American Journal of Plant Sciences, 2010, 1, 55-68
doi:10.4236/ajps.2010.12008 Published Online December 2010 (
Copyright © 2010 SciRes. AJPS
Biotechnology and Plant Disease Control-Role of
RNA Interference
Shabir H. Wani1*, Gulzar S. Sanghera2, N. B. Singh3
1Biotechnology Laboratory, Central Institute of Temperate Horticulture, Srinagar, Jammu and Kashmir, India; 2Rice Research &
Regional Station (SKUAST-K) Khudwani, Anantnag Jammu and Kashmir, India; 3Department of Plant Breeding and Genetics, COA,
Central Agricultural University, Imphal, Manipur, India.
Email: *
Received July 10th, 2010; revised August 24th, 2010; accepted September 3rd, 2010.
Development of crop varieties which are resistant against many economically important diseases is a major challenge
for plant biotechnologists worldwide. Although much progress in this area has been achieved through classical genetic
approaches, this goal can be achieved in a more selective and robust manner with the success of genetic engineering
techniques. In this regard , RNA interference (RNAi) has emerged as a powerful modality for battling some of the most
notoriously challenging diseases caused by viruses, fungi and bacteria. RNAi is a mechanism for RNA-guided regula-
tion of gene expression in which double-stranded ribonucleic acid (dsRNA) inhibits the expression of genes with com-
plementary nucleotid e sequences. The application of tissu e-specific or inducible gene silencing in comb ination with the
use of appropriate promo ters to silence several genes simultaneously will result in protection of crops against destruc-
tive pathogens. RNAi application has resulted in successful control of many economically important diseases in plants.
Keywords: RNAi, Viruses, Fungi, dsRNA, Gene Silencing
1. Introduction
Plant diseases are a threat to world agriculture. Signifi-
cant yield losses due to the attack of pathogen occur in
most of the agricultural and horticultural crop species.
More than 70% of all major crop diseases are caused by
fungi [1]. Plant diseases are usually handled with appli-
cations of chemicals. For some diseases, chemical con-
trol is very effective; but it is often non-specific in its
effects, killing beneficial organisms as well as pathogens.
Chemical control may have undesirable effects on health,
safety and cause environmental risks [2]. Traditional
plant breeding methods have been used to develop culti-
vars resistant to various diseases. However, this process
is time-consuming and limited availability of genetic
resources for most of the crops has left little room to
continued improvement by these means. There are many
reasons for the limited genetic resources available for
breeding [3]. Two of the most important ones are: 1) loss
of gene pools occurring during the domestication and
breeding of crop plants [4] and 2) many of the natural
gene traits that may be beneficial in one plant tissue such
as seeds and fruits, may be deleterious in other plant tis-
sues such as vegetative tissues [5,6]. Over the past few
decades, breeding possibilities have been broadened by
genetic engineering and gene transfer technologies in-
cluding gene mapping and identification of the genome
sequences of model plants and crops. Modern technolo-
gies such as trancriptomics, proteomics, and metabolom-
ics are now proved to be important in understanding
plant metabolic pathways and the role of key genes asso-
ciated with their regulation. This can facilitate new in-
sights into the complex metabolite neighborhoods that
give rise to a given phenotype and may allow discovery
of new target genes to modify a given pathway. Such
genes can then be subject to new metabolic engineering
efforts and applications.
During the last decade, our knowledge repertoire of
RNA-mediated functions has been greatly increased with
the discovery of small non-coding RNAs which play a
central part in a process called RNA silencing. Ironically,
the very important phenomenon of co-suppression has
recently been recognized as a manifestation of RNA in-
terference (RNAi), an endogenous pathway for negative
post-transcriptional regulation. RNAi has revolutionized
the possibilities for creating custom “knock-downs” of
gene activity. RNAi operates in both plants and animals,
and uses double stranded RNA (dsRNA) as a trigger that
Biotechnology and Plant Disease Control-Role of RNA Interference
Copyright © 2010 SciRes. AJPS
targets homologous mRNAs for degradation or inhibiting
its transcription or translation [7,8], whereby susceptible
genes can be silenced. This RNA-mediated gene control
technology has provided new platforms for developing
eco-friendly molecular tools for crop improvement by
suppressing the specific genes which are responsible for
various stresses and improving novel traits in plants in-
cluding disease resistance. It has emerged as a method of
choice for gene targeting in fungi [9], viruses [10,11],
bacteria [12] and plants [13] as it allows the study of the
function of hundreds of thousands of genes to be tested
[14]. It can silence a gene throughout an organism or in
specific tissues [15], offer the versatility to partially si-
lence or completely turn off genes, work in both cultured
cells and whole organisms and can selectively silence
genes at particular stages of the organism’s life cycle
[16]. Methods that introduce dsRNA into plant and ani-
mal cells have been enormously successful for decreas-
ing cognate gene expression in vivo [17,18]. Due to all
these elegant and unique features of RNAi, our review
specifically focuses on 1) the current knowledge of
RNAi concept and 2) its pathways and induction in
plants and explore the possibilities for the applications of
this technology in the development of disease resistance
2. Mechanism of RNAi
‘RNA interference’ refers collectively to diverse
RNA-based processes that all result in sequence-specific
inhibition of gene expression at the transcription, mRNA
stability or translational levels. It has most likely evolved
as a mechanism for cells to eliminate foreign genes. The
unifying features of this phenomena are the production of
small RNAs (21-26 nucleotides (nt) that act as specific
determinants for down-regulating gene expression [17,
19,20] and the requirement for one or more members of
the Argonaute family of proteins [21]. RNAi operates by
triggering the action of dsRNA intermediates, which are
processed into RNA duplexes of 21-24 nucleotides by a
ribonuclease III-like enzyme called Dicer [22-24]. Once
produced, these small RNA molecules or short interfer-
ing RNAs (siRNAs) are incorporated in a multi-subunit
complex called RNA induced silencing complex (RISC)
[21,25]. RISC is formed by a siRNA and an endonucle-
ase among other components. The siRNAs within RISC
acts as a guide to target the degradation of complemen-
tary messenger RNAs (mRNAs) [21,25]. The host ge-
nome codifies for small RNAs called miRNAs that are
responsible for endogenous gene silencing. The dsRNAs
triggering gene silencing can originate from several
sources such as expression of endogenous or transgenic
antisense sequences, expression of inverted repeated se-
quences or RNA synthesis during viral replication [26].
When dsRNA molecules produced during viral replica-
tion trigger gene silencing, the process is called vi-
rus-induced gene silencing (VIGS) [27]. One interesting
feature of RNA silencing in plants is that once it is trig-
gered in a certain cell, a mobile signal is produced and
spread through the whole plant causing the entire plant to
be silenced [28]. After triggering RNA silencing, the
mobile signaling molecules can be relay-amplified by
synthesis of dsRNAs on the primary cleavage of product
templates or by their cleavage into secondary siRNAs.
This amplification leads to the transitory nature of si-
lencing reaction that may spread along the mRNA,
though initiated by a locally targeted single siRNA [29]
and spreads in both the 5´and 3´ directions [25]. This
bi-directional transition further have been witnessed by a
process where both the 5´ and 3´ cleavage products of the
initial target RNA act as aberrant mRNAs to trigger
dsRNA synthesis [30], and induce secondary silencing
reactions. This silencing process is also enhanced by the
enzymatic activity of the RISC complex, mediating mul-
tiple turnover reactions [31,25]. Furthermore, production
of the secondary siRNAs leads to enrichment of silencing
via its spread from the first activated cell to neighboring
cells, and systemically through the system [32]. The
cell-to-cell spread can be mediated as passive spread of
the small RNAs via plasmodesmata or by the silencing
signal complex which is between 27 and 54 kDa [33].
The systemic spread in phloem is mediated by the 24 nt
siRNAs [32], unloading of the systemic signal appears to
be mediated via plasmodesmata, since it does not spread
into meristematic cells [26]. The discovery of RNA-
binding protein (PSRP1) in the phloem and its ability to
bind 25 nt ssRNA species add further to the argument
that siRNAs (24-26 nt) are the key components for sys-
temic silencing signal. The extent of cell-to-cell move-
ment is dependent on the levels of siRNAs produced at
the site of silencing initiation, but is independent of the
presence of siRNA target transcripts in either source or
recipient cells [34].
3. Methods to Induce RNAi in Plants
One of the biggest challenges in RNAi research is the
delivery of the active molecules that will trigger the
RNAi pathway in plants. In this system, a number of
methods for delivery of dsRNA or siRNA into different
cells and tissue include transformation with dsRNA-
forming vectors for selected gene(s) by an Agrobacte-
rium mediated transformations [19,35], delivery cognate
dsRNA of uidA GUS (β-glucuronidase) and TaGLP2a:
GFP (green fluorescent protein) reporter genes into sin-
gle epidermal cells of maize, barley and wheat by parti-
cle bombardment [36], introducing a Tobacco rattle virus
(TRV)-based vector in tomato plants by infiltration [37],
Biotechnology and Plant Disease Control-Role of RNA Interference
Copyright © 2010 SciRes. AJPS
delivery of dsRNA into tobacco suspension cells by
cationic oligopeptide polyarginine-siRNA complex; in-
fecting plants with viral vectors that produce dsRNA [38]
and delivery of siRNA into cultured plant cells of rice,
cotton and slash pine for gene silencing by nanosense
pulsed laser-induced stress wave (LISW) [40]. Among
these the most reliable and commonly used approaches
for delivery of dsRNA to plants cells are agroinfiltration,
micro-bombardment and VIGS. These are discussed in
the following sections.
3.1. Agroinfiltration
Agroinfiltration is a powerful method to study processes
connected with RNAi. The injection of Agrobacterium
carrying similar DNA constructs into the intracellular
spaces of leaves for triggering RNA silencing is known
as agroinoculation or agroinfiltration [41]. In most cases
agroinfiltration is used to initiate systemic silencing or to
monitor the effect of suppressor genes. In plants, cyto-
plasmic RNAi can be induced efficiently by agroinfiltra-
tion, similar to a strategy for transient expression of
T-DNA vectors after delivery by Agrobacterium tumefa-
ciens. The transiently expressed DNA encodes either an
ss- or dsRNA, which is typically a hairpin (hp) RNA.
The infiltration of hairpin constructs are especially effec-
tive, because their dsRNA can be processed directly to
siRNAs, while the constructs expressing ssRNA can also
be useful to induce silencing [42-45] and for dissecting
the mechanism of gene silencing, especially concerned
with its suppressors, systemic silencing signal and also
for simple protein purification [42-45]. Besides, they
provide a rapid, versatile and convenient way for
achieving a very high level of gene expression in a dis-
tinct and defined zone.
3.2. Micro-Bombardment
In this method, a linear or circular template is transferred
into the nucleus by micro-bombardment. Synthetic
siRNAs are delivered into plants by biolistic pressure to
cause silencing of GFP expression. Bombarding cells
with particles coated with dsRNA, siRNA or DNA that
encode hairpin constructs as well as sense or antisense
RNA, activate the RNAi pathway. The silencing effect of
RNAi is occasionally detected as early as a day after
bombardment, and it continues up to 3 to 4 days of post
bombardment. Systemic spread of the silencing occurred
2 weeks later to manifest in the vascular tissues of the
non-bombarded leaves of Nicotiana benthamiana that
were closest to the bombarded ones. After one month or
so, the loss of GFP expression was seen in non-vascular
tissues as well. RNA blot hybridization with systemic
leaves indicated that the biolistically delivered siRNAs
induced due to de novo formation of siRNAs, which ac-
cumulated to cause systemic silencing [29].
3.3. Virus Induced Gene Silencing (VIGS)
Modified viruses as RNA silencing triggers are used as a
mean for inducing RNA in plants. Different RNA and
DNA viruses have been modified to serve as vectors for
gene expression [46,47]. Some viruses, such as Tobacco
mosaic virus (TMV), Potato virus X (PVX) and TRV,
can be used for both protein expression and gene silenc-
ing [48-51]. All RNA virus-derived expression vectors
will not be useful as silencing vectors because many have
potent anti-silencing proteins such as TEV (Tobacco e tch
virus), that directly interfere with host silencing machin-
ery [48,52]. Similarly, DNA viruses have not been used
extensively as expression vectors due to their size con-
straints for movement [53]. However, a non-mobile
Maize streak Virus (MSV)-derived vector has been suc-
cessfully used for long-term production of protein in
maize cell cultures [48]. Using viral vectors to silence
endogenous plant genes requires cloning homologous
gene fragments into the virus without compromising viral
replication and movement. This was first demonstrated in
RNA viruses by inserting sequences into TMV [54], and
then for DNA viruses by replacing the coat protein gene
with a homologous sequence [53]. These reports used
visible markers for gene silencing phytoene desatu-
rase( PDS) and chalcone synthase (CHS), providing a
measure of the tissue specificity of silencing as these
have been involved in carotenoid metabolic pathway.
The PDS gene acts on the antenna complex of the thyla-
koid membranes, and protects the chlorophyll from
photooxidation. By silencing this gene, a drastic decrease
in leaf carotene content resulted into the appearance of
photobleaching symptom [55,56]. Similarly, over ex-
pression of CHS gene causes an albino phenotype instead
of producing the anticipated deep orange color [57]. As a
result, their action as a phenotypic marker helps in easy
understanding of the mechanism of gene silencing. Table
1 shows some general characteristics for currently avail-
able virus-derived gene silencing vectors. Most viruses
are plus-strand RNA viruses or satellites, whereas To-
mato golden mosaic virus (TGMV) and Cabbage leaf
curl virus (CaLCuV) are DNA viruses. Though RNA
viruses replicate in the cytoplasm DNA viruses replicate
in plant nuclei using the host DNA replication machinery.
Both types of viruses induce diffusible, homol-
ogy-dependent systemic silencing of endogenous genes.
However, the extent of silencing spread and the severity
of viral symptoms can vary significantly in different host
plants and host/virus combinations. With the variety of
viruses and the diversity of infection patterns, transmis-
sion vectors, and plant defenses it is not surprising that
viruses differ with respect to silencing [58]. Because the
Biotechnology and Plant Disease Control-Role of RNA Interference
Copyright © 2010 SciRes. AJPS
continuing development of virus-based silencing vectors
can extend VIGS to economically important plants, it is
useful to consider some of the characteristics of success-
ful VIGS vectors.
4. RNAi in Plant Disease Management
Despite substantial advances in plant disease manage-
ment strategies, our global food supply is still threatened
by a multitude of pathogens and pests. This changed
scenario warrants us to respond more efficiently and ef-
fectively to this problem. The situation demands judi-
cious blending of conventional, unconventional and fron-
tier technologies. In this sense, RNAi technology has
emerged as one of the most potential and promising
strategies for enhancing the building of resistance in
plants to combat various fungal, bacterial, viral and
nematode diseases causing huge losses in important ag-
ricultural crops. The nature of this biological phenome-
non has been evaluated in a number of host-pathogen
systems and effectively used to silence the action of
pathogen. Many of the examples listed below illustrate
the possibilities for commercial exploitation of this in-
herent biological mechanism to generate disease-resistant
plants in the future by taking advantage of this approach.
4.1. Management of Plant Pathogenic Fungi
RNA-mediated gene silencing (RNA silencing) is used as
a reverse tool for gene targeting in fungi. Homology-
based gene silencing induced by transgenes (co-suppres-
sion), antisense, or dsRNA has been demonstrated in
many plant pathogenic fungi, including Cladosporium
fulvum [59], Magnaporthae oryzae [60-62], Venturia
inaequalis [63], Neurospora crassa [64], Aspergillus
nidulans [65], and Fusarium graminearum [9] (Table 1),
whether it is suitable for large scale mutagenesis in fun-
gal pathogens remains to be tested. Hypermorphic
mechanism of RNA interference implies that this tech-
nique can also be applicable to all those plant pathogenic
fungi, which are polyploid and polykaryotic in nature,
and also offers a solution to the problem where frequent
lack of multiple marker genes in fungi is experienced.
Simultaneous silencing of several unrelated genes by
introducing a single chimeric construct has been demon-
strated in case of Venturia inaequalis [63]. HCf-1, a gene
that codes for a hydrophobin of the tomato pathogen C.
fulvum [66], was co-suppressed by ectopic integration of
homologous transgenes. Transformation of Cladospo-
rium fulvum with DNA containing a truncated copy of
the hydrophobin gene HCf-1 caused co-suppression of
hydrophobin synthesis in 30% of the transformants. The
co-suppressed isolates had a hydrophilic phenotype,
lower levels of HCf-1 mRNA than wild type and contain
multiple copies of the plasmid integrated as tandem re-
peats at ectopic sites in the genome. The transcription
rate of HCf-1 in the co-suppressed isolates was higher in
the untransformed strains, which suggested that silencing
acted at the post-transcriptional level. This was due to
ectopic integration of the transgene next to promoters
which initiate transcription to form antisense RNA and
that this in turn determines down-regulation of HCf-1.
But gene silencing was not associated with DNA cyto-
sine methylation [59]. Similarly, the silencing of cgl1
and cgl2 genes using the cgl2 hairpin construct in
Cladosporium fulvum has also been reported [67], though
the effect was possibly restricted to highly homolougous
genes (exons of cgl 1 and cgl 2 are 87% identical).
However, the less homologus cgl 3 (53% overall identity
to cgl 2) was not affected as the target specificity always
depends upon the actual sequence alignment and more
over, short regions of high density that led to unwanted
off-targets effects. Such a strategy could be exploited for
protecting the consumable products of vegetables and
fruits crops from the post-harvest diseases caused by
different plant pathogens in future.
Hairpin vector technology resulted in simultaneous
high frequency silencing of a green fluorescent protein
(GFP) transgene and an endogenous trihydroxynaphtha-
lene reductase gene (THN) in Venturia inaequalis [63]
GFP transgene, acting as easily detectable visible marker
while the trihydroxynaphthalene reductase gene (THN)
playing role in melanin biosynthesis. High frequency
gene silencing was achieved using hairpin constructs for
the GFP or the THN genes transferred by Agrobacterium
(71 and 61%, respectively). THN-silenced transformants
exhibited a distinctive light brown phenotype and main-
tained the ability to infect apple. Silencing of both genes
with this construct occurred at a frequency of 51% of all
the transformants. All 125 colonies silenced for the GFP
gene were also silenced for THN [63]. Similarly, multi-
ple gene silencing has been achieved in Cryptococcus
Table 1. RNAi effects on targeted region in some fungal
plant pathogens.
Pathogen Targeted
region Result References
oryzae eGFP
Sequence specific
degradation of
falvum cgl 1 and cgl 2Blocking disease
infection spread [67]
inverted repeats - [63]
graminearum - - [9]
graminis Mlo Immunity [36]
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neoformans using chimeric hairpin constructs [39] and in
plants using partial sense constructs [68]. The first effort
towards the systematic silencing of Magnaporthe grisea,
a causal organism of rice blast was carried by using the
enhanced green florescent protein gene as a model [60].
To assess the ability of RNA species to induce silencing
in fungus, plasmid construct expressing sense, antisense
and hairpin RNA were introduced into an eGFP-ex-
pressing transformants. The fluorescence of eGFP in the
transformants was silenced much more efficiently by
hairpin RNA of eGFP than by other RNA species. In the
silenced transformants, the accumulation of eGFP
mRNA was drastically reduced. But not methylation of
coding or promoter region was involved. The small in-
terfering RNA molecules of 19-23 nucleotides were ob-
served in both sense and antisense strands of eGFP gene
[60]. Later on a protocol for silencing the mpg1 and
polyketide synthase-like genes was also developed [9].
Mpg1 gene is a hydrophobin gene which is essential for
pathogenicity as it act as a cellular relay for adhesion and
trigger for the development of appressorium [69]. Their
work on this host-pathogen system revealed that they
were successfully able to silence the above mentioned
genes at varying degrees by pSilent-1-based vectors in
70–90% of the resulting transformants. Ten to fifteen
percent of the silenced transformants exhibited almost
‘‘null phenotype’’. This vector was also efficiently ap-
plicable to silence a GFP reporter in another ascomycete
fungus Colletotrichum lagenarium [9]. A novel high-
throughput approach for gene function analysis using
RNAi, which provides an alternative to the gene
knock-out by homologous recombination was also de-
scribed [70]. The authors developed an RNA silencing
vector, pSilent-Dual1 (pSD1) that carries two convergent
dual promoters, the Aspergillus nidulans tryptophan
promoter (PtrpC) and the A. nidulans glyceraldehyde-3-
phosphate dehydrogenase promoter (Pgpd). Both pro-
moters have been used to drive constitutive gene expres-
sion in a large number of filamentous fungi. A multi-
cloning site (MCS) has been inserted between two pro-
moters. The greatest merit of the pSD1 system over oth-
ers, such as hpRNA or intron spliced hair-pin RNA (ih-
pRNA) silencing system is that it allows a single step
cloning for generation of an RNAi construct. To facilitate
efficient screening for silenced transformants, gfp gene
was incorporated the into pSD1 system [70]. It allows
expression of a chimeric RNA and assessment of gene
silencing efficiency by utilizing a recipient strain that
produces GFP and therefore, fluoresces green when us-
ing epifluorescence microscopy. A main bottleneck of
this system is its lower silencing efficiency compared
with hpRNA or ihpRNA-expressing RNA-silencing vec-
tors. Formation of dsRNA in the pSD1 system requires
physical annealing of two different RNA molecules in
the target cells while that in the hpRNA systems is
achieved by self-folding of inverted repeats within RNA
molecule. The difference in dsRNA formation between
the systems can be a major cause of the different silenc-
ing efficiencies. The authors generated a series of
knock-down mutants of almost all known calcium related
genes in the genome of M. oryzae and examined for
phenotypical defects. Gene knock-down requires rela-
tively short stretches of sequence information. This is a
major advantage for phytopathogens for which there is
little sequence information available. As RNAi works at
the mRNA level, its efficacy is not compromised by the
presence of non-transformed nuclei or multicopy genes
due to aneuploidy [71]. RNAi causes only a partial re-
duction, but not a complete loss of, in gene expression.
Partial gene suppression is considered a main drawback
of RNAi. However, it could be a merit where the effect
of an essential gene on a phenotype is of interest. Gene
knock-down offers a more convenient and effective tool,
especially in combination with an inducible promoter
that allows gene expression to be diminished at specific
stages during development [72]. Another disadvantage of
gene knock-down is, as it requires only a short sequence,
that genes other than those targeted might be silenced.
This causes unexpected changes in gene expression pat-
terns (off-target effects). Testing for the possibility of
off-target effects is simpler for phytopathogen species for
which complete genome sequence data are available but
remains elusive for those phytopathogens whose ge-
nomes have not been sequenced [71]. In another study,
RNA interference (RNAi) strategy was used to specifi-
cally knockdown 59 individual rice genes encoding puta-
tive LRR-RLKs, and a novel rice blast resistance-related
gene (designated as OsBRR1) was identified by screen-
ing T0 RNAi population using a weakly virulent isolate
of Magnaporthe oryzae, Ken 54-04. Wild-type plants
(Oryza sativa L. cv. ‘Nipponbare’) showed intermediate
resistance to Ken 54-04, while OsBRR1 suppression
plants were susceptible to Ken 54-04 [73]. Furthermore,
OsBRR1-overexpressing plants exhibited enhanced re-
sistance to some virulent isolates (97-27-2, 99-31-1 and
zhong 10-8-14). OsBRR1 expression was low in leaves
and undetectable in roots under normal growth condi-
tions, while its transcript was significantly induced in
leaves infected with the blast fungus (Ken 54-04) and
was moderately affected by ABA, JA and SA treatment.
Overexpression or RNAi suppression of OsBRR1did not
cause visible developmental changes in rice plants.
4.2. Management of Plant Pathogenic Bacteria
One of the striking examples of bacterial disease man-
agement where RNAi showed a remarkable type of gene
Biotechnology and Plant Disease Control-Role of RNA Interference
Copyright © 2010 SciRes. AJPS
regulation has been documented [12]. They developed a
crown gall disease management strategy that targets the
process of tumourogensis (gall formation) by initiating
RNAi of the iaaM and ipt oncogenes. Expression of
these genes is a prerequisite for wild type tumor forma-
tion. Transgenic Arabidopsis thaliana and Lycopersicon
esculentum transformed with RNAi constructs, targeting
iaaM and ipt gene(s) showed resistance to crown gall
disease. Transgenic plants generated through this tech-
nology contained a modified version of these two bacte-
rial gene(s) required to cause the disease and was the first
report to manage a major bacterial disease through RNAi.
The extra genes recognize and effectively shut down the
expression of the corresponding bacterial gene during
infection, thus preventing the spread of infection. The
incoming bacteria could not make the hormones needed
to cause tumors and plants deficient in silencing were
hyper-susceptible to A. tumefaciens [28]. Successful in-
fection relied on a potent anti-silencing state established
in tumors whereby siRNA synthesis is specifically inhib-
ited. The procedure can be exploited to develop broad-
spectrum resistance in ornamental and horticultural
plants which are susceptible to crown gall tumorigenesis.
This approach can be advocated for the effective man-
agement of those pathogens which multiply very rapidly
and results in tumor formation such as Albugo candida,
Synchytrium endobioticum, Erwinia amylovora etc. The
natsiRNA (nat-siRNAATGB2) was strongly induced in
Arabidopsis upon infection by Pseudomonas syringae pv
tomato and down-regulates a PPRL gene that encodes a
negative regulator of the RPS2 disease resistance path-
way. As a result, the induction of nat-siRNAATGB2
increases the RPS2-mediated race-specific resistance
against P. syringae pv tomato in Arabidopsis [74]. Re-
cently, the accumulation of a new class of sRNA, 30 to
40 nucleotides in length, termed long-siRNAs (lsiRNAs),
was found associated with P. syringae infection. One of
these lsiRNAs, AtlsiRNA-1, contributes to plant bacterial
resistance by silencing AtRAP, a negative regulator of
plant defense [75]. A Pseudomonas bacterial flagellin
derived peptide is found to induce the accumulation of
miR393 in Arabidopsis. miR393 negatively regulates
mRNAs of F-box auxin receptors, resulting in increased
resistance to the bacterium (P. syringae), and the over-
expression of miR393 was shown to reduce the plant’s
bacterial titer by 5-fold [76].
4.3. Management of Plant Pathogenic Viruses
Antiviral RNAi technology has been used for viral dis-
ease management in human cell lines [77-80]. Such si-
lencing mechanisms (RNAi) can also be exploited to
protect and manage viral infections in plants [19,81]. The
effectiveness of the technology in generating virus resis-
tant plants was first reported to PVY in potato, harbour-
ing vectors for simultaneous expression of both sense
and antisense transcripts of the helper-component pro-
teinase (HC-Pro) gene [82]. The P1/HC-Pro suppressors
from the potyvirus inhabited silencing at a step down
stream of dsRNA processing, possibly by preventing the
unwinding of duplex siRNAs, or the incorporation into
RISC or both [83]. The utilization of RANi technology
has resulted in inducing immunity reaction against sev-
eral other viruses in different plant-virus systems (Table
2). In phyto-pathogenic DNA viruses like geminiviruses
Table 2. Effects of targeted region of RNAi in various plant-
virus systems.
Host system Virus Targeted
region References
N. benthamiana,
M. esculenta African cassava
mosaic virus pds, su,
cyp79d2 [98]
Barley, wheat Barley stripe
mosaic virus pds [99-101]
Soybean Bean pod
mottle virus Pds
Actin [102-104]
Barley, rice,
Brome mosaic
virus pds, actin 1,
rubisco activase[105]
Arabidopsis Cabbage leaf
curl virus gfp, CH42, pds [56]
P. sativum Pea early
browning virus pspds, uni, kor [106]
N. benthamianaPoplar mosaic
virus gfp [107]
N. benthamiana,
S. tuberosum Potato virus X pds, gfp [108,109]
tabacum Satellite tobacco
mosaic virus Several genes [110]
N. benthamiana,
N. tabacum Tobacco
mosaic virus pds, psy [48]
N. benthamiana,
tomato, Solanum
species, Chilli
pepper, opium
poppy, Aquilegia
rattle virus
Rar1, EDS1,
pds, rbcS, gfp [111-115]
N. benthamianaTomato bushy
shunt virus gfp [116]
N. benthamianaTomato golden
mosaic virus su, luc [117]
N. benthamiana,
N. glutinosa,
N. tabacum
Tomato yellow
leaf curl China
b DNA satellite
pcna, pds,
su, gfp [118]
(Modified after [119])
Biotechnology and Plant Disease Control-Role of RNA Interference
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non-coding intergenic region of Mungbean yellow mo-
saic India virus (MYMIV) was expressed as hairpin con-
struct under the control of the 35S promoter and used as
biolistically to inoculate MYMIV-infected black gram
plants and showed a complete recovery from infection,
which lasted until senescence [84]. RNAi mediated si-
lencing of geminiviruses using transient protoplast assay
where protoplasts were co-transferred with a siRNA de-
signed to replicase (Rep)-coding sequence of African
cassava mosaic virus (ACMV) and the genomic DNA of
ACMV resulted in 99% reduction in Rep transcripts and
66% reduction in viral DNA [85]. It was observed that
siRNA was able to silence a closely related strain of
ACMV but not a more distantly related virus. More than
40 viral suppressors have been identified in plant viruses
[86]. Results from some of the well-studied virus sup-
pressors indicated that suppressors interfere with sys-
temic signaling for silencing [44]. During last few years,
the p69 encoded by Turnip yellow mosaic viru s has been
identified as silencing suppressors that prevented host
RDR-dependent secondary dsRNA synthesis [87]. P14
protein encoded by aureus viruses suppressed both virus
and transgene-induced silencing by sequestering both
long dsRNA and siRNA without size specificity [88].
Multiple suppressors have been reported in Citrus
tristeza virus where p20 and coat protein (CP) play im-
portant role in suppression of silencing signal and p23
inhibited intracellular silencing [27]. Multiple viral
components, viral RNAs and putative RNA replicase
proteins were reported for a silencing or suppression of
Red clover necrotic mosaic virus [89]. In this case, the
RNA silencing machinery deprived of DICER-like en-
zymes by the viral replication complexes appears to be
the cause of the suppression. Pns10 encoded by Rice
dwarf virus suppressed local and systemic S-PTGS but
not IR-PTGS suggesting that Pns10 also targets an up-
stream step of dsRNA formation in the silencing pathway
[90]. A 273-bp (base pair) sequence of the Arabidopsis
miR159 a pre-miRNA transcript expressing amiRNAs
was used against the viral suppressor genes P69 and
HC-Pro to provide resistance against Turnip yellow mo-
saic virus and Turnip mosaic virus infection, respectively
[91]. In addition, a dimeric construct harboring two
unique amiRNAs against both viral suppressors con-
ferred resistance against these two viruses in inoculated
Arabidopsis plants. Similarly, a different amiRNA vector
was used to target the 2 b viral suppressor of the Cu-
cumber mosaic virus (CMV), a suppressor that interacted
with and blocked the slicer activity of AGO1 had also
shown to confer resistance to CMV infection in trans-
genic tobacco [92]. A strong correlation between virus
resistance and the expression level of the 2 b-specific
amiRNA was shown for individual plant lines. It is evi-
dent from above-mentioned reports that the RNA com-
ponents, such as single strand template RNA, dsRNA
and/or siRNA of the silencing pathways are the preferred
targets of most viral suppressors. However, plant viruses
are known to have evolved a counter-silencing mecha-
nism by encoding proteins that can overcome such resis-
tance [34,93]. These suppressors of gene silencing are
often involved in viral pathogenicity, mediate synergism
among plant viruses and result in the induction of more
severe disease. Simultaneous silencing of such diverse
plant viruses can be achieved by designing hairpin struc-
tures that can target a distinct virus in a single construct
[93]. Contrarily, the RNAi system may cause an increase
in the severity of viral pathogenesis and/or encode pro-
teins, which can inactivate essential genes in the RNAi
machinery [94] that helps them in their replication in the
host genome [17]. Transgenic rice plants expressing
DNA encoding ORF IV of Rice tungro bacilliform virus
(RTBV), both in sense and in anti-sense orientation, re-
sulting in the formation of dsRNA, were generated. Spe-
cific degradation of the transgene transcripts and the ac-
cumulation of small RNA were observed in transgenic
plants. In RTBV-ODs2 line, RTBV DNA levels gradu-
ally rose from an initial low to almost 60% of that of the
control at 40 days after inoculation [95]. For the effective
control of PRSV and Papaya leaf-distortion mosaic virus
(PLDMV), an untranslatable chimeric construct contain-
ing truncated PRSV YK CP and PLDMV P-TW-WF CP
genes has been transferred into papaya (Carica papaya
cv. ‘Thailand’) by Agrobacterium-mediated transforma-
tion via embryogenic tissues derived from immature zy-
gotic embryos of papaya [96]. Based on sequence profile
of silencing suppressor protein, HcPro, it was that
PRSV-HcPro acts as a suppressor of RNA silencing
through micro RNA binding in a dose dependent manner.
In planta expression of PRSV-HcPro affects develop-
mental biology of plants, suggesting the interference of
suppressor protein in micro RNA-directed regulatory
pathways of plants. Besides facilitating the establishment
of PRSV, it showed strong positive synergism with other
heterologous viruses as well [97].
4.4. Management of Plant Parasitic Nematodes
Several major plant parasitic nematodes such as the root-
knot (Meloidogyne spp.) and cyst (Heterodera spp.)
along with other minor nematodes cause significant
damage to important agricultural crops such as legumes,
vegetables and cereals in most parts of the world. There-
fore, a natural, eco-friendly defense strategy that delivers
a cost-effective control of plant parasitic nematodes is
needed urgently which is difficult to achieve through
conventional approaches. However, the origin of RNAi
technology from classical C. elegans studies has shown
Biotechnology and Plant Disease Control-Role of RNA Interference
Copyright © 2010 SciRes. AJPS
the ways and means to explore the possibilities of this
mechanism for protecting plants from nematode damage.
In this context, two approaches have been advocated i.e.,
1) relies on targeting plant genes that are involved with
the infection process and 2) targets the essential genes
within the nematode. RNAi can be induced in C. elegans
by feeding it dsRNA, hence it was reasoned that ex-
pressing hpRNAs containing sequences of vital nema-
tode genes in the host plant might deliver dsRNA to a
feeding nematode to incapacitate or kill it.
After the demonstration of gene silencing using siRNA
duplexes in the nematode [22], the use of RNAi has rap-
idly emerged as the technique for plant nematologists to
put their efforts, especially for nematode management in
agriculture. RNAi-mediated suppression of a gene plays
an indispensable role in hampering the nematode devel-
opment and may adversely affect the progression of
pathogenesis in direct or indirect ways. There are accu-
mulating evidences for the efficacy of RNAi in plant
parasitic nematode management and a wide range of
genes have been targeted for silencing in cyst and root-
knot nematode species (Table 3).
RNAi in the context of phyto-parasitic nematodes was
used as early as the beginning of this century, when
stimulation of oral ingestion by second-stage juveniles of
cyst nematodes H. glycines, G. pallida [120] and root-
knot nematode M. incognita was achieved by using
octopamine [121]. Later on, resorcinol- and serotonin-
inducing dsRNA uptake by second stage juvenile of M.
incognita was found to be more effective than octopi-
mine [122]. The genes targeted by RNAi to date are ex-
pressed in a range of different tissues and cell types. The
ingested dsRNA can silence genes in the intestine [120,
123], female reproductive system [124], sperm [120,
125], and both subventral and dorsal oesophageal glands
[121,122,126,127,]. Uptake of dsRNA from the gut is a
proven route to systemic RNAi in C. elegans. The sys-
temic nature of RNAi in plant parasitic nematodes fol-
lowing ingestion of dsRNA suggests that they share
similar uptake and dispersal pathways. However, RNAi
of a chitin synth ase gene expressed in the eggs of Meloi-
dogyne artiella was achieved by soaking intact eggs
contained within their gelatinous matrix in a solution
containing dsRNA [128]. The enzyme plays a key role in
the synthesis of the chitinous layer in the eggshell. De-
pletion of its transcript by RNAi led to a reduction in
stainable chitin in eggshells and a delay in hatching of
juveniles from treated eggs. Similarly, RNAi targeting
for cysteine proteinase transcripts did not reduce para-
sitic population of established nematodes on plants but
result into the alteration of their sexual fate in favour of
males at 14 days after invasion [120]. On the other hand
H. glycines exposed to dsRNA corresponding to a protein
Table 3. RNAi effect on targeted region of plant parasitic
Nematode Targeted region RNAi effect
M. incognita Cysteine proteinase
Delayed development,
Decrease in established
nematodes population
Dual oxidase
Decrease in established
nematodes population and
Splicing factor,
Reduction in gall formation
and Female nematode
Secreted peptide
16D 10
Reduction in gall
formation and established
nematode population
H. glycines Cysteine
Increased male:
female ratio.
C-type lectin
Reduction in established
nematodes population
Major sperm
protein Reduction in mRNA level
Decrease in established
nematodes population and
increase in male:
female ratio.
β-1,4-endoglucanase Decrease in established
nematodes population
Pectate lyase,
Chorismate mutase
Increase in male:
female ratio.
Secreted peptide
Decrease in established
nematode population
G. pallida Cysteine
Increase in male:
female ratio.
FMR Famide-like
peptides Motility inhibited
G. rostochiensisChitin synthase Delay in egg hatch
β-1,4-endoglucanase Decrease in established
nematodes population
Secreted amphid
Reduction in invasion
ability to locate and
invade plant roots
Suc transporter
Reduction of female
nematode development
(Modified after [132])
with homology to C-type lectins did not affect sexual fate,
but 41% fewer nematodes were recovered from the
plants. But treatment with dsRNA corresponding to the
major sperm protein (MSP) had no effect on nematode
development or sexual fate 14 days after treatment. In
Biotechnology and Plant Disease Control-Role of RNA Interference
Copyright © 2010 SciRes. AJPS
addition to this, reduction in transcript abundance for
targeted mRNAs in the infective juvenile and for MSP
transcripts when males reached sexual maturity and
sperm are produced was observed [120]. In further ex-
tension of such types of experiments showed efficient
FITC uptake by soaking M. incognita, 90-95% of indi-
viduals swallowed the dye when the target was a dual
oxidase (an enzyme comprised with a peroxidase domain
EF-hands and NADPH oxidase domain and potentially
involved in extracellular matrix development). The effect
of RNAi was observed when root knot nematode (RKN)
juveniles were fed on dual oxidase-derived dsRNA, the
reduction in the number and size of established females
at 14 and 35 days post- infection with an overall reduc-
tion of 70% in egg production was observed [121]. RNAi
has also been induced for a chitin synthase gene that is
expressed in the eggshells of M. artiella after soaking its
developing eggs in a dsRNA [128]. Heterodera schachtii
induces syncytial feeding structures in the roots of host
plants, and this requires the up-regulation of Suc trans-
porter genes to facilitate increased nutrient flow to the
developing structure. Targeting these genes and down-
regulating them with RNA silencing resulted in a sig-
nificant reduction of female nematode development
[129]. Indeed, tobacco plants transformed with hpRNA
constructs against two such root-knot nematode genes
have shown such an effect: the target mRNAs in the
plant parasitic nematodes were dramatically reduced, and
the plants showed effective resistance against the parasite
[130]. In another study mRNA abundances of targeted
nematode genes were specifically reduced in nematodes
feeding on plants expressing corresponding RNAi con-
structs. Furthermore, this host-induced RNAi of all four
nematode parasitism genes led to a reduction in the
number of mature nematode females. Although no com-
plete resistance was observed, the reduction of develop-
ing females ranged from 23% to 64% in different RNAi
lines [131]. These observations demonstrate the rele-
vance of the targeted parasitism genes during the nema-
tode life cycle and more importantly, suggest that a vi-
able level of resistance in crop plants may be accom-
plished in the future by using RNAi technology against
cyst nematodes.
5. Conclusions and Future Prospects
RNAi and miRNA technologies of gene silencing are
newly developed genomics tools that have great advan-
tages over antisense and co-suppression due to their
higher silencing efficiency and shorter time requirements
for screening. These technologies are particularly useful
in conjunction with the practice of gene or pathway dis-
coveries through nutritional genomics, trancriptomics,
proteomics and metabolomics in plants to improve hu-
man health. The RNA silencing has ability to reduce
gene expression in a manner that is highly sequence spe-
cific as well as technologically facile and economical.
Therefore, this technique has great potential in agricul-
ture specifically for nutritional improvement of plants
and the management of mascotous plant diseases. How-
ever, the major obstacles hindering its immediate appli-
cations include selection of targeting sequences and in
the delivery of siRNA. The key issues are: 1) how to
select silencing targets for a particular disease, and 2)
how to efficiently deliver siRNAs into specific cell types
in vivo. Tissue or organ-specific RNAi vectors have al-
ready been proven to be useful for targeted gene silenc-
ing in specific plant tissues and organs with minimal in-
terference with the normal plant life cycle. New genera-
tion RNAi and miRNA vectors have been developed
with high silencing accuracy and fewer side effects in
plants. Genetic engineering of highly nutritional food
crops requires both gene silencing and counter-silencing
technologies. Besides, RNAi technology can be consid-
ered an eco-friendly, biosafe and ever green technology
as it eliminates even certain risks associated with devel-
opment of transgenic plants carrying first generation
constructs (binary vectors and sense and antisense genes).
As witnessed from earlier strategies for obtaining viral
resistant plants, the expression of protein product from
the transgene of interest risked hetero-encapsidation
through protein-protein interactions between target and
non-target viral gene product, resulted in the develop-
ment of a non-aphid transmissible strain of Zucchini yel-
low mosaic virus to aphid-transmissible strain from a
transgene expressing a plum pox capsid protein. Since
RNAi triggers the formation of dsRNA molecules that
target and facilitate the degradation of the gene of inter-
est as well as the transgene itself to avoid problems aris-
ing from the synthesis of gene sequences as well as non-
coding regions of gene, thus limiting undesirable recom-
bination events. Keeping in view the potentialities of
RNAi technology this technology has emerged to combat
plant pathogens in the near future as it has already added
new dimensions in the chapter of plant disease manage-
ment. Further, development of vectors that can suppress
the RNAi pathway but overexpress transgenes in a tis-
sue-specific manner will revolutionize this field. Such
vectors could be based on various viral RNA silencing
suppressors and their derivatives. Future directions will
focus on developing finely tuned RNAi-based gene si-
lencing vectors that are able to operate in a temporally
and spatially controlled manner. However, a better and
comprehensive understanding of RNAi would allow the
researchers to work effectively and efficiently in order to
improve crop plants nutritionally and manage various
mascotous intruders of crop plants.
Biotechnology and Plant Disease Control-Role of RNA Interference
Copyright © 2010 SciRes. AJPS
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