Journal of General Virology (2007), 88, 1624–1633
DOI 10.1099/vir.0.82662-0
Infectivity, pseudorecombination and mutagenesis
of Kenyan cassava mosaic begomoviruses
Simon E. Bull,14 Rob W. Briddon,11 William S. Sserubombwe,13
Kahiu Ngugi,2 Peter G. Markham1 and John Stanley1
Correspondence
Rob W. Briddon
robbriddon@nibge.org
Received 24 October 2006
Accepted 23 January 2007
1
Department of Disease and Stress Biology, John Innes Centre (JIC), Colney Lane, Norwich
NR4 7UH, UK
2
Kenya Agricultural Research Institute, Katumani Applied Biotechnology Laboratory, PO Box 340,
Machakos, Kenya
Cloned DNA-A and DNA-B components of Kenyan isolates of East African cassava mosaic virus
(EACMV, EACMV-UG and EACMV-KE2), East African cassava mosaic Kenya virus (EACMKV)
and East African cassava mosaic Zanzibar virus (EACMZV) are shown to be infectious in cassava.
EACMV and EACMKV genomic components have the same iteron sequence (GGGGG) and can
form viable pseudorecombinants, while EACMZV components have a different sequence
(GGAGA) and are incompatible with EACMV and EACMKV. Mutagenesis of EACMZV has
demonstrated that open reading frames (ORFs) AV1 (encoding the coat protein), AV2 and AC4
are not essential for a symptomatic infection of cassava, although mutants of both ORF AV1 and
AV2 produce attenuated symptoms in this host. Furthermore, ORF AV1 and AV2 mutants
were compromised for coat protein production, suggesting a close structural and/or functional
relationship between these coding regions or their protein products.
INTRODUCTION
Begomoviruses (family Geminiviridae, genus Begomovirus)
associated with cassava mosaic disease (CMD) are found
throughout sub-Saharan Africa where cassava [Manihot
esculenta (Crantz)] is the primary food crop. They are
transmitted by the whitefly Bemisia tabaci (Gennadius) and
are considered to be one of the most damaging vectorborne pathogens of any African crop, with estimated losses
in excess of US$1.5 billion a year (Thresh et al., 1994).
Several distinct species are associated with CMD, namely
African cassava mosaic virus (ACMV), East African
cassava mosaic virus (EACMV), East African cassava
mosaic Cameroon virus (EACMCV), East African cassava
mosaic Kenya virus (EACMKV), East African cassava mosaic
Malawi virus (EACMMV), East African cassava mosaic
Zanzibar virus (EACMZV) and South African cassava
mosaic virus (SACMV) in Africa, and Sri Lankan cassava
mosaic virus (SLCMV) and Indian cassava mosaic virus
(ICMV) in the Indian subcontinent (Stanley et al., 2005).
3Deceased 30 July 2004.
4Present address: Department of Biology and Biochemistry, University
of Bath, Claverton Down, Bath, Avon BA2 7AY, UK.
1Present address: Plant Biotechnology Division, National Institute for
Biotechnology and Genetic Engineering, Jhang Road, Faisalabad,
Pakistan.
Sequences of primers used are available as supplementary material in
JGV Online.
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All of these begomoviruses have bipartite genomes
comprising a DNA-A component required for replication
and encapsidation and a DNA-B component required
for virus movement (Hanley-Bowdoin et al., 1999). Both
components have a highly conserved intergenic common
region (CR) containing a stem–loop structure with an
invariant nonanucleotide motif (TAATATTAC) and bidirectional, partially overlapping open reading frames (ORFs)
AV1 and AV2 on the virion-sense strand and AC1–AC4 on
the complementary-sense strand that are typical of Old
World bipartite begomoviruses. The ACMV coat protein
(CP), encoded by ORF AV1 (Townsend et al., 1985), is
principally involved in the encapsidation of the DNA
components for transmission by the whitefly vector
(Briddon et al., 1990; Liu et al., 1998). It has also been
implicated in subcellular targeting (Unseld et al., 2001,
2004), but is not essential for systemic infection in
Nicotiana benthamiana (Stanley & Townsend, 1986;
Ward et al., 1988), although the extent of CP involvement
in virus systemic movement may depend on virus–host
adaptation (Pooma et al., 1996). Additionally, ACMV CP
mutants characteristically accumulate only low levels of
single-stranded (ss) DNA (Stanley & Townsend, 1986). The
function of the upstream, overlapping ORF AV2 product is
uncertain. ACMV ORF AV2 mutants remain infectious in
N. benthamiana (Etessami et al., 1989) and those of tomato
leaf curl New Delhi virus (ToLCNDV) produced attenuated symptoms and accumulated low levels of viral DNA
(Padidam et al., 1996). Similarly, the function of ORF AC4
0008-2662 G 2007 SGM Printed in Great Britain
Infectivity of Kenyan cassava begomoviruses
is not clear, since disruption of the coding sequence in
ACMV had no effect on the phenotype in N. benthamiana
(Etessami et al., 1991). However, there is compelling
evidence to suggest that ACMV AC4 may counter a plant
defence mechanism initiated by expression of the replication-associated protein (Rep) that would otherwise lead to
severe necrosis and plant death (van Wezel et al., 2002).
In addition, ACMV AC4 has been shown to be an RNAsilencing suppressor protein that binds to small interfering
and micro RNAs and induces developmental abnormalities
in transgenic plants (Chellappan et al., 2005). This suggests
a role in cell-cycle control, which has been proposed for the
beet curly top virus (BCTV) C4 positional homologue
(Latham et al., 1997). The fact that the AC4 proteins of
EACMCV and ICMV do not show similar silencing
suppression activity (Vanitharani et al., 2004) may reflect
the considerable diversity observed within AC4 sequences
of distinct begomovirus species (Bull et al., 2006).
Infectious clones of ACMV were produced from diseased
cassava over two decades ago (Stanley, 1983; Stanley & Gay,
1983) and remain the only examples to originate from
Kenya. Although ACMV DNA-A is capable of limited
systemic spread in N. benthamiana in the absence of
DNA-B (Klinkenberg & Stanley, 1990), both components
are required for a systemic symptomatic infection. The
clones induced a severe downward leaf-curling phenotype
and stunted growth when mechanically inoculated to N.
benthamiana. However, despite replicating in cassava leaf
discs (Zhang & Gruissem, 2003), the cloned ACMV
components failed to systemically infect cassava as a
consequence of defects in the CP and DNA-B component
(Briddon et al., 1998; Liu et al., 1998). Since these studies,
only a few other clones have been successfully inoculated
to cassava, namely an ACMV isolate from Nigeria (Briddon
et al., 1998), SACMV (Berrie et al., 2001), SLCMV
(Saunders et al., 2002) and ICMV (Rothenstein et al.,
2005).
Pseudorecombination occurs during mixed infections in
the field and provides a means for the generation of new
viruses by the exchange of genomic components. For
example, Pita et al. (2001) showed that EACMV-UG2
DNA-A is capable of trans-replicating EACMV-UG3 DNAB, representing the first demonstration of infectivity of
EACMV clones to cassava. The resulting symptoms were
particularly severe and this pseudorecombinant has
dominated the initially identified EACMV-UG1 isolate in
Uganda (Zhou et al., 1997). Synergistic interactions also
arise between viruses, as seen in N. benthamiana plants coinoculated with Cameroon isolates of ACMV and EACMV
that showed more severe symptoms than in plants infected
with either virus alone (Fondong et al., 2000), as well as
between EACMV-UG2 and ACMV-[UG] (Harrison et al.,
1997; Pita et al., 2001) and in natural infections between
EACMV and ACMV from Nigeria (Ogbe et al., 2003).
Frischmuth et al. (1993) showed that pseudorecombination
compatibility between ACMV and ICMV components is
http://vir.sgmjournals.org
restricted by trans-replication rather than an inability to
spread throughout the plant. Rep-binding motifs, referred
to as iterons (Argüello-Astorga et al., 1994) and located in
the CR 59 of the nonanucleotide motif, are crucial in
determining the compatibility of genomic components for
trans-replication. The interaction between Rep and the
iteron is highly specific, usually preventing any functional
interaction between components of distinct begomovirus
species (Fontes et al., 1992, 1994a; Orozco et al., 1998;
Chatterji et al., 2000). However, this incompatibility can
be overcome by exchange of intergenic region sequences,
which has been shown to occur frequently between
begomoviruses and their associated components, both
experimentally (Roberts & Stanley, 1994; Saunders et al.,
2001) and in the field (Saunders et al., 2002).
We have recently undertaken a comprehensive investigation of the epidemiology of begomoviruses associated with
CMD in Kenya (Bull et al., 2006). During the course of this
study, we cloned numerous full-length genomic components of EACMV and its distinct strains EACMV-UG and
EACMV-KE2, as well as EACMKV and EACMZV. Here,
we demonstrate the biological activity of selected clones
and investigate their compatibility in pseudorecombination
experiments using both the experimental host N. benthamiana and the natural host cassava. Finally, we have
undertaken a mutational analysis on ORFs AV1, AV2 and
AC4 to investigate their contribution to CMD.
METHODS
Plant inoculation and maintenance. N. benthamiana and tissue-
cultured cassava ‘Ebwanateraka’ (kindly provided by M. N. Maruthi,
Natural Resources Institute, Medway, UK) were biolistically inoculated with cloned genomic components as described by Briddon et al.
(1998). The plants were maintained in insect-free glasshouses at
the JIC at 25–30 uC and with supplementary lighting to give a 16 h
photoperiod. Symptoms of CMD in cassava were classified by visually
assessing the plant and designating a score between 0 (no symptoms)
and 5 (severe symptoms). All viruses were maintained and manipulated under DEFRA licence PHL 185A/4538 (7/2003).
Plasmid constructs. Full-length DNA-A and DNA-B components
of the Kenyan begomoviruses EACMZV-[K18], EACMV-[K24],
EACMV-KE2[K48], EACMKV-[K261] and EACMV-UG[K282] (Bull
et al., 2006) were used in this analysis.
Site-directed mutagenesis. Point mutations were introduced into
AV1, AV2 and AC4 coding sequences of the cloned EACMZV-[K18]
DNA-A component using a QuikChange site-directed mutagenesis kit
(Stratagene) and the overlapping primers shown in Supplementary
Table S1 (available in JGV Online). Nonsense codons were introduced to replace codons encoding tyrosine177 in ORF AV1 (mutV1),
tyrosine24 (mutV2), leucine6 (mutV2A) and glutamine95 (mutV2B) in
ORF AV2 and serine66 in ORF AC4 (mutC4) (Fig. 1).
Detection of viral DNA. DNA was extracted from newly emerging
and/or symptomatic leaves approximately 15 days post-inoculation
(days p.i.) using a Nucleon Phytopure plant DNA extraction kit
(Amersham Biosciences). Samples (10 mg) were fractionated on
1 % agarose gels in TNE buffer [40 mM Tris/HCl (pH 7.5), 200 mM
sodium acetate, 20 mM EDTA], transferred to Hybond-NX membranes
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S. E. Bull and others
Fig. 1. Maps of EACMZV DNA-A and DNA-B components. For
simplicity, the circular genomic components are depicted as linear
maps. The position and orientation of ORFs (light grey) and
mutated ORFs (dark grey) are shown relative to the common
region (black box). The mutations introduced into ORFs AV1, AV2
and AC4 are indicated. CP, Coat protein; TrAP, transcriptional
activator protein; REn, replication enhancer protein; Rep, replication-associated protein; NSP, nuclear shuttle protein; MP, movement protein.
Fig. 2. Symptoms induced in N. benthamiana and cassava by
Kenyan begomoviruses.
(Amersham Biosciences) and hybridized to [a-32P]dCTP-labelled
EACMV DNA-A or DNA-B probes produced using a Random
Primer DNA labelling kit (Gibco-BRL) and a NucTrap probe
purification column (Stratagene).
A PCR-based approach to isolate full-length DNA-A and DNA-B
components for nucleotide sequencing has been described by Bull
et al. (2006).
Analysis of CP expression. Proteins were extracted from N.
benthamiana and cassava as described by von Arnim et al. (1993) and
fractionated on 12 % polyacrylamide gels (Laemmli, 1970) before
transfer to nitrocellulose membrane (Whatman - Schleicher &
Schuell) using a semi-dry transfer cell (Bio-Rad). CP was detected
by immunolabelling using polyclonal antiserum raised against purified ACMV (Stanley & Townsend, 1986).
RESULTS
Infectivity of cognate DNA-A and DNA-B
components
Cloned components of EACMV, EACMV-UG, EACMVKE2, EACMKV and EACMZV all induced downward leaf
curling and stunted growth in N. benthamiana (Fig. 2).
Infection by EACMZV was slightly more aggressive than
either EACMV or EACMV-KE2, whereas EACMV-UG
caused only mild curling and slight stunting. N. benthamiana plants infected with EACMKV developed an intermediate phenotype. Disease symptoms became apparent
between 9 and 10 days p.i. for the more aggressive isolates
(EACMZV, EACMV and EACMV-KE2) and between 11
and 15 days p.i. for EACMKV and EACMV-UG (Table 1).
Southern blot hybridization confirmed the presence of
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both virus components in systemically infected tissues
(data not shown).
The cloned components of the representative viruses were
also infectious in cassava, inducing typical disease symptoms (yellow mosaic and leaf distortion) between 10 and
15 days p.i. (Table 1). Symptom severity was overall
commensurate with that seen in N. benthamiana, with
EACMV-UG giving a markedly mild phenotype and
EACMZV, EACMV, EACMV-KE2 and EACMKV inducing
severe symptoms (Fig. 2) that continued to be expressed in
new growth following ratooning, reflecting the aggressive
nature of these virus clones in cassava. ACMV-infected
cassava plants frequently recover (symptoms reducing in
severity or disappearing entirely) due to post-transcriptional gene silencing (PTGS), while plants infected with
the more aggressive EACMCV do not recover and cannot
silence it by PTGS (Chellappan et al., 2004). The regrowth
following ratooning frequently shows a change in symptom
severity for viruses exhibiting the recovery phenotype. The
lack of this suggests that EACMZV, EACMV, EACMV-KE2
and EACMKV are viruses of the aggressive type that can
overcome PTGS. In all cases, symptom severity associated
with the cloned components resembled the phenotype
associated with the field isolates from which the viruses
were isolated.
Infectivity of pseudorecombinants
Pseudorecombinants produced by exchanging components
of EACMV, EACMV-KE2, EACMKV and EACMV-UG all
Journal of General Virology 88
Infectivity of Kenyan cassava begomoviruses
Table 1. Infectivity of cognate DNA-A and DNA-B components of representative begomovirus species and strains sampled from
districts throughout Kenya
Isolate
N. benthamiana
Infected/inoculated
EACMZV-[K18]
EACMV-[K24]
EACMV-KE2[K48]
EACMV-UG[K282]
EACMKV-[K261]
Cassava
days p.i.
Infected/inoculated
days p.i.
Severity*
Field severity*
10
9
10
15
11
2/2
2/2
2/2
2/2
2/2
10
11
12
15
12
5
5
5
3
4
5
4
5
3
3
12/16
18/20
15/16
10/15
16/20
*Severity rating based on 0 (asymptomatic) to 5 (severe mosaic disease and leaf distortion).
gave a disease phenotype in N. benthamiana (Table 2).
Plants infected with EACMV DNA-A gave a range of
symptoms depending on the DNA-B component with
which it was associated (Fig. 3). The most severe phenotype
was produced with EACMV-KE2 DNA-B, the least severe
with EACMV-UG DNA-B and EACMKV DNA-B gave an
intermediate phenotype. Similarly, plants infected with
EACMV-KE2 DNA-A produced severe symptoms with
EACMV DNA-B, intermediate symptoms with EACMKV
DNA-B and least severe symptoms with EACMV-UG
DNA-B. However, symptoms in N. benthamiana induced
by either EACMKV or EACMV-UG DNA-A components
with any other DNA-B component were largely indistinguishable, comprising mild stunting and leaf curling. In
contrast, all pseudorecombinants that contained either the
DNA-A or DNA-B component of EACMZV failed to give
symptoms in N. benthamiana. However, a low level of
EACMZV DNA-A was detected by Southern blotting when
this component was co-inoculated with either EACMV,
EACMV-KE2 or EACMV-UG DNA-B. PCR-mediated
isolation, cloning and sequencing of a full-length DNA-A
component confirmed its presence in the asymptomatic
tissues. Furthermore, Southern blotting and PCR amplification using component-specific primers (Bull et al., 2006)
failed to detect DNA-B components in newly emerging
leaves. Subsequent Southern blot analysis of 18 asymptomatic N. benthamiana plants inoculated with EACMZV
DNA-A alone showed that the component was present at
low levels in newly emerging leaves of three plants by 20
days p.i. (data not shown) and its integrity was confirmed
by PCR amplification of the full-length component and
sequence analysis. Our finding that EACMZV DNA-A can
spread systemically in N. benthamiana following biolistic
delivery is consistent with an earlier observation using
agroinoculation of ACMV DNA-A (Klinkenberg & Stanley,
1990).
Table 2. Infectivity of pseudorecombinants
Infectivity is given as the number of plants infected/number inoculated. Symptom abbreviations: mild leaf curl (mlc); severe leaf curl (slc); mild
stunting (mst); severe stunting (sst); mosaic (m). ND, Not done; Nb, N. benthamiana.
DNA-B
component
DNA-A component
EACMZV
EACMV
EACMV-KE2
EACMKV
EACMV-UG
Nb
cassava
Nb
cassava
Nb
cassava
Nb
cassava
Nb
cassava
12/14
sst/slc
0/10*
2/2
m
0/20
ND
0/32
ND
0/20
ND
0/32
ND
ND
EACMV-KE2
0/32*
ND
EACMKV
0/20
ND
EACMV-UG
0/32*
ND
8/10
sst/slc
9/10
slc/mst
8/10
slc/mst
9/9
slc/mst
2/2
m
2/2
m
2/2
m
2/2
m
8/10
slc/mst
12/14
sst/slc
10/10
mst/mlc
19/20
mst/mlc
2/2
m
2/2
m
2/2
m
2/2
m
9/9
mst/mlc
9/10
mst/mlc
6/6
mst/mlc
9/10
mst/mlc
2/2
m
2/2
m
2/2
m
2/2
m
8/10
mst/mlc
15/20
mst/mlc
5/10
mst/mlc
7/14
mst/mlc
2/2
m
2/2
m
2/2
m
2/2
m
EACMZV
EACMV
*Asymptomatic plants show limited systemic movement of DNA-A but not DNA-B.
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S. E. Bull and others
severe downward leaf curling and stunted growth by
12 days p.i., comparable to the wild-type infection (Fig.
4a). In addition, two inoculated cassava plants displayed a
mosaic phenotype and leaf distortion, also at approximately 12 days p.i., although symptoms were slightly
milder than in plants infected with the wild-type virus.
Southern blot analysis revealed a marked reduction in the
level of ssDNA in N. benthamiana (Fig. 4b) as well as in
cassava when compared with the wild-type infection.
Sequence analysis of three full-length DNA-A clones
isolated from two cassava plants and one N. benthamiana
plant confirmed that the mutation had been retained in
vivo and that no other nucleotide changes had occurred.
Western blot analysis showed that CP was expressed in N.
benthamiana and cassava plants infected with wild-type
virus, but not with mutV1 (Fig. 4c), confirming that the
mutation had disrupted CP expression.
Mutation of ORF AV2
Fig. 3. Symptoms induced in N. benthamiana and cassava by
pseudorecombinants produced by exchanging DNA-A and DNA-B
components of Kenyan begomoviruses.
The pseudorecombinants that were infectious in N.
benthamiana also induced mosaic disease symptoms in
cassava, although they were generally less severe than those
induced by DNA-A and DNA-B components derived from
the same species or strain (Fig. 3; Table 2). EACMV-UG,
shown to be the least virulent of the viruses in both N.
benthamiana and cassava, also gave relatively mild symptoms in cassava as a component of a pseudorecombinant.
Other pseudorecombinants produced very similar phenotypes that often varied throughout the plant, with some
shoots showing signs of recovery and others displaying a
yellow mosaic. Furthermore, the symptoms sometimes
increased in severity as the plant aged. This was more likely
to occur in plants infected with pseudorecombinants that
initially gave a more severe mosaic phenotype (for
example, EACMV-KE2 DNA-A and EACMKV DNA-B),
whereas other pseudorecombinants (for example,
EACMKV DNA-A and EACMV-UG DNA-B) were likely
to show signs of recovery.
Mutation of ORF AV1
The EACMZV CP coding sequence (257 aa) was disrupted
by replacing tyrosine177 with an in-frame nonsense codon
in ORF AV1, downstream of the overlapping ORF AV2
(mutant mutV1; Fig. 1). All ten N. benthamiana plants coinoculated with mutV1 and EACMZV DNA-B exhibited
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The EACMZV AV2 coding sequence (118 aa) was initially
disrupted by replacing tyrosine24 with an in-frame nonsense codon in ORF AV2, upstream of the overlapping
ORF AV1 (mutant mutV2; Fig. 1). Four out of ten N.
benthamiana plants inoculated with mutV2 and EACMZV
DNA-B expressed symptoms by 12 days p.i., which were
significantly milder than in plants infected with the wildtype virus (Fig. 4a). Plants also showed some variation in
phenotype, ranging from slight downward leaf curling to
moderate stunting of growth. Two cassava plants inoculated with mutV2 displayed mosaic disease symptoms that
were milder than in plants infected with the wild-type
virus. Southern blot analysis showed variable and generally
low levels of viral DNA accumulation in N. benthamiana
and cassava, although both ssDNA and double-stranded
DNA (dsDNA) forms accumulated in cassava (Fig. 4b).
Sequence analysis of a full-length DNA-A clone isolated
from a N. benthamiana plant exhibiting mild leaf curling
(clone piv2pl5) confirmed that the mutation had been
retained in vivo and that no other nucleotide changes had
occurred. However, a clone isolated from a plant exhibiting
mild leaf curling and stunting (clone piv2pl1) retained the
introduced mutation, but also had a C to T transition at
position 2681 in the CR. Furthermore, a clone isolated
from a plant exhibiting slightly more severe symptoms
(clone piv2) had a T to G transversion at position 2732 in
the CR and an A to G transition at position 2372 in the
overlapping AC1/AC4 ORFs. The latter did not affect the
Rep sequence, but caused a phenylalanine39 to serine
substitution in AC4. The plant exhibiting a relatively high
level of viral DNA accumulation (Fig. 4b) was not analysed
and consequently we cannot rule out the possibility that
the mutation had reverted on this occasion. A full-length
DNA-A clone isolated from cassava showed no alterations
other than the introduced mutation in ORF AV2.
To investigate the phenotypes of the mutants produced in
vivo and to ensure that they represent biologically active
Journal of General Virology 88
Infectivity of Kenyan cassava begomoviruses
Fig. 4. (a) Infectivity of EACMZV mutants in N.
benthamiana and cassava. Plants were either
healthy (H) or infected with wild-type (WT)
virus or mutants mutV1 (V1), mutV2 (V2),
mutV2A (V2A), mutV2B (V2B) and mutC4
(C4). (b) Southern blot analyses of DNA
extracted from N. benthamiana and cassava
plants, probed for EACMZV DNA-A. The
positions of viral single-stranded (ss) and
supercoiled (sc) DNA forms are indicated.
(c) Western blot analysis of EACMZV coat
protein expression in N. benthamiana (N) and
cassava (C). Proteins were extracted from
healthy plants or plants infected with wild-type
virus and mutants mutV1, mutV2, mutV2A and
mutV2B. The positions of size markers (kDa)
are indicated.
components, DNA-A clones piv2pl5, piv2pl1 and piv2 were
co-inoculated with EACMZV DNA-B to N. benthamiana.
Clone piv2 produced leaf curling and stunting by 12 days
p.i. which was initially more severe than the symptoms
caused by piv2pl1 and piv2pl5, consistent with an influence
of the additional mutations on phenotype, although
symptoms induced by all three clones became indistinguishable by 20 days p.i.
Due to the overlapping nature of ORFs AV1 and AV2, the
possibility that the mutation in mutV2 could affect CP
expression was considered. To test this, protein extracts
from infected N. benthamiana and cassava plants were
analysed by Western blotting. Unexpectedly, mutV2infected plants contained no detectable accumulation of
CP (Fig. 4c), implying that the mutation indeed had a
detrimental effect on CP expression. This problem was
addressed by designing two additional ORF AV2 mutants
in which in-frame nonsense mutations replaced either
leucine6 (mutV2A) or glutamine95 (mutV2B), the latter
mutation located within the overlapping virion-sense
ORFs, although the CP coding sequence remained
unaffected (Fig. 1). When co-inoculated with EACMZV
DNA-B, both mutants induced leaf curling and stunted
growth in N. benthamiana by 13 days p.i. Symptoms were
more severe for mutV2B, although both mutants induced
less severe symptoms than the wild-type virus (Fig. 4a).
Viral DNA accumulation was unaffected by the mutation
in mutV2A while the accumulation of mutV2B, particularly the ssDNA, was significantly reduced (Fig. 4b). Once
again, Western blot analysis failed to detect CP in protein
extracts from N. benthamiana plants infected with either of
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these ORF AV2 mutants (Fig. 4c), indicating that both
were compromised for CP production.
Mutation of ORF AC4
The AC4 coding sequence (85 aa) was disrupted by
replacing serine66 with an in-frame nonsense codon
(mutant mutC4; Fig. 1). The mutation did not alter the
amino acid encoded by the overlapping ORF AC1. All ten
N. benthamiana plants co-inoculated with mutC4 and
EACMZV DNA-B developed severe wild-type symptoms
by 12 days p.i. Furthermore, symptoms in two inoculated
cassava plants were indistinguishable from those in plants
infected with the wild-type virus (Fig. 4a). Southern blot
analysis showed some variation in viral DNA accumulation
in N. benthamiana plants, although mutC4 could accumulate to almost wild-type levels. MutC4 and wild-type
virus accumulated to similar levels in cassava (Fig. 4b).
Sequence analysis of full-length DNA-A components
isolated from two N. benthamiana plants and two cassava
plants showed that the mutation had been retained in vivo
and that no other nucleotide changes had occurred.
DISCUSSION
Using a biolistic delivery system, we have shown that
cloned DNA-A and DNA-B genomic components of
EACMV (including two distinct strains, EACMV-KE2
and EACMV-UG), EACMKV and EACMZV, representative of the begomovirus population associated with CMD
in Kenya (Bull et al., 2006), are infectious in cassava.
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S. E. Bull and others
Demonstration of their biological activity validates these
virus isolates as functional representatives of their respective species and strains, providing an important resource
for the molecular analysis of viral and host factors that
contribute to the disease. In addition, the ability to
reproduce the disease in cassava using a relatively
inexpensive delivery system offers a convenient method
to screen cassava for CMD resistance under controlled
conditions, without the need to transmit the begomoviruses using the whitefly vector B. tabaci. The development of cassava breeding lines is a time-consuming process
and farmers are often reluctant to move to new, introduced
stocks to monitor disease incidence (Thresh & Cooter,
2005). It is anticipated that the availability of infectious
cloned components representative of the begomovirus
population will allow preliminary screening prior to the
initiation of costly and time-consuming multiplication and
field trials.
The mild and severe phenotypes associated with EACMVUG and EACMZV, respectively, in naturally infected
cassava, reproduced using their cloned components, were
not commensurate with published reports. EACMZV
infection was associated with a mild phenotype in cassava
growing in Zanzibar (Maruthi et al., 2002, 2004) and
EACMV-UG with severe symptoms in Uganda (Gibson
et al., 1996; Zhou et al., 1997; Pita et al., 2001). The reason
for these differences in symptoms is unknown but may
simply reflect selected sampling of plants and clones.
Certainly, EACMV-UG[K282] was the only isolate of this
particular strain that was infectious to both N. benthamiana and cassava. It is also worth noting that EACMVUG[K282] originates from Machakos (central Kenya),
whereas all other EACMV-UG isolates were from districts
along the border with Uganda (Bull et al., 2006).
Comparative analyses involving additional EACMV-UG
isolates from Kenya and the construction of infectious
clones of EACMZV from Zanzibar may help to resolve this
issue. We have also demonstrated that cloned components
of the recombinant viruses EACMV-KE2 and EACMKV
produce severe symptoms in cassava. This is in keeping
with the realization that begomoviruses associated with
CMD have a propensity for recombination that can result
in a severe phenotype (Gibson et al., 1996; Zhou et al.,
1997; Fondong et al., 2000; Pita et al., 2001).
The dissemination of begomoviruses provides the opportunity for mixed infections, allowing the exchange of genomic components and recombination to play an important
role in diversification of the population (Padidam et al.,
1999). Indeed, we have recently demonstrated that the
DNA-B components of Kenyan begomoviruses associated
with CMD segregate into two main groups, although
distinct strains are not necessarily confined to a single
group, suggesting that component exchange has occurred
within the population (Bull et al., 2006). The production of
viable pseudorecombinants by reassortment of genomic
components is generally restricted to virus strains.
Consistent with this, we have demonstrated that pseudorecombinants produced by exchange of components of
EACMV, EACMV-KE2 and EACMV-UG were infectious
in N. benthamiana, although pseudorecombinants between
these viruses and EACMKV, a distinct species, were also
infectious. In these experiments, symptom severity was
generally defined by the DNA-B component, consistent
with previous observations using strains of ACMV and
tomato golden mosaic virus (Stanley et al., 1990; Morris
et al., 1991; von Arnim & Stanley, 1992). A productive
infection is usually defined by the ability of DNA-A to
trans-replicate DNA-B. Reiterated sequences (iterons 1–3)
that contribute to Rep binding and the initiation of viral
DNA replication were identified in the CRs of the Kenyan
viruses (Fig. 5). Iteron sequence variation occurs between
isolates and even between components of a single isolate,
but all were similarly positioned and located upstream of
the ubiquitous nonanucleotide motif (TAATATTAC) as
described by Argüello-Astorga et al. (1994). Interestingly,
the 39 core sequence of iteron 2, which is believed to play a
Fig. 5. Alignment of DNA-A and DNA-B common region sequences of selected Kenyan begomoviruses. Iteron core sequences
are highlighted and their orientation is indicated by arrows. The TATA box for Rep expression is highlighted in bold. Spaces (-)
have been introduced to align the motifs.
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Journal of General Virology 88
Infectivity of Kenyan cassava begomoviruses
crucial role in Rep binding (Fontes et al., 1994a), was identical between EACMV strains and EACMKV (GGGGG). In
addition, this core sequence occurs as either an exact or
partial repeat in both iteron 1 and the 59 core sequence of
iteron 2, and as an inverted repeat in iteron 3. This high
level of conservation explains the compatibility of their
components for replication. However, the 39 core sequence
of EACMZV (GGAGA) differs from those of EACMV and
EACMKV, and a 59 core sequence does not occur in
EACMZV, suggesting why EACMZV was unable to form
viable pseudorecombinants with these viruses. Indeed, the
arrangement of EACMZV iterons is more similar to isolates
of ACMV that also have the core sequence GGAGA. Despite
this, pseudorecombinants constructed between EACMZV
and ACMV were not infectious in N. benthamiana (data not
shown), indicating that factors other than a conserved iteron
core sequence are necessary for trans-replication compatibility (Fontes et al., 1994b).
We have investigated the contribution of CP, AV2 and AC4
proteins to the disease phenotype in cassava using EACMZV
DNA-A mutants. The results indicate that EACMZV CP
expression is not essential for symptomatic infection of
cassava, although the CP mutant accumulated only low levels
of ssDNA and produced slightly attenuated symptoms.
Similarly, a reduction in ssDNA accumulation has been
reported for ACMV CP mutants in N. benthamiana (Stanley
& Townsend, 1986; Etessami et al., 1989) as well as for
isolates of bean golden yellow mosaic virus, ToLCNDV and
tomato yellow leaf curl virus (Azzam et al., 1994; Padidam
et al., 1995, 1996; Wartig et al., 1997). This may be
attributable to a reduction of ssDNA being sequestered into
virions in the absence of CP expression. Although ACMV CP
has been shown to facilitate nuclear import/export and
transport of the virus to the cell periphery (Unseld et al.,
2001), it is likely that the ORF AV1 mutant remains viable
due to functional redundancy between CP and DNA-Bencoded BV1 (Pooma et al., 1996).
Previous reports have demonstrated that ACMV ORF
AV2 is not essential for infection in N. benthamiana (Ward
et al., 1988; Etessami et al., 1989). Here, we have shown
that EACMZV ORF AV2 mutants are infectious in N.
benthamiana and cassava, although symptoms in both
hosts were attenuated and levels of viral DNA accumulation were reduced. Mild symptoms and a reduction in the
level of viral DNA were also associated with ToLCNDV
ORF AV2 mutants (Padidam et al., 1996). Despite introducing point mutations at different positions within three
ORF AV2 mutants, none of which affected the CP coding
sequence, it was surprising to find that all mutants were
unable to express detectable levels of CP. The fact that
ssDNA accumulation was not significantly reduced for
mutV2A indicates a phenotype distinct from that of the CP
mutant, which is not attributable simply to a reduction in
CP expression. It has been shown that mutants of the
positional homologue (ORF V2) in BCTV produce an
asymptomatic infection associated with elevated levels of
dsDNA (Stanley et al., 1992; Hormuzdi & Bisaro, 1993). In
http://vir.sgmjournals.org
this way, disruption of EACMZV ORF AV2 could have an
indirect effect on CP accumulation by limiting the amount of
ssDNA available for encapsidation, although the accumulation of ssDNA in plants infected with mutV2A suggests that
this is not the case. The possibility that AV2 protein either
plays a direct role in the control of CP expression or prevents
CP turnover cannot be ruled out. Nonetheless, it is likely that
expression of these two overlapping ORFs will be closely
coordinated and it is conceivable that even subtle alterations
could have significant effects on the spatial and/or temporal
accumulation of CP, AV2 and viral DNA levels. It is worth
noting that, although ACMV DNA-A virion-sense transcripts have been mapped (Townsend et al., 1985), it is far
from clear how CP expression occurs, particularly as a lowabundance transcript maps across the CP ORF while the
major transcript maps across both virion-sense ORFs. Hence,
it is also possible that the ORF AV2 mutations could impact
on CP expression by affecting transcription or transcript
processing.
ACMV AC4 has been implicated in counteracting the plant
hypersensitive response to infection (van Wezel et al.,
2002), and has also been shown to suppress RNA silencing
and induce developmental abnormalities in transgenic
plants (Chellappan et al., 2005). Furthermore, the AC4
protein homologue in monopartite begomoviruses and
curtoviruses is an important symptom determinant and
may be involved in virus movement (Rigden et al., 1994;
Latham et al., 1997; Rojas et al., 2001). Despite this,
disruption of ACMV ORF AC4 had no effect on phenotype
in N. benthamiana (Etessami et al., 1991). Here, we have
demonstrated that an EACMZV ORF AC4 mutant is
infectious, not only in N. benthamiana but also in cassava,
without a significant change in phenotype. This suggests
that considerable variation in AC4 function may exist
between distinct begomoviruses or that there is an element
of functional redundancy between EACMZV AC4 and
other viral proteins (Vanitharani et al., 2004), reflecting the
considerable diversity observed within AC4 sequences of
distinct species (Bull et al., 2006).
ACKNOWLEDGEMENTS
We are indebted to Dr M. N. Maruthi for the provision of tissuecultured cassava (‘Ebwanateraka’). This work was funded by the
European Union INCO-DEV programme (project numbers
ICA4CT2000 30001). The authors formerly at the JIC acknowledge
the support of the BBSRC.
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