Role of Elicitors in Inducing Resistance in Plants against Pathogen Infection: A Review

Disease control is largely based on the use of fungicides, bactericides, and insecticides—chemical compounds toxic to plant invaders, causative agents, or vectors of plant diseases. However, the hazardous effect of these chemicals or their degradation products on the environment and human health strongly necessitates the search for new, harmless means of disease control. There must be some natural phenomenon of induced resistance to protect plants from disease. Elicitors are compounds, which activate chemical defense in plants. Various biosynthetic pathways are activated in treated plants depending on the compound used. Commonly tested chemical elicitors are salicylic acid, methyl salicylate, benzothiadiazole, benzoic acid, chitosan, and so forth which affect production of phenolic compounds and activation of various defense-related enzymes in plants. Their introduction into agricultural practice could minimize the scope of chemical control, thus contributing to the development of sustainable agriculture. This paper chiefly highlights the uses of elicitors aiming to draw sufficient attention of researchers to the frontier research needed in this context.


Introduction
Plants are challenged by a variety of biotic stresses like fungal, bacterial, or viral infections. is lead to a great loss to plant yield. ere are various options available for the farmers to protect their crop from the disease. Some options include development of resistant cultivars, biological control, crop rotation, tillage, and chemical pesticides. Nearly all chemical pesticides or fungicides have a direct antibiotic principle. But their use at commercial level is uneconomical, application is cumbersome, and some are proved to be carcinogenic. erefore, considerable efforts have been accomplished to devise environmental-friendly strategies for the check of plant diseases and thus to save mankind from health hazard [1].
Plants can activate separate defense pathways depending on the type of pathogen encountered [2]. Jasmonic acid (JA) and ethylene dependent responses seem to be initiated by necrotrophs, whereas salicylic acid (SA) dependent response is activated by biotrophic pathogens. e mechanisms responsible for this differential recognition and response may involve crosstalk among these three different signal transduction pathways: JA, ethylene, and SA. e better understanding of plant signalling 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 [3]. Different types of elicitors have been characterized, including carbohydrate polymers, lipids, glycopeptides, and glycoproteins. In plants, a complex array of defense response is induced aer detection of microorganism via recognition of elicitor molecules released during plant-pathogen interaction. 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 pathogenesis-related (PR) proteins, some of which possess antimicrobial properties [4]. AOS lead to hypersensitive response (HR) [5] 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 [6].

Host Pathogen Interaction
Resistance in plant species is oen divided into host-or nonhost-speci�c resistance. Host-speci�c resistance involves interactions between speci�c host and pathogen genotypes, which give a pathogen race-speci�c resistance. Nonhost resistance, shown by a whole plant species against a speci�c parasite or pathogen, is the most common form of resistance in plants towards the majority of potential pathogens [7]. e biochemical changes that occur during infection are very similar in host and nonhost resistant plants [8]. Disease spreads only in susceptible plants (compatible interactions) which are unable to recognize the pathogen or respond too slowly [2].
e hypersensitive response is triggered by the plant when it recognizes a pathogen. e identi�cation of a pathogen typically occurs when avirulence (Avr) gene products, secreted by pathogen, bind to or indirectly interact with the product of a plant resistance (R) gene (gene for gene model). When both the R gene and corresponding Avr genes are present, recognition occur, which lead to active resistance of the plant and avirulence of the pathogen. If either Avr gene in the pathogen or R gene in the host is absent or is mutated, no recognition will occur and outcome will be a compatible reaction and disease [9]. As a result of putative binding of these two partners, a signal transduction cascade is activated and lead to the activation of a variety of plant defense responses. e defense responses are associated with restriction of pathogen growth. R gene products are highly polymorphic and many plants produce several different types of R gene products, enabling them to act as a receptor of Avr proteins produced by many different pathogens [7].

Hypersensitive Response (HR)
. Direct physiological contact between the host and infecting parasite is obviously necessary for the activation of HR. e HR was �rst described by Stakman [10] to describe rapid host cell death in resistant wheat plants upon infection by rust fungi. Hypersensitivity is a rapidly developing defense reaction induced in incompatible host by a plant pathogen, which results in the death of a limited number of host cells and a concomitant localization of the pathogen. Some investigators have described the HR as resembling the process of apoptosis, the principal manifestation of programmed cell death in many animal cell types [11]. is de�nition has now expanded to include defense gene expression in addition to cell death [7]. e HR is analogous to the innate immune response found in animals. HR provides resistance to biotrophic pathogens that obtain their energy from living cells [12].

Generation of Reactive Oxygen Species (ROS)
. e �rst report on the rapid generation of ROS during plantpathogen interactions was by Doke [13] in Phytophthora infestans-potato interaction. In studies involving bacteria and cell suspensions in the incompatible interaction, there are two phases of ROS production, termed as "oxidative burst". Phase 1 is rapid, transient, and nonspeci�c, whereas phase 2 occurs later and yields a much higher concentration of ROS [14]. is speci�c, biphasic response is proposed to be an important component of plant defense [15] because in compatible interactions only the �rst phase is induced [16]. e two distinct phases of the oxidative burst are seen only when an R gene and an Avr gene are both present, for example, with transgenic tomato plants differing only in the presence or absence of the R gene, Pto, and the bacterial pathogen, Pseudomonas syringae pv. tomato, with the avr gene, avrPto. is con�rms that the second phase of the oxidative burst is associated with disease resistance [17]. e earlier defense responses are the opening of speci�c ion channels across the plasma membranes, the rapid production of AOS, such as O 2 − and H 2 O 2 , known as the oxidative burst or phosphorylation or dephosphorylation of speci�c proteins [18]. ese initial reactions are the prerequisite for initiation of the signalling network that will trigger the overall defense response [19].

Sources of ROS.
ROS are toxic intermediates that are generated through the sequential one electron reduction steps of molecular oxygen [20]. Various enzyme systems have been proposed as the source of ROS in plants. An NADPH oxidase system similar to that of mammalian systems or a pH-dependent cell wall peroxidase may be two sources of oxidative burst [21]. If NADPH oxidase activity is a ROS generating system, O 2 − should be the initial product produced, however the O 2 − generated is usually rapidly dismutated to H 2 O 2 via SOD. erefore, in most systems H 2 O 2 appears to be the major ROS that accumulates. Under physiological conditions, the �rst reduction of O 2 forms the superoxide anion (O 2 − ) and hydroperoxyl radical (HO 2 • ), the second step forms hydrogen peroxide (H 2 O 2 ), and the third step produces hydroxyl radical (OH • ). OH • and O 2 − possess very short half lives. Uncharged H 2 O 2 is more stable, whereas OH • cannot migrate in solution and instead reacts locally, notably with molecular targets by modifying their structure and activity. H 2 O 2 as well as OH • can react with polyunsaturated lipids in membranes forming lipid peroxides, which can lead to biological membrane destruction [22].

Role of ROS in Plant Disease
Resistance. ROS species such as O 2 − , OH • , and H 2 O 2 are commonly produced under stress conditions and are strong oxidizing species that can rapidly attack all types of biomolecules and damage. For the protection from oxidative damage, plant cells contain both oxygen radical detoxifying enzymes such as catalase, peroxidase, and superoxide dismutase, and nonenzymatic antioxidants such as ascorbate peroxidase and glutathione-S-transferase [55]. ese enzymes play a crucial role in the protection of plant cells from oxidative damage at the sites of enhanced ROS generation [56]. e cooperative function of these antioxidants plays an important role in scavenging ROS and maintaining the physiological redox status of organisms [57].

Systemic Acquired Resistance (SAR).
Host plants can be protected against further pathogen attack if they have survived earlier infection by phytopathogenic viruses, bacteria, or fungi. It appears that the �rst infecting pathogen immunizes the plant against further infections by homologous pathogens, even though the plant may not carry gene determining cultivar-speci�c resistance. e readiness of the plant to repel subsequent pathogen attacks spread throughout the whole plant. is response is called systemic acquired resistance (SAR). e development of SAR is oen associated with various cellular defense responses, such as synthesis of PR proteins, phytoalexins and accumulation of AOS, rapid alterations in cell wall, and enhanced activity of various defense related enzymes [58].

Sequence of Events
Associated with the Establishment of SAR. e onset of SAR in noninfected plant organs is triggered by the phloem mobile signal which is released following pathogen infection. e signal travels throughout the plant and transduced in target tissues. Following signal transduction, resistance is maintained for several days and weeks and this is likely due to de novo gene expression. e biochemical changes that occur during SAR can be divided into two phases, that is, initiation and maintenance. Physiological changes during initiation phase may be transient and short lived, but during maintenance a quasisteady state should exist.

Elicitors and Their Mode of Action
Originally the term elicitor was used for molecules capable of inducing the production of phytoalexins, but it is now commonly used for compounds stimulating any type of plant defense [59][60][61]. Eventually, the induction of defense responses may lead to enhanced resistance. is broader de�nition of elicitors includes both substances of pathogen origin (exogenous elicitors) and compounds released from plants by the action of the pathogen (endogenous elicitors) [59,62]. Elicitors are classi�ed as physical or chemical, biotic or abiotic, and complex or de�ned depending on their origin and molecular structure (Table 1).
Elicitors may be divided into two groups, "general elici-tors� and "race speci�c elicitors�. �hile general elicitors are able to trigger defense both in host and nonhost plants, race speci�c elicitors induce defense responses leading to disease resistance only in speci�c host cultivars. A complementary pair of genes in a particular pathogen race and a host cultivar determines this cultivar speci�c (gene-for-gene) resistance. us, a race speci�c elicitor encoded by or produced by the action of an avirulence gene present in a particular race of a pathogen will elicit resistance only in a host plant variety carrying the corresponding resistance gene. e absence of either gene product will oen result in disease [19,[63][64][65][66][67]. In contrast, general elicitors signal the presence of potential pathogens to both host and nonhost plants [61]. e nonspeci�c nature of general elicitors is relative, however, and some of these are only recognized by a restricted number of plants [68].
Recent studies have indicated remarkable similarities between the defense mechanisms triggered by general elicitors and the innate immunity of animals, and it is tempting to speculate that the recognition of general elicitors subsequently leads to plant innate immunity [69]. Elicitors act as signal compounds at low concentrations, providing information for the plant to trigger defense, distinguishing elicitors from toxins, which may act only at higher concentrations and/or affect the plant detrimentally without active plant metabolism [62]. Elicitor signal transduction mechanism which activates plant primary immune response is shown in Figure 1.

Commercialization
Alternatives to fungicides in plant protection have arisen with the discovery of disease resistance inducers of biotic and abiotic origins that induce a localized or systemic resistance in susceptible plants, which become resistant to subsequent infections. Depending on their efficacy, these compounds can be used in �elds either alone or in combination with fungicides.
Many compounds have been commercially released in some countries as a plant health promoter of annual crops under the name Bion or Actigard [70]. e SA-dependent defense pathway can be activated by treatment of plants with chemical inducers such as benzo (1,2,3)-thiadiazole-7carbothioic acid-S-methyl ester (acibenzolar-S-methyl, ASM or BTH, Bion) developed as a potent SAR activators which do not only possess antimicrobial properties, but instead increase the crop resistance to diseases by activating SAR signal transduction pathways in several plant species. BTH is a chemical analogue of SA and has been used successfully to induce resistance to a wide range of diseases on �eld crops. e nonprotein amino acid -aminobutyric acid (BABA) protects numerous plants against various pathogens. Several products have also been used as inducers of resistance in plants against pathogens, including chitosan [71,72], salicylic acid analogues [24,73,74], living or processed fungal products [75], and seaweed extracts [76]. Certain synthetic compounds with no direct antimicrobial effect such as 2,6dichloroisonicotinic acid (INA) and potassium salts has been reported to induce SAR in plants [77]. Table 2 shows the list of various elicitors used and their effects on different plant species.

Conclusion
e use of elicitors in crop protection and pest management is still in the very early stages of use as a new control method, and thus the current experiences come from experimental trials, and not yet from large scale agricultural use. At least the following advantages of using elicitor treatments have been reported or can be expected: (1) reduced damage from insects, fungi, pests, and herbivores, (2) reduced environmental hazards as elicitors affect directly the crop plant, and their acute toxicity to other organisms is lower than that of pesticides, (3) as protective agrochemicals, elicitors can be applied with the current spraying technology, (4) elicitor treatments could be an alternative to genetically modi�ed (GM) plants for better attraction of natural enemies of pest organisms on cultivated plants [78], (5) elicitor-treated plants bear lower ecological risks than GM plants [79].

Brassica napus
Methyl jasmonate Accumulation of indolyl glucosinolates in the leaves. e predominant components of the response were 3-indolylmethyl-and 1-methoxy-3-indolylmethylglucosinolates, which together comprised 90% of the total glucosinolates in treated leaves. [23] 2

Oryza sativa
Benzothiadiazole BTH protected wheat systemically against powdery mildew infection by affecting multiple steps in the life cycle of the pathogen. e onset of resistance was accompanied by the induction of a number of wheat chemically induced (WCI) genes, including genes encoding a lipoxygenase and a sulfur-rich protein. [24] 3

Lycopersicon esculentum, Commelina communis
Oligogalacturonic acid (OGA) and chitosan ese elicitors reduced the size of the stomatal aperture. OGA not only inhibited light-induced stomatal opening, but also accelerated stomatal closing in both species; chitosan inhibited light-induced stomatal opening in tomato epidermis. [28] 7

Lycopersicon esculentum
Salicylic acid Induced the synthesis of some stress proteins, such as PR proteins, which leads to increased chilling tolerance and resistance to pathogens, thereby decreasing the incidence of decay. [30] 9

Lilium
Benzoic acid Modi�ed the growth, stress tolerance, anatomy and morphology of eatable and ornamental species. [31] 10 Programmed cell death in oat.

Parsley
Glycoprotein from Phytophthora sojae Synthesis of phytoalexin and activation of defense genes in parsley. [49] 34 Soybean

Syringolids from
Pseudomonas syringae Activation of hypersensitive response. [49] 35 Tobacco Fatty acid amino acid conjugates from Lepidopterans Synthesis of monoterpenes leading to activation of indirect defense in tobacco. [49] 36 Arabidopsis Bacterial toxin, for example, coronatine from Pseudomonas syringae Acivation of defense genes in Arabidopsis. [54] 37

Arabidopsis, tomato
Sphinganine analogue mycotoxins from Fusarium moniliforme Programmed cell death and activation of defense genes in Arabidopsis and tomato. [49]