Rice yellow mottle virus

W. Bakker
State Agricultural College, Prof. H. C. van Hallstraat 1, Groningen, The Netherlands

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 Bakker (1970, 1974).

An RNA-containing virus with isometric particles about 25 nm in diameter, infecting species of Gramineae only. It is readily transmissible by inoculation of sap and also by several chrysomelid beetles. Reported in rice from Kenya.

Main Diseases

In rice (Oryza sativa) the virus causes a yellow or orange leaf discolouration, stunting and sterility. Infected plants are found first near the bunds (Fig.1, Fig.2) but soon thereafter whole fields may be affected.

Geographical Distribution

Reported from the Kisumu area near Lake Victoria in Western Kenya (Bakker, 1974).

Host Range and Symptomatology

Host range is narrow, being restricted to species in the Gramineae, mainly in the tribes Oryzeae and Eragrostideae. Symptoms, when induced, consist initially of systemic mottling. Some grass species develop symptoms soon after mechanical inoculation of sap and are used as experimental hosts (Bakker, 1974).

Diagnostic species

Oryza sativa (rice). Symptoms are induced in seedlings of all rice varieties tested. About a week after inoculation yellow dots appear at the bases of the youngest systemically infected leaves. The dots elongate parallel to the veins; leaves formed later are mottled (Fig.4) and often spirally twisted. Plants are stunted, form few tillers and no panicles. In cv. Basmati 217 early symptoms are difficult to discern.

Oryza punctata. Young systemically infected leaves show only a few elongated yellowish dots.

Dinebra retroflexa. Mild mottling and yellowish-green discolouration of young systemically infected leaves.

Phleum arenarium. Obvious mottling of the systemically infected leaves (Fig.3) which later turn yellow and die.

Propagation species

O. sativa cv. Sindano is a suitable plant for maintaining cultures and a good source of virus for purification. Other rice varieties are probably suitable too.

Assay species

No known local lesion host. O. sativa cvs. Sindano and IR 20 are suitable for insect transmission tests.

Strains

None found.

Transmission by Vectors

Transmissible by chrysomelid beetles belonging to the subfamilies Criocerinae, Cryptocephalinae, Galerucinae (Sesselia pusilla), Halticinae (Chaetocnema spp.) and Hispinae (Trichispa sericea). The long-horned grasshopper Conocephalus merumontanus also transmits the virus (Bakker, 1974). Chaetocnema pulla (Fig.5) carries the virus in the field. S. pusilla, C. pulla and T. sericea sometimes acquire the virus in 15 min and also sometimes inoculate in 15 min. Percentage of insects transmitting increases with increasing length of acquisition and inoculation feeds up to 2-3 days. S. pusilla and C. pulla retain the virus usually for 2-3 days, and sometimes for 8 and 5 days respectively, whereas T. sericea retains the virus for one day only (Bakker, 1974).

Transmission through Seed

Apparently not seed-transmitted in rice.

Transmission by Dodder

No information.

Serology

The virus is strongly immunogenic, inducing antibody titres of up to 1/4096. It forms only one band of precipitate in agar gel-diffusion tests using 1% agar and antiserum prepared by a series of intravenous and intramuscular injections.

Relationships

The virus cannot be placed in an established group of viruses. No serological relationship was detected between rice yellow mottle virus and any of several other isometric viruses, including cocksfoot mottle and phleum mottle, which also are beetle-transmitted in grasses (Serjeant, 1967; Catherall, 1970).

Stability in Sap

In sap from rice cv. Sindano tested on seedlings of the same variety, the thermal inactivation point (10 min) is about 65°C, and the dilution end-point is 10^-6. The infectivity of sap diluted with 0.01 M phosphate buffer pH 7.0 is retained for 99 days at 20°C or for 260 days at 4°C. The virus survives for at least a year in leaf tissue stored at 4°C above CaCl₂ (Bakker, 1974).

Purification

The virus is stable in vitro and easy to purify (Bakker, 1970, 1974). Use fresh or deep-frozen young rice leaves, harvested 10-12 days after infection. Homogenize small pieces in 0.1 M phosphate buffer pH 5.0 + 0.2% 2-mercaptoethanol (1 g of leaves/20 ml of buffer). Squeeze through cloth. Emulsify extract with 0.5 vol chloroform (5 min) and centrifuge at low speed. To each 100 ml of the aqueous phase add 20 g (NH₄)₂SO₄ while stirring, and centrifuge at low speed. Repeat this procedure with the supernatant fluid but this time retain the pellets after low speed centrifugation. Resuspend them in a small amount of buffer pH 5.0 and dialyse against the same buffer. Clarify by one cycle of differential centrifugation (20 min at 6600 g, 100 min at 78,500 g). Suspend the sediment from high speed centrifugation in 0.01 M phosphate buffer pH 7.0. Dialyse against the same buffer and centrifuge at low speed. Work at room temperature during precipitation of the virus with (NH₄)₂SO₄ and during the first dialysis, otherwise at 4°C. Yield is about 1 mg virus/g of leaves.

Properties of Particles

Sedimentation coefficient (s°₂₀) in 0.01 M phosphate buffer pH 7.0: about 109 S. No accessory viral components are found by analytical centrifugation. Purified virus in water or buffer does not withstand freezing.

Absorbance at 260 nm (1 mg/ml, 1 cm light path): 6.5.

A₂₆₀/A₂₈₀: 1.54.

Particle Structure

Particles are isometric, about 25 nm in diameter (Fig.6). No obvious surface structure is revealed in phosphotungstate negative stain. Some particles are penetrated by phosphotungstate.

Particle Composition

Nucleic acid: RNA, single-stranded, M. Wt about 1.4 x 10⁶; constitutes about 23% of particle weight. Molar percentages of nucleotides: G29; A21; C25; U25.

Protein: no information.

Relations with Cells and Tissues

In rice cv. Sindano the virus is found free in the cytoplasm of epidermis and mesophyll cells. Aggregates of virus particles are common in mesophyll cells of young leaves. Long flexuous tubules (Fig.7), inclusions with electron-dense centres and aggregates of fibrils are observed by electron microscopy (Bakker, 1974).

Notes

The virus is easily differentiated from other known viruses occurring naturally in rice (Ou, 1972) by being mechanically transmissible. Among other viruses affecting grasses it resembles cocksfoot mottle (Serjeant, 1967) and phleum mottle (Catherall, 1970; Benigno & A’Brook, 1972) viruses in having beetle vectors and in several other properties. Although it resembles phleum mottle virus in infecting Phleum arenarium, it differs in being unable to infect P. bertolonii and P. pratense.

References

1. Benigno & A’Brook, Ann. appl. Biol. 72: 43, 1972.
2. Bakker, Neth, J. Pl. Path. 76: 53, 1970.
3. Bakker, Agric. Res. Reports 829: 152 pp., 1974.
4. Catherall, Pl. Path. 19: 101, 1970.
5. Serjeant, Ann. appl. Biol. 59: 31, 1967.
6. Ou, in Rice Diseases, Commonwealth Mycological lnstitute, Kew, 368 pp., 1972.

Figure 1

Field infected rice cv. Sindano.

Figure 2

Field infected rice cv. Sindano.

Figure 3

Systemic symptoms in mechanically infected Phleum arenarium.

Figure 4

Mottling of rice cv. Sindano leaves induced by mechanical inoculation.

Figure 5

Adult of the chrysomelid beetle, Chaetocnema pulla.

Figure 6

Virus particles from a purified preparation stained with phosphotungstate. Bar represents 100 nm.

Figure 7

Flexuous tubules in epidermal cell of mature leaf of RYMV-infected rice cv. Sindano. Bar represents 500 nm.