I.INTRODUCTION
Plants are challenged by a variety of biotic stresses like fungal, bacterial, or viral infections.
However, plants are able to defend themselves with different mechanisms simultaneously at the
local and systemic scale (Ali et al., 2018). The first layer of their active defence occurs at the
cellular level and is called pathogen-associated molecular pattern PAMP triggered immunity
(PTI). Plants are capable of a pathogen molecular pattern recognition to prevent their further
infection.Nevertheless, plant pathogens have developed a mechanism to overturn PTI. They
produce effectors and insert them inside the plant cell. At this point, the hypersensitive response
(HR) may occur and block the harmful pathogen mainly by cell apoptosis.
Once the plant reacts to the pathogen, signals are released that trigger resistance in adjacent cell
as well as distant tissue (Ryals et al., 1994). This system is called Systemic Acquired Resistance
(SAR) and salicylic acid is the main signalling molecule involved. A later reaction named
Induced Systemic Resistance also occurs and is triggered by beneficial microorganisms with
jasmonic acid and ethylene as the main signalling hormones. ethylene, and SA. The better
understanding of plant signaling pathways has led to the discovery of natural and synthetic
compounds called elicitors that induce similar defense responses in plants as induced by the
pathogen infection (Meenakshi and Baldev, 2013).
Following elicitor perception, the activation of signal transduction pathways generally lead to
the production of active oxygen species (AOS), phytoalexin biosynthesis, reinforcement of plant
cell wall associated with phenyl propanoid compounds, deposition of callose, synthesis of
defense enzymes, and the accumulation of pathogenesisrelated (PR) proteins, some of which
possess antimicrobial properties. AOS lead to hypersensitive response (HR) in plants which is a
localized or rapid death of one or few cells at the infection site to delimit the pathogen growth.
Following the activation of HR, uninfected distal parts of the plant may develop resistance to
further infection, by a phenomenon known as systemic acquired resistance (SAR), which is
effective against diverse pathogens, including viruses, bacteria, and fungi (Meenakshi and
Baldev, 2013).
Plants elaborate a vast array of natural products, many of which have evolved to confer selective
advantage against microbial attack. Recent advances in molecular technology, aided by the
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enormous power of large-scale genomics initiatives, are leading to a more complete
understanding of the enzymatic machinery that underlies the often complex pathways of plant
natural product biosynthesis. Meanwhile, genetic and reverse genetic approaches are providing
evidence for the importance of natural products in host defence. Metabolic engineering of natural
product pathways is now a feasible strategy for enhancement of plant disease resistance(R.A.
Dixon,2001). Some other alternative control strategies of currently emerging plant diseases are
based on the use of resistance inducers that induce resistance by a priming mechanism.
In contrast to foliar diseases, relatively little is known about the nature of root defenses against
the soil borne pathogens. In addition to pathogens, plant roots interact with a plethora of nonpathogenic and symbiotic microorganisms. Therefore, a good understanding of how plant roots
interact with the microbiome would be particularly important to engineer resistance to root
pathogens without negatively altering root-beneficial microbe interactions.
II UNDERSTANDING PLANT IMMUNITY
2.1 Distinct uses of an integrated signaling network rather than distinct
networks
Plant immunity is controlled by a complex signaling network. Pattern-triggered immunity (PTI)
and effector-triggered immunity(ETI) are two modes of plant immunity defined by the types of
molecules recognized by plants as indicators of a pathogen attack (Jones and Dangl 2006).
Recognition of molecular patterns characteristic of microbes (microbe-associated molecular
patterns [MAMPs]) by plant pattern-recognition receptors (PRR), which are typically localized
in the plant cell membrane, triggers PTI (Boller and Felix 2009). Recognition of pathogen
effectors by plant resistance (R) proteins, which are typically localized inside the plant cell,
triggers ETI (Jones and Dangl 2006). Although the characteristics of immunity associated with
PTI and ETI are different, many molecular responses are associated with both PTI and ETI
(Abramovitch et al. 2006; Navarro et al. 2004; Tao et al. 2003; Zipfel et al. 2006). The latter fact
suggests overlap in the signaling machinery betweenPTI and ETI.
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PTI and ETI are complementary.
Molecular patterns that trigger PTI are conserved among pathogens and benign microbes (Boller
and Felix 2009). If strong immune responses were activated rapidly without distinguishing
pathogens from benign microbes, the responses would be excessive in many cases and carry
fitness costs. Therefore, PTI responses have likely been selected for weakness in early stages due
to the low specificity in pathogen recognition (Fig. 1A). This possible requirement for a slow
start suggests that enhancing PTI in crop plants in a nonspecific manner may not be beneficial.
Under experimental conditions, PTI-enhanced plants may only be tested with pathogens,
revealing enhanced immunity without much fitness cost. However, in the field where many
benign microbes are present, the plants may suffer substantial fitness costs. In addition to the
possible requirement for a slow start in PTI, plants have a way to evaluate the effectiveness of
early PTI responses (Fig. 1A). For example, continuous or increasing MAMP signaling may be
interpreted as an inadequacy of early responses. If the early responses are sufficient, plants can
abort further, unnecessary immune responses. If the early responses are not sufficient, plants may
use the four-sector network to amplify the signal for stronger responses in a later stage. This
signal amplification likely involves positive feedback loops involving multiple sectors (Shah
2003); hence, synergistic interactions among the sectors were observed with PTI (Tsuda et al.
2009). Two notions about PTI, the requirement for a slow start and the late-stage synergistic
sector interactions, can explain why PTI is vulnerable to attack by adapted pathogens (Fig. 1B).
A slow start of PTI can give adapted pathogens opportunities to effectively interfere with PTI
signaling: initial low concentrations of effectors can easily suppress weak PTI signaling. The late
signaling sectors are vulnerable to effector perturbations because the synergistic interactions
among the sectors can be disrupted by inactivation of one of the sectors. In contrast to MAMPs,
effectors are hallmarks of potent pathogens, not benign microbes; therefore, plants have likely
been selected to activate strong immune responses in ETI immediately after recognition by an R
protein of a very low concentration of the corresponding effector (Fig. 1C). Because plants
rapidly generate strong signals, it is difficult for a low concentration of effectors to interfere with
ETI signaling in phase I. Because the signal is already strong, the signal does not need to be
amplified in phase II. Instead, the phase II signaling sectors, which highly overlap with the late
stage sectors in PTI, can be used for network compensation.
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2.2 Microbial patterns and plant pattern recognition:
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In this scheme, the ultimate amplitude of disease resistance or susceptibility is proportional to
[PTI – ETS1ETI]. In phase 1, plants detect microbial/pathogen-associated molecular patterns
(MAMPs/ PAMPs, red diamonds) via PRRs to trigger PAMP-triggered immunity (PTI). In phase
2, successful pathogens deliver effectors that interfere with PTI, or otherwise enable pathogen
nutrition and dispersal, resulting in effector-triggered susceptibility (ETS). In phase 3, one
effector (indicated in red) is recognized by an NB-LRR protein, activating effector-triggered
immunity (ETI), an amplified version of PTI that often passes a threshold for induction of
hypersensitive cell death (HR). In phase 4, pathogen isolates are selected that have lost the red
effector, and perhaps gained new effectors through horizontal gene flow (in blue)—these can
help pathogens to suppress ETI. Selection favours new plant NB-LRR alleles that can recognize
one of the newly acquired effectors, resulting again in ETI.
2.3How do plants achieve immunity?
Plant pathogens have developed a mechanism to overturn PTI. They produce effectors and
insert them inside the plant cell. At this point, the hypersensitive response (HR) may occur and
block the harmful pathogen mainly by cell apoptosis.Once the plant reacts to the pathogen,
signals are released that trigger resistance in adjacent cell as well as distant tissue (Ryals et al.,
1994). This system is called Systemic Acquired Resistance (SAR) and salicylic acid is the main
signalling molecule involved. A later reaction named Induced Systemic Resistance also occurs
and is triggered by beneficial microorganisms with jasmonic acid and ethylene as the main
signalling hormones . When a pathogen infects a plant, infected and non-infected tissues both
product SA. But its production and its mechanisms of action are lightly different. First, in the
infected plant part, pathogenactivated signal indirectly generates a necrosis which indirectly
activates EDS1. Then, a cascade involving gene/protein interactions stimulates SA production.
However, in noninfected tissues, the necrosis generates another cascade. Then, in infected
tissues, SA activates PR gene expression by two pathways. In the first one, SA activates NPR1
gene which triggers the production of TGA factors. The second pathway involves Ethylene
and/or Jasmonic acid. In non-infected tissues, the second pathway is absent. SA can also control
its own production by regulating upstream genes. NPR1 also repress the gene/protein cascade.
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2.3.1 Mechanisms of Salicylic Acid in plants and its self-regulating/down-regulating
When a pathogen infects a plant, infected and non-infected tissues both product SA. But its
production and its mechanisms of action are lightly different. First, in the infected plant part,
pathogenactivated signal indirectly generates a necrosis which indirectly activates EDS1. Then, a
cascade involving gene/protein interactions stimulates SA production. However, in noninfected
tissues, the necrosis generates another cascade. Then, in infected tissues, SA activates PR gene
expression by two pathways. In the first one, SA activates NPR1 gene which triggers the
production of TGA factors. The second pathway involves Ethylene and/or Jasmonic acid. In noninfected tissues, the second pathway is absent. SA can also control its own production by
regulating upstream genes. NPR1 also repress the gene/protein cascade.
2.4Plant Natural Products and Plant disease resistance
plants produce a remarkably diverse array of over 100,000 low-molecular-mass natural products,
also known as secondary metabolites. Secondary metabolites are distinct from the components of
intermediary (primary) metabolism in that they are generally nonessential for the basic metabolic
processes of the plant. Most are derived from the isoprenoid, phenylpropanoid, alkaloid or fatty
acid/polyketide pathways (Fig. 1). This rich diversity results in part from an evolutionary process
driven by selection for acquisition of improved defence against microbial attack or insect/animal
predation. However, such diversity has made it difficult to apply conventional molecular and
genetic techniques to address the functions of natural products in plant defence, or to improve
plant disease resistance using metabolic pathway engineering. Related plant families generally
make use of related chemical structures for defence (for example, isoflavonoids in the
Leguminosae, sesquiterpenes in the Solanaceae), although some chemical classes are used for
defensive functions across taxa (for example, phenylpropanoid derivatives). Some species
produce a broad range of antimicrobial compounds. For example, cocoa, when infected by the
vascular wilt fungus Verticillium dahliae, accumulates the pentacyclic triterpene arjunolic acid,
two hydroxylated acetophenones and, most unusually, elemental sulphur , the only known
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inorganic antimicrobial agent produced by plants1. Most antimicrobial plant natural products
have relatively broadspectrum activity, and specificity is often determined by whether or not a
pathogen has the enzymatic machinery to detoxify a particular host product2. Accumulation of
inducible antimicrobial compounds is often orchestrated through signal-transduction pathways
linked to perception of the pathogen by receptors encoded by host resistance genes.
2.5Phytoalexin and Phytoanticipin
The simplest functional definitions recognize phytoalexins as compounds that are synthesized de
novo (as opposed to being released by, for example, hydrolytic activity) and phytoanticipins as
pre-formed inflectional inhibitors10. However, the distinction between phytoalexin and
phytoanticipin is not always obvious. Some compounds may be phytoalexins in one species and
phytoanticipins in others. A good example is the methylated flavanone sakuranetin which
accumulates constitutively in leaf glands of blackcurrant, but which is a major inducible
antimicrobial metabolite in rice leaves11. In cases where a constitutive metabolite is produced in
larger amounts after infection, its status as a phytoalexin would depend on whether or not the
constitutive concentrations were sufficient to be antimicrobial. In vivo antimicrobial activity is
inherent in the definition of a phytoalexin or phytoanticipin, but it is this feature that has proven
most problematical to determine directly in the absence of methods to genetically modify the
host plant’s natural product profiles. In most cases, concentrations of phytoalexins have not been
measured specifically in the cells that are in direct contact with the invading microorganism. One
exception is a careful study of the cellular concentrations of sesquiterpenoid phytoalexins in
leaves of cotton varieties responding to the bacterial pathogen Xanthomonas campestris pv.
malvacearum, in which it was shown that phytoalexin levels in and around the challenged cells
were significantly higher than would be required to effectively inhibit the growth of the pathogen
in vitro.
2.6 Gene Silencing as defense responses against virus
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RNA silencing is a host defence mechanism targeted against invasive or mobile RNA elements,
such as viruses or transposable retro-elements, leading to sequence-specific RNA degradation. It
is highly conserved in plants and animals and known as post-transcriptional gene silencing
(PTGS) in plants. PTGS was discovered in plants, and a closely related phenomenon, RNA
interference, is known to occur in a wide range of organisms including Caenorhabditis elegans
and Neurospora crassa.( Chicas A,2001) This strategy is able to adapt to all sorts of viruses,
because its specificity is dictated by the sequence of the viral genome itself. Another feature of
PTGS is that a signal that triggers silencing is not restricted to individual plant cells, but can
spread from the site of infection, generating a response in more distant tissues. Silencing can be
triggered in plants by replicating viruses, double-stranded RNA molecules, and foreign genes
(transgenes) that allow the production of high levels of normal or aberrant messenger RNAs 9
Matzke MA,2001) which become dsRNA by host-encoded RNA-dependent RNA polymerase
activity. These dsRNAs are cleaved by Dicer-like enzymes into short interfering RNAs (siRNAs)
of between
21 and 26 nt in length, which then promote RNA degradation by forming a
multicomponentRNA- induced silencing complex that destroys cognatemRNA.
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III STRATEGIES FOR PLANT DISEASEMANAGEMENT
3.1Synthetic Plant Inducers
Salicylic acid (SA) has been characterized as a phytohormone when scientists discovered his role
in flowering induction on voodoo lily. Later on during the 70s, it was known that applying SA to
tobacco plants induces defense gene expression and enhances virus resistance. Still, SA’s role on
plant resistance was only demonstrated in 1990(Dempsey et al., 2017).
Salicylic acid (2 hydroxybenzoic acid, opposite chemical formula) is a phenolic compound
synthetized by plants. SA is a critical hormone involves in many aspects of plant growth,
development and disease resistance.
INA(2,6- dichloroisonicotinic acid) BTH(Acibenzolar-S-methyl): Those two molecules have
been studied and deeply tested to identify if they can be used as a plant activator.Phytotoxicity
has been revealed for INA molecule, even at very low dose, involving that those SA’s analogue
can not be used. However, acibenzolar-S-methyl (BTH) has been developed because it fulfils all
the criteria of a plant activator: it has no direct effect on pathogens and its mode of action is
distinctly different from any other conventional fungicide (Gullino et al., 2000). Acibenzolar-Smethyl is the main compound of a plant activator product that has been homologated. Due to his
mode of action, it has to be applied preventively. Moreover, a lag-time is required between the
application moment and the fully effective plant resistance. For most of the crops, this time
ranges between 3 and 7 days. The product is diluted in water and sprayed directly on the plant
leaves.Two products are available to immunize plants from fungal diseases and these are
Messenger from Eden Bioscience and Elexa from Safer science and are considered as low risk
biochemical pesticides. Sphingolipids play a key role in plant defense towards different lifestyle
pathogens by modulating cell death, ROS accumulation and jasmonate signaling pathway.
Sphingolipids are emerging as second messengers in programmed cell death and plant defense
mechanisms. However, their role in plant defense is far from being understood, especially
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against necrotrophic pathogens. Sphingolipidomics and plant defense responses during
pathogenic infection were evaluated in the mutant of long-chain base phosphate (LCB-P) lyase,
encoded by the AtDPL1 gene and regulating LCB/LCB-P homeostasis. Atdpl1 mutants exhibit
tolerance to the necrotrophic fungus Botrytis cinerea but susceptibility to the hemibiotrophic
bacterium Pseudomonas syringae pv. tomato (Pst)(Magnin-Robert Maryline1 et al. 2016)
3.2Systemic Acquired Resistance (SAR) and it’s Application in Crop Plants Improvement
to Biotic Stresses:
3.2.1 Application of SAR on Fababean
Exogenous applications of salicylic acid (SA) and benzothiadiazole (BTH) solutions have been
used in fababean to induce systemic acquired resistance (SAR) to rust (Uromyces viciaefabae),
ascochyta blight (Ascochyta fabae) and broomrape (Orobanche crenata). Both SA and BTH
solutions were effective inducing SAR to U. viciae-fabae and A. fabae on susceptible accessions
under controlled conditions, although SA was less effective than BTH for A. fabae. BTH
treatments reduced the infection of all pathogens studied under field conditions in susceptible
accessions, and rust infection was also reduced by SA applications. Moderately resistant
accessions became immune to ascochyta blight with BTH treatment, and showed a lower degree
of infection to rust after SA or BTH treatments. No effect was observed in the highly resistant
accessions. Chemical induction of systemic resistance may provide an additional method for
controlling fababean diseases to be considered in an integrated diseases management (Sillero et
al., 2012).
3.2.2 Application of SAR on Chickpea (Cicer arietinum L.)
Fungal diseases are the most important biotic limiting the growth of Chickpea (Cicer
arietinum L.). Salycilic acid application is known as a plant hormone that has the role of
signal in responses of defense, whose the acquired systemic resistance. The study was aimed to
evaluate the affectivity of some concentrations of Salicylic acid (SA) against the
phytopathogenic fungus (Fusarium roseum) on two chickpea genotypes (ILC 3279 and FLIP
8555). Results showed that the inhibitory effect of (SA) on the development of Fusarium roseum
increased linearly with increasing the concentration. The colony diameter reduced significantly
at 200, 250 mg/ l. Additionally, the results showed that the different treatment of (SA) were
effective in reducing the disease infection and it could advice to utilize the Salicylic Acid as
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stimulating agent to decrease the degree of infection by Fusarium diseases by immersing the
seeds of Chickpea in the mentioned concentration for 24 hours before the planting. Also, it could
sprinkle the plantlets before a sufficient period of the infection by pathogens. This can
significantly contribute in limiting the appearance and development of diseases (Noura
et al., 2016).
3.2.3 Priming of plant resistance by natural compounds
Priming is a mechanism which leads to a physiological state that enables plants to respond more
rapidly and/or more robustly after exposure to biotic or abiotic stressThe “primed” state has been
related to increased, more efficient activation of the defense response and enhanced resistance to
challenging stress (Conrath, 2009). depending on the situation. Over the years, a range of
chemical treatments has proven capable of triggering IR, mostly through the priming mechanism.
The first to be identified were synthetic SA analogs, such as 2,6-dichloroisonicotinic acid and its
methyl ester (both referred to as INA), and benzo (1,2,3)thiadiazole-7-carbothioic acid Smethyl
ester (BTH), which triggers SAR (Oostendorp et al., 2001; Conrath et al., 2002). A wide range of
cellular responses has been reported to be potentiated by these compounds, including alterations
in ion transport across the plasma membrane, synthesis and secretion of antimicrobial secondary
metabolites (phytoalexins), cell wall phenolics and lignin-like polymers, and activation of
various defense genes (Conrath, 2009). Non-protein amino acid β-aminobutyric acid (BABA)
has received plenty of attention given its versatility, and its priming for different defense
responses dependent on distinct hormones pathways and upon different challenging stresses
(Conrath, 2009). This is remarkable because synthetic chemicals tend to prime SA-dependent
immunity, as illustrated by the identification of priming-active compounds called imprimatins in
synthetic library screening (Noutoshi et al., 2012). to priming, including oligosaccharides,
glycosides, amides, vitamins, carboxylic acids, and aromatic compounds. In general, natural
compounds tend to be better tolerated by plants than most of the synthetic compounds tested, but
there is still concern about toxicity (Iriti et al., 2010; Noutoshi et al., 2012).
Many natural compounds have been claimed to be plant growth promoters, plant activators or
plant defense inducers, among other names.Recently, it has been demonstrated that thiamine can
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modulate the cellular redox status to protectArabidopsis against Sclerotinia sclerotiorum at early
stages of infection (Zhou et al., 2013). Early in the pathogenesis, thiamine can effectively
alleviate the inhibition of host reactive oxygen species (ROS) generation by Sclerotinia-secreted
oxalate.
Thiamine can also induce cell wall fortifications with callose/lignin to prevent oxalate
diffusion. Further reports in other plants are consistent with the central role of ROS, particularly
H2O2 in vitamin-IR.
The exogenous application of riboflavin primed bean, but not tomato plants, The exogenous
application of riboflavin primed bean, but not tomato plants, accelerates H2O2 generation after
Botrytis cinerea infection. H2O2 is a signaling molecule involved in cell wall modification, gene
expression regulation and cross-talk with various defense pathways (Azami-Sardooei et al.,
2010). Riboflavin-IR also correlates with JA-dependent pathway activation by priming for
enhanced lipoxygenase (LOX) activity.
Phenolics play a role in cell wall fortification, and also show antimicrobial and antioxidant
activity (Taheri and Tarighi, 2010, 2011).
Para-aminobenzoic acid (PABA) is a cyclic amino acid belonging to the vitamin B group. Field
experiments have proven that it is capable of enhancing resistance against Cucumber mosaic
virus and Xanthomonas axonopodis by inducing SAR, while simultaneously improving plant
yield (Song et al., 2013). This contrasts with BTH which, in the same study, reduced disease
severity, but produced shoot length shortening and significant fruit weight reduction when
compared to PABA and control treatments.
Menadione sodium bisulfite (MSB) is a vitamin K3 derivative known to be a growth regulator
(Rama Rao et al., 1985). Borges et al. (2003a) found that MSB protects rape plants (Brassica
napus) from the fungus Leptosphaeria maculans by stimulating ROS production, but without
inducing PR1. Borges et al. (2003b, 2004) also demonstrated that MSB protects banana from
Panama disease caused by Fusarium oxysporum and that MSB primes phytoalexin accumulation.
Chitosan is a polymeric deacetylated derivative of chitin that is naturally present in some fungi
cell walls, and has various deacetylation degrees and molecular weights. Although it performs
several antimicrobial activities, its main contribution to reduce plant disease is to enhance plant
defenses (El Hadrami et al., 2010). Chitosan has also been reported to improve growth and yield
(Reddy et al., 1999; Kim et al., 2005; Cho et al., 2008). It is a potent general elicitor of proven
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efficiency in a wide range of experiments with different host plants and pathogens (Iriti et al.,
2010). Iriti and Faoro (2009) pointed out that chitosan can directly activate systemic resistance or
can prime the plant for a more efficient defense response upon challenge, depending on dose, by
considering the different cytotoxicity thresholds for each chitosan derivative and plant. The
diverse mechanisms of action of chitosan have been studied, which include oxygen-species
scavenging and antioxidant activities, as well as octadecanoid pathway activation (reviewed in El
Hadrami et al., 2010). Despite these studies however, experiments which specifically address the
role of priming in the complex chitosan-plant interaction framework are still scarce.
Oligogalacturonides (OGs) are plant cell wall pectin-derived oligosaccharides which consist in
linear chains of α-(1-4)-linked D-galacturonic acid with a degree of polymerization between 10
and 25, which can be methyl-esterified or acetylated depending on the source plant. They are
considered endogenous elicitors, and the degree of methylation and acetylation has been found to
affect the activation of defense responses (Osorio et al., 2008; Randoux et al., 2010). OG
treatment has been reported to induce a range of defense responses, like accumulation of
phytoalexins, β-1,3- glucanase and chitinase, or generation of ROS by triggering nitric oxide
(NO) production (Rasul et al., 2012). Exogenous treatments with OGs protect grapevine leaves
against necrotrophic pathogen Botrytis cinerea infection in a dose-dependent manner (Aziz et al.,
2004). In Arabidopsis, OGs increase resistance to Botrytis cinerea independently of JA-, SA-,
and ethylene (ET)-mediated signaling. Azelaic acid (AA) has been suggested to be a phloemmobile signal that primes SA-induced defenses (Jung et al., 2009; Shah, 2009).
3.3Genetic approaches to natural product function
One approach for addressing phytoalexin or phytoanticipin function in vivo has been to take
advantage of the power of gene-knockout technology in microbes to disrupt pathogen genes that
encode enzymes known to detoxify the host plant’s antimicrobial compounds. For example,
saponins are widely occurring, constitutively expressed, glycosylated steroids, steroidal alkaloids
or triterpenes, many of which have antimicrobial activity in vitro. Mutants of the fungal pathogen
of oat roots, Gaeumannomyces graminis, that had lost the enzyme avenacinase were no longer
able to detoxify the triterpene saponin avenacin; they lost pathogenicity on oats, but retained full
pathogenicity on wheat, which does not produce saponins13. This indicates that avenacin, which
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is localized to the epidermal cells that would represent one of the first barriers to infection by G.
graminis, is a key determinant of resistance in this particular host–pathogen system. Likewise,
disruption of the MAK1gene in the fungal pathogen Nectria haematococcaleads to inability to
detoxify the pterocarpan phytoalexin maackiain and reduced virulence of the fungus on
chickpea. In this case the effect was incomplete, and host factors in addition to maackiain are
therefore involved in resistance.
Targeted transgenic approaches allow for evaluation of effects of directly altered phytoalexin
profiles, and such approaches have been undertaken in the case of stilbenoids and isoflavonoids.
Introduction of a novel phytoalexin, resveratrol, into alfalfa by constitutive expression of a
grapevine stilbene-synthase gene resulted in reduced symptoms following infection by the leaf
spot pathogen Phoma medicaginis21 (Fig. 2a, b). Constitutive overexpression of isoflavone Omethyltransferase (IOMT) in transgenic alfalfa resulted in more rapid and increased production
of the pterocarpan phytoalexin medicarpin
after infection by P. medicaginis, resulting in
amelioration of symptoms22 . Taken together, the results of genetic analyses indicate that
phytoalexins and phytoanticipins can indeed be effective in contributing to resistance in vivo, but
more than one individual compound or class of compound may be necessary to impart resistance,
which is consistent with the multi-component nature of plant defence responses. Clearly, the
diversity of plant natural products and host–pathogen combinations makes it impossible to make
any general conclusions in regard to the multitude of systems not yet analysed.
There have recently been a number of successful applications of genetic and genomic approaches
to identify genes of plant natural product biosynthesis. For example, comparative EST database
mining was used to identify the 2-HIS cytochrome P450 that catalyses the entry-point reaction
into isoflavonoid phytoalexin biosynthesis23,24, based on the predicted expression of this
enzyme for production of isoflavonoids in developing seeds and elicited cell cultures. Mass
sequencing of complementary DNA libraries corresponding to metabolically specialized cells
has been used to identify several of the genes of monoterpene biosynthesis and the associated
Rhomer pathway for formation of the isoprenoid precursor isopentenyl diphosphate in mint
glandular trichomes25,26. Tagging with the Mutator transposable element was used to map and
subsequently clone the first enzyme specific for DIMBOA biosynthesis in maize15 . Virtually all
the enzymes of flavonoid and isoflavonoid biosynthesis the early steps of mono-, sesquiand
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diterpene biosynthesis29, and the formation of DIMBOA15 have now been isolated and
functionally characterized presents an overview of these and other pathways leading to
phytoalexins or phytoanticipins, indicating how they are inter-related through primary
metabolism, and highlighting some of the genes that have been cloned so far. Although
information derived from large-scale genomics initiatives is invaluable, it can be potentially
difficult to mine these data to construct whole pathways from gene sequence information alone.
With few exceptions (for example, the BX1 indole-3-glycerol phosphate lyase and four
subsequent cytochrome P450 enzymes (BX2–BX5) of DIMBOA biosynthesis, genes encoding
enzymes in natural product pathways are not closely linked in plants, making genetic
characterization of pathways more difficult than in bacteria or fungi. However, bioinformatic
tools are available for identifying genes encoding members of specific classes of enzymes, such
as cytochrome P450s, O-methyltransferases (OMTs), terpene cyclases and polyketide synthases
from large EST projects. In addition, knowledge of transcript expression patterns (for example,
upregulated after pathogen attack) from DNA-microarray or more conventional membranehybridization approaches can sufficiently be helpful for combating the problem.
3.4 Post-transcriptional gene silencing in controlling Viruses of various economic crops
RNA silencing is a host defence mechanism targeted against invasive or mobile RNA elements,
such as viruses or transposable retro-elements, leading to sequence-specific RNA degradation.
This strategy is able to adapt to all sorts of viruses, because its specificity is dictated by the
sequence of the viral genome itself. Another feature of PTGS is that a signal that triggers
silencing is not restricted to individual plant cells, but can spread from the site of infection,
generating a response in more distant tissues .
Silencing can be triggered in plants by replicating viruses, double-stranded RNA molecules, and
foreign genes (transgenes) that allow the production of high levels of normal or aberrant
messenger RNAs [Meins F Jr (2000)], which become dsRNA by host-encoded RNA-dependent
RNA polymerase activity. These dsRNAs are cleaved by Dicer-like enzymes into short
interfering RNAs (siRNAs) of between 21 and 26 nt in length, which then promote RNA
degradation by forming a multicomponentRNA- induced silencing complex that destroys
cognatemRNA [Waterhouse PM,(2001)]. A PTGS-based strategy to control gemeniviruses was
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demonstrated when tobacco and tomato plants were transfected with dsRNA derived from the
AC1 gene ofAfrican cassava mosaic virus and transgenes developed from the intergenic region
(IR) and the Rep gene of TYLCV plants were highly resistant to cotton leaf curl virus and
TYLCV, respectively, [Yang Y(2004)].
IV LIMITATIONS:
•
The diversity and apparent complexity of biosynthetic pathways of plant natural products
are
seen as a barrier to progress in advancing the understanding of phytoalexin
function, and in developing technologies to improve resistance by pathway engineering.
•
Many crop plants are susceptible to pathogens because of years of selective breeding
leading to removal of natural products found in their more resistant.
•
The large numbers of genes that may have to be transferred, and coordinately regulated,
to introduce effective antimicrobial activity
•
SA synthetic analogue products may be useless against HR and SAR suppressive plant
pathogen as some microorganisms have mechanisms that block the HR, no matter the SA
concentration in the plant.
•
Abiotic and biotic factors, as well as the physiological state of the plant, affect the
efficiency of the SA synthetic analogues products.
•
The expensive cost of SA synthetic-analogue products may prevent their use at a large
scale.
•
Interactions between SA and other plant hormones (JA mainly) in the HR/SAR
establishment are complex to understand.
•
efficient against every kind of pathogens because they are designed to enhance the global
plant resistance.
•
Molecular basis of priming has recently started to be unraveled, but is still poorly
understood.
•
The major challenges for plant molecular biologists in RNAi research are (1) how to
select silencing targets for a particular disease and (2) how to efficiently deliver
siRNAs into specific cell types in vivo.
Understanding plant immunity and approaches in plant disease resistance
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V. Scope and Future Directions:
•
Metabolic engineering of natural product pathways is now a feasible strategy for
enhancement of plant disease resistance.
•
Transferred DNA-(T-DNA) activation tagging has recently been applied to the
characterization of transcriptional regulators of natural product pathways
and this
approach, by which regulatory genes can be identified without the need to understand the
individual enzymatic steps of the pathway, offers an exciting opportunity to develop new
molecular tools for pathway engineering for improved plant defense.
•
Constitutive strategy for PR gene expression-the expression pattern of Pathogenesis
related genes can be considered as markers for salicylic acid (SA)- dependent SAR.
•
Economically feasible SA analogues and Environmentally low risk compounds can
serve as a scope to overcome the possibilities of pathogen suppresion,environmental
hazards and high cost of chemicals. .
•
The study into natural plant inducers has helped to unravel the complex mechanisms
underlying the IR phenomenon
•
RNAi: a revolution in the field of plant molecular genetics that it has enormous potential
for engineering control of gene expression, as well as for the use of a tool in functional
genomics.
•
Beneficial Microbes be a potential source of research in triggering Induced resistance.
•
Bioinformatics analysis of large-scale plant genomic and expressed sequence tag (EST)
databases3 is beginning to reveal how new enzymes of natural product biosynthesis may
have arisen through processes of gene duplication and mutation. This provides the
genetic variation that leads to continued elaboration of new chemical structures that will
be selected for if they impart a significant advantage in plant defense.
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VI. CONCLUSION
•
Recent advances in molecular technology, genetic and reverse genetic approaches are
providing evidence for the understanding complex importance of natural products in host
defense,thus unraveling the complex interaction of plant immunization is been made
possible by applying the knowledge of quantitative genetic variation.
•
Studies into natural plant inducers as priming agents has helped to unravel the complex
mechanisms underlying the IR phenomenon.
•
With the isolation of some genes like NPR1/NIM,WIPK and PIN2, immense
opportunities has been unfolded how these gene products function and gene families
evolve.
•
Salicylic acid as the key molecule in SAR has encouraged scientist to produce various
synthetic analogues and natural plant products for triggering SAR. But there are some
limitations regarding its application and it interactions with other plant hormones.
•
RNA-silencing-based approaches is a very efficient knockdown technology in plants
against plant pathogens.RNA silencing is an area of intense investigation that is leading
to exciting new discoveries in the fields of control of gene expression, development and
host defense.
Understanding plant immunity and approaches in plant disease resistance
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I.INTRODUCTION
1-2
II.UNDERSTANDINGPLANT IMMUNITY
2.1 Distinct uses of an integrated signaling network
rather than distinct networks
2-3
2.2 Microbial patterns and plant pattern recognition:
3-4
2.3How do plants achieve immunity?
4-6
2.4Plant Natural Products and Plant disease resistance
6-7
2.5Phytoalexin and Phytoanticipin
7
7-8
2.6 Gene Silencing as defense responses against virus
IIISTRATEGIES FOR PLANT DISEASEMANAGEMENT
3.1Synthetic Plant Inducers
9-10
3.2Systemic Acquired Resistance (SAR) and it’s
Application in Crop Plants Improvement to Biotic
Stresses:
10-11
3.3Genetic approaches to natural product function
11-13
3.4 Post-transcriptional gene silencing in controlling
Viruses of various economic crops
13-15
IV LIMITATIONS:
16
V. Scope and Future Directions:
17
VI. CONCLUSION
18
Understanding plant immunity and approaches in plant disease resistance
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Understanding plant immunity and approaches in plant disease resistance
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