Plant Hormones: Role in Alleviating Biotic Stress

Abstract

Plant hormones play a critical role in regulating plant developmental processes. Jasmonic acid, salicylic acid and brassinosteroids have been recently added to the list of plant hormones apart from auxins, gibberellins, cytokinins, abscisic acid and volatile hormone ethylene. Besides their regulatory role in plant development, plant hormones, ethylene, Jasmonic acid and salicylic acid play key roles in the plant defense response while as auxins, gibberellins, abscisic acid, cytokinins and brassinosteroids are known to modulate their effects. For an effective response to biotic stresses, the signaling pathways of different hormones are integrated at different levels enabling crosstalk between them. In this chapter, I will analyze how plant hormones signal defense response and interact with each other through crosstalk to regulate plant defense.

1. Introduction

Plant productivity is threatened by biotic and abiotic stress. In order to feed the world population of over 7 billion at the moment, productivity needs to be safeguarded against biotic and abiotic stresses. Biotic stress is caused due to attacks of viruses, bacteria, fungi, nematodes and other pathogens and pests. Pathogens are usually categorized into biotrophs and necrotrophs. Although the former penetrate the epidermal cells, multiply inside the intercellular spaces and feed on the living host tissue the latter kill the host cells and then feed on the cell remains. Biotrophs are mostly host-specific, the nectrophs have a broader host range [1]. Agricultural intensification has already led to increased soil pollution and land degradation problems. Therefore, understanding the natural mechanisms of defense in plants against various kinds of stresses is important to exploit it in a sustainable and environment-friendly manner. Of the various mechanisms plants have developed to combat biotic stress, hormones are of primary importance. Plant hormones are biochemicals that are synthesized at one location in plants and bring about the desired effect at the same or different location, at unimaginably low concentrations. Plant hormones are diverse in their chemical nature and biological functions derived from amino acids (IAA, ethylene), lipids (Jasmonic acid), from the isoprenoid (cytokinins, gibberellins, abscisic acid etc) and chorismate (salicylic acid) pathways (Figure 1). There are many biomolecules that have been added to the list of plant or phytohormones of late, which include jasmonic acid (JA), salicylic acid (SA), strigolactones (SL), brassinosteroids (BR) and peptides, besides auxins (IAA), gibberellins (GA), abscisic acid (ABA), cytokinins (CK) and ethylene (ET) that have been there since a long time. Salicylic acid, jasmonic acid and ethylene play very important roles in plant biotic stress response [2] while as, auxins, abscisic acid and gibberellins etc. modulate it. An overview of the important roles of major plant hormones is presented in Table 1. The role of hormones in plant growth and development is largely known and mechanisms of their biosynthesis have been elucidated in the majority of the cases, what remains to be fully understood is their mediation of the defense response in plants. In this chapter, I discuss how these hormones mediate the plant defense response and also assess how their effects are modulated by other hormones.

Figure 1.

Pathways of hormone biosynthesis.

Hormone	Nature	Stress	Mechanism	References
Ethylene	Alkene	Biotic Stress (Necrotrophic pathogens and herbivores)	ISR; Interactions with JA; modulates JA/SA antagonism; induces defense genes such as Plant Defensin1.2 (PDF1.2) or THI2.1 (thionin), Vegetative Storage Protein2 etc.	[3]
Jasmonic acid	Sesquiterpene	Biotic stress	Interacts with many other hormones to mediate stress response; ISR	[4, 5]
		Abiotic stress	It activates the antioxidant system, causes accumulation of amino acids, and soluble sugars and regulates stomatal opening and closing	[6]
Salicylic acid	Phenolic compound	Abiotic stress	Increase antioxidant activity	[7]
		Biotic stress	JA/ SA antagonism perfects pathogen specific response; mediates SAR; also involved together with MAPK signaling in resistance to aphids	[3, 8, 9]
Gibberellins	Diterpenoid	Abiotic stress	Degradation of DELLAs	[10]
		Biotic stress; Reverse the inhibitory effect of different stress conditions in seed germination and seedling establishment	Affect the relative strength of SA/JA signaling; Induction of salicylic acid (SA)-dependent defense pathway	[3, 10, 11]
		Modulate SA Biosynthesis	Through degradation of DELLAs	[10]
Auxins	Tryptophan derivative	Drought stress	By modulating root architecture, ABA (abscisic acid)-responsive genes expression, and ROS metabolism	[12]
		Biotic stress	Contributes to SAR	[8]
			Interaction with other hormones	[13]
Abscisic acid	Isoprenoid	Biotic stress	Influences the Central backbone (SA-JA/ET) of plant defense	[3]
		Abiotic stress	Stomatal closure; reduction in ROS levels	[3]
Cytokinins	Isopentenyladenine derivative	Biotic stress	Through direct interactions of CK-signaling components with the Central phytohormonal immunity backbone; interact with SA to induce defense responses via WRKY45 and NPR1; Phytoalexin accumulation	[3, 14]

Table 1.

Role of various plants hormones in biotic and abiotic stress response.

2. The major players in the plant defense

Ethylene (ET): This is a gaseous hormone that is responsible for various functions in plants notably fruit ripening, flower senescence and abscission of leaves etc. In dark-grown seedlings ET causes inhibition of hypocotyls and root elongation, radial swelling of hypocotyls and exaggeration of the apical hook this is commonly called as the triple response [15, 16]. The role of ET in plant stress is very well known [17, 18, 19], it favors stress resistance over growth thereby increasing stress tolerance [20, 21, 22, 23]. ET acts in cooperation with JA to present an effective defense response against necrotrophic and chewing insects [1, 24]. ET response involves a battery of ET receptors which, for example, in Arabidopsis (Arabidopsis thaliana) include ETHYLENE RESPONSE 1 (ETR1), ETR2, ETHYLENE RESPONSE SENSOR 1 (ERS1), ERS2 and ETHYLENE INSENSITIVE 4 (EIN4) [24, 25, 26, 27, 28]. Mutations in these lead to ET insensitivity and increased susceptibility to the necrotrophic pathogens [24, 29, 30]. ein3 and eil1 (ETHYLENE INSENSITIVE LIKE) double mutants are completely ethylene insensitive and these lacks the triple response, pathogen resistance and the ability to fully suppress ctr1 mutation [15, 31, 32]. CONSTITUTIVE TRIPLE RESPONSE (CTR) is the negative regulator of the ET pathway in absence of ET. When ET binds to ER anchored EIN2-protein receptors, it causes dephosphorylation of the latter. This leads to cleavage of the C-terminal domain (EIN2-C) of the EIN2 [1, 15, 33, 34]. EIN2-C moves to the nucleus and triggers EIN3 and EIN3-like transcription factors, eliciting ET-mediated response [15]. Transcription of ETHYLENE INSENSITIVE3 (EIN3) activates defense response by inducing expression of ERF1 [19, 35]. Repression of EIN2 by CONSTITUTIVE TRIPLE RESPONSE (CTR1) is released after perception of ET by ETHYLENE RESPONSE 1 (ETR1) [19, 36]. ET release stabilizes EIN3/EIL1 levels [15]. At the same time, ET also decreases levels of EBF1/2 (EIN3 BINDING F-BOX 1) protein through suppression of translation of its mRNA in the cytosol promoted by EIN2-C [15, 37, 38] and by EIN2-dependent proteasomal degradation of EBF1/2 proteins [15, 39]. EIN3 leads to EBF1/2 expression providing a negative feedback loop to the ET signaling [1].

Jasmonic acid (JA): Methyl jasmonate and its free acid jasmonic acid are collectively known as jasmonates. Jasmonic acid is the better known of the two while JA-Ile is the active form [1]. JA is a cyclopentane fatty acid that is synthesized from linolenic acid which is a common constituent of plant cell membranes [40]. JAs play a vital role in the various plant developmental processes including flowering, fruiting, senescence and secondary metabolism. These are known to be critically important in plant defense and abiotic stress response [41, 42, 43]. JA activates the antioxidant system, causes the accumulation of amino acids, and soluble sugars and regulates the stomatal opening and closing during abiotic stress [6, 44]. JA interacts with SA, ET and ABA during plant defense response [1, 4, 45]; its interactions with auxins, gibberellins and cytokinins during important development processes like root, stamen, hypocotyl, xylem development etc. are also well known [40]. The JAs affect both plant development and plant stress-resistance [46, 47].

Coronatine Insensitive Receptor (COI1) and JAZ (Jasmonate-ZIM domain) proteins mediate JA-signaling pathway [24, 48, 49]. The others involved in JA signaling include JASMONATE INSENSITIVE 1/MYC2 (JIN1/MYC2) and several members of the APETALA2/ETHYLENE-RESPONSIVE FACTOR (AP2/ERF) family [19, 50]. COI1 forms a part of the E3 –ubiquitin degradation complex as SCF ^COI1 complex. The SCF consists of Skp-1/Cullin/F-box. JAZ is a repressor of the JA response. SCF ^COI1 complex binds with JAZ repressors at higher JA concentrations and this leads to ubiquitination and degradation of JAZ mediated by 26S proteasome [1]. The JA signaling pathway may follow two paths one is the MYC pathway and another is the ERF pathway. Wounding and insect feeding induces the MYC branch which further involves MYC2, MYC3 and MYC4 - basic helix–loop–helix leucine zipper transcription factors [1]. In absence of JA-Ile JAZ proteins interact with JIN1/MYC2 and inhibit transcriptional regulation of JA-responsive genes [19]. Interaction of JAZ with MYC proteins competitively inhibits their interaction with the MED25 subunit of the Transcriptional Mediator Complex [1, 51]. This causes the expression of several JA responsive genes including VSP2 (vegetative storage protein), JA synthesis gene LOX2 and JA signaling repressor JAZ genes. ERF pathway is stimulated by necrotrophic pathogens. As the name indicates this branch is regulated by ET; AP2/ERF-domain transcription factors ORA59 and ERF (ERF1, ERF2, ERF5 and ERF6) control this branch. ORA59 and ERF1 bind to GCC-box motif through ERF domain and activate the expression of PDF1.2 which is the marker gene of this pathway [1, 19, 52, 53, 54, 55]. The mode of interaction between JAZ and ERFs is not known. EIN3 directly interacts with JAZ which represses the expression of ORA59 and ERF1 [1, 56]. Given the two branches MYC and ERF are induced under different kinds of pathogen attacks these two are mutually antagonistic [1].

Coi1 mutants lacking JA response are more susceptible to necrotrophic pathogens including Botrytis cinerea, Pythium irregulare, Alternaria brassicicola and other pathogens [24, 57, 58, 59, 60]. Susceptibility to herbivores is increased by mutations stabilizing JA e.g., JAZ1∆3A mutation increases susceptibility to Spodoptera exigua [24, 61]. The fine-tuning of JA-mediated defense response mediated via MYC2 is achieved by post-translational phosphorylation at thr328 residue that makes it unstable leading to its degradation by plant Ubox protein (PUB10) that works as E3 ligase facilitating MYC2 turn over [19, 62].

Salicylic acid (SA): this is a phenolic acid hormone that plays important role in the regulation of plant growth, fruit ripening and development. It is involved in pathogenesis-related protein expression [63, 64]. It may be synthesized through the shikimic acid pathway either via the isochorismate branch or phenylalanine ammonia-lyase branch. Salicylic acid regulates the expression of genes encoding molecular chaperones, heat shock proteins, antioxidants and those involved in the biosynthesis of secondary metabolites, alcohol dehydrogenases and cytochrome P450 [64, 65]. In recent years, SA has been increasingly implicated in the plant defense response [10, 66]. Increased SA biosynthesis improves plant tolerance to salt, oxidative and heat stress [10] and it is synthesized in response to pathogen attack [19]. Meaning thereby it has a role to play in biotic as well as the abiotic stress response. SA also leads to systemic acquired resistance (SAR) - defense response to a secondary pathogen infection far and wide in the plant after it has been exposed to a pathogen previously [19]. SA is accumulated in the plant tissue before SAR is initiated [24]. During SAR there is oxidative burst which is followed by increased levels of antioxidants to neutralize the harmful effects of the reactive oxygen species [24]. Mutations in SA-related genes compromise plant immunity to pathogens and diminish the expression of anti-microbial proteins [19, 67]. sid2–1 Arabidopsis mutants with impaired SA biosynthesis show reduced pathogen resistance [68]. In transgenic Arabidopsis plants expressing bacterial SA hydroxylase gene nahG, which causes the conversion of SA to catechol, SAR is not activated instead PR gene expression is activated [24, 69, 70]. SA effects SAR by affecting the expression of various genes including PAL and priming genes, it activates phytoalexin and auxin signaling-related pathways, it also effects the deposition of callose and phenolic products [46, 71]. SA induces resistance against biotrophic and hemibiotrophic pathogens including Hyaloperonospora arabidopsidis and Pseudomonas syringae.

SA signaling involves NPR1 (non-expressor of pathogenesis-related (PR) genes, a protein with ankyrin repeat [8, 72]. NPR1 is an oligomer formed by intramolecular disulfide bridges under uninduced conditions [8, 73]. SA induces de-oligomerization of NPR1 releasing active monomers which migrate to the cell’s nucleus inducing expression of PR genes [8, 74]; only the monomeric forms can interact with the TGA (TGACG binding) transcription factors which are bZIP proteins [8, 19, 75]. This facilitates the binding of TGA transcription factors with promoters of NPR1 dependent genes [8, 76]. The triple mutant tga2 tga5 tga6 does not respond to SA and does not have SAR [8, 77]. Both NPR1 and TGA undergo nitrosylation which increases the DNA binding ability of the latter. Thiol S-nitrosylation on the other hand causes oligomerization of the NPR1 leading to its inactivation [8, 74]. NPR1 undergoes phosphorylation and proteasome-degradation thereby allowing its turnover [78]. NRR3/4 also interacts with TGA and mutants nrp3/4 over accumulate NPR1 leading to faulty SAR [8]. The binding of NPR3 and NPR4 with Cullin 3 ubiquitin E3 ligase causes SA-dependent NPR1 degradation [8, 79]. The binding of NPR with SA causes a conformational change in the NPR1 required for NPR1 dependent PR gene expression. NPR is also important in epigenetic effect-dependent trans-generational immunity in plants [8, 80]. Pathogen resistance in Monocots is enhanced by over expression of NPR1 [8, 81].

3. The modulators of the plant defense

Auxins (IAA): The role auxins play in plant growth and development is very well known however, their involvement in plants’ response to the biotic stress has only begun to be elucidated [64, 82]. Auxins control apical dominance, tropic responses, development of vascular cambium, organ patterning, flower and fruit development [13]. Auxin/indole acetic acid (Aux/IAA), Auxin response factors (ARF), TOPLESS (TL) proteins are the transcriptional regulators that affect cell-specific transcription of auxins and are involved in auxin signaling [83, 84, 85, 86, 87]. Research on ARF has led to their identification and characterization from several pants including Arabidopsis [88], maize [89], rice [85, 90], poplar [91], tomato [87, 92, 93], Chinese cabbage [94], sorghum [95] and banana [83, 96].

Important auxin-responsive genes include Aux/IAA, GH3 and SAUR gene families. In a study by Ghanshyam and Jain [13], 154 auxin-induced and 161 auxin-repressed genes were reported to express differentially under the biotic stress-induced by Magnaporthe grisea and Striga hermonthica. 62 of the auxin-induced genes were common to both the pathogens while others showed a specific response, 55 to M. grisea and 37 to S. hermonthica. In the category of auxin-repressed genes, 16 genes showed response to both the pathogens while others showed a specific response, 10 to M. grisea and 35 to S. hermonthica. Altered expression in auxin genes has also been reported in cotton in response to Fusarium oxysporum f. sp. vasinfectum infection [13, 97]. Botrytis cinerea infection in arabidopsis causes down-regulation of all the auxin-responsive genes [98]. Repression of auxin-mediated signaling through micro-RNAs leads to resistance against P. syringae in arabidopsis. Various pathogens operate by modulating auxin levels in planta to enhance the host susceptibility [99, 100]. Exogenous IAA increased susceptibility to Xanthomonas oryzae pv. oryzae due to cell wall loosening effects of auxins [101]. The P. syringae type III effector AvrRpt2 causes altered auxin levels and modified auxin-related phenotypes and decreases resistance against Pst DC3000 in Arabidopsis plants lacking expression of RPS2 [24, 102]. In this case, susceptibility was found to be directly related to the auxin levels. Another study conducted by Naseem et al. [103], also supports that auxins, JA and ABA increase the host’s susceptibility. Naseem and Dandekar, [104] propose a working model of the interaction between auxins and CK stating that the pathogens increase auxin levels increasing disease susceptibility by decreasing SA- and CK-based disease response while as CK pretreatment influences auxin synthesis and transport thereby increasing resistance [24].

Auxins down-regulate jasmonic acid biosynthesis genes in Arabidopsis [13, 105]. GH3.5 acts as a bifunctional modulator in auxin and SA signaling [106]. Overexpression of GH3.5 leads to accumulation of SA and accumulation of pathogenesis-related −1 gene (PR-1 gene) product while the capacity of systemic acquired resistance (SAR) is compromised in gh3.5 mutants [106]. Resistance of arabidopsis against X. oryzae is increased due to the over expression of GH3–8 [101].

Abscisic acid (ABA): ABA is a sesquiterpene that is synthesized from carotenoids [107]. ABA is usually known as “stress hormone”, it regulates a wide range of processes to increase a plant’s stress tolerance [23, 108, 109]. It is known to influence the expression of 10% of the protein-coding genes in the events of stress [64, [110]. Important components in ABA signaling include PYR/PYL/RCAR etc. [107]. This hormone oversees important functions in plants that include, seed dormancy, accumulation of nutrient reserves in the developing seeds, desiccation tolerance and arrest of embryonic development during seed maturation [111]. It also plays important role in the protein synthesis and synthesis of some osmolytes [112]. ABA may positively or negatively modulate defense response depending on the type of the pathogen [113, 114, 115, 116]. Impaired biosynthesis or signaling ABA mutants in Arabidopsis(abi1–1, abi2–1, aba1–6, aba2–12, aao3–2, and pyr1pyl1pyl2pyl4) and tomato (sitiens) showed increased resistance to B. cinerea, P. syringae, Fusarium oxysporum, Plectosphaerella cucumerina and Hyaloperonospora parasitica [117, 118, 119, 120, 121, 122]. Antagonistic interactions between ABA and major plant defense hormones including ET, JA and SA have been reported and it has been found that 65% of the genes upregulated and 30% of the genes down-regulated in aba1–6 mutants are those that are affected (up-or down-regulated) by ET, JA or SA treatment [119, 122, 123]. The genes constitutively up/down-regulated in these mutants were also found to differentially express upon infection by P. cucumerina, indicating their role in the defense. ABA also plays role in expression of R genes [124]. ABA deficient plants are more susceptible than wild types to pathogens Alternaria brassicicola, Ralstonia solanacearum and Pythium irregulare [57, 125, 126]. JA biosynthesis needs ABA in Arabidopsis for P. irregulare resistance [57]. However, negative interaction between the two is known in the case of the P. cucumerina [122]. It has been reported that inoculation of Arabidopsis with avirulent strains of R. solanacearum makes the plant resistant to the virulent strains of the bacterium and the resistance is mediated through ABA, hence, this hormone could be used for controlling wilt induced by this pathogen [114].

Cytokinins (CK): The most important aspect of cytokinin function in plants is the maintenance of the identity of stem cells thus cytokinins affect the basic aspects of the growth and development of plants [40, 127]. CK was first identified as a hormone affecting cell division in tissue culture conditions and now its role in regulating the cell cycle is well known [23, 128]. Various functions regulated by cytokinins include inhibition of lateral root initiation and leaf senescence [23, 129, 130, 131, 132] differentiation of vascular tissue (phloem and metaxylem in roots [23, 133, 134], morphogenic differentiation in expanding leaves and regulation of their cell division [23, 135, 136, 137] Cytokinins are derivatives of isopentenyladenine; zeatin is a common CK [40, 127, 138] which exists in two forms -cis and trans-zeatin. Trans-zeatin is more active [40, 139, 140] and is produced by isopentenyl transferases and cytochrome P450CYP735A1 and CYP735A2 [40, 127]. Activity and homeostasis of cytokinins are regulated by their degradation or conjugation with glucose and amino acids, and CK oxidase which cleaves cytokinins [40, 139, 140].

The role of cytokinins in plant defense was first recorded from tobacco plants with down-regulated S-adenosyl homocysteine hydrolases; the plants had higher resistance to tobacco mosaic virus, cucumber mosaic virus, potato virus X, and potato virus Y and also showed increased levels of CK and higher levels CK-related developmental defects [24, 141]. Cytokinin deficient plants have higher stress tolerance [40, 43, 142, 143, 144]. Several cytokinin receptors, histidine phosphotransfer proteins and transcription factors mediate CK signaling. Three histidine kinases (AHK2, AHK3, and AHK4/WOODEN LEG) working as cytokinin receptors have been identified in Arabidopsis [127, 145]. Cytokinins cause autophosphorylation of the conserved histidine residues in these kinases [40]. The phosphate is transferred to the histidine phosphotransfer proteins (AHPs) through aspartate residue. Phosphorylated AHPs move to the cellular nucleus activating B-TYPE ARABIDOPSIS RESPONSE REGULATOR (ARR) transcription factors causing transcription of cytokinin response genes [146, 147]. Both the environmental factors as well as the JA levels in plants affect the components of the cytokinin response system [40, 148, 149, 150, 151]. It is believed that JA may be controlling CK response through MYC2 by promoting AHP expression [40]. Thus interactions between JA and CK might occur at the levels of signaling response elements [40]. Many studies indicate that CKs affect the plant defense response mediated by SA and JA. CK is believed to affect priming in SAR and affects the synthesis of SA and PR proteins [24, 152, 153, 154]. Exogenous supply and internal increased levels of CK increase the JA levels to hasten the defense reaction in wounded plants [24, 155, 156]. The mechanism employed by CK for disease protection is different in different plants e.g., in solanaceous plants, it increases the ratio of phytoalexin to pathogen restricting pathogen development [24]. JA accumulation increases CK ribosides in potato [24, 157]. Several genes involved in regulating CK levels in plants including IPT and CKX are seen as purported targets in enhancing plant disease resistance [24, 143, 158, 159]. Stabilized CK levels in transgenic arabidopsis plants lead to improved resistance to Verticillium longispoum [19, 160]. The interaction of ARABIDOPSIS RESPONSE REGULATOR 2 (ARR2), with TGA3 to promote plant defense response in an NPR 1 dependent manner is known [19, 160]. Likewise, rice resistance to M. grisea increases due to interaction between SA and CK in an OsNPR1 and WRKY45-dependent manner [14, 19]. Thus the role of cytokinins in the plant disease response cannot be over-emphasized [19, 152, 160, 161].

Gibberellins (GA): Plant developmental processes including, seed development and seed germination, seedling growth, root proliferation, trichome initiation, determination of leaf size and shape, flower induction and development, pollination, fruit expansion etc. are mediated by gibberellins [162, 163]. Gibberellins are tetracyclic diterpenoid carboxylic acids; only a few of many known GAs, notably GA1 and GA4 act as plant hormones [164]. Gibberellins help plants to maintain their internal homeostasis by enabling control over their osmotic and water levels [162]. The mechanism of gibberellin action is relatively better understood in comparison to the other phytohormones. These work by bringing about the degradation of DELLA transcription factors via E3-ubiquitin-ligase [10, 165, 166]. In some plants loss of function mutations in DELLAs has been reported to improve the resistance of plants to biotic stress through SA dependent pathways, meaning thereby GAs, work in biotic as well as an abiotic stress response [10, 11, 167]. During seed germination and seedling establishment, exogenous GA reverses the inhibitory effect of the different stress factors and it also improves SA biosynthesis thereby, improving plant stress response [10]. GA antagonistically interacts with JA to mediate plant growth and defense response which involves direct interaction between DELLA and JAZ proteins [40, 168, 169]. Of the various JAZ proteins, osJAZ9 has been reported to be the key protein in mediating these interactions [40, 170]. The overall plant growth is the result of the fine balance between stress response and developmental process which is mediated through well-regulated JA/GA balance [34, 150, 171]. The “relief of repression” model explains this antagonistic interaction very well. It postulates that DELLAs and JAZ interact with each other leaving MYC2 free to mediated JA-dependent response under conditions of low GA while in presence of an adequate concentration of GA, DELLA is degraded by E3-ubiquitinylation mediated by GA leaving JAZ free to interact with MYC2 and attenuating the JA-mediated response [46]. Thus, JA/GA interact antagonistically [168]. The fact that JA promotes transcription of RGA3 (Repressor of GA1–3) and that MYC2 directly binds to their promoter further lends support to this model [40, 172].

Brassinosteroids (BR): First discovered from Brassica napus, BRs are polyhydroxy steroidal compounds about 70 different types of which have been isolated so far [46, 173]; only a few of them including brassinolide, 28-homobrassnolide and 24-epibrassinolide are actively engaged in the plant development [64, 174]. These are widely distributed in different plant organs including pollen, flower buds, vascular cambium, fruits, leaves, roots and shoots [64, 175]. These are also involved in modulating JA signaling and in the JA-dependent plant defense response. These affect many plant functions and alleviate the effects of hypoxia and unfavorable effects of various environmental stressors [46]. BRs are mainly seen as the hormones that alleviate abiotic stress, however, there are reports that these modulate the pattern-triggered immunity (PTI) (discussed in the next section) in Arabidopsis [176, 177].

4. Pathogen recognition reactions

A set of conserved pathogen proteins are important for plants to recognize the infection, these Microbe-Associated Molecular Patterns (MAMP), also called as Pathogen-Associated Molecular Patterns (PAMP), are recognized and bound by Pattern Recognition Receptors (PRRs) present in the host cell plasma membrane. This MAMP-PRR binding triggers an immune response called as MAMP Triggered Immunity (MTI) [24, 178, 179]. Microbes synthesize effectors which interfere with MTI and help pathogens evade recognition by the host immune system increasing their virulence and making plants susceptible to the pathogen and deregulating the host immunity, this process is known as Effector Triggered Susceptibility (ETS) [24, 180]. Bacteria acquire large repertoires of type III Effectors (T3E) and inject them through a syringe-like type III secretion system into their host plant. Xanthomonas sp. secretes Transcription activator-like (TAL) effectors, such as AvrBs3 secreted by Xanthomonas axonopodis pv. Vesicatoria, which after finding their way into the plant cell nucleus affects host gene expression [177, 181, 182, 183]. Auxin is a potential target for bacterial effectors. Effector proteins AvrBs3 1–5 have been reported to upregulate UPA1–5 [177, 184]. Induction of UPA20, a TAL target leads to cell hypertrophy indicating auxin accumulation [177, 185]. In Arabidopsis lines lacking the gene that recognizes T3E the bacterial effector AvrRpt2, a cysteine protease, triggers the auxin signaling pathway. Transgenic plants expressing AVrRpt2 accumulate higher auxin levels and constitutively express auxin signaling [177]. Thus, AVrRpt2 enhances bacterial virulence by affecting auxin signaling [177, 186]. An auxin signaling pathway is the preferred target of phytoplasmas [177, 187]. Candidatus phytoplasma asteris effector TENGU leads to dwarfism and abnormal organogenesis in reproductive parts leading to flower sterility. In transgenic Arabidopsis plants, many auxin-related genes including Aux/IAA, SAUR, GH3 and PIN families were found to be downregulated indicating TENGU effector mediated disruption in the auxin signaling in the host plants [177, 188]. Similarly, Ustilago maydis hijacks the SA biosynthesis pathway in the maize plants to express its virulence [177]. Effector–triggered immunity (ETI) counters ETS in a gene-for-gene resistance mechanism [24, 189]. This leads to hypersensitivity response i.e., localized cell death in the infected region [24, 190, 191]. Hypersensitivity response leads to the activation of SAR [24]. The type of defense response depends upon the type of pathogens e.g., biotrophic pathogens are contained by programmed cell death which is mediated by SA [24]. On the other hand, the necrotrophs benefit from the cell death, the defense response, in this case, entails secretion of antibacterial/fungal compounds and accumulation of proteins that have antimicrobial properties such as defensins [24, 192, 193, 194]. JA-ET and SA, because of the inherent difference in the defense response these engage in, are antagonistic [24, 195, 196].

5. Hormone crosstalk in defense

Hormone crosstalk is the interaction of various plant hormones in a highly complex yet ordered manner [197, 198]. Crosstalk between various hormones is intensive in defense response [1, 5, 40, 45, 199]; it is mediated through regulatory proteins, hormone receptors, protein kinases, transcription factors etc. involved in hormone biosynthesis, degradation or signaling [5, 10, 40, 200, 201]. Hormonal cross-talk becomes increasingly important when plants are exposed to multiple pathogen stress simultaneously. Plants have to trade-off between defense and growth, therefore, the impact of individual hormones may not be as important as the overall interaction (positive or negative) among them. Numerous studies bring to light the flexible and coordinated interplay between growth and stress-related hormones especially JA and GA in regulating plant defense response [40, 150, 171, 202]; antagonistic interaction between SA and JA has been well researched [177, 203], the crosstalk between GA and SA was known only recently [10, 167]. Crosstalk enables pathogenesis-related genes affect response to abiotic stress [10, 204]. Crosstalk aids plants to gear up their defense system against various kinds of pathogens; however, all the aspects of this phenomenon in plant defense are not known.

6. Molecular Mechanism of hormone crosstalk

Phosphorylation Cascade is a common second messenger which integrates various hormone responses. JAZ and DELLA proteins mediate the antagonistic interactions between JA and GA [198, 205, 206]. A great deal of information exists on how DELLAs interact with JAZ proteins [168, 169]. GA response in Arabidopsis is suppressed by direct interaction of its transcription-factors-like PIF (phytochrome interacting factors) with the DELLAs including GIBBERELLIC ACID INSENSITIVE (GAI), REPRESSOR OF GA (RGA), RGA-like1 (RGL1), RGL2 and RGL3 thus, DELLAs act as negative regulators of the GA response [198, 207]. DELLAs binding with JAZ1 leaves MYC2 free to initiate JA signal response thereby enabling JA-responsive transcription [19, 168]. In a contrary situation, higher GA levels attenuate JA signaling by degrading DELLAs thereby allowing interaction of MYC2 with JAZ1 [40, 198, 208]. GA-related transcription factors like PIF3 are repressed at higher JA levels as JA stabilizes DELLA proteins through JAZ degradation [169, 198]. JA/GA antagonism in rice is mediated by an interaction between OsJAZ9 and DELLA proteins, namely, SLENDER RICE 1 (SLR1) [170, 198]. DELLAs also repress SA biosynthesis as well signaling affecting the balance between JA and SA [10, 167]. JA leads to selection of defense overgrowth in the events of pathogen attack by interfering with GA-mediated degradation of DELLAs [19, 169, 209]. In DELLA quadruple mutants (mutant lacking GAI, RGA, RGL1 and RGL2 proteins) expression of PR1 and PR2 is increased which makes them more resistant to hemibiotrophs, however, delayed induction of PDF1.2 a JA/ET dependent gene marker in such mutants leaves them susceptible to necrotrophs [19, 167]. By way of controlling DELLAs GAs indirectly control the SA/JA balance. The molecular mechanism of antagonistic interaction of SA with ET and JA pathways is largely unknown [1]. NPR1, however, is at the core of most of these antagonistic interactions. A WRKY70 transcription factor is another key player in the hormone crosstalk. WRKY33 is a positive regulator of JA-dependent genes but a repressor of SA pathway, therefore, wrky33 mutants show upregulated expression of several SA-regulated genes including SID2/ICS1, EDS5/SID1, PAD4, EDS1, NIMIN1, PR1, PR2, PR3. SA induction leads to down-regulated JA signaling and increased susceptibility of these mutants to necrotrophic fungi [177, 210]. When over-expressed it leads to constitutive expression of SA-responsive PR genes and repression of JA responsive PDG1.2 gene [19, 211]. Likewise, Arabidopsis mpk4 (MAP kinase 4) knockout mutants exhibit constitutive SAR, higher expression of PR genes but an impaired expression of JA-responsive PDF1.2 and THI2.1 genes [19, 196]. Synergistic interactions between SA and JA have also been reported especially at their lower concentrations and when both the defense responses are triggered together [19, 196, 211]. MED16 which positively regulates SA-induced defense response negatively regulates JA/ET signaling pathway [177, 212]. Some strains of P. syringae produce phytotoxin coronatine (COR), a mimic of the JA-Ile and this suppresses SA signaling [177, 213, 214]. This is the reason for the lower virulence of the strains of P. syringae that have impaired production of COR on wild Arabidopsis plants but not on SA deficient plants [177, 215]. JA and ET cooperate in comparison with the JA and SA where interactions are mostly antagonistic, e.g., JA and ET both stabilize EIN3 thereby leading to the defense of roots against necrotrophic pathogens [19, 56]. Both JA and ET activate the expression of ERF1 which in turn activates PR genes [19, 216]. However, in cases of herbivore and insect attacks the two pathways may interact antagonistically e.g., JA–activated MYC2 interacts with ET-stabilized EIN3 and represses its downstream activity. In turn, EIN3 represses MYC2 thereby repressing JA-mediated defense response against herbivores [19, 217].

7. Concluding remarks and future perspectives

Allocation of resources and energy to defense in absence of threat would constrain growth and developmental processes [177, 218, 219, 220]. Therefore, a hormone-based defense mechanism in plants evolved to prevent loss of resources in absence of stress [177, 220] slowing down the potential adaptation of putative attackers to the biochemical defense system of plants [177, 220]. During priming plants subjected to pathogen attack respond more strongly to subsequent pathogen attacks, resources here are not committed until the threat returns making priming a relatively cost-effective defense strategy [177, 221]. Moreover, the primed plants treated with a low, non-effective concentration of defense hormones also respond better to the pathogen attack than the non-primed ones [177, 221]. Priming has parallels with the trans-generational defense in plants, such as SA-dependent SAR and JA-dependent inherited defense as trans-generational priming has been described in some plants [80, 177, 214]. Epigenetically inherited changes can strongly affect the defense response including priming in plants [80, 177, 222].

Our understanding of plant defense response has considerably improved in the past few years due to modified and transgenic plants species [177]. Transgenic plants constitutively expressing some hormones have been reported to show improved resistance to pathogens [177, 223, 224]. But, such an “effective” resistance response is also known to incur the costs paid in terms of altered development e.g. dwarfism, development of spontaneous lesions in different organs, accelerated pace of senescence, delayed flowering, sterility and lower seed output [177, 223, 224, 225].

Dissecting hormone response specifically in the event of a pathogen attack is complicated by the complex regulatory pathways interconnecting at several different levels. In nature, a plant has to deal with both abiotic and biotic stresses therefore, its response to the environment, in general, has to be concerted and balanced. Ideally, a plant resistant to biotic or abiotic stress should not be hampered in terms of its growth, development and overall productivity. It is a generally conceded fact that the traditional methods of crop improvement have reached their peak and are now leveling off. Thus, molecular and genetic engineering methods provide reliable alternative means of crop improvement. Phytohormone engineering is seen as a new opportunity to maintain susceptible crop production, especially in the climate change scenario. Elucidating the path of signal transduction in stress response is an important step in manipulating the role of phytohormones in stress response. In the future plant defense response mediated by hormones should be studied under field conditions with model crop plants so that a better picture of the effectiveness of the hormone-mediated disease control, associated trade-offs in growth and development parameters, and impact on the performance of the plants are brought to light. A clear understanding of the hormone homeostasis at the molecular level is required to manipulate it and use it as a tool for effective defense against crop pathogens.