323
December 1986
Family: Geminiviridae
Genus: Begomovirus
Species: Mungbean yellow mosaic virus
Acronym: MYMV


Mung bean yellow mosaic virus

Y. Honda
National Agriculture Research Center, Yatabe, Tsukuba, Ibaraki 305, Japan

M. Ikegami
NODAl Research Institute, Tokyo University of Agriculture, Setagaya-ku, Tokyo 156, Japan

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

Disease first described by Nariani (1960). Virus particles first observed by Thongmeearkom et al. (1981a) and purified by Honda et al. (1983).

A virus with geminate particles, c. 30 x 18 nm, containing two species of circular single-stranded DNA. The host range is largely confined to species of Leguminosae. The virus is transmitted by the whitefly, Bemisia tabaci, in the persistent (circulative) manner, and by inoculation with sap. The virus causes serious losses of yield in mung bean and blackgram.

Main Diseases

Causes a yellow mosaic disease of mung bean (Vigna radiata) starting as small yellow specks along the veinlets and spreading over the lamina; the pods become thin and curl upward (Fig.1). In blackgram (Vigna mungo) there are two types of yellow mosaic symptom depending upon the variety: ‘yellow mottle’ (generalized yellowing of the leaves) and ‘necrotic mottle’ (yellowing restricted to small spots which turn necrotic) (Nene, 1973; Nair et al., 1974). In India the virus causes more severe yellow mosaic disease in blackgram than in mung bean (Williams et al., 1968); however, in Thailand the disease is common in mung bean but is seldom observed in blackgram under natural conditions (Honda et al., 1983).

Geographical Distribution

Distributed in south Asia. Reported from India (Nariani, 1960), West Pakistan (Ahmad & Harwood, 1973), Sri Lanka (Shivanathan, 1977) and Thailand (Thongmeearkom et al., 1981b).

Host Range and Symptomatology

The virus from Thailand has been transmitted by mechanical inoculation, but only to Canavalia ensiformis, Glycine max, Phaseolus angularis, P. lunatus, P. vulgaris, Vigna mungo and V. radiata, all of which are members of the Leguminosae (Honda et al., 1983). The virus reported from India has not been transmitted by mechanical inoculation but has been transmitted by the whitefly vector, not only to several species in the Leguminosae (Nariani, 1960) but also to Brachiaria romosa (Gramineae) and Cosmos bipinnatus, Eclipta alba and Xanthium strumarium (Compositae) (Nene et al., 1971; Nene, 1973; Rathi & Nene, 1974).

Diagnostic species
Glycine max (soybean). Small yellow specks along the veinlets developing into yellow mosaic.

Phaseolus vulgaris (French bean). Young systemically infected leaflets show downward curling without yellow mosaic (Fig.4). Irregular chlorotic spots develop rarely.

Vigna radiata (mung bean). Irregular chlorotic spots along the veinlets developing into yellow mosaic (Fig.2, Fig.3). The first emerging trifoliolate leaves often show severe downward curling.

Propagation species
Phaseolus vulgaris cv. Top Crop grown at about 30°C in high humidity is a good source of virus for purification. Systemically infected leaves are harvested within 2 weeks after mechanical inoculation.

Assay species
No local lesion host is known. Infectivity of purified virus preparations may be assayed by determining the proportion of young seedlings of Vigna radiata that become infected following mechanical inoculation. A similar whole plant assay in V. radiata is used to measure the frequency of transmission by whitefly vectors.

Strains

Host range, symptomatology and mechanical transmission suggest that there may be relationships among the isolates of mung bean yellow mosaic virus from India, West Pakistan, Sri Lanka and Thailand. The Thai isolate of the virus is transmissible by mechanical inoculation and its host range appears to be restricted to seven species in the Leguminosae (Honda et al., 1983). On the contrary, other isolates from India, West Pakistan and Sri Lanka are not sap-transmissible (Nene, 1972; Ahmad & Harwood, 1973; Shivanathan, 1977). An Indian isolate had a wider host range than Thai isolates including certain species in the Leguminosae, Compositae and Gramineae (Nene, 1972). The serological relationships among these presumed isolates of mung bean yellow mosaic virus from various countries are not yet understood.

Transmission by Vectors

Transmissible by the whitefly, Bemisia tabaci (Nariani, 1960; Ahmad & Harwood, 1973; Shivanathan, 1977; Thongmeearkom et al., 1981b). Acquisition and inoculation by adults can each be effected in a minimum time of 15 min. The latent period is less than 4 h (Nair, 1971). A single viruliferous adult can transmit the virus. The most efficient female and male adults in a population can retain infectivity for 10 days and 3 days, respectively. Neither female nor male adults can retain infectivity throughout the life span. Female adults are over three times more efficient as vectors than males (Rathi, 1972). Nymphs of Bemisia tabaci can acquire the virus from diseased leaves (Nene, 1972). The virus does not pass through eggs of B. tabaci (Rathi, 1972; Ahmad & Harwood, 1973).

Transmission through Seed

Not seed-transmitted in mung bean or soybean (Y. Honda, unpublished data).

Serology

An antiserum against the Thai isolate had titres of 1/160 in the tube precipitin test and at least 1/409,600 in a ‘Western‘ immunoblotting technique and dot immunobinding assay (Honda, 1986).

Relationships

Serological relationships are found between mung bean yellow mosaic virus and bean golden mosaic virus by Ouchterlony double diffusion tests (M. Ikegami, unpublished data), dot immunobinding assay and ‘Western’ immunoblotting (Honda, 1986), although the host range and symptomatology of the two viruses are different (Honda et al., 1983). Both viruses react in the ‘Western’ immunoblotting technique with antisera against other whitefly-transmitted geminiviruses, including African cassava mosaic, tobacco leaf curl and tomato yellow leaf curl viruses (Y. Honda, unpublished data). In Ouchterlony double diffusion tests, mung bean yellow mosaic virus forms a single band of precipitate with antiserum against African cassava mosaic virus (Y. Honda & M. Ikegami, unpublished data), and a relationship between the two viruses can also be shown by immunosorbent electron microscopy (I. M. Roberts & B. D. Harrison, unpublished results).

Stability in Sap

In infected Vigna radiata sap, the thermal inactivation point (10 min) is 40-50°C, the dilution end-point is between 10-2-10-3, and the longevity in vitro is 1-2 days at 20°C (Honda et al., 1983). Infectivity is retained for at least 4 years in frozen dehydrated leaves or frozen leaves (Y. Honda & M. Ikegami, unpublished data).

Purification

Homogenize systemically infected leaves in 0.1 M potassium phosphate, pH 7.8, containing 0.1% thioglycollic acid, 10 mM sodium diethyldithiocarbamate and 1 mM sodium ethylenediamine-tetraacetate (2 ml/g tissue). Clarify by adding 0.5 vol. chloroform and centrifuging at low speed. Add NaCl to 0.2 M and polyethylene glycol (PEG), M. Wt 6000, to 6% (w/v) and centrifuge at 15,000 g for 30 min. Dissolve the precipitate in 0.1 M potassium phosphate, pH 7.8. Clarify by low-speed centrifugation, then centrifuge at 125,000 g for 90 min and resuspend the pellets in potassium phosphate buffer containing 6% PEG and 0.2 M NaCl. Layer the preparations onto PEG discontinuous reverse solubility gradients (Honda et al., 1983), then centrifuge in a Hitachi RPS 25 swinging bucket rotor at 15,000 g for 30 min. Recover the opaque band and concentrate the virus by high-speed centrifugation. Final purification is by sucrose density gradient centrifugation. Yield is less than 1 mg virus particles/kg leaf (Honda et al., 1983).

Properties of Particles

The geminate particles form a single band in sucrose density gradients (Honda et al., 1983).

A260/A280 : 1.3-1.4.

Particle weight (daltons): 4.0 x 106.

Particle Structure

The virus particles are geminate, c. 30 x 18 nm; the protein shells can perhaps be regarded as formed by the fusion of two quasi-icosahedra (Fig.5). The particles are stable in 2% sodium phosphotungstate (pH 3.5) or 2% uranyl acetate without any previous fixation (Honda et al., 1983).

Particle Composition

Nucleic acid: Circular single-stranded (ss) DNA, about 20% of particle weight. M. Wt c. 0.8 x 106, estimated by electron microscopy (Fig.7; Ikegami et al., 1985). Sedimentation coefficient (s20,w) at pH 7.0, i = 0.1: 16 S (Ikegami et al., 1985). Restriction endonuclease mapping shows that there are two major DNA species of similar size (Fig.8; Morinaga et al., 1985). DNA-1 has 2715 nucleotides (22.5% G, 27.1% A, 20.7% C, 29.7% T; M. Wt 0.84 x 106) and DNA-2 has 2637 nucleotides (20.6% G, 28.7% A, 19.0% C, 31.7% T; M. Wt 0.81 x 106) (Morinaga et al., 1985).

Protein: One species of polypeptide in the particles, of M. Wt c. 27,500, estimated by SDS/polyacrylamide gel electrophoresis (Honda, 1986).

Genome Properties

A double-stranded (ds) DNA, probably a replicative intermediate, isolated from infected leaves of Top Crop bean is a circular molecule and has sequences complementary to those of viral DNA. Both the dsDNA from leaves and the ssDNA from virus particles are infective in Top Crop bean when assayed by mechanical inoculation (Ikegami et al., 1984). The two genomic DNA species have different sequences except for a ‘common region’ of about 200 nucleotides which is almost identical in the two molecules (Morinaga et al., 1985). The ‘common region’ contains a possible stem-loop structure of 34 nucleotides with a G + C-rich 11 base-pair stem and an A + T-rich 12-nucleotide loop (Morinaga et al., 1985). The loop sequence includes the sequence TAATATTAC.

Relations with Cells and Tissues

In Vigna radiata leaf cells, the virus particles often form loose aggregates (Fig.6) that sometimes fill almost completely the nuclei of infected phloem cells. In V. radiata, hypertrophied nucleoli, aggregates of virus particles and fibrillar bodies appear in the nuclei of phloem cells as early as two days before symptom appearance. Virus particles are sometimes scattered in distribution but they occasionally form aggregates having a paracrystalline or double cylindrical arrangement in the vacuoles of infected sieve elements (Thongmeearkom et al., 1981a).

Notes

Among four viruses presumed to be mung bean yellow mosaic virus, from India, West Pakistan, Sri Lanka and Thailand, virus purification (Honda et al., 1983) and nucleotide sequencing (Morinaga et al., 1985, 1987) have been reported only for the Thai isolate. Comparisons of the sequence homology among the four viruses could be useful in determining which are isolates of mung bean yellow mosaic virus and which are separate viruses. The DNA-1 of mung bean yellow mosaic virus has substantial sequence homology with the corresponding DNA species of bean golden mosaic virus but there is relatively little homology between DNA-2 and the smaller of the two genome species of bean golden mosaic virus. Also the common regions, except for the stem-loop sequence, of the two DNA species of mung bean yellow mosaic virus are different in sequence from those of bean golden mosaic virus (Morinaga et al., 1987).

References

  1. Ahmad & Harwood, Pl. Dis. Reptr 57: 800, 1973.
  2. Honda, Ann. phytopath. Soc. Japan 52: 553, 1986.
  3. Honda, Iwaki, Saito, Thongmeearkom, Kittisak & Deema, Pl. Dis. 67: 801, 1983.
  4. Ikegami, Morinaga & Miura, Virus Res. 1: 507, 1984.
  5. Ikegami, Yazaki, Honda, Iwaki, Fujii, Morinaga & Miura, Microbiol. Immunol. 29: 783, 1985.
  6. Morinaga, Arai, Miura & Ikegami, Abstr. 33rd Meeting Soc. Japanese Virologists, p. 347, 1985.
  7. Morinaga, Ikegami, Shimotohno & Miura, Microbiol. Immunol. 31: 147, 1987.
  8. Nair, Ph.D. Thesis, Uttar Pradesh Agricultural University, Pantnagar, India, 1971. In Nene, 1972.
  9. Nair, Nene & Naresh, Indian Phytopath. 27: 256, 1974.
  10. Nariani, Indian Phytopath. 13: 24, 1960.
  11. Nene, A Survey of Viral Diseases of Pulse Crops in Uttar Pradesh, Res. Bull. No. 4, 191 pp., G.B. Pant University of Agriculture & Technology, Pantnagar, India, 1972.
  12. Nene, Pl. Dis. Reptr 57: 463, 1973.
  13. Nene, Naresh & Nair, Indian Phytopath. 24: 415, 1971.
  14. Rathi, Ph.D. Thesis, G.B. Pant University of Agriculture & Technology, Pantnagar, India 1972. In Nene, 1972.
  15. Rathi & Nene, Indian Phytopath. 27: 429, 1974.
  16. Shivanathan, Trop. Agric. Res. Ser. (Trop. Agric. Res. Center, Japan), No. 10, p. 65, 1977.
  17. Thongmeearkom, Honda, Saito & Syamananda, Phytopathology 71: 41, 1981a.
  18. Thongmeearkom, Kittipakorn & Surin, Thai J. agric. Sci. 14: 201, 1981b.
  19. Williams, Grewal & Amin, Pl. Dis. Reptr 52: 300, 1968.


Figure 1

Upward curling symptoms of naturally infected Vigna radiata pods.

Figure 2

Irregular chlorotic spots along the veinlets in V. radiata leaf at an early stage of infection.

Figure 3

Yellow mosaic symptoms in V. radiata leaf at late stage of infection.

Figure 4

Severe downward curling symptoms in Phaseolus vulgaris cv. Top Crop.

Figure 5

Purified virus preparation stained with 2% uranyl acetate. Bar represents 500 nm.

Figure 6

Ultrathin section of infected V. radiata phloem cell, showing an aggregate of virus particles (V) in the nucleus. Bar represents 1 µm.

Figure 7

Electron micrograph of mung bean yellow mosaic virus DNA (smaller molecule) and fX 174 DNA (larger molecule). Bar represents 500 nm.

Figure 8

Physical map of two genome DNA species of mung bean yellow mosaic virus (courtesy T. Morinaga).