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
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 10
5;
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 T
m 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
- Allen & Dale, Ann. appl. Biol. 98: 451, 1981.
- Allen, Palukaitis & Symons, Australas. Pl. Path. 10: 31, 1981.
- Alper, Bar-Joseph, Salomon & Loebenstein, Phytoparasitica 6: 15, 1978.
- da Graça & van Vuuren, Res. Rep. S. Afr. Avocado Growers Ass. 4: 81, 1980.
- Desjardins, Drake, Atkin & Bergh, Calif. Agric. 33: 14, 1979.
- Desjardins, Drake & Swiecki, Pl. Dis. 64: 313, 1980.
- Gross, Domdey, Lossow, Jank, Raba & Alberty, Nature, Lond. 273: 203, 1978.
- Gross, Krupp, Domdey, Raba, Jank, Lossow, Alberty & Sänger, Eur. J. Biochem. 121: 249, 1982.
- Haseloff & Symons, Nucleic Acids Res. 9: 2741, 1981.
- Horne & Parker, Phytopathology 21: 235, 1931.
- Mohamed & Thomas, J. gen. Virol. 46: 157, 1980.
- Palukaitis & Symons, J. gen. Virol. 46: 477, 1980.
- Palukaitis, Hatta, Alexander & Symons, Virology 99: 145, 1979.
- Palukaitis, Rakowski, Alexander & Symons, Ann. appl. Biol. 98: 439, 1981.
- Symons, Nucleic Acids Res. 9: 6527, 1981.
- Visvader, Gould, Bruening & Symons, FEBS Lett. 137: 218, 1982.
- Wallace & Drake, Phytopathology 52: 237, 1962.
Range of symptoms on avocado cv. Hass including variegation and
distortion of leaves and a typical fruit lesion.
Yellow streaks on the stem of avocado cv. Hass.
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).
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).
Proposed secondary structure of sequence J02020 (Symons, 1981).
