min or by dilution to 5 x 10^-4 (Webb & Scott, 1965). Type W isolates retained infectivity after storage for at least 6 years in dried leaf pieces over CaCl₂ under vacuum at 4°C (D. Purcifull, unpublished data).

Purification

Purification of virus particles (method 1). (Gonsalves & Ishii, 1980). Inoculate Cucumis metuliferus accession 2459 (Yeh et al., 1984) or zucchini squash at cotyledonary stage and harvest systemically infected leaves after 21-25 days. Homogenise in a blender with 0.5 M potassium phosphate, pH 7.5 (2 ml/g tissue), containing 0.01 M ethylenediamine- tetraacetate (EDTA) and 0.1% sodium sulphite. Add 0.5 ml each of chloroform and carbon tetrachloride per g tissue during homogenisation. Centrifuge homogenate at 10,000 g for 10 min. Recover the aqueous phase and, to each 100 ml, add 8 g of polyethylene glycol (PEG) M. Wt 6000, stir 1-4 h at 6°C, centrifuge at 10,000 g for 10 min and resuspend in 20 ml 0.1 M potassium phosphate containing 0.01 M EDTA, pH 7.0 (PE buffer). Centrifuge at 10,000 g for 10 min, and re-precipitate the virus from the supernatant fluid by adding PEG and NaCl to final concentrations of 5% and 0.3 M, respectively. Centrifuge at 10,000 g for 10 min, and resuspend the pellet in 5 ml PE buffer. Further purify by dissolving 0.15 g Cs₂SO₄ in each ml of virus suspension, layer on a cushion of 53% (w/w) Cs₂SO₄ in PE buffer, centrifuge 18-22 h at 30,000 rev/min at 6°C. Collect virus-containing zone, dilute with 2 vol. PE buffer, remove host material by centrifugation at 10,000 g for 10 min, and recover the supernatant fluid, which contains the virus. Dialyse against PE buffer overnight to remove caesium salts. Further purification can be achieved by recentrifugation in Cs₂SO₄. Yields are about 5 mg per 100 g tissue.

Purification of virus particles (method 2). (Modification of method used by Purcifull & Hiebert (1979) for a type W isolate). Homogenise Small Sugar pumpkin tissue (100 g), collected 3-4 weeks after inoculation, in cold 200 ml 0.5 M potassium phosphate buffer (pH 7.5) (PB) containing 0.1% sodium sulphite, 50 ml chloroform and 50 ml carbon tetrachloride. Centrifuge at 1000 g for 5 min. Re-extract the pellet with 100 ml PB and centrifuge again. Combine the supernatant fluids and centrifuge at 13,000 g for 15 min. The resulting pellet can be used for purifying cylindrical inclusions (Hiebert, Purcifull & Christie, 1984a). To the supernatant fluid (which contains the virus), add Triton X-100 to 1% (v/v), PEG (M. Wt 6000-8000) to 4% (w/v) and NaCl to 100 mM, and stir for 1 h at 4°C. Centrifuge at 10,000 g for 10 min. Resuspend the pellets in 50 ml 20 mM PB with the aid of a tissue grinder. Centrifuge at 12,000 g for 10 min and discard the pellet. Re-precipitate virus particles from the supernatant fluid by adding PEG to 8% (w/v) and NaCl to 100 mM and stir for 0.5 h. Centrifuge at 12,000 g for 10 min. Resuspend the pellets in 5-10 ml 20 mM PB with the aid of a tissue grinder. Layer the resuspended material over 30% (w/w) CsCl or Cs₂SO₄ in 20 mM PB and centrifuge at 140,000 g for 16-18 h at 5°C. Collect the virus-containing zone by droplet fractionation, dilute with an equal volume of 20 mM PB and then centrifuge at 12,000 g for 10 min. Recover the virus from the supernatant fluid by PEG precipitation as before. The yield of virus particles is about 5-10 mg/100 g tissue. Clarification with n-butanol and the use of 10 mM EDTA in the buffer during purification (both useful for watermelon mosaic virus 2 purification) are detrimental to type W isolates (E. Hiebert, unpublished data).

Purification of cylindrical (pinwheel) inclusion (CI) protein. (Yeh, 1984). Propagate the virus in zucchini squash. Monitor the development and abundance of CI by using light microscopy of epidermal strips stained with luxol brilliant green-calcomine orange (Christie & Edwardson, 1977). Collect the tissue approximately 25-30 days after inoculation. Homogenise each 100 g tissue in a blender for 2 min in 200 ml 0.5 M potassium phosphate buffer, pH 7.5, containing 0.25% sodium sulphite. Add 50 ml chloroform and 50 ml carbon tetrachloride and homogenise for 1 min. Centrifuge at 165 g for 5 min save the upper aqueous phase and re-extract the pellets with 100 ml buffer. Centrifuge at 165 g for 5 min and combine the supernatant fluid with the aqueous phase already retained. Centrifuge at 1000 g for 5 min, discard the pellets, and centrifuge the supernatant fluid at 10,000 g for 20 min. The supernatant fluid after this step can be used for virus purification. To purify the CI, resuspend the pellets in 50 ml 0.05 M potassium phosphate buffer, pH 8.2, containing 0.1% 2-mercaptoethanol (2-ME), with the aid of a homogeniser. Add Triton X-100 to a final concentration of 5%, stir 1 h at 4°C, and centrifuge at 38,000 g for 15 min. Resuspend the pellets in 5 ml 0.02 M potassium phosphate buffer, pH 8.2, containing 0.1% 2-ME using a homogeniser. Repeat treatment with Triton X-100 and high speed centrifugation, and resuspend pellet in 2 ml 0.02 M Tris-HCl, pH 8.2.

CI proteins can be assayed by SDS-polyacrylamide gel electrophoresis. Further purify the CI protein by preparatory slab gel electrophoresis (Laemmli, 1970), degrading the CI by boiling for 2 min in 0.33 vol. buffer (0.4 M Tris-HCl, pH 6.8, 10% SDS, 20% 2-ME, 20% sucrose and 0.005% bromophenol blue). Centrifuge for 5 min at low speed to remove undenatured material, conduct electrophoresis, and then make the band visible by soaking the gel in 0.2 M KCl for c. 10 min at 4°C. Excise the band, rinse three times for 5 min in cold distilled water and store frozen. To elute protein, thaw gels, cut in 1 mm slices, place in 10 vol. buffer (0.05 M Tris-HCl, pH 7.9, 0.1% SDS, 0.1 mM EDTA and 0.15 M NaCl), and stir at room temperature for 6-8 h. Remove liquid and save; repeat process. Pool the two eluates, filter through paper to remove gel pieces, place sample on ice, and precipitate the CI protein by adding trichloracetic acid to a final concentration of 20% (w/v). After 10-15 min centrifuge at 12,000 g for 10 min, rinse the pellet with anhydrous ethyl ether, resuspend in 6.0 M guanidine-HCl in 0.02 M Tris-HCl, pH 8.2. Store the CI protein frozen or at 6°C with sodium azide.

Purification of amorphous inclusion body (AI) protein. (De Mejia, 1984). Inoculate zucchini squash in cotyledonary stage and harvest systemically infected tissue 4-6 weeks later. Homogenise the tissue in a blender in 1 ml extraction buffer (EB) (0.1 M Tris, pH 7.5, 0.5% sodium sulphite) per g tissue. Filter through 2 layers of cheesecloth, layer over 30 ml 20% sucrose in EB, and centrifuge at 4000 g for 5 min (Sorvall GSA rotor). Resuspend the pellets in cold EB, add Triton X-100 to a final concentration of 5%, and stir 1 h at 4°C. Centrifuge at 4000 g for 5 min through 15 ml 40% sucrose in EB, and resuspend the pellets in EB. Repeat the Triton X-100 treatment and centrifugation in 40% sucrose two more times, and resuspend in a small volume of 0.1 M Tris, pH 7.5. Monitor presence of AI in extracts by light microscopy of preparations stained with 0.15% phloxine in water. Store inclusions frozen. Purify inclusion protein by preparatory gel electrophoresis (Laemmli, 1970), dissociating the inclusions by boiling in 0.1 M Tris-HCl, pH 6.8, containing 2.5% SDS, 5% 2-ME and 5% sucrose. Detect AI protein in the gel by pre-labelling a portion of the AI protein with dansyl chloride (Talbot & Yphantis, 1971). Excise the gel band, crush with mortar and pestle, add 10 vol. water, freeze at -20°C for several hours, incubate at room temperature for several hours, and centrifuge at 1000 g for 5 min. Re-extract the pellet, re-centrifuge, combine the two supernatant fluids, filter through 0.45 µm Millipore membrane, and freeze-dry to remove water. Resuspend dried residue in 2 ml deionized water and dialyse for 8 h against deionized water. Store at -20°C. Yields up to 35 A₂₈₀ units per kg tissue are obtained.

Properties of Particles

A₂₆₀/A₂₈₀: 1.2 (uncorrected for light-scattering; E. Hiebert, unpublished data).

Buoyant density in CsCl: 1.32 g/cm³ (De La Rosa & Lastra, 1983).

Particle Structure

Particles are flexuous filaments (Fig.10) c. 12 nm in diameter and 760-800 nm long (Herold & Weibel, 1962; Purcifull & Edwardson, 1967; Purcifull & Hiebert, 1979; Russo et al., 1979; De La Rosa & Lastra, 1983).

Particle Composition

Nucleic acid: RNA, single-stranded (De La Rosa & Lastra, 1983). RNA extracted from virus particles by the procedure of Brakke & Van Pelt (1970) sediments at 39 S, like that of other potyviruses (E. Hiebert, unpublished data).

Protein: Subunit M. Wt 3.6-3.65 x 10⁴. Two smaller products, which have M. Wt of 3.1-3.4 x 10⁴ and 2.6-2.7 x 10⁴ have been reported, and preparations stored 2 months contained primarily the smaller form. Presumably, the smaller form arises by proteolytic degradation (Purcifull & Hiebert, 1979; Gonsalves & Ishii, 1980; Baum, 1980).

Genome Properties

The RNA molecules of papaya ringspot virus types P and W are translated efficiently in vitro, and antisera to virus-specified structural and inclusion body proteins have been used to analyse the translation products (De Mejia, 1984; Hiebert, Thornbury & Pirone, 1984b; De Mejia, Hiebert & Purcifull, 1984; M. De Mejia, E. Hiebert & D. Purcifull, unpublished data; L. Quiot-Douine, D. Purcifull & E. Hiebert, unpublished data). The translation product profile is similar to those described for tobacco etch and pepper mottle viruses (Dougherty & Hiebert, 1980) except that products reactive with antiserum to the 49 x 10³ M. Wt nuclear inclusion protein of tobacco etch virus are not detected. The proposed genetic map of the papaya ringspot virus genome (Fig.11) is based on the in vitro translation analysis and its similarity to the tobacco etch virus analysis. In the rabbit reticulocyte lysate system, the predominant in vitro translation product, mapped near the 5' end, has an estimated M. Wt of 110 x 10³ (for a type W isolate), 112 x 10³ (for a type P isolate), or 114 x 10³ (for the Guadeloupe isolate). This 110-114 K product is thought, by comparison with those formed in the wheatgerm system, to be a ‘read-through’ product consisting of a 40-60 K protein (not yet identified in vivo) and a 51 K protein, which reacts with antiserum to the AI protein of a type W isolate and to the helper component protein of tobacco vein mottling virus (M. de Mejia, E. Hiebert & D. Purcifull, unpublished data). Peptide mapping showed that the predominant (M. Wt 110 x 10³) product of the type W isolate was distinct from that (M. Wt 107 x 10³) of a watermelon mosaic virus 2 isolate (E. Hiebert, unpublished data) even though both products are immunoprecipitated by antiserum to the helper component of tobacco vein mottling virus (Hiebert et al., 1984b).

Relations with Cells and Tissues

Isolates of types P and W both typically induce two types of cytoplasmic inclusion in their hosts: cylindrical inclusions (CI) (Fig.5 and Fig.7) and amorphous inclusions (AI) (Fig.6 and Fig.7). The CI are striated, with a periodicity of 5 nm, and are assocated with scrolls (Purcifull & Edwardson, 1967; Edwardson, Purcifull & Christie, 1968; Zettler et al., 1968; Martelli & Russo, 1976). The CI aggregate to form fibrous structures that are clearly visible by light microscopy of epidermal strips stained with luxol brilliant green-calcomine orange (Fig.7). They contain a protein of M. Wt 6.9-7.0 x 10⁴ (Baum, 1980; Yeh, 1984) which is distinct serologically from host protein and virus particle protein.

The AI have been observed in tissues of papaya and cucurbits infected with various isolates (Edwardson, 1974; Martelli & Russo, 1976; Christie & Edwardson, 1977; Russo et al., 1979). However, the AI were not detected in tissues infected with one type W isolate, even though the AI protein was detected serologically (M. De Mejia, R. Christie, D. Purcifull & E. Hiebert, unpublished data). In some reports, the AI have been referred to as irregular cytoplasmic inclusions (Christie & Edwardson, 1977). The AI often are visible in the same cells as the CI (Martelli & Russo, 1976). The staining reactions of the inclusions indicate they contain both RNA and protein (Martelli & Russo, 1976; Christie & Edwardson, 1977). A major protein isolated from the AI has a M. Wt of 5.1 x 10⁴; an antiserum to this protein reacts with AI in situ in immunofluorescence tests, and with the predominant in vitro translation product (mapped near the 5' end) of viral RNA (M. De Mejia, E. Hiebert & D. Purcifull, unpublished data; De Mejia et al., 1984).

Notes

The mosaic disease of papaya caused by papaya ringspot virus (Ishii & Holtzmann, 1963) should not be confused with that caused by papaya mosaic virus, which is a potexvirus (Purcifull & Hiebert, 1971). The two viruses can be distinguished readily by particle morphology (De Bokx, 1965), by the types of intracellular inclusion they induce in their hosts (Zettler et al., 1968; Christie & Edwardson, 1977), and by serological tests (Fig.9) (De Bokx, 1965; D. Purcifull, unpublished data).

An aphid-transmitted, filamentous virus (c. 760 nm long) is associated with yellow mottling and necrosis of papaya in Okinawa (Yonaha, 1977); its relationship to papaya ringspot virus is not clear. Other viruses that infect papaya naturally (Cook, 1972) include tobacco ringspot (Lambe, 1963; Lana, 1980), tomato spotted wilt (Trujillo & Gonsalves, 1967), and one or more rhabdoviruses (Lastra & Quintero, 1981; Wan & Conover, 1981), all of which can be distinguished from papaya ringspot virus on the basis of particle morphology.

Papaya ringspot virus isolates can be distinguished from watermelon mosaic virus 2 (Webb & Scott, 1965; Purcifull et al., 1984b) by inclusion body morphology (Edwardson, 1974; Christie & Edwardson, 1977; Russo et al., 1979) and by serology and host range (Webb & Scott, 1965; Purcifull & Hiebert, 1979; Russo et al., 1979).

The following four potyviruses that also cause diseases of cucurbits can be distinguished from papaya ringspot virus on the basis of SDS-immunodiffusion tests: bean yellow mosaic virus (Provvidenti & Uyemoto, 1973; Purcifull & Hiebert, 1979); a potyvirus from cucurbits in Morocco (Fischer & Lockhart, 1974; Purcifull & Hiebert, 1979; Baum, Purcifull & Hiebert, 1979); zucchini yellow fleck virus (Vovlas, Hiebert & Russo, 1981); and zucchini yellow mosaic virus (Lisa et al., 1981). The last-named virus has been observed in cucurbits in various parts of the world (Lecoq, Lisa & Dellavalle, 1983; Lisa & Lecoq, 1984), sometimes in mixed infections with other potyviruses, including type W isolates of papaya ringspot virus (Purcifull et al., 1984a).

Numerous other viruses cause diseases of cucurbits (Lovisolo, 1980). Cucumber mosaic virus (Francki, Mossop & Hatta, 1979) and squash mosaic virus (Campbell, 1971) can be distinguished from papaya ringspot virus by host reactions (Grogan, Hall & Kimble, 1959; Milne et al., 1969), by SDS-immunodiffusion tests (Purcifull, Christie & Lima, 1981; Purcifull et al., 1984a) and by light microscopic examination of the types of inclusion they induce in host cells (Christie & Edwardson, 1977).

References

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Figure 1

Mottle induced in papaya leaf by a type P isolate.

Figure 2

Distortion induced in papaya leaf by a type P isolate, showing lobes reduced to veins and small portions of laminar tissue.

Figure 3

Local lesions induced in Chenopodium quinoa by a type P isolate (courtesy of S.-D. Yeh).

Figure 4

Ringspots in papaya fruit of naturally infected plant (courtesy of S. Garnsey).

Figure 5

Electron micrograph of cylindrical (pinwheel) and scroll inclusions in cytoplasm of pumpkin leaf cell infected by a type W isolate. Bar represents 250 nm.

Figure 6

Electron micrograph of amorphous (irregular) inclusion body in cytoplasm of pumpkin leaf cell infected by a type W isolate. Bar represents 1 µm.

Figure 7

Light micrograph of pumpkin epidermal cell infected by a type W isolate, showing amorphous inclusion (ai) and a group of cylindrical (pinwheel) inclusions (ci) in the cytoplasm. Bar represents 10 µm. (Photomicrograph courtesy of R. G. Christie.)

Figure 8

Mosaic in pumpkin leaf infected by a type W isolate.

Figure 9

Sodium dodecyl sulphate (SDS)-immunodiffusion tests illustrating close relationship of types P and W isolates of papaya ringspot virus and their distinction from certain other viruses. A = antiserum to a type W isolate. The other wells contain SDS-treated leaf sap: 1 = type W isolate in squash; 2= type P isolate in squash; 3 = healthy squash; 4 = watermelon mosaic virus 2 in squash; 5 = zucchini yellow mosaic virus in squash; 6 = type P isolate in papaya; 7 = healthy papaya; 8 = papaya mosaic virus (potexvirus) in papaya.

Figure 10

Electron micrograph of filamentous particles of a type P isolate, mounted in uranyl acetate (micrograph courtesy of S. R. Christie). Bar represents 200 nm.

Figure 11

Proposed genetic map of papaya ringspot virus (type P, Florida isolate). The M. Wt of the gene products are presented above the map and three of the products are identified below the map. The 112k product is serologically related to tobacco vein mottling virus helper component and the 57k product is serologically related to the 54k nuclear inclusion protein of tobacco etch virus.