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Jasmonate Biochemical Pathway

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Science Signaling  16 Feb 2010:
Vol. 3, Issue 109, pp. cm3
DOI: 10.1126/scisignal.3109cm3

Abstract

Plants possess a family of potent fatty acid–derived wound-response and developmental regulators: the jasmonates. These compounds are derived from the tri-unsaturated fatty acids α-linolenic acid (18:3) and, in plants such as Arabidopsis thaliana and tomato, 7(Z)-, 10(Z)-, and 13(Z)-hexadecatrienoic acid (16:3). The lipoxygenase-catalyzed addition of molecular oxygen to α-linolenic acid initiates jasmonate synthesis by providing a 13-hydroperoxide substrate for formation of an unstable allene oxide by allene oxide synthase (AOS). This allene oxide then undergoes enzyme-guided cyclization to produce 12-oxophytodienoic acid (OPDA). These first steps take place in plastids, but further OPDA metabolism occurs in peroxisomes. OPDA has several fates, including esterification into plastid lipids and transformation into the 12-carbon prohormone jasmonic acid (JA). JA is itself a substrate for further diverse modifications, including the production of jasmonoyl-isoleucine (JA-Ile), which is a major biologically active jasmonate among a growing number of jasmonate derivatives. Each new jasmonate family member that is discovered provides another key to understanding the fine control of gene expression in immune responses; in the initiation and maintenance of long-distance signal transfer in response to wounding; in the regulation of fertility; and in the turnover, inactivation, and sequestration of jasmonates, among other processes.

Description

This record contains general information about the Jasmonate Biochemical Pathway collected across species.

Jasmonates: Potent lipid regulators

The jasmonate signaling pathway performs critical roles in plant defense, development, and metabolism and was named for methyl jasmonate (MJ), a compound derived from scented jasmine flowers long used in the perfume industry. MJ is the methyl ester of jasmonic acid (JA), a 12-carbon fatty acid derivative (1) that is only one of many jasmonate isomers (see Fig. 1 for the structures of jasmonate and its precursors). In most plants, the nonvolatile JA is more readily detected than is the volatile methyl ester, and JA conjugated to amino acids, such as l-isoleucine, is thought to be the most important endogenous biologically active member of this family of closely related fatty acid–derived regulators. Vick and Zimmerman (2) demonstrated that JA is derived from the unsaturated fatty acid α-linolenic acid (18:3), an octadecanoid that is abundant in the cellular membranes of higher plants. In many plants, including Arabidopsis thaliana (3), JA may also be synthesized from 7(Z), 10(Z), 13(Z)-hexadecatrienoic acid (16:3) through a hexadecanoid pathway. Because JAs play critical roles in plant defense, some pathogens produce virulence factors that mimic biologically active jasmonates. The metabolic interrelationships of jasmonates are shown in Fig. 2.

Fig. 1

The biosynthetic intermediates (outlined in black) and biologically active members (outlined in blue) of the JA family of signaling lipids. JA is outlined in red. Newly characterized molecules include 12-hydroxy-jasmonoyl-l-isoleucine (12HOJAIle), 12-carboxyjasmonoyl-l-isoleucine (12HOOCJA-I), and jasmonoyl-l-tryptophan (JA-Trp). ACC indicates 1-aminocyclopropane-1-carboxylic acid, and GSH is glutathione.

Fig. 2

Pathway image captured from the dynamic graphical display of the information in the Connections Maps available 12 January 2010. This updated version of the pathway includes 12-hydroxy-jasmonoyl-l-isoleucine (12HOJAIle), 12-carboxyjasmonoyl-l-isoleucine (12HOOCJA-I), and jasmonoyl-l-tryptophan (JA-Trp). Please see the pathway (About Connections Map) for a key to the colors and symbols, for details about the pathway components, and to access the underlying data.

Jasmonate biosynthesis

The synthesis of biologically active JA-isoleucine (JA-Ile) takes place in three cellular compartments. Conversion of 18:3 to the 18-carbon cyclic precursor 12-oxophytodienoic acid (OPDA) takes place in plastids, OPDA is converted to JA in peroxisomes, and JA is conjugated to amino acids in the cytosolic compartment (Table 1).

Table 1 Enzymes involved in jasmonate synthesis in A. thaliana. TAIR ID indicates Arabidopsis Information Resource identification code.
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In addition to all of the intermediates in JA-Ile synthesis, Arabidopsis contains “arabidopsides.” These are galactolipids in which OPDA and dnOPDA are esterified in the plastid primarily to monogalactosyl diacyl glycerol (MDGD) and to digalactosyl diacyl glycerol (DGDG). Arabidopsides, which appear to be Arabidopsis-specific, are rapidly synthesized in response to wounding (46). The bulk of the arabidopside pool is made by LOX2 (At3g45140, 7).

Currently, it is not known whether free or esterified fatty acids are the in vivo substrates that initiate jasmonate synthesis. The enzyme13-lipoxygenase (LOX) converts 18:3 and 16:3 into their hydroperoxide forms [13-hydroperoxylinolenic acid (13-HPOTrE) and 11(S)-hydroperoxy-7(Z), 9(E), 13(Z)-hexadecatrienoic acid (11-HPHTrE), respectively]. Allene oxide synthase (AOS) catalyzes the formation of unstable allene oxide intermediates [linolenic acid 12-allene oxide (12,13-EOTr) and hexadecenoic acid 11-allene oxide (10,11-EHTr), respectively] (8), which undergo enzyme-directed cyclization [mediated by allene oxide cyclase (AOC)] to form the cyclopentenone 12-oxophytodienoic acid (OPDA, 18 carbons) and its 16-carbon homolog, dinor-oxo-phytodienoic acid (dnOPDA).

AOS, which is a cytochrome P450 also known as CYP74, rearranges the oxygens in 13-HPOTrE to form allene oxide, a highly unstable compound. In water, this compound decomposes to form a mixture of racemic OPDA as well as α- and γ-ketols. In cells, however, homotrimeric AOC effectively directs the rearrangement of the allene oxide to 9S,13S-OPDA. The crystal structure of Arabidopsis AOS shows that Phe137 stabilizes the carbon-centered radical intermediate, which is essential for AOS activity (9). Mutation of this residue to a Leu resulted in the enzyme having hydroperoxide lyase activity. Related members of the CYP74 family of cytochrome P450s exist in a few metazoans, including coral (Cnidaria), Trichoplax adhaerens (the only extant member of Placozoa), and lancelets (Cephalochordata). Some phytoprostanes, which are non–enzymatically derived oxygenated fatty acids, are cyclopentenones that have OPDA-like structures. They can activate the expression of genes involved in detoxification in plants (10).

The cyclopentenone ring in the AOS- and AOC-catalyzed products OPDA and dnOPDA is then reduced to form a cyclopentanone {3-oxo-2[2(Z)-pentenyl]-cyclopentane with 8-, 6-, or 4-carbon chains (OPCs)}, which undergoes β-oxidation to form JA (2, 11). This series of peroxisomal reactions begins with the enzyme OPR3, a member of the ubiquitous old yellow enzyme (OYE) family that crystallizes as a dimer. The product of this reaction, known as OPC:8, is then subject to three rounds of β-oxidation to produce JA. In tomato, a single point mutation in acyl–coenzyme A oxidase (LeACX1) abolished JA production and wound responses (12).

After β-oxidation, JA is transported to the cytosol, where it can then be conjugated to amino acids, methylated to form the volatile derivative MJ, or metabolized to another volatile compound, (Z)-jasmone. Another route of JA modification is hydroxylation and subsequent sulfonation (13). Biological activity requires that JA is conjugated to a hydrophobic amino acid, such as isoleucine. JA-Ile is a major biologically active jasmonate that is perceived by the COI1-JAZ complex (14) [see the Arabidopsis Jasmonate Signaling Pathway (About Connections Map) for details]. Fonseca et al. (15) showed that, in Arabidopsis, (+)-7-iso-jasmonoyl isoleucine [(+)-7-iso-JA-Ile] is biologically active in vivo and is the most effective epimer in promoting the interaction of the signaling components COI1 and JAZ9. Epimerization of JAs may produce inactive isomers and may be an in vivo mechanism to terminate JA/JA-Ile activity (1517).

Temporal and spatial accumulation of jasmonates in response to wounding

JA synthesis in response to wounding is induced very rapidly. Within less than 3 min of wounding, JA is detected in leaves distal to a wound. JA and other weakly polar, and presumably biologically active, jasmonates accumulate in the lamina, the flattened region of the leaves (18).

The highly polar JA clearance products, such as hydroxyjasmonates (HOJAs), and a dicarboxy JA-isoleucine derivative 12-carboxyjasmonoyl-l-isoleucine (12HOOCJA-Ile) appear to be concentrated in regions rich in vasculature, such as petioles. These biologically inactive compounds are detectable starting about 20 to 45 min after wounding (18).

Regulation of JA biosynthesis

Mechanisms regulating JA biosynthesis in response to wounding and during development are largely unknown. During the wound response, in some plants, the amount of JA in wounded leaves greatly exceeds that in resting leaves. Arabidopsis microRNA319a (miR319a) is the first described example of a small noncoding RNA (ncRNA) that regulates a hormone biosynthesis pathway. miR319a interacts with transcripts encoding transcription factors of the TCP (teosinte branched1/CYCLOIDEA/PCNA) family (chiefly TCPs 2, 3, 4, 10, and 24), which regulate several genes that encode components of JA biosynthesis, notably LOX2 and, potentially, LOX3, LOX4, AOC1 (At3g25760), and AOS. Overexpression of a version of miR319 that acts as a constitutive repressor of the TCP transcription factors substantially reduces wound-induced JA accumulation (19). Experiments in tobacco also provide evidence for ncRNAs in regulation of JA biosyntheis: Down-regulation of transcripts encoding RNA-directed RNA polymerase reduces the abundance of wound-inducible JA (20). Most of the enzymes involved in JA biosynthesis have been crystallized (Table 1), and exploration of their posttranslational regulation is an open field of investigation. In Arabidopsis, mutation of the gene encoding the PP2C-type protein phosphatase AP2C1 increased JA synthesis after wounding compared with that in wild-type plants (21), suggesting that protein phosphorylation plays a role in negatively regulating one or more steps of JA production.

Jasmonates in defense: Controlling and coordinating direct and indirect defenses

Treatment of plant tissues with JA triggers the accumulation of diverse groups of proteins referred to as JA-inducible proteins (22). The observation that genes encoding wound-inducible proteinase inhibitors in tomato were MJ-inducible (23) initiated an assessment of the contribution of JAs to wound-regulated defense gene expression, with the result that many defense genes are known to be jasmonate-regulated. The JA pathway plays roles in direct defense (where the plant defends itself) against arthropods and pathogens (2429) and is important for resistance to the biotrophic fungal pathogen powdery mildew, Erysiphe cichoracearum (30). The JA pathway also contributes to regulation of the production of many low molecular mass compounds (e.g., arabidopsides and glucosinolates in Arabidopsis) that are involved in plant defense responses (31).

JA signaling is also implicated in nonpathogenic infectious processes, such as the induced systemic resistance (ISR) triggered by nonpathogenic rhizobacterial infection (32). It is increasingly apparent that JA signaling plays diverse roles in mutualistic symbioses (33).

Although it is clear that the absence of JA production compromises defense against many organisms (34), OPDA itself may also function as a regulator in defense and perhaps in other processes (35).

Volatile JA derivatives serve as long-distance communication signals between other plants and other organisms. MJ, for example, can act as a signal within and possibly between plants (23). In laboratory experiments, (Z)-jasmone released by plants can attract or repel insects, as well as activate transcription of target genes in other plants (36, 37). In addition to themselves acting as volatile signals, JAs affect the production of many other volatiles, such as monoterpenes, crucial signals that control the behavior of herbivorous arthropods and their predators. In this way, jasmonates have roles in indirect defense whereby plants call in other organisms that act to defend the plant by attacking herbivores. The JA pathway thus controls both direct and indirect defenses and coordinates their activities (38).

Jasmonates in reproduction and development

An essential role for JA in reproduction was uncovered when Arabidopsis mutants unable to perceive JA exhibited male sterility (39). A role of jasmonate in this process was confirmed by using a triple mutant unable to synthesize JA (40). In Arabidopsis, JA is not only vital for pollen development but also plays a role in anther elongation and timing of pollen release (dehiscence) (41, 42). In tomato, JA production is essential for full female fertility (43). Jasmonates also function to limit petal size in Arabidopsis (44)

Developmental roles for the pathway are not restricted to flowers. For example, trichome production is to some extent dependent on JA signaling (45, 46), and, in the climbing plant Bryonia dioica, tendril coiling, which requires mechanotransduction, may also be under the control of the JA pathway (47). In the absence of JA production, Arabidopsis seeds are larger than those of the wild type (48). The JA-conjugate, jasmonoyl-l-tryptophan (JA-Trp), functions as an auxin signaling inhibitor in roots (49).

Updates to the pathway

In the 2009 update, 12-hydroxy-jasmonoyl-l-isoleucine (12HOJAIle), 12-carboxyjasmonoyl-l-isoleucine (12HOOCJA-I), and jasmonoyl-l-tryptophan (JA-Trp) were added to the pathway. For a historic representation of the pathway before these new elements were added, see Fig. 3.

Fig. 3

Historic pathway image captured from the dynamic graphical display of the information in the Connections Maps available 12 July 2006. Please see the pathway (About Connections Map) for a key to the colors and symbols, for details about the pathway components, and to view the most current information.

Pathway Details

URL: About Connections Map

Scope: Canonical

Acknowledgments

This work is supported by the Swiss National Science Foundation (proposal 3100A0_122441) and by SystemsX.ch.

References and Notes

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