Indian J. Virol. (January–June 2013) 24(1):27–34
DOI 10.1007/s13337-012-0125-9
ORIGINAL ARTICLE
Biological and Molecular Characterization of Cucumber mosaic
virus Subgroup II Isolate Causing Severe Mosaic in Cucumber
Reenu Kumari • Pooja Bhardwaj • Lakhmir Singh
Aijaz A. Zaidi • Vipin Hallan
•
Received: 5 September 2012 / Accepted: 29 December 2012 / Published online: 17 January 2013
Ó Indian Virological Society 2013
Abstract Cucumber mosaic virus (CMV) has a wide host
range causing severe damage in many important agricultural
and ornamental crops. Earlier reports showed the prevalence
of CMV subgroup I isolates in India. However, some recent
reports point towards increasing incidence of subgroup II
isolates in the country. The complete genome of a CMV
isolate causing severe mosaic in cucumber was characterized
and its phylogenetic analysis with other 21 CMV isolates
reported worldwide clustered it with subgroup II strains. The
genome comprised of RNA 1 (3,379 nucleotides), RNA 2
(3,038 nucleotides) and RNA 3 (2,206 nucleotides). The
isolate showed highest homology with subgroup II isolates:
95.1–98.7, 87.7–98.0, and 85.4–97.1 % within RNA1,
RNA2, and RNA3, respectively. RNA1 and RNA2 were
closely related to the Japanese isolate while RNA3 clustered
with an American isolate. Host range studies revealed that
isolate showed severe mosaic symptoms on Nicotiana spp.
and Cucumis spp. The isolate induced leaf deformation and
mild filiform type symptoms in tomato. To best of our
knowledge this is the first report of complete genome of
CMV subgroup II isolate from India.
Keywords Cucumber mosaic virus Subgroup II
Host range
A. A. Zaidi—deceased.
Electronic supplementary material The online version of this
article (doi:10.1007/s13337-012-0125-9) contains supplementary
material, which is available to authorized users.
R. Kumari P. Bhardwaj L. Singh A. A. Zaidi
V. Hallan (&)
Plant Virology Lab, Institute of Himalayan Bioresource
Technology, Palampur 176061, HP, India
e-mail: rnaivi@gmail.com
Introduction
Cucumber mosaic virus (CMV) is a devastating plant virus
having worldwide distribution in temperate and tropical
areas. The virus was first reported in 1916 [11, 22] and since
then reported to cause disease in a variety of economically
important agricultural and ornamental crops. The virus has
widest host range infecting over 1,200 species from 100
plant families. CMV is an icosahedral virus approximately
28–30 nm in diameter and belongs to the genus Cucumovirus in the family Bromoviridae. The virus is rapidly
transmitted by more than 80 aphid species in a non-persistent manner [31].
CMV is a tripartite virus having three plus sense, single
stranded RNA molecules encased in separate particles.
RNA1 and RNA2 encodes for the protein 1a and 2a,
respectively which forms the replicase complex [31].
N-terminal region of 1a protein contains putative methyltransferase domain [39] and C-terminal region shows
sequence similarity to viral helicases [16]. RNA 2 encodes
for another protein 2b, which is expressed from the subgenomic RNA 4A and acts as suppressor of gene silencing
[4, 5, 27]; plays a role in long distance movement of the
virus [9, 23, 43, 54] and also behave as pathogenicity
determinant [10, 41, 43]. A recent finding suggested that
this suppressor protein is indirectly involved in aphid
transmission [56]. A study on mechanism of 2b action
showed that the suppressor specifically recruits AGO4
small RNAs and directly interacts with PIWI and PAZ
domains of AGO4 affecting its slicer activity, causing
hypomethylation of its loci [18]. RNA3 encodes for two
proteins: movement protein (MP) located at 50 end and coat
protein which is expressed from another ORF through
subgenomic RNA 4. MP plays an important role in cell to
cell movement of the virus. While coat protein is required
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for encapsidation of the genomic RNAs into virus particles,
important in aphid transmission [7, 30] and affecting
symptom expression [42, 49]. CMV strains are divided into
two subgroups I and II [3, 31, 33] and subgroup I is subdivided into IA and IB based on nucleotide sequence of the
30 untranslated region of RNA3 [37]. Nucleotide sequence
similarity among IA and IB subgroup strains is 92–94 %.
Subgroup IA and II have worldwide distribution and IB is
restricted to Asia only [38].
Broad host range of CMV from subgroup I and II have
been identified in India, which includes: Chrysanthemum
[45]; Ocimum sanctum and Zinnia elegans [34]; Amaranthus
tricolor, Datura innoxia and Hyoscyamus muticus [44];
Gerbera [53]; Vanilla [28]; Pelargonium [52]; Piper longum
and Piper betel [19]; Rouvolfia serpentine [35]; Jatropha
curcas [36]; Catharanthus roseus [40]; Daucas carrota [1];
Banana [24, 29]; Lycopersicum esculentum [15, 25, 47];
Gladiolus [13]. Although many CMV strains were isolated
from India, but incomplete information is available at the
genome level about these isolates. Till date only one complete
genome from subgroup I have been reported [25] in India.
Here we report molecular and biological characterization of
the CMV subgroup II isolate, further information obtained
can be utilised for the development of improved diagnostics
and finding out symptom and virulence determinants.
Materials and Methods
Mechanical Transmission and Host Range Studies
Young infected leaves of Cucumber plants (Cucumis sativus)
showing symptoms of chlorosis and mottling were crushed in
chilled 0.1 M potassium phosphate buffer, pH 8.0 in a sterile
mortar. The crushed sap was filtered through muslin cloth and
then filtrate was mechanically inoculated by making gentle
abrasion on the leaves of Nicotiana glutinosa using carborundum powder. Then plants were maintained at 22–25 °C
under glass house conditions and after 1 week young leaves
were analysed for presence of virus. For further study, pure
culture of the virus was also maintained on the N. glutinosa
plants by single lesion transfer and aphid transmission.
Host range studies were carried out by mechanical inoculation of pure culture of the virus on N. glutinosa, N. clevelandii,
N. benthamiana, N. rustica, Chenopodium amaranticolor, C.
quinoa, L. esculentum cv. Pusa ruby, different varieties of C.
sativus and Capsicum annum plants. The plants were observed
for symptom development 3 weeks post inoculation.
Cloning and Sequencing of the Genome
Total RNA was extracted from infected leaf tissue
(100 mg) of N. glutinosa by using TRIZOL reagent
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(Invitrogen, Thermo Fisher Scientific, USA). The integrity
and quality of the total RNA was checked on 1 % agarose
gel and quantified by nanodrop2000 (Thermo Fisher Scientific, USA).
For the amplification of RNA1, 2 and 3 specific primers
were designed by multiple alignments of sequences of
different isolates of CMV (Supplementary Table 1) available in NCBI database. First strand cDNA synthesis was
carried out with *4 lg total RNA. Total RNA was
denatured along with 1.0 ll reverse primer (10 pmol/ll) at
72 °C for 2 min, followed by addition of 5 ll 59 first
strand buffer, 0.2 ll ribonuclease inhibitor (40 U/ll),
1.5 ll 40 mM dNTPs and 0.5 ll MMLV-RT (200 U/ll) in
a total reaction of 25 ll. Reaction was performed at 42 °C
for 60 min followed by incubation at 80 °C for 5 min.
Amplification was done using Taq DNA polymerase
(Bangalore Genei, Bengaluru, India) in a thermocycler (Gstorm, Somerton Biotechnology centre, Somerset, UK)
with initial denaturation of 1 min at 95 °C followed by 35
cycles of denaturation at 94 °C for 40 s, annealing and
extension temperature being specific for particular amplicon and finally an extension time of 10 min was given at
68 °C (Supplementary Table 1). PCR reaction contained
5 ll of 109 Taq buffer A, 2 ll of forward primer and
reverse primer each (10 pmol/ll), 2.5 ll of dNTPs, 1 ll
cDNA and 3 U of Taq polymearse (3 U/ll) in total reaction of 50 ll. PCR products were ligated into pGEMT-easy
vector (Promega, Madison, USA) according to the manufacturer’s instructions and then transformed into E. coli
DH5a. Recombinant colonies were screened by restriction
digestion. Positive clones were sequenced in automated
DNA sequencer (ABI PRISM 3130xl genetic analyzer)
using ABI prism Big Dye Terminator v3.1 ready Reaction
Cycle Sequencing Kit (Applied Biosystems, USA) by primer walking. For each amplicon, at least three clones were
sequenced. Assembled sequences of RNA1, 2 and 3 were
submitted to the GenBank database (HE650150,
HE613667, and HE583224 respectively) and the isolate
was named as CMV-SG.
Phylogenetic Analysis
Sequences of 32 isolates of CMV belonging to subgroup I
and II were retrieved from the NCBI database (http://www.
ncbi.nlm.nih.gov) and compared using MEGA 5.05 version
(http://www.megasoftware.net). Phylogenetic relationships
between these isolates were inferred from the nucleotide
sequence alignment by maximum likelihood (Juke canter
and 1,000 non-parametric bootstrap replicates). To check the
possibility of recombination within CMV isolates we used
RDP3 program with default settings. Sequence identity
percentage was calculated using Bioedit sequence alignment
editor version 7.1.3.0 [17].
Biological and Molecular Characterization of Cucumber mosaic virus
Results
CMV-SG isolate developed mosaic symptoms on the
inoculated cucumber (Summer Beauty, Summer Green and
Veer varieties; Fig. 1b, d), N. glutinosa (Fig. 1m, n; mock
inoculated and infected) and N. rustica plants 4–5 days
post inoculation (dpi). Concentric chlorotic lesions were
found in N. clevelandii (Fig. 1o). Plants showed severe
stunting in all tested varieties of cucumber (Fig. 1a, c). In
case of tomato, mosaic pattern appeared 5–6 days post
inoculation which later became systemic and newly
emerging leaves showed downward leaf curling and mild
filiform like symptoms (Fig. 1e mock inoculated, f, g
infected). In case of Chenopodium quinoa, chlorotic lesions
were observed after 3 dpi which later became necrotic
(Fig. 1k, l; mock inoculated and infected) whereas in chilli,
mild mosaic symptoms appeared 7 dpi and plant showed
stunting (Fig. 1h–j; mock inoculated and infected). Cryoelectron microscopy of the purified virus revealed the
presence of *28 nm isometric particles typical of CMV
(Supplementary Fig. 1). SDS-PAGE and western blot
analysis of purified viral particles showed the presence of
dimeric *52 kDa and monomeric *26 kDa coat protein
(CP) as major band (Supplementary Fig. 2).
The sequence analysis revealed that RNA1, 2 and 3
comprised of 3,379, 3,038, and 2,206 nucleotides, respectively. Location of various ORFs on various RNAs was
established. It can be seen that the ORF 1a codes for 993
amino acids (aa) protein, known to be involved in replication. RNA2 consisted of two overlapping ORFs, 2a and
2b. 2a encodes for RNA dependent RNA polymerase
whereas 2b protein functions as viral suppressor. RNA3
codes for 280 aa movement protein (3a) and 219 aa long 3b
(CP).
Complete genome sequences of 31 different isolates
were selected worldwide and compared with CMV-SG
isolate (Supplementary Table 2–4). It showed highest
homology with subgroup II isolates viz. 95.1–98.7,
87.7–98.0, and 85.4–97.1 % with RNA1, RNA2, and
RNA3, respectively whereas it showed 71.3–75.6,
67.5–69.6, and 65.0–72.0 % homology with subgroup I
isolates for respective RNAs. RNA1 of CMV-SG
(HE650150) showed maximum nucleotide identity 98.7
and 98.6 % with American (AF416899), Japanese
(AB176849) and Australian isolates (AF198101) from
subgroup II. RNA2 also shared maximum identity of 98 %
with American (AF416900) and Japanese (AB176848)
isolates. Similarly, RNA3 shared 97 % sequence similarity
with Japanese (AB176847) and Chinese (EF202397) isolates. It was observed that the 30 UTR of viral RNAs were
conserved and showed 96 % identity in case of RNA1 and
2, 88 % in RNA1 and 3 and 87 % identity in RNA2 and 3.
Comparison with complete RNA3 sequences of Indian
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isolates showed that CMV-SG isolate showed 95.8 and
94.3 % nucleotide identity with EU642567 (carrot) and
JF279605 (Tss-In) isolates, respectively. Based on coat
protein sequence analysis, CMV-SG isolate showed 99 and
96.9 % identity with isolates from geranium (AJ866272)
and lily (AJ585086), respectively and 98.1 % identity with
Aligarh isolate from Ocimum sanctum (EU600216). Phylogenetic analysis of 22 CMV isolates revealed that RNA1
(Fig. 2a) and RNA2 (Fig. 2b) clustered with Japanese
isolate while RNA3 clustered with American and Australian isolates from subgroup II (Fig. 2c). RDP analysis
didn’t show any recombination event among CMV isolates.
Amino acid sequence comparison of 1a protein with other
isolates showed some unique changes at following positions: serine at 223; tyrosine at 460; phenylalanine at 624
and glutamic acid at 904 when compared to members of
subgroups I and II. In 2a protein: glycine at 53 position;
arginine at 206; glycine at 451; cysteine at 594; asparagine
at 600; proline at 657; cysteine at 688 and alanine at 788
positions. In 2b protein histidine is substituted with glutamine at aa 14 and asparagine at position 32 with serine
(Fig. 3). There were two substitutions in case of coat
protein at aa 10 serine with glycine and at aa 106 histidine
with arginine. 3a protein also showed three substitutions at
aa 42 (glycine with serine), at aa 158 (phenylalanine with
tyrosine) and at aa 253 (asparagine with serine). The isolate
showed maximum changes in RNA2 (8 substitutions in 2a
protein and 2 substitutions in 2b protein). Nuclear localization signal motif (NLS1), KRRRRR; NLS2, RRAR and
putative phosphortylation motif KSPSE were conserved in
2b. The substitution at aa 14 histidine with glutamine is
unique among subgroup II isolates but represented by
subgroup I isolate and other substitution at position 32 is
present before NLS2 sequence. The 30 UTR were conserved
among RNA1, 2 and 3.
Discussion
In India, CMV isolates have been reported from several
regions. Mainly, subgroup I isolates has been reported and
there are few reports of subgroup II isolates [1, 15, 47, 53].
The isolate under study showed up to 99 and 76 % similarity to the subgroup II and I isolates, respectively.
Depending on the host and virus strain, CMV is known to
cause variable symptoms, including necrotic or chlorotic
lesions, mild to severe mosaic, stunting, leaf deformation
and shoestring formation [14]. Subgroup I strains shows
severity in terms of symptom and disease development on
tobacco [55]. The present isolate (CMV-SG) induced
chlorotic lesions becoming necrotic in local lesion assay
host, Chenopodium quinoa 3 dpi. Mosaic symptoms and
systemic infection was observed in all Nicotiana species
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Fig. 1 Symptoms induced by the CMV on different host plants.
Cucumis sativus (a, c) stunting shown by infected plants of C. sativus
var. veer and var. summer green respectively in comparison to
control. b, d chlorosis and mosaic on leaves after 2 weeks post
inoculation; Lycopersicum esculentum (var. Pusa Ruby) e mock
inoculated, infected f chlorotic patches on leaves, g leaf deformation
and curling; Capsicum anumm h stunted growth in infected plant,
i mock inoculated leaf, j infected leaf showing chlorotic patches;
Chenopodium quinoa k mock inoculated plant, l chlorotic necrotic
lesions on leaves; Nicotiana glutinosa m mock inoculated leaf, n leaf
showing mosaic pattern and lesions; Nicotiana clevelandii o infected
leaf showing presence of concentric chlorotic lesions
(N. glutinosa, N. clevelandii, N. benthamiana and N. rustica). Lesions and concentric chlorotic rings were observed
on N. glutinosa and N. clevelandii respectively. Severe
chlorotic, mosaic and stunting symptoms developed on
various cucumber varieties (Fig. 1b, d). In newly developing tomato leaves, viral isolate induced symptoms
ranging from severe chlorotic patches, downward curling
and reduced leaf lamina to mild filiformism. Previously,
shoestring like leaves symptoms were reported only with
some subgroup I strains [2, 20, 32, 46, 48]. From India also
there was report of mosaic symptoms on tomato by subgroup II strain [47] and now in a recent observation it was
found that shoestring symptoms were not restricted to
subgroup I. A subgroup II isolate Tss-In developed shoestring symptoms on tomato [15]. Cryoelectron microscopy
of purified virus showed the presence of *28 nm isometric
particle size, its SDS-PAGE and western blotting showed
CP of *26 kDa and its dimeric form at *52 kDa. Similar
kind of pattern was previously observed in case of CMVamaranthus strain [34].
Phylogenetic analysis with 21 different isolates of CMV
(selected worldwide) at the nucleotide level clustered
RNA1 and 2 with Japanese isolate and RNA3 with
American and Australian isolates. At the amino acid level
1a and MP protein showed close resemblance to Japanese
isolate, 2a and 2b to USA isolate, and coat protein to
Australian isolate. Changes in the replicase protein and
suppressor protein can affect the infectivity of the virus
[26, 50, 54]. Reassortment is a natural mechanism which
leads to genetic variation and new strain emergence in
multipartite RNA viruses [6]. New CMV strains emerged
due to reassortment among subgroup I and II strains [8].
From India, 15 complete RNA3 sequences (including
present isolate) and 70 complete coat protein and 27
movement protein sequences have been submitted. Based
on the comparison of 15 RNA3 sequence and their phylogenetic analysis it was found to be closely related to an
isolate infecting carrot (EU642567; Aligarh isolate) an
isolate closely related to Australian isolate, AF198103 [1].
Also an isolate (IA) from India showing 99 % identity to
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Biological and Molecular Characterization of Cucumber mosaic virus
31
Fig. 2 Phylogenetic
relationships of complete RNA1
(a), RNA2 (b) and RNA3
(c) sequences of present CMV
isolate with 21 different isolates
of CMV, determined using the
maximum-likelihood method
based on the Jukes-Cantor
model (implemented in
MEGA5, with 1000 maximumlikelihood bootstrap replicates)
CP gene of USA isolate has been reported [12]. A subgroup
II isolate from Ocimum sanctum was found closely related
to European isolate, EU191025 [24].
The adaptation of the virus to the new environment and
the host plants leads to the emergence of new isolates. In
CMV there was report of specific adaptation to the host
plant in case of soybean isolate from Indonesia [21]. So far
from India complete genome of CMV from subgroup II has
not been reported and resemblance of all reported isolates
with Asian and European isolates is based on only RNA3
and CP phylogenetic analysis. In our study CMV isolate
showed resemblance to Japanese and American isolate and
thus showing its mixed origin which might be making it
more stable in this type of environment. The isolate is very
competitive in its infection on different host plants taken
during the study and thus affecting their growth. Previously
from India reports of CMV subgroup-I incidence were
prevalent, but now reports belonging to incidence of sub
group-II are increasing. Similar results were also reported
from China [51]. These results indicate that sub group-II
strains were evolving in such a manner that increases their
successful infection rate in field. Thus the present study
reports the first complete genome of CMV subgroup II
from India, information related to complete genome of an
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R. Kumari et al.
Fig. 3 Alignment of 2b protein
from subgroup I and II showing
two arginine rich nuclear
localization regions (NLS1 and
NLS2), putative
phosphorylation motif
(indicated by bar and putative
phosphorylated serine residues
at position 40 and 42) and
substitution site at position 14
and 32 in the CMV-SG isolate
Indian isolate from subgroup II can be utilised for the better
diagnostic development and understanding the host pathogen relationship.
Acknowledgments The authors are thankful to the Director,
Council of Scientific & Industrial Research, Institute of Himalayan
Bioresource Technology, Palampur, Himachal Pradesh India for
providing the necessary facilities to carry out the work. We also thank
Dr. Avnesh Kumari and Dr. Madhu Sharma for cryoelectron
microscopy work. A fellowship to PB and RK, financial support from
the Department of Biotechnology, Government of India and Council
of Scientific & Industrial Research are duly acknowledged. This is
IHBT publication number 3432.
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