Bearded iris mosaic virus
O. W. Barnett
Clemson University, Clemson, South Carolina, USA
A. A. Brunt
Glasshouse Crops Research Institute, Littlehampton, Sussex, England
Contents
Introduction
Disease described by
Brierley & McWhorter (1936)
and virus partially characterized by
Barnett, de Zoeten & Gaard (1971).
A virus with filamentous particles c. 12 x 750 nm, prevalent in many species
of rhizomatous
iris. It is transmissible to iris and one other iridaceous species by
sap-inoculation, and by aphids
after brief acquisition feeds. A typical but apparently distinct member of the
potyvirus group.
Distribution probably world-wide.
Main Diseases
Causes conspicuous chlorotic markings on the leaves and flower ‘breaking’
in intolerant cultivars
of bearded (Eupogon and Oncocyclus groups), crested (Evansia group) and
beardless (Apogon group)
rhizomatous iris, but inconspicuous leaf chlorosis or even symptomless infection
in other cultivars.
Symptoms are usually more severe in plants grown at low temperatures (16°C),
or in those also
containing
cucumber mosaic virus
(
Brunt, 1968;
Brunt, 1975;
Barnett, de Zoeten & Gaard, 1971).
Geographical Distribution
Reported from the USA
(
Brierley & McWhorter, 1936),
Japan
(
Fukushi, 1932)
and Europe
(
Moore, 1949).
Distribution probably world-wide, but the identity of the virus
concerned in early records is
uncertain because, until 1971, mosaic diseases in bulbous and rhizomatous irises
were generally assumed
to be caused by the same virus
(
Barnett, de Zoeten & Gaard, 1971).
Host Range and Symptomatology
The virus is only known to occur naturally in rhizomatous irises.
It is transmissible by aphids or
by sap-inoculation to one of three other iridaceous species
(
Belamcanda chinensis although not
Gladiolus or
Freesia spp.), but it infects none of 30 species from
7 dicotyledonous
families
(
Barnett, de Zoeten & Gaard, 1971;
Brunt, 1975).
-
Diagnostic species
- Belamcanda chinensis.
Commonly-occurring isolates of the type strain
induce a few spreading
necrotic lesions in inoculated leaves after c. 14 days and severe chlorosis
in the first few
systemically-infected leaves
(Fig.5)
but symptomless infection in leaves produced later; avirulent
isolates induce inconspicuous or symptomless systemic infection, but virulent
isolates cause conspicuous
leaf chlorosis, severe stunting, leaf necrosis and death of infected plants
(Brierley & Smith, 1948;
Barnett, de Zoeten & Gaard, 1971).
- Iris spp. Intolerant cultivars of rhizomatous iris produce
conspicuous chlorotic mosaic on
the leaves
(Fig.1),
and short-lived flowers which are often ‘broken’
(Fig.4)
or have ‘tear-drop’
markings
(Fig.7);
some tolerant cultivars remain almost symptomless, others have faintly chlorotic
leaves but normal flowers. The leaves of I. tectorum (crested iris) often
develop chlorotic or
water-soaked ringspot and line patterns
(Fig.2).
-
Propagation species
- Cultures are best maintained in Belamcanda chinensis which is also a
good source of inoculum
and of virus for purification. This species is especially convenient because
it germinates and grows
quickly, and seedlings are readily infected.
-
Assay species
- Belamcanda chinensis:
by recording the proportion of inoculated plants
that become infected.
Strains
Three strains of bearded iris mosaic virus have been recognized by the
severity of the symptoms they
induce in
Belamcanda chinensis (O. W. Barnett, unpublished information).
Type strain. In B. chinensis this strain induces a few local necrotic
lesions followed
initially by conspicuous systemic leaf chlorosis but leaves produced later are often
symptomless. Common
in bearded Eupogon irises (e.g. Iris pumila, I. ricardi).
Avirulent strain. In B. chinensis this strain induces distinct necrotic or
chlorotic
stripes and/or ringspots in inoculated leaves
(Fig.3)
but almost symptomless infection in
systemically-infected leaves. Isolated from I. susiana (bearded Oncocyclus iris)
with difficulty.
Virulent strain. In B. chinensis, isolates of this strain from
I. spuria
(beardless iris) induce severe systemic leaf chlorosis
(Fig.6)
and death. Symptoms in I. spuria
and B. chinensis are indistinguishable from those previously attributed
to ‘beardless iris
mosaic virus’
(Brierley & Smith, 1948).
Transmission by Vectors
Transmitted efficiently by some races of the aphids
Macrosiphum euphorbiae
and
Myzus
persicae after acquisition feeds of 3-5 min
(
Brierley & Smith, 1948;
Barnett, de Zoeten & Gaard, 1971;
O. W. Barnett, unpublished data).
Transmission through Seed
Not seed-borne in
Belamcanda chinensis
(
Barnett, de Zoeten & Gaard, 1971).
Transmission by Dodder
No report.
Serology
The virus is a good immunogen; antiserum collected 6 weeks after injecting a rabbit
intra-muscularly
with
c. 0.45 mg of virus emulsified in Freund’s complete adjuvant
reacted in micro-precipitin
tests to a homologous titre of 1/2048.
Relationships
The virus has many properties in common with
potyviruses,
but is serologically unrelated to similar
viruses commonly infecting iridaceous species
(
iris mild mosaic,
iris severe mosaic,
I. fulva mosaic,
bean yellow mosaic,
turnip mosaic,
freesia mosaic)
and to four other viruses of the same group
(
Barnett, de Zoeten & Gaard, 1971;
Brunt, 1975).
Stability in Sap
In
Belamcanda chinensis sap, the thermal inactivation point is 50°C,
dilution end-point
10
-3, and infectivity is lost at 27°C after 24-36 h
(
Barnett, de Zoeten & Gaard, 1971).
Purification
Yields of
c. 5 mg virus/kg of
Belamcanda chinensis leaf tissue
are obtainable by the
following procedure
(
Barnett, de Zoeten & Gaard, 1971).
Homogenize each 1 g infected leaves in
2 ml of extractant at pH 8.9 containing 0.1 M borate buffer,
0.01 M sodium ethylenediamine-tetraacetate
and 0.02 M 2-mercaptoethanol. Remove coarse debris, shake the fluid with 0.5 vol
chloroform and
centrifuge for 10 min at 8000
g. Sediment virus from the aqueous phase by
centrifugation for 90-120 min at 100,000
g, and allow pellets to
resuspend for 18-24 h
in 0.1 M borate buffer at pH 8.9 containing 0.01 M sodium ethylenediamine-tetraacetate.
Adjust the
solution to pH 4.8 with 0.05 M citric acid, collect pellets by centrifugation and
shake gently for
5-6 h in 0.1 M borate buffer at pH 8.9. Purify the virus further by exclusion
chromatography in 2%
agarose beads and concentrate by ultrafiltration.
Similar or higher yields of mostly unaggregated virus are obtained as described by
Huttinga (1973).
Homogenize each 1 g infected leaves in 3 ml 0.1 M Tris-thioglycollate buffer
(pH 8.9) and 1.6 ml of
a 1:1 mixture of carbon tetrachloride and chloroform, subject the aqueous phase
to one or two cycles
of differential centrifugation (1.5 h at 44,000 g; 10 min at
5000 g), and
further purify the virus by rate zonal sucrose density gradient centrifugation.
Properties of Particles
Sedimentation coefficient (
s°
20, w):
c. 150 S; some
preparations may also contain faster sedimenting virus aggregates
(A. A. Brunt, unpublished information).
A260/A280: 1.12;
A260(max)/A248(min): 1.15 (both values after
correction for light-scattering).
Particle Structure
Particles
(
Fig.10)
are slightly flexuous filaments mostly
c. 750 x 12 nm. Particles
mounted in uranyl formate often have a central canal and show the helical arrangement
of the protein
subunits.
Particle Composition
Nucleic acid: 5% of particle weight (estimated spectrophotometrically).
Protein: c. 95% of particle weight; one type of polypeptide of
M. Wt 33,000 (A. A. Brunt,
unpublished information).
Relations with Cells and Tissues
In ultrathin sections of infected iris leaves and petals, the virus particles are
often scattered
throughout the cytoplasm, but in
Belamcanda chinensis cells they are also sometimes
found in
membrane-enclosed ovoid masses
c. 8 µm in diameter
(
Fig.9).
Infected cells of
Belamcanda chinensis leaves, but not iris petals, also contain inclusions
(
Fig.8)
seen in
sections as pinwheels, bundles and laminated aggregates
(
Barnett, de Zoeten & Gaard, 1971).
Notes
In particle morphology, bearded iris mosaic virus is indistinguishable from
iris mild mosaic
and
iris severe mosaic viruses,
which commonly infect bulbous irises, and from
Iris fulva mosaic
virus, which is reported to infect some species of rhizomatous iris. The two viruses
infecting bulbous
irises are serologically distinct from bearded iris mosaic virus
(
Brunt, 1968;
Brunt, 1973;
Barnett, de Zoeten & Gaard, 1971).
Similarly,
Iris fulva mosaic virus
(
Brierley & Smith, 1948;
Travis, 1957)
is serologically distinct from bearded iris mosaic virus and differs also in infecting
Amaranthus caudatus and
Chenopodium quinoa, and having cytoplasmic
inclusions which can
be seen in leaf sections as scrolls and tubes as well as pinwheels, bundles and
laminated aggregates
(O. W. Barnett, unpublished information).
Recent tests suggest that ‘beardless iris mosaic virus’
(Brierley & Smith, 1948;
Travis, 1957)
is a virulent strain of bearded iris mosaic virus (O. W. Barnett, unpublished),
but further tests are
needed to establish whether the two are synonymous.
Rhizomatous irises infected with bearded iris mosaic virus in Britain commonly
also contain
cucumber mosaic virus
(Brunt, 1968;
Barnett, de Zoeten & Gaard, 1971)
or, probably rarely,
broad bean wilt virus
(Bailiss, Brunt & Dale, 1975);
tobacco ringspot virus
also occurs occasionally in rhizomatous
iris in the USA
(Travis, 1957;
Travis & Brierley, 1957)
and Britain
(Brunt, 1972).
Unlike bearded
iris mosaic virus, these three viruses have isometric particles and infect a wide
range of plant
species; moreover, all three are readily purified and identified by serological tests
and by their
physico-chemical properties.
References
- Bailiss, Brunt & Dale, Pl. Path. 24: 60, 1975.
- Barnett, de Zoeten & Gaard, Phytopathology 61: 926, 1971.
- Brierley & McWhorter, J. agric. Res. 53: 621, 1936.
- Brierley & Smith, Phytopathology 38: 574, 1948.
- Brunt, Ann. appl. Biol. 61: 187, 1968.
- Brunt, Rep. Glasshouse Crops Res. Inst. 1971: 116, 1972.
- Brunt, CMI/AAB Descriptions of Plant Viruses 116, 4 pp., 1973.
- Brunt, Acta hort. 47: 45, 1975.
- Fukushi, Trans. Sapporo Nat. Hist. Soc. 12: 130, 1932.
- Huttinga, Neth. J. Pl. Path. 79: 125, 1973.
- Moore, Bull. Minist. Agric. Fish Fd, Lond. 117: 140, 1949.
- Travis, Phytopathology 47: 454, 1957.
- Travis & Brierley, Pl. Dis. Reptr 41: 524, 1957.
Typical mosaic symptoms in leaf of naturally infected bearded iris.
Chlorotic ringspot symptoms in leaf of Iris tectorum.
Chlorotic ring-and-line patterns induced in inoculated Belamcanda chineosis
leaves by the avirulent strain from Iris susiana.
Colour ‘breaking’ in bearded iris flower.
Necrotic local lesions (arrow) and systemic leaf chlorosis in
Belamcanda
chinensis infected with bearded iris mosaic virus (type strain).
Severe systemic chlorosis in leaf of Belamcanda chinensis
infected with the virulent
strain from Iris spuria.
‘Tear-drop’ markings in infected bearded iris flower.
Thin section of Belamcanda chinensis leaf showing
pinwheels and laminated
aggregates. Bar represents 500 nm.
Thin section of Belamcanda chinensis leaf showing a membrane-bound
bundle of virus particles. Bar represents 500 nm.
Virus particles mounted in potassium phosphotungstate. Bar represents 250 nm.
