254
July 1982
Family: Avsunviroidae
Genus: Avsunviroid
Species: Avocado sunblotch viroid
Acronym: ASBVd


Avocado sun-blotch viroid

J. L. Dale
Department of Primary Industries, Indooroopilly, Queensland, 4068, Australia

R. H. Symons
Biochemistry Department, University of Adelaide, South Australia, 5001, Australia

R. N. Allen
Agricultural Research Centre, Wollongbar, New South Wales, 2480, Australia

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 Horne & Parker (1931); viroid nature of infective agent established by Palukaitis et al. (1979).

An infective single-stranded circular RNA molecule of 247 nucleotides. Mechanically transmitted with difficulty to avocado and cinnamon. No direct vector transmission. Reported from most avocado-growing areas.

Main Diseases

Causes sunken yellow or red streaks on fruit of avocado (Persea americana) rendering them unmarketable. Also induces yellow, orange or white streaks and spots on the stem and petioles, variegation and distortion of leaves (Fig.1, Fig.2).

Geographical Distribution

Recorded in many avocado-growing countries including Australia, Israel, Peru, South Africa, USA and Venezuela.

Host Range and Symptomatology

Transmitted by graft inoculation only to members of the family Lauraceae including Persea americana, Cinnamomum camphora, Cinnamomum zeylanicum and Ocotea bullata (da Graça & van Vuuren, 1980).

Diagnostic and assay species

Persea americana (avocado) cv. Hass. Graft inoculated seedlings show yellow, orange or white streaks or spots on the stems and petioles after 2 months to 3 years and may also show variegation and distortion of the leaves (Fig.1, Fig.2). Symptom development is accelerated by incubating at high temperatures (30-32°C). Plants can also be infected by slashing the stem with a razor blade wetted with a partially purified preparation (Desjardins, Drake & Swiecki, 1980).

Because of the length of time to symptom expression, avocado sunblotch viroid (ASBV) can be more conveniently diagnosed by nucleic acid hybridization with complementary DNA (Palukaitis et al., 1981; Allen & Dale, 1981).

Propagation species

Persea americana is the only host from which the viroid has been purified. Guatamalan cultivars (e.g. Hass) are the most suitable because of the lower viscosity of the aqueous phase of extracts obtained by phenol treatment.

Strains

Minor differences in nucleotide sequence were observed between two isolates (Palukaitis et al., 1981).

Transmission by Vectors

None reported except one instance of probable pollen transmission via bees (Desjardins et al., 1979) - see below.

Transmission through Seed

Transmitted through seed at a very high frequency (80-100%) in symptomlessly infected trees and the resultant infected seedlings are also symptomless. However, the frequency of seed transmission in trees displaying symptoms is low (<5%) and the resultant infected seedlings show symptoms (Wallace & Drake, 1962). Under experimental conditions, low frequencies (1.8-3.1%) of pollen transmission via bees (Apis mellifera) were obtained (Desjardins et al., 1979).

Relationships

Avocado sunblotch viroid (247 nucleotide residues, M. Wt 0.8 x 105; Symons, 1981) is considerably smaller than potato spindle tuber viroid (359 residues; Gross et al., 1978) and chrysanthemum stunt viroid (356 residues; Haseloff & Symons, 1981) but it has 18% sequence homology with them (Symons, 1981). It is also smaller than citrus exocortis viroid (371 residues; Visvader et al., 1982; Gross et al., 1982) but is similar in size to the smallest RNA component of cadang-cadang viroid (J. Haseloff, N. Mohamed and R. H. Symons, unpublished data).

Purification

The viroid can be purified from infected avocado leaves by a procedure described for the purification of chrysanthemum stunt viroid from chrysanthemum leaves (Palukaitis & Symons, 1980). The method involves the initial preparation of a partially purified nucleic acid extract containing mostly low M. Wt RNA and DNA, followed by electrophoresis of this extract in polyacrylamide slab gels under non-denaturing conditions. The viroid band, located by staining, is eluted electrophoretically from the gel and further purified by electrophoresis in polyacrylamide slab gels under denaturing conditions (7 M urea); circular viroid molecules are separated from linear ones at this stage. Highly purified circular and linear viroid molecules are eluted electrophoretically from the gel. Residual contaminating polyacrylamide can be removed by sedimentation of the viroid in a sucrose gradient (Palukaitis et al., 1981). Circular viroid molecules purified by this method are infective (Allen, Palukaitis & Symons, 1981).

Properties of Infective Nucleic Acid

Shown to consist of RNA by treatment with DNase I and pancreatic RNase and by sequence determination. When spread under completely denaturing conditions, most molecules seen in the electron microscope were circular (Palukaitis et al., 1979). Thermal denaturation in 15 mM NaCl, 1.5 mM trisodium citrate, pH 7.0, indicated a hyperchromicity of 21% and a Tm of 38°C (Palukaitis et al., 1979).

The molecule has M. Wt of 0.8 x 105 and contains 247 nucleotides (Fig.4) with a base composition of G 20.6: A 27.5: C 17.4: U 34.4 (Symons, 1981). The proposed secondary structure (Fig.5) is a covalently closed single-stranded circular molecule, 67% base-paired; 34% of the base pairs are G:C, 52% A:U and 14% G:U (Symons, 1981).

Relations with Cells and Tissues

The viroid can be isolated from leaves, petioles and stems of infected avocado trees (Fig.3), both from those that are symptomlessly infected and from those displaying sunblotch symptoms. Mohamed & Thomas (1980) reported that most viroid is found in the ‘chloroplast’ fraction and none in either the ‘cytoplasmic’ or ‘mitochondrial’ fractions.

Notes

Avocado sunblotch is the only viroid known to infect avocado. A tobamovirus related to tobacco mosaic virus was isolated from symptomlessly infected avocado seedlings in Israel, but its distribution and importance is unknown. It can be readily distinguished from avocado sunblotch viroid because it is mechanically transmissible to Datura stramonium, Chenopodium amaranticolor and Nicotiana glutinosa (Alper et al., 1978).

References

  1. Allen & Dale, Ann. appl. Biol. 98: 451, 1981.
  2. Allen, Palukaitis & Symons, Australas. Pl. Path. 10: 31, 1981.
  3. Alper, Bar-Joseph, Salomon & Loebenstein, Phytoparasitica 6: 15, 1978.
  4. da Graça & van Vuuren, Res. Rep. S. Afr. Avocado Growers Ass. 4: 81, 1980.
  5. Desjardins, Drake, Atkin & Bergh, Calif. Agric. 33: 14, 1979.
  6. Desjardins, Drake & Swiecki, Pl. Dis. 64: 313, 1980.
  7. Gross, Domdey, Lossow, Jank, Raba & Alberty, Nature, Lond. 273: 203, 1978.
  8. Gross, Krupp, Domdey, Raba, Jank, Lossow, Alberty & Sänger, Eur. J. Biochem. 121: 249, 1982.
  9. Haseloff & Symons, Nucleic Acids Res. 9: 2741, 1981.
  10. Horne & Parker, Phytopathology 21: 235, 1931.
  11. Mohamed & Thomas, J. gen. Virol. 46: 157, 1980.
  12. Palukaitis & Symons, J. gen. Virol. 46: 477, 1980.
  13. Palukaitis, Hatta, Alexander & Symons, Virology 99: 145, 1979.
  14. Palukaitis, Rakowski, Alexander & Symons, Ann. appl. Biol. 98: 439, 1981.
  15. Symons, Nucleic Acids Res. 9: 6527, 1981.
  16. Visvader, Gould, Bruening & Symons, FEBS Lett. 137: 218, 1982.
  17. Wallace & Drake, Phytopathology 52: 237, 1962.


Figure 1

Range of symptoms on avocado cv. Hass including variegation and distortion of leaves and a typical fruit lesion.

Figure 2

Yellow streaks on the stem of avocado cv. Hass.

Figure 3

Electrophoresis of nucleic acids on 5% polyacrylamide gels under non-denaturing conditions. Partially purified nucleic acid extracts from (A) chrysanthemum with stunt viroid; (B) avocado with sunblotch viroid (low viroid concentration); (C) healthy avocado; (D) avocado with sunblotch viroid; (E) purified avocado sunblotch viroid (Palukaitis et al., 1979).

Figure 4

Primary nucleotide sequence. The circular RNA is presented as a linear sequence with residue 1 corresponding to the left hand end of the secondary structure model of Fig.5 (Symonds, 1981).

Figure 5

Proposed secondary structure of sequence J02020 (Symons, 1981).