2.1. Physical and Chemical Defenses

Due to the constant exposure to a broad range of pathogens, plants have evolved complicated immune systems, including different aspects of defense mechanisms, to protect against pathogen colonization [1,7,11,12,13,14]. Firstly, the primary mechanical and structural barriers prevent the physical penetration of pathogens. The plant cuticle coating the surface of epidermal cells is a protective layer against biotic stresses, and it consists of lipids, hydrocarbon polymers, and waxes [15]. Stomata localized on the leaf surface are natural openings for pathogen entry, and plant cells can close them when challenged by pathogens, such as for instance, Pseudomonas syringae [16]. Lenticels are also natural pores that are easily invaded by pathogens. For instance, the area per fruit surface of lenticels positively correlates with plant susceptibility toward Botryosphaeria dothidea, which is an apple disease [17]. The importance of the cell wall as a mechanical barrier was elaborated well by the enhanced susceptibility toward pathogens of plants impaired in defense-induced lignification, which can strengthen the cell wall [18]. Recently, defense-induced lignification was found to be regulated by the Arabidopsis SG2-type R2R3-MYB transcription factor, MYB15 [19]. Secondly, for chemical defense, plants produce non-specific secondary antimicrobial chemicals to counteract pathogen invasion. Phytoalexins, acting as antimicrobial toxins, can be synthesized de novo very rapidly in response to pathogen infection [20]. The synthesis of camalexin, as a major phytoalexin in Arabidopsis, can be regulated by two major enzymes: Arabidopsis cytochrome P450 monooxygenase 71A13 (CYP71A13), and Arabidopsis cytochrome P450 monooxygenase 71B15/ phytoalexin-deficient 3 (PAD3). The increased susceptibility of the mutant pad3 toward Alternaria brassicicola was caused by camalexin biosynthesis impairment [21]. Phytoanticipins are constitutively produced metabolites against pathogens in plants [22]. Glucosinolates, as well-characterized phytoanticipins, play a pivotal role in defense against herbivores, bacteria, and fungi [23,24]. Glucosinolates are produced in the trichomes of Arabidopsis, and their biosynthesis is regulated by the basic helix–loop–helix (bHLH) transcription factors (TFs): MYC2, MYC3, and MYC4 [25]. Plant defensins are small cysteine-rich peptides that are active against bacteria and fungi [26,27,28]. The importance of defensins in killing pathogens are well-proven in many plant species, including Arabidopsis [29], Solanum lycopersicum (tomato) [30], and Nicotiana tabacum [31].

Physical and chemical defenses play significant roles in defending pathogens. However, in the future, the signals from pathogens that direct these responses should receive more research attention.

2.2. Basal Disease Resistance Regulated by Pattern Recognition Receptors

PTI takes the main role in the combat against pathogens as a basal resistance for susceptible hosts, and in non-host resistance [7]. PAMPs comprise conserved structure components, such as flagellin and fungal chitin, as well as conserved elongation factor, from pathogens, for instance, elongation factor Tu (EF-Tu) [32,33]. Flagellin 22 (flg22), a 22-amino-acid peptide, is detected by flagellin-sensing 2 (FLS2), encoding a leucine-rich repeat receptor kinase (LRR-RK) [34]. The alteration of FLS2 conformation, due to its interaction with flg22, causes an association with brassinosteroid insensitive 1(BRI1)-associated receptor kinase 1 (BAK1), thus transducing the signal [35]. Furthermore, β-aminobutyric acid (BABA)-responsive L-type lectin receptor kinase-VI.2 (LecRK-VI.2) is necessary for flg22-induced PTI, through an unknown mechanism [36]. Chitin produced by the fungal cell wall is detected by chitin elicitor receptor kinase 1 (CERK1), which is another receptor-like kinase that is able to regulate pathogen defense [37]. Upon chitin recognition, CERK1 can associate with, and phosphorylate, the receptor-like cytoplasmic kinase (RLCK) family protein, avrpphb susceptible 1 (PBS1)-like kinases (PBL27). PBL27 further interacts with and phosphorylates mitogen-activated protein kinase kinase kinase 5 (MAPKKK 5), thus activating the downstream MAPK cascade in Arabidopsis [38]. Similarly, OsCERK1 phosphorylates OsRLCK185, which is an ortholog of PBL27 in rice. Then, OsRLCK185 phosphorylates OsMAPKKK 11 and OsMAPKKK 18, which are rice orthologs of MAPKKK 5, transducing the downstream MAPK signaling [39,40]. EF-Tu is recognized by the elongation factor-Tu receptor (EFR), which is another LRR-RK [41]. EFR physically interacts with BAK1, thereby rendering a transduction for defense signaling [42,43]. Endogenous elicitors produced from damaged plant cell fragments from pathogens are characterized as damage-associated molecular patterns (DAMPs). The recognition of DAMPs resembles the recognition of PAMPs. These endogenous elicitors, including cell wall fragments, are released by microbial hydrolysis, cutin monomers, and short peptides, such as systemin and hydroxyproline-containing glycopeptides (HypSys), as well as the 23-amino-acid peptide 1 (PEP1) [44,45,46]. Similar to the perception of flg22, PEP1 is recognized by PEP1 receptor 1 (PEPR1) and PEP1 receptor 2 (PEPR2) [44]. Botrytis-induced kinase 1 (BIK1), belonging to the receptor-like cytoplasmic kinase (RLCK) family, directly interacts with, and is phosphorylated by PEPR1, and likely PEPR2, upon ethylene (ET) treatment. The positive role of PEPR1, PEPR2, and BIK1 in mediating ethylene (ET)-induced defense signaling was proven by Jian-Ming Zhou et al. [47]. In addition, another study indicated that PEPRs and the ET pathway play an additive role in PTI in general [48]. Interestingly, the above-mentioned FLS2, EFR, and PEPRs can be internalized into endosomes requiring BAK1. Endocytosis participates in regulating FLS2-mediated immunity, including stomatal closure and callose deposition [49].

In Arabidopsis, more than 600 RLKs exist [50], forming one of the biggest protein families. Most of them have the characteristics of an extracellular domain, a transmembrane domain, and a kinase domain facing the cytoplasm [50]. They play diverse roles in plant activities, including defense [51]. The above-mentioned pattern receptors: FLS2, LecRK-VI.2, EFR, CERK1, and PERPs all belong to receptor-like kinases (RLKs), suggesting an important role of RLKs in mediating defense via binding to specific ligands [52]. FLS3, another RLK, can recognize the epitope of flagellin, and it is termed flgII-28 in Solanum pimpinellifolium (tomato) [53]. In Oryza sativa (rice), kinase XA21 mediates the resistance of Xanthomonas [54]. Receptor-like proteins (RLPs) lacking the kinase motif, and thus different from RLKs, have been implicated in the involvement of defense in Solanum pimpinellifolium (tomato), Gossypium barbadense (cotton), and Brassica napus [55,56,57]. Fifty-seven putative RLPs were reported to exist in Arabidopsis [58]; RLP23 specifically recognizes nlp20, which is a PAMP from necrosis and ethylene-inducing peptide1-like proteins (NLPs) [59]. The RLP protein Ve1 mediates the recognition of Fusarium oxysporum in tomato [60]. Recently, the transcriptional regulation of receptor-like protein genes has been systemically analyzed after different stresses and hormone treatments in Arabidopsis [61]. RLP20, RLP21, RLP22, and RLP40 are responsive to Pseudomonas syringae infection [62]. However, whether these RLPs play roles in recognizing pathogen epitopes requires more research.

2.3. Virulent Effector-Induced Plant Defense

During the long-term battle between pathogens and plants, pathogens have evolved a mechanism involving a type III secretion system that is used to inject effectors to interfere with an unspecific PTI, in order to achieve a successful infection. Many virulent effectors are known to suppress PTI [62,63]. It has been shown that a Pseudomonas syringae type III effector can suppress PTI by the downregulation of the putative secreted cell wall and defense proteins [64]. Avirulent Pseudomonas syringae pv tomato strain DC3000-specific B (AvrptoB), as an E3 ubiquitin ligase from Pseudomonas syringae, can target the FLS2 receptor for degradation in Arabidopsis and mitogen-activated protein kinase 2 in N. benthamiana [65]. In addition, hypersensitive response and pathogenicity (Hrp) outer protein (hop) M 1 (HopM1), a conserved P. syringae virulence protein, was found to interact with Arabidopsis HopM interactor 7 (AtMIN7), which is an immunity-related vesicle traffic regulator, and interfere with its stability via proteasome degradation to suppress PTI [66]. Recently, HopM1 triggers an oxidative burst and stomatal immunity in an AtMIN7-independent manner; however, it most likely acts on AtMIN10 [67]. Generally, the virulent effectors function as the eukaryotic enzymes, such as ubiquitin ligases, phosphatases, and proteases, to regulate immune signaling [68,69,70]. To overcome the virulence of effectors from pathogens, plants, in turn, establish ETI by recognizing virulent effectors via corresponding receptor resistance (R) proteins in a specific gene-to-gene manner, which is commonly accompanied by a hypersensitive response in the form of rapid cell death, which limits the spread of the pathogen from infection sites. Increased SA accumulation is accompanied by the hypersensitive response in ETI [71,72]. The specific recognition of variable virulent effectors is regulated by many R proteins. The effectors recognized by R proteins become nontoxic for the hosts, since R proteins can detoxify the virulence of the effector s. Thus, an avirulent pathogen is a pathogen carrying an effector, which can be recognized by R proteins in plants. The receptor proteins mainly contain nucleotide-binding (NB) domains and LRRs, which can mainly be classified into two subclasses: Toll-interleukin-1 receptor-nucleotide binding-leucine-rich repeat (TIR-NB-LRR) proteins and coiled-coil (CC)-NB-LRR proteins. Perception by NB-LRR proteins can be mediated either through direct interaction with effectors, or by the accessory proteins, whose conformations can be altered by their associations with effectors. Then, the activation of defense signaling is conducted by the alteration of NB-LRR proteins conformations [73]. However, R protein-regulated ETI mostly exists in biotrophic pathogens, and it seldom exists in necrotrophic pathogens. Only one example of R protein-regulated ETI, in that the protein Toll/interleukin 1 receptor triggered a broad defense against several necrotrophs, was reported [74]. Conversely, a specific protein was often found to trigger the disease susceptibility in necrotrophs, which was designated as effector-triggered susceptibility [75].

Future research studies will reveal more novel strategies that are employed by pathogen effectors, which can be exploited to control plant disease.

3.2. The JA Pathway in Plant Defense

The JA pathway mainly protects plants against necrotrophic pathogens and wounding. The plant hormone JA is a lipid-derived compound, and it is synthesized through the oxylipin biosynthetic pathway. The JA synthesis starts with the precursor α-linolenic acid, which is released from membrane lipids in chloroplast. The key synthetic enzymes include a 13-lipoxygenase (13-LOX), a 13-allene oxide synthase (13-AOS), an allene oxide cyclase (AOC), and an OPDA-specific reductase (OPR3). There are many conjugation forms of JA, including jasmonoyl-l-isoleucine (JA-Ile), methyl jasmonate (MeJA), jasmonoyl -1-aminocyclopropane-1-carboxylic acid (JA-ACC), jasmonoyl-1-O-β-glucose (JA-Glc), and 12-hydroxyjasmonic acid sulphate (12-HSO4-JA). However, there is evidence showing that only MeJA and JA-Ile are the active forms [116]. Upon stress, JA can be rapidly conjugated to amino acids such as isoleucine by jasmonate-resistant 1 (JAR1), leading to the major biologically active form JA-Ile. JA-Ile can then bind to the F-box protein coronatine insensitive 1 (COI1), leading to the conformation change of COI1. These conformation changes allow for the association of COI1 with the jasmonate zinc-finger expressed in inflorescence meristem (ZIM) (JAZ)-domain transcriptional repressor proteins. Inositolpentakis phosphate 5 (InsP5) acts as a co-receptor for JA-Ile to stabilize the association of JAZ-COI1 [117]. COI1 functions in the E3-ligase S phasekinase-associated protein 1 (SKP1)-Cullin-F-box complex SCF^COI1 and directs the degradation of JAZs, causing the activation of the JA response. In Arabidopsis, there are 12 JAZ members (JAZ1–JAZ12). Under normal conditions, JAZ proteins repress the activity of positive transcriptional regulators by binding to them. Co-repressors are recruited to coordinate the suppression with JAZ proteins. The recruitment of Topless (TPL) is achieved through the novel interactor of JAZ (NINJA), containing an ethylene response factor (ERF)-associated amphiphilic repression (EAR) motif (Figure 2). TPL has been shown to interact with histone deacetylases (HDCAs) to deacetylate histones at promoters or interrupt the Mediator-RNA polymerase II complex, thus leading to the suppression of transcription [118] (Figure 2). There are two major branches of JA signaling downstream of JAZ repressors: the MYC branch, which is responsible for the wounding response, and the ERF branch, which is associated with necrotrophic pathogen resistance (Figure 2). The MYC branch, controlled by MYC-type transcriptional factors, directs wounding responses, including the expression of Vegetative storage protein 2 (VSP2), which is the JA-responsive marker gene. MYC2 recruits MED25, which is one subunit of the plant mediator complex to initiate transcription [119]. The activity of MYC2 could be regulated by the proteasome degradation of JAZs due to the lost and gained repression from JAZs. Furthermore, MYC2 was recently reported to be regulated at the protein level by ubiquitination and deubiquitination [120,121]. MYC2 is the major transcription factor regulating the JA pathway, while MYC3 and MYC4 activate the JA response additively to MYC2, although it is still unclear how MYC3 and MYC4 modulate the JA pathway [122]. MYC2 prefers binding to the G-box sequence (5′-CACGTG-3′). Consistently, the G-box sequence of the promoter is required for the induction of the early-responsive JA response gene JAZ2, which is regulated by MYC2. The ERF branch, containing the ethylene response factor 1 (ERF1) and octadecanoid-responsive Arabidopsis AP2 59 (ORA59), controls the JA-responsive marker gene plant defensin 1.2 (PDF1.2), which is usually regulated by the ET pathway in response to necrotrophic pathogen infection (Figure 2). Interestingly, the MYC branch and ERF branch were found to be antagonistic to each other. The myc2 mutant was more resistant to the infection of the necrotrophic pathogen Alternaria brassicicola, which was probably due to the mediated ERF responses [123]. When the MYC branch was suppressed by ORA59 overexpression, plants were more susceptible to P. rapae larvae, which feed on the plants, causing a wounding response [124]. MYC2 negatively regulates genes in the other JA-responsive branch, including ERF1, ORA59, and PDF1.2. However, the suppression mechanism is still unclear [123]. Interestingly, MYC2 regulates some NAC domain-containing TF genes ANAC109 and ANAC055, which have been shown to positively regulate vegetative storage protein (VSP) genes, but negatively regulate PDF1.2 [125]. Recently, two transcription factors ethylene insensitive 3 and EIN3-like1 (EIL1) upstream of ERF1 and ORA59 were found to be the synergistic convergence knot, which was regulated by both the JA and ET pathways (Figure 2). EIN3 and EIL1 are activated by the release of repression from JAZ and HDA6 in the JA pathway; however, they are positively regulated by the essential regulator ethylene insensitive 2 (EIN2) in the ET pathway. EIN2 can stabilize EIN3 and EIL1, via preventing them from being degraded by EIN3-binding F box protein 1 and 2 (EBF1 and EBF2). EBF1 and EBF2 are two F-box proteins mediating the proteasome degradation [126] (Figure 2). Except for the classical COI1-JAZ-MYC2 and COI1-JAZ-EIN3 models, JAZs could bind other transcriptional regulators that are responsive to developmental and environmental cues. JAZs could bind JA-associated MYC2-like 1(JAM1), JAM2, and JAM3. However, unlike MYC2, JAM1, JAM2, and JAM3 negatively regulate the JA pathway through binding to the target sequences of MYC2 as the transcriptional repressors. Probably, these negative regulators were required to fine-tune the overshooting JA response [127,128,129] (Figure 2).

Open in a separate window

Figure 2

Model of biosynthesis and signal transduction of jasmonic acid (JA) and ethylene (ET). JA and ET can synergistically activate ethylene insensitive 3 (EIN3) and ethylene insensitive 1 (EIL1), which are positive regulators of ERF1 and octadecanoid-responsive Arabidopsis AP2 59 (ORA59), leading to the induction of plant defensin 1.2 (PDF1.2). The active form jasmonoyl-l-isoleucine (JA-Ile) can be recognized by coronatine insensitive 1 (COI1) and the co-receptor Inositolpentakis phosphate 5 (InsP5), causing the degradation of jasmonate ZIMs (JAZs). There are two antagonistic branches in the JA response: the MYC branch and the ERF branch. JAZ proteins can suppress the activity of MYC2, the positive regulator of Vegetative storage protein 2 (VSP2), by enrolling the negative regulator Topless (TPL) via the novel interactor of JAZ (NINJA). MYC2 can directly interact with mediator 25 (MED25, which is a subunit of the mediator. MYC2 can activate the expression of NAC domain containing proteins (ANACs) to upregulate VSP2. MYC3 and MYC4 can activate JA response additively with MYC2 by binding to the G-box. JAZs could also suppress the activity of jasmonate associated MYC2 like proteins (JAMs), which are a group of transcriptional repressors that could bind to the MYC2 target sequence G-box to negatively regulate the JA response. The MYC2 protein is regulated at protein levels. MYC2 can be ubiquitinated during the JA response for degradation. Thus, JA-mediated defense is suppressed. Conversely, MYC2 can be inversely deubiquitinated to stay stable, triggering a positive regulation on JA response. In the ERF branch, JAZs can inhibit the activity of EIN3 and EIN3-like 1 (EIL1) by direct association with them and enrollment of the histone deacetylase 6 (HDA6). In the ET pathway, there are five ethylene receptors localized in the ER membrane. A copper ion interacts with the ethylene-binding domain. The interaction is required for high-affinity ethylene-binding activity. By binding to ET, constitutive triple response 1 (CTR1) is deactivated by ET receptors. CTR1 can inhibit EIN2, which activates EIN3 and EIL1 by preventing their degradation of them via EIN3-binding F box protein 1 and 2 (EBF1 and EBF2). Activation (closed arrowhead), suppression (), and important genes are indicated.

3.3. The Ethylene Pathway in Plant Defense

The plant hormone ET is a gaseous hormone, playing key roles in many physiological processes. The biosynthesis of ET originates from S-adenosyl methionine (SAM), which is produced by SAM synthetase from methionine and ATP. 1-Aminocyclopropane-1-carboxylic acid (ACC), the precursor of ET, can be synthesized by ACC oxidase from SAM and converted to ET. It induces the ripening of the fruits or leaf abscission. However, ET also plays an active role in plant defense. For example, in Arabidopsis, ET potentiates the expression of PR1 via an unknown mechanism [130] (Figure 3). There are five ethylene-responsive receptors, which can be divided into two subgroups: the first group contains ethylene response 1 (ETR1) and ethylene response sensor 1 (ERS1), which are characterized by histidine (His) kinase activity; the second group contains ETR2, ERS2, and EIN4, which are characterized by serine (Ser)/threonine (Thr) kinase activity in vitro [131]. These five ethylene receptors are localized on the endoplasmic reticulum (ER) membrane, and they play redundant roles in recognizing ET (Figure 2). A copper ion first associates with the ethylene-binding domain, and is required for high-affinity ethylene-binding activity [132]. In non-challenged cells, these receptors without the association of ET ligands can activate constitutive triple response 1 (CTR1), which negatively regulates EIN2 by phosphorylation [133]. EIN2 can inhibit the degradation of EIN3 and EIL1 by the 26S proteasome, and the degradation process requires EBF1 and EBF2 (Figure 2). EIN3 and EIL1 are positive transcription factors, which regulate ORA59 and ERF1. Upon ethylene binding, CTR1 is deactivated by ethylene receptors such as ETR1, causing a release of the repression on the EIN2, thus stabilizing the EIN3 and EIL1. The JA pathway and ET pathways share the same branch, starting from EIN3 and EIL1. EIN3 and EIL1 can be regulated by both the JA and the ET pathways; therefore, they are synergistic knots of the JA and ET pathways (Figure 2) [126]. The mutant cev1 of gene constitutive expression of VSP1 (CEV1) constitutively activates JA and ET signaling, suggesting a common regulation of both pathways [134]. ET can act on the ERF branch of the JA pathway, but antagonizes the MYC branch. Pathogenesis-related proteins PR3 and PR4 are the marker genes of the ET response. PDF1.2 requires both the JA and ET pathways to be induced, and therefore is a marker gene for both pathways.

Open in a separate window

Figure 3

Interactions between the SA and the JA/ET pathways. ET can synergistically induce PR1 via an unknown mechanism. SA and JA/ET are mostly antagonistic to each other. On the left-hand (SA) side, the cytosolic localization of NPR1 is sufficient for the suppression of the JA/ET response. Some components that are regulated by NPR1, such as WRKY62, TGAs, and glutaredoxin 480 (GRX480), can confer the suppression on the JA response. Some WRKYs such as WRKY50, WRKY51, and WRKY70, can suppress the JA response independent from NPR1. The suppression of the JA pathway by the SA pathway is downstream of JA synthesis, having no influence on JAZs accumulation; however, it results in the degradation of ORA59. On the right-hand (JA) side, JA can suppress the SA response by inducing ANACs, which are regulated by MYC2. ANACs can inhibit isochorismate synthase 1 (ICS1); however, they can enhance the basal transcript level of the SA methyl transferase 1 (BSMT1) and UDP-glucosyltransferase 74F2 (UGT74F2) to elevate SA accumulation. Furthermore, EIN3 and EIL1 can suppress ICS1 activity by directly binding to the promoter of ICS1. Activation (closed arrowhead), suppression (), and important genes are indicated.

3.4. SA-JA Cross-Talks

The mutually antagonistic effect between the SA and JA pathways are quite well-known. The suppression of the SA pathway on the JA pathway has been extensively studied [1,77]. NPR1 acts as a crucial modulator in the SA-mediated suppression of JA signaling. It has been reported that the nuclear localization of NPR1 is required for the activation of SA response, but not for the SA-mediated suppression of the JA response [135]. This indicates that cytosolic NPR1 is capable of suppressing the JA pathway. Furthermore, nuclear NPR1 is required for the expression of transcription (co)factors that suppress JA-dependent gene expression such as glutaredoxin 480 (GRX480), TGAs, and WRKYs [116,136,137] (Figure 3), also suggesting the suppression of the JA response also in a NPR1 nucleus-dependent manner. GRX480 plays a role in SA–JA cross-talk, by suppressing the JA-mediated response, in an NPR1-dependent manner [138]. Additionally, the expression of the marker gene PDF1.2 in the mutant npr1, after the pharmacological application of SA and MeJA, showed that SA suppression on the JA pathway is dependent on NPR1 [139].

Many other regulators have been reported to play roles in the cross-talk of SA and JA pathways, such as the WRKY TFs including WRKY70, WRKY50, WRKY51, WRKY33, and WRKY75, which exert antagonistic influences on SA–JA communication [71,113,114,136,140,141]. Among them, WRKY70, WRKY50, and WRKY51 show suppression of the JA response through an NPR1-independent mechanism [118] (Figure 3), even though they can be regulated by NPR1. Interestingly, ET can bypass NPR1-dependency to render the suppression of SA on JA [139]. Moreover, the exogenous application of SA was reported to suppress the JA pathway through the influence of both ORA59 expression and protein stability, which targets GCC-box motifs in JA-responsive promoters [137]. However, another recent study indicated that the insects’ egg-triggered SA/JA antagonism is independent from ORA59; however, it relies on the protein stability of MYC2, MYC2, and MYC4 [136].

The influence of the JA pathway on the SA pathway is less clear. The inhibition of the SA pathway by the JA pathway is through the activation of three homologous NAC TF genes: ANAC019, ANAC055, and ANAC072 (Figure 3). These can also be activated by the phytotoxin coronatine (COR), which requires SCF^COI1 and MYC2. These TFs exert an inhibitory effect via the suppression of ICS1, which is a positive regulator of SA synthesis, and they activate the basal transcript level of the SA methyl transferase 1 (BSMT1), which transforms SA into the inactive SA ester (Figure 3). Despite the well-known antagonistic effect, the synergistic actions of the SA and JA pathways have also been frequently reported [135,142,143,144,145,146]. It has been reported that both SA and JA-mediated responses are activated by the epiphytic fungus Pseudozyma aphidis [146]. A low light ratio of low red:far-red light compromises both SA and JA-dependent responses in Arabidopsis, although the mechanism is still unclear [147]. In the enhanced disease resistance 1 glucan synthase-like 5 (edr1 gsl5) double mutant, SA and JA marker genes were synergistically activated, although the single gene mutations edr1 and gsl5 only prime the SA response [148]. Exogenous application of low concentrations of both SA and JA can synergistically upregulate both the SA and JA pathways [145].

Reactive oxygen species (ROS), NO, and Mediator, which is a multi-protein complex that functions as a transcriptional co-activator in all of the eukaryotes, are believed to positively regulate both SA and JA pathways. NO was established to have a positive influence on the defense against both biotrophic and necrotrophic pathogens. Ozone exposure to plants generated reactive oxygen species (ROS), and this could trigger all of the SA, JA, and ET responses. ET synthesis preceded both SA and JA production [149]. The loss of function of hemoglobin 1 (GLB1), encoding hemoglobin, led to the enhanced resistance to both biotrophs and necrotrophs, which is concomitant with elevated NO accumulation after both infections [150]. The mutation of Mediator subunit 16 (MED 16) blocked both the induction of JA and SA responses [151]. Furthermore, flagellin (flg22), regulating PTI, can enhance SA, JA, and ET pathways [152,153]. The overall fitness of plants is dramatically impaired, which is concomitant with the activation of the defense responses. The equilibrium of both pathways requires tight control (Figure 3). The regulation of the SA–JA equilibrium is still unclear yet, and it requires more study.