Insights into the role of jasmonic acid-mediated defenses against necrotrophic and biotrophic fungal pathogens

Abstract

Jasmonic acid (JA) is a natural hormone regulator involved in development, responses against wounding and pathogen attack. Upon perception of pathogens, JA is synthesized and mediates a signaling cascade initiating various defense responses. Traditionally, necrotrophic fungi have been shown to be the primary activators of JA-dependent defenses through the JA-receptor, COI1. Conversely, plants infected with biotrophic fungi have classically been associated with suppressing JA-mediated responses. However, recent evidence has shown that certain biotrophic fungal species also trigger activation of JA-mediated responses and mutants deficient in JA signaling show an increase in susceptibility to certain biotrophic fungal pathogens. These findings suggest a new role for JA in defense against fungal biotrophs. This review will focus on recent research advancing our knowledge of JA-dependant responses involved in defense against both biotrophic and necrotrophic fungi.

Keywords

jasmonic acid (JA) / methyl jasmonate (MeJA) / biotrophic fungi / necrotrophic fungi / COI1

Introduction

Plants are under constant bombardment from a wide array of environmental pressures. Unlike mammals, plants lack an adaptive immune response and are unable to produce memory cells to specific stimuli. To respond to these environmental pressures, plants rely on a highly diverse set of receptors to activate an intricate innate immune system. These receptors can perceive a variety of signals: from external signals produced by pathogens such as elicitors to internally produced systemic regulatory signals such as phytohormones. These receptors can play a role in a number of different functions such as perception, signal amplification, or both. In plant defense responses, there are three major pathways that are mediated by the phytohormones jasmonic acid (JA), salicylic acid (SA) and ethylene (ET). Synergistic or antagonistic interactions between these three pathways allow the plant to finely tune responses to a specific pathogen (Penninckx et al., 1998; Thomma et al., 1998; Takahashi et al., 2004). The SA pathway is unregulated in response to the biotrophic pathogens Golovinomycetes cichoracerum and Pseudomonas syringae (Murray et al., 2007; Fabro et al., 2008). In contrast, the JA- and ET-mediated pathways are specifically unregulated in response to necrotrophic fungi such as Botrytis cinerea (Berrocal-Lobo et al., 2002; Pena-Cortes et al., 2004; Glazebrook, 2005). Though each of these pathways plays an important role in resistance against several classes of plant pathogens, this review will focus on the role of JA-mediated signaling in fungal defense.

Jasmonic acid is a natural hormone regulator that induces proteinase inhibitor proteins in response to wounding, pathogen attack and development (Gfeller et al., 2010). Jasmonates are derived from a family of oxygenated fatty acids collectively known as oxylipins, which can be made internally from enzymatic reactions within the plant cell (Wasternack and Kombrick, 2010) or derived externally from a few fungal species (Miersch et al., 1991). JA is synthesized by the conversion of α-linolenic acid to 12-Oxo-phytodienoic acid (OPDA) by a series of enzymatic reactions in the chloroplast (Kazan and Manners, 2008). OPDA is then transported to the peroxisomes where it undergoes a series of β-oxidation reactions to generate JA (Vick and Zimmerman et al., 1984; Wasternack, 2007; Kazan and Manners, 2008).

Pathogen attack and wounding utilize their own unique receptors in order to trigger JA-mediated responses. Once JA is generated, it diffuses from the peroxisome into the cytosol where it can undergo subsequent reactions to form various JA derivatives. As illustrated in Figure 1, JA can be converted into volatile methyl jasmonate by the enzyme jasmonic methyl transferase (JMT). The addition of a methyl group to JA increases its lipoxygenase activity and methyl jasmonate (MeJA) has been shown to elicit JA-mediated responses by exogenous application (Avdiushko et al., 1995; Berrocal-Lobo et al., 2010). Exogenous application of MeJA has been shown to enhance resistance against several necrotrophic fungal species. For example, wheat (Triticum aestivum) pre-treated with MeJA showed delayed symptom development against Fusarium pseudograminearum (Desmond et al., 2005) and enhanced resistance to infection by Stagonospora nodorum (Jayaraj et al., 2004). JA and ET application has also been shown to enhance maize resistance to the necrotrophic pathogens Rhizopus microspores and Colletotrichum graminicola (Schmelz et al., 2011).

Additionally, JA can be conjugated to isoleucine by the JA amino acid synthetase, JAR1 (Staswick and Tiryaki, 2004; Fig. 1). JA and its derivatives then migrate to the nucleus where they are bound by the JA receptor, CORONATINE INSENSITIVE1 (COI1). COI1 contains 16 leucine-rich repeats (LRR) and an F-box motif, an important factor in protein ubiquitination (Xie et al., 1998). Once activated COI1 associates with other proteins forming a large complex (SCF-COI1) that acts as an E3 ubiquitin ligase specifically ubiquitinating JA-repressor proteins (Fig. 1). These repressor proteins, known as JASMONATE ZIM DOMAIN (JAZ) proteins, negatively regulate JA-responsive genes by interacting with MYC2, a JA-regulatory transcription factor (Chini et al., 2007; Fig. 1). Once activated by JA, the SCF-COI1 complex targets JAZ proteins for ubiquitination and subsequent protein degradation by the 26S proteasome (Chini et al., 2007; Thines et al., 2007). Following degradation of the JAZ proteins, MYC2 activates the transcription of several JA-responsive genes that respond to a variety of stimuli though not all JA-responsive genes are regulated by MYC2.

Jasmonic acid acts in defense against necrotrophs via a complex group of signaling molecules

Necrotrophic fungi are organisms that kill host tissue and feed on the remains (Glazebrook et. al., 2003). In response to infection by a biotrophic pathogen, a plant will typically undergo cell death in order to deter further tissue infection and rid itself of the pathogen. Upon necrotroph infection, however, cell death appears to aid the pathogen in its attack so plants have evolved an alternate mechanism of defense that is mediated by JA. JA and its associated signaling pathway respond strongly to infection by necrotrophic fungi and a defense response is initiated that is more effective than resistance gene mediated responses or SA-mediated defenses (Spoel et al., 2007). JA dependent signaling causes increased jasmonic acid synthesis and initiates expression of defense effector genes such as plant defensin 1.2 (PDF1.2), thionin 2.1 (thi2.1) and vegetative storage protein (VSP) (Glazebrook, 2005).

Despite the fact that the effects of JA on plants have been well characterized, the precise mechanisms by which JA acts are not fully understood (Xu et al., 2002). In Arabidopsis, all known activities of JA appear to require the function of the COI1 gene (Glazebrook 2005). COI1 encodes an F-box protein that binds to JA and its associated proteins along with AtCUL1, AtRBX1, and SKP1-like proteins (ASK1 and ASK2) to form an active SCF/COI1 ubiquitin ligase complex that is involved in protein degradation (Xu et al., 2002). Since COI1 is an essential component in regulating JA-specific responses many studies have characterized its role in fungal defense by testing the response of coi1 mutants against a variety of necrotrophic fungi such as Alternaria brassicicola (Table 1). In Arabidopsis JA biosynthesis is upregulated after infection with A. brassicicola and synthesis of JA-induced defense genes is amplified in a COI1-dependent manner (van Wees et al., 2003). Additionally, coi1 Arabidopsis mutants display an increased susceptibility to the necrotrophic fungi A. brassicicola, Botrytis cinerea, and Plectosphaerella cucumerina (Thomma et al., 1998). The Arabidopsis triple mutant fad3 fad7 fad8, a JA biosynthetic mutant that is deficient in JA and its precursor metabolites, also shows an increase in susceptibility to A. brassicicola (McConn and Browse, 1996; Stintzi et al., 2001) and the JA-signaling mutant jar1 (JA-insensitive) has been shown to have increased susceptibility to B. cinerea (Kachroo and Kachroo, 2009). Reductions in JA-mediated defense responses in Arabidopsis have also been experimentally linked to an increase in susceptibility to the necrotrophs Fusarium oxysporum f. sp. Lycopersici (Thaler et al., 2004), B. cinerea (Thomma et al., 1998), Pythium mastophorum (Vijayan et al., 1998), Pythium irregulare (Staswick and Tiryaki, 2004), and A. brassicicola (Penninckx et al., 1996; Thomma et al., 1998). In tomato, the jasmonate-deficient mutant, deficient in jasmonate1 (def1), shows a greater susceptibility to F. oxysporum (Thaler et al., 2004), Verticillium dahlia (Thaler et al., 2004) and B. cinerea (AbuQamar et al., 2008). Furthermore, the tomato homolog of Arabidopsis coi1, known as jasmonate insensitive1 (jai1), and the tomato acyl-coA oxidase (acx1) mutant, which is deficient in JA biosynthesis, both show an increase in susceptibility to B. cinerea (AbuQamar et al., 2008). The wheat phytochrome and flowering time1 (pft1) mutation, which is required for JA-mediated defense signaling and encodes the MEDIATOR25 subunit of Mediator, is more susceptible to F. oxysporum (Kidd et al., 2009).

Some mutations in JA biosynthesis and signaling result in resistance to necrotrophs

The studies discussed above show that mutations in JA biosynthesis and signaling reduce plant resistance to necrotrophic fungi. This is expected since JA-mediated responses are known to play an important role in defense against these pathogens. However, several recent studies have shown that some mutations linked to JA signaling or synthesis can lead to an increase in resistance to several necrotrophic fungi (Thatcher et al., 2009; Makandar et al., 2010; Table 1). Mutations in JASMONATE INSENSITIVE1/MYC2 (JIN1/MYC2) are known to exhibit resistance to both B. cinerea and P. cucumerina (Kachroo and Kachroo, 2009). The tomato mutant def1 has also been shown to have increased resistance to Alternaria alternate (Egusa et al. 2009). Additionally, mutations in OPDA REDUCTASE3 (opr3), coi1 and JASMONATE RESISTANT1 (jar1) have been shown to exhibit a hyper-resistant response when infected with Fusarium graminearum (Makandar et al., 2010). Taken together these results suggest that these genes may contribute in some way to pathogen susceptibility.

Mutations in OPR3 have also been shown to be more resistant to A. brasscicola (Stintzi et al., 2001). This mutant contains a T-DNA insertion in the second intron of OPR3 and accumulates OPDA but is blocked in JA biosynthesis and induction of JA-responsive genes. It has previously been described as null mutant and its resistance to A. alternate was attributed to high levels of OPDA though the role of OPDA in defense was unknown (Stintzi and Browse, 2000; Stintzi et al., 2001). In a recent paper, Chehab et al. (2011) conducted a detailed characterization of the opr3 mutant in an attempt to better understand OPDA’s role in defense. Interestingly, the authors determined that opr3 plants did accumulate detectable levels of JA when infected with the virulent pathogen B. cinerea but do not produce JA in response to wounding or insect attack (Chehab et al. 2011). The authors go on to show that in opr3 plants infected with B. cinerea full-length transcripts of OPR3 were present though at a significantly reduced level when compared to wild type (Chehab et al., 2011). They argue that the observed resistance to B. cinerea is due to the accumulation of JA in response to the fungus and that opr3 is not a true loss-of-function mutant. The authors speculate that under certain conditions, such as fungal infection, opr3 mutants are able to remove the T-DNA insertion via splicing to produce full-length functional transcripts. The exact mechanism that allows the mutant to regain function, however, remains to be determined (Chehab et al., 2011).

JA stimulates phytoalexin production in response to necrotroph infection

Phytoalexins are a large class of antimicrobial compounds that are produced in response to pathogens. Several recent studies suggest that accumulation of phytoalexins is important for defense against necrotrophic pathogens. In Arabidopsis, camalexin is the major phytoalexin involved in the inhibition of pathogen growth and encodes the PHYTOALEXIN DEFICIENT3 (PAD3) gene (Zhou et al., 1999). A study conducted by Ferrari et al. (2007) showed that functional PAD3 is required for resistance to B. cinerea in Arabidopsis. Mutants in PAD3 were also shown to be more susceptible to A. brassicicola (Thomma et al. 1999; van Wees et al., 2003; Table 1). Camalexin production in Arabidopsis has most commonly been observed in plants infected with bacterial pathogens but JA is known to activate camalexin production in other plant species (Thomma et al. 1999, Zhou et al. 1999). These data point to a possible interaction between JA and PAD3 though additional experiments are needed. The opr3 mutant was also shown to accumulate camalexin to near wild type levels in response to infection with B. cinerea (Chehab et al. 2011). The authors speculate that JA production may be required in some cases for the synthesis of camalexin (Chehab et al. 2011). In maize, both JA and ET were shown to induce the accumulation of kauralexins, a class of ent-kaurane–related diterpenoid phytoalexins that are induced by pathogens (Schmelz et al., 2011). Kauralexin B3 accumulation was shown to enhance maize resistance to the necrotrophic pathogens Rhizopus microspores and Colletotrichum graminicola (Schmelz et al., 2011).

JA also acts in resistance to biotrophic fungal pathogens

The JA signaling pathway has traditionally been linked to defense responses against necrotrophic fungi leading to necrosis and death of the plant. In contrast, biotrophic fungi require living plant tissue to flourish; therefore, obligate biotrophic fungi must establish a connection with their host to obtain nutrients and produce reproductive structures. Once an infection is established, fungal associated elicitors can trigger defense responses in the plant (Ramonell et al., 2005) that include the upregulation of several defense transcripts like PDF1.2 (Penninckx et al., 1996) and Pathogenesis Related-1 (PR-1; Yalpani et al., 1991). Work by Fabro et al., (2008) showed that in Arabidopsis infected with the fungal biotroph, G. cichoracearum (powdery mildew), endogenous levels of both SA and JA were increased (Fabro et al., 2008). The SA-insensitive mutant, noninducible pr1 (npr1-1) and jar1, were more susceptible to G. cichoracearum compared to wild type plants six to eight days post inoculation (Fabro et al., 2008; Table 2). An increase in susceptibility to G. cichoracearum was also observed in coi1-16 mutants (Kloek et al., 2001; Ellis et al., 2002). In contrast a constitutive JA signaling mutant, constitutive expression of vegetative storage protein1 (cev1), showed increased resistance to Erysiphe orontii and G. cichoracearum (Ellis and Turner, 2001; Ellis et al., 2002). Unlike other constitutive JA-signaling mutants (Hilpert et al., 2001) cev1 activates JA-dependent signaling and represses SA-mediated defenses (Ellis et al., 2002). The CEV1 gene encodes cellulose synthase CESA3 and has reduced cellulose levels in addition to being compromised in the jasmonate pathway. The increased resistance to powdery mildew observed in cev1 mutants strongly suggests that the resistance is due to the activation of JA-mediated defenses only and is not dependent on SA-mediated responses. Exogenous application of MeJA to barley induced systemic protection against the virulent barley powdery mildew strain, Blumeria graminis f. sp. hordei (Walters et al., 2002). Interestingly JA-mediated defenses appear to contribute to resistance against the powdery mildews but not against other classes of biotrophic fungal pathogens. For example, in tomato the JA-deficient mutant def1 showed no difference in the production of fungal conidiophores compared to the wild type when inoculated with the fungal biotrophs Cladosporium fulvum and Oidium neolycopersici (Thaler et al., 2004; Table 2). Taken together, these studies indicate that JA signaling plays a role in resistance against the powdery mildews but little is known about the molecular mechanisms underlying this resistance.

Crosstalk between the JA, ET and SA signaling pathways in fungal defense

The magnitude of interplay between the major defense pathways is becoming clearer as research studies continue to expand our understanding of these signaling pathways. JA-mediated signaling appears to work in concert with ET-mediated responses and the expression of PDF1.2 depends on both hormones (Farmer et al., 2003; Guo and Ecker, 2004). In the cev1 mutant, JA is produced at very high levels leading to the constitutive activation of JA-responsive genes including PDF1.2 (Ellis et al., 2002). PDF1.2 expression was repressed in both cev1/coi1-1 and cev1/ethylene response factor1 (etr1) double mutants indicating that cev1 plants require both JA and ET signaling to induce PDF1.2 expression (Ellis et al., 2002). Further when infected with G. cichoracearum, cev1 mutants exhibit enhanced resistance compared to wild type plants (Table 2). The enhanced resistance shown in cev1 mutants against G. cichoracearum is likely due to the constitutive expression of JA-regulated defenses (Ellis et al., 2002).

Activation of SA-mediated responses most often results in the inhibition of JA/ET signaling and vice versa (Gupta et al., 2000; Kunkel and Brooks, 2002). However, there are some genes that may be induced by both SA and JA (Kunkel and Brooks, 2002; Glazebrook et al., 2003). The induction of both SA and JA-mediated signaling has been shown to synergistically enhance resistance to G. cichoracearum as well as bacterial and insect pathogens (Ellis et al., 2002). During haustorium formation in G. cichoracearum, 70 genes were found to have an altered expression pattern during the infection process and several of the genes identified are known to be sensitive to both SA and JA (Fabro et al., 2008). Infection of Arabidopsis by G. cichoracearum results in the early induction of specific SA-mediated responses (Frye er al., 2001) and in JA-mediated induction of PDF1.1, PDF1.2, and PDF1.3 18 h post inoculation (Zimmerli et al., 2004). In jar1-1 and npr1-1 mutants infected with G. cichoracearum, 17 genes showed altered expression patterns (Fabro et al., 2008) including PDF1.1. Interestingly, four of the genes identified play a role in energy metabolism and it has been shown that photo-responsive proteins can influence the JA-mediated pathway (Fabro et al., 2008; Frenkel et al., 2009). It is possible that these genes might play a role in reprogramming metabolism and energy allocation necessary for induction of JA-responsive processes during fungal infection (Fabro et al., 2008).

Several studies have focused on the role of a mitogen-activated protein kinase, MPK4 that induces JA/ET signaling responses and represses SA-mediated defense (Petersen et al., 2000; Brodersen et al., 2006; Kazan and Manners, 2008; Gao et al., 2008; Wang et al., 2009). MPK4 plays an important role in the MAP kinase cascade that is induced by pathogen infection and fungal elicitors such as chitin (Wan et al., 2008). In mpk4/NahG plants, the JA responsive PDF1.2 and THI2.1 genes were not induced by treatment with MeJA despite the fact that both are expressed at normal levels in NahG plants alone suggesting that MPK4 is required for the induction of JA-responsive genes (Petersen et al., 2000). Interestingly, mpk4 mutants show an enhanced resistance to the hemibiotrophic bacterial pathogen, Pseudomonas syringae pv. tomato DC3000 (Qiu et al., 2008; Cui et al., 2010) though no studies have been conducted to analyze the resistance phenotype of mpk4 mutants to fungal biotrophs such as G. cichoracearum.

Another important signaling molecule is WRKY70, a transcription factor that has been shown to regulate JA- and SA-mediated signaling after pathogen infection (Li et al., 2004,2006). Overexpression lines of WRKY70 had an increased resistance to G. cichoracearum, but enhanced susceptibility to A. brassicicola. In contrast, loss-of-function wrky70 plants were more susceptible to G. cichoracearum (Li et al., 2006). Taken together these data illustrate that WRKY70 is necessary to mediate the balance between SA- and JA-induced defense responses against different fungal pathogens (Li et al., 2006).

Conclusions and future directions

JA and its derivatives play diverse roles in several important plant processes such as development, stress responses and pathogen defense. Classically, JA has been shown to be a fundamental component in initiating defense signaling against necrotrophic fungi but mounting recent evidence suggests that JA also plays a role in resistance against specific types of biotrophic fungi. Though the evidence for JA’s role in defense against biotrophic fungi is compelling more studies are needed to verify and expand on existing data. To date, the JA-deficient mutants, jar1 and coi1, have been shown to increase susceptibility to the biotroph G. cichoracearum. (Ellis et al., 2002; Fabro et al., 2008). Additionally, the JA-constitutive mutant, cev1, shows an increased resistance to G. cichoracearum (Ellis et al., 2002). More studies are needed that focus on the resistance of other JA-deficient mutants or mutants that are impaired in JA signaling or biosynthesis to G. cichoracearum and other biotrophic fungi. For example, inoculating JA mutants that are impaired at various points in JA signaling would allow a more precise determination of components of the pathway that are responsible for biotroph resistance. Analysis of the fad3 fad7 fad8 triple mutant and opr3 could be used to pinpoint the role of JA and OPDA biosynthesis with roles in the defense response though studies with opr3 would have to be carefully monitored because of the potential for JA biosynthesis under certain conditions (Chehab et al., 2011). Studies using jin1/myc2 transcription factor double mutants would also provide some insight (Lorenzo et al., 2004). Oxidative stress tolerance and wound response are positively regulated by JIN1, but pathogen defense is negatively regulated by JIN1 (Dombrecht et al., 2007; Kazan and Manners, 2008). By inoculating jin1 mutants with a biotrophic pathogen, one could evaluate the function of JA-mediated gene expression in response to biotroph infection. By using combinations of these mutants in infection studies we may be able to develop a more accurate picture of the role of JA in defense against biotrophic fungi.

This data, however, raises an additional question: is resistance against biotrophs mediated directly by the JA pathway or is JA-mediated defense influenced by crosstalk with other known defense pathways? Previous work has shown that the fungal elicitor chitin activates a MAP kinase cascade (Wan et al., 2008) that includes MPK4 and MPK4 is known to activate JA-mediated signaling (Petersen et al., 2000; Brodersen et al., 2006). However, no studies have tested the defense phenotype of mpk4 mutants against biotrophic fungi. A previous study showed that a deficiency in the JA-signaling pathway lead to an increase in susceptibility to G. cichoracearum (Fabro et al., 2008). Could activated MPK4 be turning on JA-mediated signaling? Studies are needed to observe the expression of JA-responsive genes in inoculated mpk4 mutants. Crosstalk between defense pathways is known to allow the plant to finely tune its responses to a variety of pathogens, but further study is needed to fully understand these mechanisms in defense.

In addition to observing the role of different JA-signaling mutants against known fungal biotrophs, studies also need to be performed to observe the defense response of common JA-deficient mutants (such as jar1 and coi1) against a larger variety of biotrophic fungi. To date, only mutants in JA signaling have been evaluated for their response to the fungal pathogens G. cichoracearum, E. orontii, C. fulvum, and O. neolycopersici (Ellis and Turner, 2001; Ellis et al., 2002; Thaler et al., 2004; Fabro et al., 2008). Furthermore, the study of the JA response in other plants (wheat, rice, barley, tomato, etc.) inoculated with various fungal biotrophs would provide a more complete picture of JA’s role in defense. For example, exogenous application of MeJA to wheat has been shown to confer resistance against insect pests and necrotrophic fungal pathogens (Desmond et al., 2005; El-Wakeil et al., 2010). Could exogenous application of MeJA to wheat also increase its resistance to the biotrophic rust fungus, Puccinia graminis var. tritici? More studies are clearly needed to explore these possibilities.

Together these data highlight the complexities of JA-mediated signaling and defense responses. Recent experimental evidence suggests that JA-mediated signaling plays a role in defense against both biotrophic and necrotrophic fungi. A more complete understanding of these defense responses and the interactions between JA-, SA- and ET-mediated defenses should provide new targets for directed improvement in plant defense against fungal pathogens. With increased food output becoming a top priority to feed a growing global population, the implications of these studies resonate far beyond the laboratory.

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