High-resolution profiling of oxylipin-containing galactolipids in Arabidopsis extracts by ultra-performance liquid chromatography/time-of-flight mass spectrometry

Wound-response jasmonate dynamics in the primary vasculature

The links between wound-response electrical signalling and the activation of jasmonate synthesis are unknown. We investigated damage-response remodelling of jasmonate precursor pools in the Arabidopsis thaliana leaf vasculature. Galactolipids and jasmonate precursors in primary veins from undamaged and wounded plants were analysed using MS-based metabolomics and NMR. In parallel, DAD1-LIKE LIPASEs (DALLs), which control the levels of jasmonate precursors in veins, were identified. A novel galactolipid containing the jasmonate precursor 12-oxo-phytodienoic acid (OPDA) was identified in veins: sn-2-O-(cis-12-oxo-phytodienoyl)-sn-3-O-(β-galactopyranosyl) glyceride (sn-2-OPDA-MGMG). Lower levels of sn-1-OPDA-MGMG were also detected. Vascular OPDA-MGMGs, sn-2-18:3-MGMG and free OPDA pools were reduced rapidly in response to damage-activated electrical signals. Reduced function dall2 mutants failed to build resting vascular sn-2-OPDA-MGMG and OPDA pools and, upon wounding, dall2 produced less jasmonoyl-isoleucine (JA-Ile) than the wild-type. DALL3 acted to suppress excess JA-Ile production after wounding, whereas dall2 dall3 double mutants strongly reduce jasmonate signalling in leaves distal to wounds. LOX6 and DALL2 function to produce OPDA and the non-bilayer-forming lipid sn-2-OPDA-MGMG in the primary vasculature. Membrane depolarizations trigger rapid depletion of these molecules. We suggest that electrical signal-dependent lipid phase changes help to initiate vascular jasmonate synthesis in wounded leaves.

Specific Changes in Arabidopsis thaliana Rosette Lipids during Freezing Can Be Associated with Freezing Tolerance

While the roles of a few specific lipids in plant freezing tolerance are understood, the effect of many plant lipids remains to be determined. Acclimation of plants to non-freezing cold before exposure to freezing temperatures improves the outcome of plants, compared to plants exposed to freezing without acclimation. Arabidopsis thaliana plants were subjected to one of three treatments: (1) "control", i.e., growth at 21 °C, (2) "non-acclimated", i.e., 3 days at 21 °C, 2 h at -8 °C, and 24 h recovery at 21 °C, and (3) "acclimated", i.e., 3 days at 4 °C, 2 h at -8 °C, and 24 h recovery at 21 °C. Plants were harvested at seven time points during the treatments, and lipid levels were measured by direct-infusion electrospray ionization tandem mass spectrometry. Ion leakage was measured at the same time points. To examine the function of lipid species in relation to freezing tolerance, the lipid levels in plants immediately following the freezing treatment were correlated with the outcome, i.e., ion leakage 24-h post-freezing. Based on the correlations, hypotheses about the functions of specific lipids were generated. Additionally, analysis of the lipid levels in plants with mutations in genes encoding patatin-like phospholipases, lipoxygenases, and 12-oxophytodienoic acid reductase 3 (opr3), under the same treatments as the wild-type plants, identified only the opr3-2 mutant as having major lipid compositional differences compared to wild-type plants.

Simple liquid chromatography-electrospray ionization ion trap mass spectrometry method for the quantification of galacto-oxylipin arabidopsides in plant samples

A simple and sensitive method to quantify five different arabidopsides by HPLC—ion trap mass spectrometry in complex plant samples was developed and validated. Arabidopsides are oxidized galactolipids first described in Arabidopsis thaliana but also produced by other plant species under stress conditions. External calibration was performed using arabidopsides purified from freeze-thawed Arabidopsis leaves. Lipids were extracted and pre-purified on an SPE silica column before HPLC–MS analysis. Arabidopsides were separated on a C18 column using a gradient of mQ water and acetonitrile:mQ water (85:15) supplemented with formic acid (0.2%) and ammonium formate (12 mM). The method was validated according to European commission decision 2002/657/CE. LOD, LOQ, linearity, intra-day and inter-day precision and accuracy, selectivity, matrix effects and recoveries were determined for the five metabolites. The established method is highly selective in a complex plant matrix. LOD and LOQ were, respectively, in the range 0.098–0.78 and 0.64–1.56 µM, allowing the arabidopside quantification from 25.6–62.4 nmol/g fresh weight. Calibration curve correlation coefficients were higher than 0.997. Matrix effects ranged from -2.09% to 6.10% and recoveries between 70.7% and 109%. The method was successfully applied to complex plant matrixes: Arabidopsis thaliana and Nasturtium officinale.

Epigenetic mapping of the Arabidopsis metabolome reveals mediators of the epigenotype-phenotype map

Identifying the sources of natural variation underlying metabolic differences between plants will enable a better understanding of plant metabolism and provide insights into the regulatory networks that govern plant growth and morphology. So far, however, the contribution of epigenetic variation to metabolic diversity has been largely ignored. In the present study, we utilized a panel of Arabidopsis thaliana epigenetic recombinant inbred lines (epiRILs) to assess the impact of epigenetic variation on the metabolic composition. Thirty epigenetic QTL (QTL^epi) were detected, which partly overlap with QTL^epi linked to growth and morphology. In an effort to identify causal candidate genes in the QTL^epi regions and their putative trans-targets, we performed in silico small RNA and qPCR analyses. Differentially expressed genes were further studied by phenotypic and metabolic analyses of knockout mutants. Three genes were detected that recapitulated the detected QTL^epi effects, providing evidence for epigenetic regulation in cis and in trans. These results indicate that epigenetic mechanisms impact metabolic diversity, possibly via small RNAs, and thus aid in further disentangling the complex epigenotype-phenotype map.

Infection by a foliar endophyte elicits novel arabidopside-based plant defence reactions in its host, Cirsium arvense

Endophytic fungi live asymptomatically within plants. They are usually regarded as nonpathogenic or even mutualistic, but whether plants respond antagonistically to their presence remains unclear, particularly in the little-studied associations between endophytes and nongraminoid herbaceous plants. We investigated the effects of the endophyte Chaetomium cochlioides on leaf chemistry in Cirsium arvense. Plants were sprayed with spores; leaf material from both subsequent new growth and the sprayed leaves was analysed 2 wk later. Infection frequency was 91% and 63% for sprayed and new growth, respectively, indicating that C. cochlioides rapidly infects new foliage. Metabolomic analyses revealed marked changes in leaf chemistry with infection, especially in new growth. Changes in several novel oxylipin metabolites were detected, including arabidopsides reported here for the first time in a plant species other than Arabidopsis thaliana, and a jasmonate-containing galactolipid. The production of these metabolites in response to endophyte presence, particularly in newly infected foliage, suggests that endophytes elicit similar chemical responses in plants to those usually produced following wounding, herbivory and pathogen invasion. Whether endophytes benefit their hosts may depend on a complex series of chemically mediated interactions between the plant, the endophyte, other microbial colonists and natural enemies.

Unusual negative charge-directed fragmentation: collision-induced dissociation of cyclopentenone oxylipins in negative ion mode

RATIONALE

Oxidized fatty acids, and particularly cyclopentenone oxylipins, are electrophilic metabolites that play diverse physiological roles. Current understanding is limited regarding how ion fragmentation provides essential information about oxylipin structures. In this work, unusual products of the collisional activation of deprotonated cyclopentenone oxylipins were investigated.

METHODS

The cyclopentenone oxylipin 12-oxo-phytodienoic acid (OPDA) and its (18)O-labeled forms were ionized using negative-ion mode electrospray ionization, and product ion tandem mass (MS/MS) spectra were generated using collision-induced dissociation (CID). CID-MS/MS spectra were also generated for several cyclopentenone prostaglandins.

RESULTS

Upon collisional activation, deprotonated cyclopentenone oxylipins 12-oxo-phytodienoic acid (OPDA) and dinorOPDA form a characteristic and dominant product ion at m/z 165 that is attributed to charge-directed hydride migration to the electrophilic enone ring followed by elimination of neutral C7H10O2 from the carboxyl end. In contrast, pseudo-MS(3) spectra of deprotonated cyclopentenone prostaglandins exhibited a different fragmentation behavior, in that cleavage near C = C bonds is directed by the carbonyl group in the nearby cyclopentenone ring.

CONCLUSIONS

Two different routes of fragmentation are proposed for cyclopentenone fatty acids with saturated and unsaturated side chains. We predict that this behavior may facilitate the identification of novel cyclopentenone oxylipins and accelerate discoveries of their biological regulatory functions.

Metabolomics reveals herbivore-induced metabolites of resistance and susceptibility in maize leaves and roots

Plants respond to herbivory by reprogramming their metabolism. Most research in this context has focused on locally induced compounds that function as toxins or feeding deterrents. We developed an ultra-high-pressure liquid chromatography time-of-flight mass spectrometry (UHPLC-TOF-MS)-based metabolomics approach to evaluate local and systemic herbivore-induced changes in maize leaves, sap, roots and root exudates without any prior assumptions about their function. Thirty-two differentially regulated compounds were identified from Spodoptera littoralis-infested maize seedlings and isolated for structure assignment by microflow nuclear magnetic resonance (CapNMR). Nine compounds were quantified by a high throughput direct nano-infusion tandem mass spectrometry/mass spectrometry (MS/MS) method. Leaf infestation led to a marked local increase of 1,3-benzoxazin-4-ones, phospholipids, N-hydroxycinnamoyltyramines, azealic acid and tryptophan. Only few changes were found in the root metabolome, but 1,3-benzoxazin-4-ones increased in the vascular sap and root exudates. The role of N-hydroxycinnamoyltyramines in plant-herbivore interactions is unknown, and we therefore tested the effect of the dominating p-coumaroyltyramine on S. littoralis. Unexpectedly, p-coumaroyltyramine was metabolized by the larvae and increased larval growth, possibly by providing additional nitrogen to the insect. Taken together, this study illustrates that herbivore attack leads to the induction of metabolites that can have contrasting effects on herbivore resistance in the leaves and roots.

A non-targeted approach for extended liquid chromatography-mass spectrometry profiling of free and esterified jasmonates after wounding

Upon wounding or herbivory, plants quickly react by activating various defense mechanisms. A major part of these defenses is thought to be regulated by the jasmonate pathway through the induction of jasmonic acid and its biologically active jasmonoyl-isoleucine conjugate. Yet, these well-known phytohormones are only two among the numerous compounds that compose the jasmonate family. Here, we describe a method based on ultrahigh-pressure liquid chromatography coupled to quadrupole-time-of-flight mass spectrometry that can potentially profile the full range of known free and esterified jasmonates in a non-targeted manner. The developed approach is illustrated by the analysis of Arabidopsis thaliana leaves after mechanical wounding.

Analysis of plant galactolipids by reversed-phase high-performance liquid chromatography/mass spectrometry with accurate mass measurement

The composition of plant membrane lipids was investigated by reversed-phase high performance liquid chromatography mass spectrometry with accurate mass measurement. The data dependent methods for the analysis of monogalactosyldiacylglycerols (MGDGs) and digalactosyldiacylglycerols (DGDGs) have been developed. The optimised chromatographic systems were based on a 2.0 mm i.d. Nucleosil C18 column with methanol/water (MGDGs) or acetonitrile/methanol/water (DGDGs) gradients. The galactolipids were ionised by electrospray operated in the positive ion mode and identified based on their MS/MS spectra. High resolution spectra with accurate masses were found to be essential for correct interpretation of the MS data. The elution order of non-oxidised MGDGs and DGDGs followed the equivalent carbon numbers. The methods were applied for detailed characterisation of the MGDGs and DGDGs in the leaves of Arabidopsis thaliana and Melissa officinalis.

Arabidopsis thaliana membrane lipid molecular species and their mass spectral analysis

Herein, current approaches to electrospray ionization mass spectrometry-based analyses of membrane lipid molecular species found in Arabidopsis thaliana are summarized. Additionally, the identities of over 500 reported membrane lipid molecular species are assembled.

Direct infusion mass spectrometry of oxylipin-containing Arabidopsis membrane lipids reveals varied patterns in different stress responses

Direct infusion electrospray ionization triple quadrupole precursor scanning for three oxidized fatty acyl anions revealed 86 mass spectral peaks representing polar membrane lipids in extracts from Arabidopsis (Arabidopsis thaliana) infected with Pseudomonas syringae pv tomato DC3000 expressing AvrRpt2 (PstAvr). Quadrupole time-of-flight and Fourier transform ion cyclotron resonance mass spectrometry provided evidence for the presence of membrane lipids containing one or more oxidized acyl chains. The membrane lipids included molecular species of phosphatidylcholine, phosphatidylethanolamine, phosphatidylglycerol, digalactosyldiacylglycerol, monogalactosyldiacylglycerol, and acylated monogalactosyldiacylglycerol. The oxidized chains were identified at the level of chemical formula and included C(18)H(27)O(3) (abbreviated 18:4-O, to indicate four double bond equivalents and one oxygen beyond the carbonyl group), C(18)H(29)O(3) (18:3-O), C(18)H(31)O(3) (18:2-O), C(18)H(29)O(4) (18:3-2O), C(18)H(31)O(4) (18:2-2O), and C(16)H(23)O(3) (16:4-O). Mass spectral signals from the polar oxidized lipid (ox-lipid) species were quantified in extracts of Arabidopsis leaves subjected to wounding, infection by PstAvr, infection by a virulent strain of P. syringae, and low temperature. Ox-lipids produced low amounts of mass spectral signal, 0.1% to 3.2% as much as obtained in typical direct infusion profiling of normal-chain membrane lipids of the same classes. Analysis of the oxidized membrane lipid species and normal-chain phosphatidic acids indicated that stress-induced ox-lipid composition differs from the basal ox-lipid composition. Additionally, different stresses result in the production of varied amounts, different timing, and different compositional patterns of stress-induced membrane lipids. These data form the basis for a working hypothesis that the stress-specific signatures of ox-lipids, like those of oxylipins, are indicative of their functions.

The Alphabet of Galactolipids in Arabidopsis thaliana

Galactolipids constitute the major lipid class in plants. In recent years oxygenated derivatives of galactolipids have been detected. They are discussed as signal molecules during leaf damage, since they accumulate in wounded leaves in high levels. Using different analytical methods such as nuclear magnetic resonance, infra-red spectroscopy, and high performance liquid chromatography/mass spectrometry (HPLC/MS) earlier reports focused on the analysis of either oxidized or non-oxidized species and needed high levels of analytes. Here, we report on the analysis of the galactolipid subfraction of the Arabidopsis leaf lipidome by an improved HPLC/MS(2)-based method that is fast, robust, and comparatively simple in its performance. Due to a combination of phase partitioning, solid phase fractionation, liquid chromatography, and MS(2) experiments this method has high detection sensitivity and requires only low amounts of plant material. With this method 167 galactolipid species were detected in leaves of Arabidopsis thaliana. Out of these 79 being newly described species. From all species the head group and acyl side chains were identified via MS(2) experiments. Moreover, the structural identification was supported by HPLC/time-of-flight (TOF)-MS and gas chromatography (GC)/MS analysis. The quantification of different galactolipid species that accumulated 30 min after a mechanical wounding in A. thaliana leaves showed that the oxidized acyl side chains in galactolipids are divided into 65% cyclopentenones, 27% methyl-branched ketols, 3.8% hydroperoxides/straight-chain ketols, 2.0% hydroxides, and 2.6% phytoprostanes. In comparison to the free cyclopentenone derivatives, the esterified forms occur in a 149-fold excess supporting the hypothesis that galactolipids might function as storage compounds for cyclopentenones. Additional analysis of the ratio of non-oxidized to oxidized galactolipid species in leaves of wounded plants was performed resulting in a ratio of 2.0 in case of monogalactosyl diacylglycerol (MGD), 8.1 in digalactosyl diacylglycerol (DGD), and 0.6 in the acylated MGD. This indicates that galactolipid oxidation is a major and rapid metabolic process that occurs class specific.

Revealing complexity and specificity in the activation of lipase-mediated oxylipin biosynthesis: a specific role of the Nicotiana attenuata GLA1 lipase in the activation of jasmonic acid biosynthesis in leaves and roots

The activation of enzymatic oxylipin biosynthesis upon wounding, herbivory and pathogen attack depends on the biochemical activation of lipases that make polyunsaturated fatty acids (PUFAs) available to lipoxygenases (LOXs). The identity and number of the lipases involved in this process remain controversial and they probably differ among plant species. Analysis of transgenic Nicotiana attenuata plants (ir-gla1) stably reduced in the expression of the NaGLA1 gene showed that this plastidial glycerolipase is a major supplier of trienoic fatty acids for jasmonic acid (JA) biosynthesis in leaves and roots after wounding and simulated herbivory, but not during infection with the oomycete Phytophthora parasitica (var. nicotianae). NaGLA1 was not essential for the developmental control of JA biosynthesis in flowers and for the biosynthesis of C(6) volatiles by the hydroperoxide lyase (HPL) pathway; however, it affected the metabolism of divinyl ethers (DVEs) early during infection with P. parasitica (var. nicotianae) and the accumulation of NaDES1 and NaLOX1 mRNAs. Profiling of lysolipids by LC-MS/MS was consistent with a rapid activation of NaGLA1 and indicated that this lipase utilizes different lipid classes as substrates. The results revealed the complexity and specificity of the regulation of lipase-mediated oxylipin biosynthesis, highlighting the existence of pathway- and stimulus-specific lipases.

12-oxo-phytodienoic acid accumulation during seed development represses seed germination in Arabidopsis

Arabidopsis thaliana COMATOSE (CTS) encodes an ABC transporter involved in peroxisomal import of substrates for β-oxidation. Various cts alleles and mutants disrupted in steps of peroxisomal β-oxidation have previously been reported to exhibit a severe block on seed germination. Oxylipin analysis on cts, acyl CoA oxidase1 acyl CoA oxidase2 (acx1 acx2), and keto acyl thiolase2 dry seeds revealed that they contain elevated levels of 12-oxo-phytodienoic acid (OPDA), jasmonic acid (JA), and JA-Ile. Oxylipin and transcriptomic analysis showed that accumulation of these oxylipins occurs during late seed maturation in cts. Analysis of double mutants generated by crossing cts with mutants in the JA biosynthesis pathway indicate that OPDA, rather than JA or JA-Ile, contributes to the block on germination in cts seeds. We found that OPDA was more effective at inhibiting wild-type germination than was JA and that this effect was independent of CORONATINE INSENSITIVE1 but was synergistic with abscisic acid (ABA). Consistent with this, OPDA treatment increased ABA INSENSITIVE5 protein abundance in a manner that parallels the inhibitory effect of OPDA and OPDA+ABA on seed germination. These results demonstrate that OPDA acts along with ABA to regulate seed germination in Arabidopsis.

Ultra performance liquid chromatography and high resolution mass spectrometry for the analysis of plant lipids

Holistic analysis of lipids is becoming increasingly popular in the life sciences. Recently, several interesting, mass spectrometry-based studies have been conducted, especially in plant biology. However, while great advancements have been made we are still far from detecting all the lipids species in an organism. In this study we developed an ultra performance liquid chromatography-based method using a high resolution, accurate mass, mass spectrometer for the comprehensive profiling of more than 260 polar and non-polar Arabidopsis thaliana leaf lipids. The method is fully compatible to the commonly used lipid extraction protocols and provides a viable alternative to the commonly used direct infusion-based shotgun lipidomics approaches. The whole process is described in detail and compared to alternative lipidomic approaches. Next to the developed method we also introduce an in-house developed database search software (GoBioSpace), which allows one to perform targeted or un-targeted lipidomic and metabolomic analysis on mass spectrometric data of every kind.

The interplay of lipid acyl hydrolases in inducible plant defense

Lipid acyl hydrolases (LAH) have received recently increased attention in the context of plant defense. Multiple structurally unrelated gene families have been annotated in Arabidopsis as encoding potential lipid deacylating enzymes with numerous members being transcriptionally activated upon biotic stress. Confirming in silico predictions, experimental data have illustrated the wide subcellular distribution of LAHs indicating they likely interact with distinct membrane systems to initiate specific cellular responses. While recombinant LAHs are active in vitro on a small set of polar lipids, precise knowledge of in vivo substrates and hydrolysis products is generally lacking. Functional analysis of a few LAHs has revealed their roles in initiating oxylipin biosynthesis, cell death execution, signalling or direct antimicrobial activity. The picture emerging is that pathogenic challenge triggers a complex network of lipid hydrolysis events across the cellular compartments resulting in changes in membrane structures and release of signal precursors involved in the building-up of an adequate immune response.

Development and validation of a liquid chromatography-electrospray ionization-time-of-flight mass spectrometry method for induced changes in Nicotiana attenuata leaves during simulated herbivory

A liquid chromatography-electrospray ionization-time-of-flight mass spectrometry (HPLC/ESI-TOF-MS) procedure was developed to characterize changes induced in Nicotiana attenuata leaves 1 h and 5 days after wounding and application of Manduca sexta elicitors. The constancy of the measurement conditions was first confirmed for 22 selected analytes spanning the entire chromatogram. Using the Profile Analysis software, we extracted 367 buckets, which were analyzed by principal component analysis and two-factorial ANOVA. One hundred seventy-three buckets were found to be statistically regulated, 128 due to time effects, and 85 due to treatment effects. In vivo 15N-isotope labeling was used to facilitate the annotation and the interpretation of the fragmentation pattern of nitrogen-containing metabolites, and a correlation analysis was performed to test mathematical relationships existing among potential in-source fragments. Additionally, tandem MS measurements of the most regulated ions are presented. Altogether, this study defines a framework for the mining and annotation of major herbivory-elicited changes in Nicotiana attenuata.

Jasmonates

ARABIDOPSIS IS A SUPERB MODEL FOR THE STUDY OF AN IMPORTANT SUBGROUP OF OXYLIPINS: the jasmonates. Jasmonates control many responses to cell damage and invasion and are essential for reproduction. Jasmonic acid (JA) is a prohormone and is conjugated to hydrophobic amino acids to produce regulatory ligands. The major receptor for active jasmonate ligands is closely related to auxin receptors and, as in auxin signaling, jasmonate signaling requires the destruction of repressor proteins. This chapter uses a frequently asked question (FAQ) approach and concludes with a practical section.

Velocity estimates for signal propagation leading to systemic jasmonic acid accumulation in wounded Arabidopsis

The wound response prohormone jasmonic acid (JA) accumulates rapidly in tissues both proximal and distal to injury sites in plants. Using quantitative liquid chromatography-mass spectrometry after flash freezing of tissues, we found that JA accumulated within 30 s of injury in wounded Arabidopsis leaves (p = 3.5 e(-7)). JA augmentation distal to wounds was strongest in unwounded leaves with direct vascular connections to wounded leaves wherein JA levels increased significantly within 120 s of wounding (p = 0.00027). This gave conservative and statistically robust temporal boundaries for the average velocity of the long distance signal leading to distal JA accumulation in unwounded leaves of 3.4-4.5 cm min(-1). Like JA, transcripts of the JA synthesis gene LIPOXYGENASE2 (LOX2) and the jasmonate response gene JAZ10.3 also accumulated to higher levels in directly interconnected leaves than in indirectly connected leaves. JA accumulation in a lox2-1 mutant plant was initiated rapidly after wounding then slowed progressively compared with the wild type (WT). Despite this, JAZ10.3 expression in the two genotypes was similar. Free cyclopentenone jasmonate levels were similar in both resting WT and lox2-1. In contrast, bound cyclopentenone jasmonates (arabidopsides) were far lower in lox2-1 than in the WT. The major roles of LOX2 are to generate arabidopsides and the large levels of JA that accumulate proximal to the wound. LOX2 is not essential for some of the most rapid events elicited by wounding.

MS-based Plant Metabolomic Approaches for Biomarker Discovery

Metabolomics, which aims at the comprehensive and quantitative analysis of wide arrays of metabolites in biological samples, is playing an increasingly important role in plant science. Various biological issues have been successfully studied by this holistic approach that includes global metabolite composition assessment, mutant characterization, taxonomy, developmental processes, stress response, interaction with environment, quality control assessment and mode of action of herbal medicine. This review summarizes the main mass spectrometry methods used for performing these studies and discusses the potential, but also the current limitations of the various approaches. The intention is not to cover exhaustively the field, which has considerably grown over about one decade, but to give a brief insight into the methods commonly employed and discuss some applications that reveal the potential of metabolomics in phytochemistry and systems biology.

Methods for the analysis of oxylipins in plants

Plant oxylipins comprise a highly diverse and complex class of molecules that are derived from lipid oxidation. The initial oxidation of unsaturated fatty acids may either occur by enzymatic or chemical reactions. A large variety of oxylipin classes are generated by an array of alternative reactions further converting hydroperoxy fatty acids. The structural diversity of oxylipins is further increased by their occurrence either as free fatty acid derivatives or as esters in complex lipids. Lipid peroxidation is common to all biological systems, appearing in developmentally regulated processes and as a response to environmental changes. The oxylipins formed may perform various biological roles; some of them have signaling functions. In order to elucidate the roles of oxylipins in a given biological context, comprehensive analytical assays are available for determining the oxylipin profiles of plant tissues. This review summarizes indirect methods to estimate the general peroxidation state of a sample and more sophisticated techniques for the identification, structure determination and quantification of oxylipins.

Oxylipins: structurally diverse metabolites from fatty acid oxidation

Oxylipins are lipophilic signaling molecules derived from the oxidation of polyunsaturated fatty acids. Initial fatty acid oxidation occurs mainly by the enzymatic or chemical formation of fatty acid hydroperoxides. An array of alternative reactions further converting fatty acid hydroperoxides gives rise to a multitude of oxylipin classes, many with reported signaling functions in plants. Oxylipins include the phytohormone, jasmonic acid, and a number of other molecules including hydroxy-, oxo- or keto-fatty acids or volatile aldehydes that may perform various biological roles as second messengers, messengers in inter-organismic signaling, or even as bactericidal agents. The structural diversity of oxylipins is further increased by esterification of the compounds in plastidial glycolipids, for instance the Arabidopsides, or by conjugation of oxylipins to amino acids or other metabolites. The enzymes involved in oxylipin metabolism are diverse and comprise a multitude of examples with interesting and unusual catalytic properties. In addition, the interplay of different subcellular compartments during oxylipin biosynthesis suggests complex mechanisms of regulation that are not well understood. This review aims at giving an overview of plant oxylipins and the multitude of enzymes responsible for their biosynthesis.