Conjugation of keto fatty acids to glutathione in plant tissues. Characterization and quantification by HPLC-tandem mass spectrometry

The Michael addition of thiols to 13-oxo-octadecadienoate (13-oxo-ODE) with implications for LC-MS analysis of glutathione conjugation

Unsaturated fatty acid ketones with αβ,γδ conjugation are susceptible to Michael addition of thiols, with unresolved issues on the site of adduction and precise structures of the conjugates. Herein we reacted 13-keto-octadecadienoic acid (13-oxo-ODE or 13-KODE) with glutathione (GSH), N-acetyl-cysteine, and β-mercaptoethanol and identified the adducts. HPLC-UV analyses indicated none of the products exhibit a conjugated enone UV chromophore, a result that conflicts with the literature and is relevant to the mass spectral interpretation of 1,4 versus 1,6 thiol adduction. Aided by the development of an HPLC solvent system that separates the GSH diastereomers and thus avoids overlap of signals in proton NMR experiments, we established the two major conjugates are formed by 1,6 addition of GSH at the 9-carbon of 13-oxo-ODE with the remaining double bond α to the thiol in the 10,11 position. N-acetyl cysteine reacts similarly, while β-mercaptoethanol gives equal amounts of 1,4 and 1,6 addition products. Equine glutathione transferase catalyzed 1,6 addition of GSH to the two major diastereomers in 44:56 proportions. LC-MS in positive ion mode gives a product ion interpreted before as evidence of 1,4-thiol adduction, whereas here we find this ion using the authentic 1,6 adduct. LC-MS with negative ion APCI gave a fragment selective for 1,4 adduction. These results clarify the structures of thiol conjugates of a prototypical unsaturated keto-fatty acid and have relevance to the application of LC-MS for the structural analysis of keto-fatty acid glutathione conjugation.

Oxylipins and Reactive Carbonyls as Regulators of the Plant Redox and Reactive Oxygen Species Network under Stress

Reactive oxygen species (ROS), and in particular H2O2, serve as essential second messengers at low concentrations. However, excessive ROS accumulation leads to severe and irreversible cell damage. Hence, control of ROS levels is needed, especially under non-optimal growth conditions caused by abiotic or biotic stresses, which at least initially stimulate ROS synthesis. A complex network of thiol-sensitive proteins is instrumental in realizing tight ROS control; this is called the redox regulatory network. It consists of sensors, input elements, transmitters, and targets. Recent evidence revealed that the interplay of the redox network and oxylipins–molecules derived from oxygenation of polyunsaturated fatty acids, especially under high ROS levels–plays a decisive role in coupling ROS generation and subsequent stress defense signaling pathways in plants. This review aims to provide a broad overview of the current knowledge on the interaction of distinct oxylipins generated enzymatically (12-OPDA, 4-HNE, phytoprostanes) or non-enzymatically (MDA, acrolein) and components of the redox network. Further, recent findings on the contribution of oxylipins to environmental acclimatization will be discussed using flooding, herbivory, and establishment of thermotolerance as prime examples of relevant biotic and abiotic stresses.

Organelles and phytohormones: a network of interactions in plant stress responses

Phytohormones are major signaling components that contribute to nearly all aspects of plant life. They constitute an interconnected communication network to fine-tune growth and development in response to the ever-changing environment. To this end, they have to coordinate with other signaling components, such as reactive oxygen species and calcium signals. On the one hand, the two endosymbiotic organelles, plastids and mitochondria, control various aspects of phytohormone signaling and harbor important steps of hormone precursor biosynthesis. On the other hand, phytohormones have feedback actions on organellar functions. In addition, organelles and phytohormones often act in parallel in a coordinated matter to regulate cellular functions. Therefore, linking organelle functions with increasing knowledge of phytohormone biosynthesis, perception, and signaling will reveal new aspects of plant stress tolerance. In this review, we highlight recent work on organelle-phytohormone interactions focusing on the major stress-related hormones abscisic acid, jasmonates, salicylic acid, and ethylene.

Color recycling: metabolization of apocarotenoid degradation products suggests carbon regeneration via primary metabolic pathways

Analysis of carotenoid-accumulating roots revealed that oxidative carotenoid degradation yields glyoxal and methylglyoxal. Our data suggest that these compounds are detoxified via the glyoxalase system and re-enter primary metabolic pathways. Carotenoid levels in plant tissues depend on the relative rates of synthesis and degradation. We recently identified redox enzymes previously known to be involved in the detoxification of fatty acid-derived reactive carbonyl species which were able to convert apocarotenoids into corresponding alcohols and carboxylic acids. However, their subsequent metabolization pathways remain unresolved. Interestingly, we found that carotenoid-accumulating roots have increased levels of glutathione, suggesting apocarotenoid glutathionylation to occur. In vitro and in planta investigations did not, however, support the occurrence of non-enzymatic or enzymatic glutathionylation of β-apocarotenoids. An alternative breakdown pathway is the continued oxidative degradation of primary apocarotenoids or their derivatives into the shortest possible oxidation products, namely glyoxal and methylglyoxal, which also accumulated in carotenoid-accumulating roots. In fact, combined transcriptome and metabolome analysis suggest that the high levels of glutathione are most probably required for detoxifying apocarotenoid-derived glyoxal and methylglyoxal via the glyoxalase pathway, yielding glycolate and D-lactate, respectively. Further transcriptome analysis suggested subsequent reactions involving activities associated with photorespiration and the peroxisome-specific glycolate/glyoxylate transporter. Finally, detoxified primary apocarotenoid degradation products might be converted into pyruvate which is possibly re-used for the synthesis of carotenoid biosynthesis precursors. Our findings allow to envision carbon recycling during carotenoid biosynthesis, degradation and re-synthesis which consumes energy, but partially maintains initially fixed carbon via re-introducing reactive carotenoid degradation products into primary metabolic pathways.

12-oxo-Phytodienoic Acid: A Fuse and/or Switch of Plant Growth and Defense Responses?

12-oxo-Phytodienoic acid (OPDA) is a primary precursor of (-)-jasmonic acid (JA), able to trigger autonomous signaling pathways that regulate a unique subset of jasmonate-responsive genes, activating and fine-tuning defense responses, as well as growth processes in plants. Recently, a number of studies have illuminated the physiol-molecular activities of OPDA signaling in plants, which interconnect the regulatory loop of photosynthesis, cellular redox homeostasis, and transcriptional regulatory networks, together shedding new light on (i) the underlying modes of cellular interfaces between growth and defense responses (e.g., fitness trade-offs or balances) and (ii) vital information in genetic engineering or molecular breeding approaches to upgrade own survival capacities of plants. However, our current knowledge regarding its mode of actions is still far from complete. This review will briefly revisit recent progresses on the roles and mechanisms of OPDA and information gaps within, which help in understanding the phenotypic and environmental plasticity of plants.

Electrophile Modulation of Inflammation: A Two-Hit Approach

Electrophilic small molecules have gained significant attention over the last decade in the field of covalent drug discovery. Long recognized as mediators of the inflammatory process, recent evidence suggests that electrophiles may modulate the immune response through the regulation of metabolic networks. These molecules function as pleiotropic signaling mediators capable of reversibly reacting with nucleophilic biomolecules, most notably at reactive cysteines. More specifically, electrophiles target critical cysteines in redox regulatory proteins to activate protective pathways such as the nuclear factor erythroid 2-related factor 2-Kelch-like ECH-associated protein 1 (Nrf2-Keap1) antioxidant signaling pathway while also inhibiting Nuclear Factor κB (NF-κB). During inflammatory states, reactive species broadly alter cell signaling through the oxidation of lipids, amino acids, and nucleic acids, effectively propagating the inflammatory sequence. Subsequent changes in metabolic signaling inform immune cell maturation and effector function. Therapeutic strategies targeting inflammatory pathologies leverage electrophilic drug compounds, in part, because of their documented effect on the redox balance of the cell. With mounting evidence demonstrating the link between redox signaling and metabolism, electrophiles represent ideal therapeutic candidates for the treatment of inflammatory conditions. Through their pleiotropic signaling activity, electrophiles may be used strategically to both directly and indirectly target immune cell metabolism.

12-Oxophytodienoic Acid Reductase 3 (OPR3) Functions as NADPH-Dependent α,β-Ketoalkene Reductase in Detoxification and Monodehydroascorbate Reductase in Redox Homeostasis

Arabidopsis (Arabidopsis thaliana) 12-oxophytodienoic acid reductase isoform 3 (OPR3) is involved in the synthesis of jasmonic acid (JA) by reducing the α,β-unsaturated double bond of the cyclopentenone moiety in 12-oxophytodienoic acid (12-OPDA). Recent research revealed that JA synthesis is not strictly dependent on the peroxisomal OPR3. The ability of OPR3 to reduce trinitrotoluene suggests that the old yellow enzyme homolog OPR3 has additional functions. Here, we show that OPR3 catalyzes the reduction of a wide spectrum of electrophilic species that share a reactivity toward the major redox buffers glutathione (GSH) and ascorbate (ASC). Furthermore, we show that 12-OPDA reacts with ASC to form an ASC-12-OPDA adduct, but in addition OPR3 has the ability to regenerate ASC from monodehydroascorbate. The presented data characterize OPR3 as a bifunctional enzyme with NADPH-dependent α,β-ketoalkene double-bond reductase and monodehydroascorbate reductase activities (MDHAR). opr3 mutants showed a slightly less-reduced ASC pool in leaves in line with the MDHAR activity of OPR3 in vitro. These functions link redox homeostasis as mediated by ASC and GSH with OPR3 activity and metabolism of reactive electrophilic species.

Protein-Ligand Fishing in planta for Biologically Active Natural Products Using Glutathione Transferases

Screening for natural products which bind to proteins in planta has been used to identify ligands of the plant-specific glutathione transferase (GST) tau (U) and phi (F) classes, that are present in large gene families in crops and weeds, but have largely undefined functions. When expressed as recombinant proteins in Escherichia coli these proteins have been found to tightly bind a diverse range of natural product ligands, with fatty acid-and porphyrinogen-derivatives associated with GSTUs and a range of heterocyclic compounds with GSTFs. With an interest in detecting the natural binding partners of these proteins in planta, we have expressed the two best characterized GSTs from Arabidopsis thaliana (At), AtGSTF2 and AtGSTU19, as Strep-tagged fusion proteins in planta. Following transient and stable expression in Nicotiana and Arabidopsis, respectively, the GSTs were recovered using Strep-Tactin affinity chromatography and the bound ligands desorbed and characterized by LC-MS. AtGSTF2 predominantly bound phenolic derivatives including S-glutathionylated lignanamides and methylated variants of the flavonols kaempferol and quercetin. AtGSTU19 captured glutathionylated conjugates of oxylipins, indoles, and lignanamides. Whereas the flavonols and oxylipins appeared to be authentic in vivo ligands, the glutathione conjugates of the lignanamides and indoles were artifacts formed during extraction. When tested for their binding characteristics, the previously undescribed indole conjugates were found to be particularly potent inhibitors of AtGSTU19. Such ligand fishing has the potential to both give new insight into protein function in planta as well as identifying novel classes of natural product inhibitors of enzymes of biotechnological interest such as GSTs.

A Strategy Combining Higher Energy C-Trap Dissociation with Neutral Loss- and Product Ion-Based MSn Acquisition for Global Profiling and Structure Annotation of Fatty Acids Conjugates

Fatty acids conjugates (FACs) are ubiquitous but found in trace amounts in the natural world. They are composed of multiple unknown substructures and side chains. Thus, FACs are difficult to be analyzed by traditional mass spectrometric methods. In this study, an integrated strategy was developed to global profiling and targeted structure annotation of FACs in complex matrix by LTQ Orbitrap. Dicarboxylic acid conjugated bufotoxins (DACBs) in Venenum bufonis (VB) were used as model compounds. The new strategy (abbreviated as HPNA) combined higher-energy C-trap dissociation (HCD) with product ion- (PI), neutral loss- (NL) based MSⁿ (n ≥ 3) acquisition in both positive-ion mode and negative-ion mode. Several advantages are presented. First, various side chains were found under HCD in negative-ion mode, which included both known and unknown side chains. Second, DACBs with multiple side chains were simultaneously detected in one run. Compared with traditional quadrupole-based mass method, it greatly increased analysis throughput. Third, the fragment ions of side chain and steroids substructure could be obtained by PI- and NL-based MSⁿ acquisition, respectively, which greatly increased the accuracy of the structure annotation of DACBs. In all, 78 DACBs have been discovered, of which 68 were new compounds; 25 types of substructure formulas and seven dicarboxylic acid side chains were found, especially five new side chains, including two saturated dicarboxylic acids [(azelaic acid (C₉) and sebacic acid (C₁₀)] and three unsaturated dicarboxylic acids (u-C₈, u-C₉, and u-C₁₀). All these results greatly enriched the structures of DACBs in VB. Graphical Abstract ᅟ.

The still mysterious roles of cysteine-containing glutathione transferases in plants

Glutathione transferases (GSTs) represent a widespread multigenic enzyme family able to modify a broad range of molecules. These notably include secondary metabolites and exogenous substrates often referred to as xenobiotics, usually for their detoxification, subsequent transport or export. To achieve this, these enzymes can bind non-substrate ligands (ligandin function) and/or catalyze the conjugation of glutathione onto the targeted molecules, the latter activity being exhibited by GSTs having a serine or a tyrosine as catalytic residues. Besides, other GST members possess a catalytic cysteine residue, a substitution that radically changes enzyme properties. Instead of promoting GSH-conjugation reactions, cysteine-containing GSTs (Cys-GSTs) are able to perform deglutathionylation reactions similarly to glutaredoxins but the targets are usually different since glutaredoxin substrates are mostly oxidized proteins and Cys-GST substrates are metabolites. The Cys-GSTs are found in most organisms and form several classes. While Beta and Omega GSTs and chloride intracellular channel proteins (CLICs) are not found in plants, these organisms possess microsomal ProstaGlandin E-Synthase type 2, glutathionyl hydroquinone reductases, Lambda, Iota and Hemerythrin GSTs and dehydroascorbate reductases (DHARs); the four last classes being restricted to the green lineage. In plants, whereas the role of DHARs is clearly associated to the reduction of dehydroascorbate to ascorbate, the physiological roles of other Cys-GSTs remain largely unknown. In this context, a genomic and phylogenetic analysis of Cys-GSTs in photosynthetic organisms provides an updated classification that is discussed in the light of the recent literature about the functional and structural properties of Cys-GSTs. Considering the antioxidant potencies of phenolic compounds and more generally of secondary metabolites, the connection of GSTs with secondary metabolism may be interesting from a pharmacological perspective.

Carotenoid oxidation products as stress signals in plants

Carotenoids are known to play important roles in plants as antioxidants, accessory light-harvesting pigments, and attractants for pollinators and seed dispersers. A new function for carotenoids has recently emerged, which relates to the response of plants to environmental stresses. Reactive oxygen species, especially singlet oxygen, produced in the chloroplasts under stress conditions, can oxidize carotenoids leading to a variety of oxidized products, including aldehydes, ketones, endoperoxides and lactones. Some of those carotenoid derivatives, such as volatile β-cyclocitral, derived from the oxidation of β-carotene, are reactive electrophile species that are bioactive and can induce changes in gene expression leading to acclimation to stress conditions. This review summarizes the current knowledge on the non-enzymatic oxidation of carotenoids, the bioactivity of the resulting cleavage compounds and their functions as stress signals in plants.

Cryptogein-induced transcriptional reprogramming in tobacco is light dependent

The fungal elicitor cryptogein triggers a light-dependent hypersensitive response in tobacco (Nicotiana tabacum). To assess the effect of light on this nonhost resistance in more detail, we studied various aspects of the response under dark and light conditions using the tobacco-cryptogein experimental system. Here, we show that light drastically alters the plant's transcriptional response to cryptogein, notably by dampening the induction of genes involved in multiple processes, such as ethylene biosynthesis, secondary metabolism, and glutathione turnover. Furthermore, chlorophyll fluorescence measurements demonstrated that quantum yield and functioning of the light-harvesting antennae decreased simultaneously, indicating that photoinhibition underlies the observed decreased photosynthesis and that photooxidative damage might be involved in the establishment of the altered response. Analysis of the isomer distribution of hydroxy fatty acids illustrated that, in the light, lipid peroxidation was predominantly due to the production of singlet oxygen. Differences in (reduced) glutathione concentrations and the rapid development of symptoms in the light when cryptogein was coinfiltrated with glutathione biosynthesis inhibitors suggest that glutathione might become a limiting factor during the cryptogein-induced hypersensitive response in the dark and that this response might be modified by an increased antioxidant availability in the light.

Nonenzymic carotenoid oxidation and photooxidative stress signalling in plants

Carotenoids play a crucial protective role in photosynthetic organisms as quenchers of singlet oxygen ((1)O(2)). This function occurs either via a physical mechanism involving thermal energy dissipation or via a chemical mechanism involving direct oxidation of the carotenoid molecule. The latter mechanism can produce a variety of aldehydic or ketonic cleavage products containing a reactive carbonyl group. One such molecule, the volatile β-carotene derivative β-cyclocitral, triggers changes in the expression of (1)O(2)-responsive genes and leads to an enhancement of photooxidative stress tolerance. Thus, besides their well-known functions in light harvesting and photoprotection, carotenoids can also play a role through their nonenzymic oxidation in the sensing and signalling of reactive oxygen species and photooxidative stress in photosynthetic organisms. Enzymic carotenoid oxidation does not seem to play a significant role in this phenomenon. Elucidation of the carotenoid-mediated (1)O(2) signalling pathway could provide new targets for improving photooxidative stress tolerance of plants.

12-oxo-phytodienoic acid-glutathione conjugate is transported into the vacuole in Arabidopsis

While exogenous toxic compounds such as herbicides are thought to be sequestered into vacuoles in the form of glutathione (GSH) conjugates, little is understood about natural plant products conjugated with GSH. To identify natural products conjugated with GSH in plants, metabolites in the Arabidopsis γ-glutamyl transpeptidase (ggt) 4 knockout mutants that are blocked in the degradation of GSH conjugates in the vacuole were compared with those in wild-type plants. Among the metabolites identified, one was confirmed to be the 12-oxo-phytodienoic acid (OPDA)-GSH conjugate, indicating that OPDA, a precursor of jasmonic acid (JA), is transported into the vacuole as a GSH conjugate.

Signaling actions of electrophiles: anti-inflammatory therapeutic candidates

Over the past several years, research on biologically relevant electrophiles has been replete with new insights, expanding our understanding of the roles electrophiles play in vivo. Importantly, many electrophiles can form reversible covalent adducts with both proteins and small-molecule thiols in cells. This post-translational protein modification has important ramifications, including changes in protein enzymatic activity, the transduction of signals within and between cells, and alterations in gene expression. Electrophiles modulate a variety of cellular signaling processes that are involved in several major diseases with inflammatory components. The electrophilic fatty-acid derivatives discussed in this work are naturally occurring products of redox reactions and enzymatic activity. Furthermore, several of these electrophilic species and their derivatives represent potential therapeutic candidates.

Roles for glutathione transferases in plant secondary metabolism

Plant glutathione transferases (GSTs) are classified as enzymes of secondary metabolism, but while their roles in catalysing the conjugation and detoxification of herbicides are well known, their endogenous functions are largely obscure. Thus, while the presence of GST-derived S-glutathionylated xenobiotics have been described in many plants, there is little direct evidence for the accumulation of similarly conjugated natural products, despite the presence of a complex and dichotomous metabolic pathway which processes these reaction products. The conservation in glutathione conjugating and processing pathways, the co-regulation of GSTs with inducible plant secondary metabolism and biochemical studies showing the potential of these enzymes to conjugate reactive natural products are all suggestive of important endogenous functions. As a framework for addressing these enigmatic functions we postulate that either: (a) the natural reaction products of GSTs are unstable and undergo reversible S-glutathionylation; (b) the conjugation products of GSTs are very rapidly processed to derived metabolites; (c) GSTs do not catalyse conventional conjugation reactions but instead use glutathione as a cofactor rather than co-substrate; or (d) GSTs are non-catalytic and function as transporter proteins for secondary metabolites and their unstable intermediates. In this review, we describe how enzyme biochemistry and informatics are providing clues as to GST function allowing for the critical evaluation of each of these hypotheses. We also present evidence for the involvement of GSTs in the synthesis of sulfur-containing secondary metabolites such as volatiles and glucosinolates, and the conjugation, transport and storage of reactive oxylipins, phenolics and flavonoids.

Nucleophile addition of reduced glutathione on 2-methyl-2-nitroso compound: a combined electron paramagnetic resonance spectroscopy and electrospray tandem mass spectrometry study

Mass spectrometry (MS) was used in conjunction with electron paramagnetic resonance (EPR) to characterize products arising from reactions between reduced glutathione (GSH) and 2-methyl 2-nitroso propane (MNP) in an oxidative medium, to evaluate the reactivity of this tripeptide as a nucleophile toward a nitroso compound. Depending on the experimental conditions, different radical species could be detected by EPR, which allowed some structural assumptions. These samples were then submitted to electrospray ionization, in both positive and negative ion modes, for structural elucidation in tandem mass spectrometry. Although the primary nitroxide products could not be detected in MS, structurally related compounds such as hydroxylamine and O-methyl hydroxylamine could be fully characterized. In the absence of light, a S-adduct was formed via a Forrester-Hepburn reaction, that is, a nucleophile addition of MNP onto the thiol function in reduced glutathione to yield a hydroxylamine intermediate, further oxidized into nitroxide. In contrast, irradiating the reaction medium with visible light could allow an inverted spin trapping reaction to take place, involving the oxidation of both MNP and GSH before the nucleophilic addition of the sulfenic acid function onto the nitrogen of MNP, yielding a so-called O-adduct. It was also found that dilution of the reaction medium with methanol for the purpose of electrospray ionization could allow nitroxides to be indirectly observed either as hydroxylamine or O-methyl hydroxylamine species.

The phytohormone precursor OPDA is isomerized in the insect gut by a single, specific glutathione transferase

Oxylipins play important roles in stress signaling in plants. The compound 12-oxophytodienoic acid (cis-OPDA) is an early biosynthetic precursor of jasmonic acid (JA), the key phytohormone orchestrating the plant anti-herbivore defense. When consumed by feeding Lepidopteran larvae, plant-derived cis-OPDA suffers rapid isomerization to iso-OPDA in the midgut and is excreted in the frass. Unlike OPDA epimerization (yielding trans-OPDA), the formation of iso-OPDA is enzyme-dependent, and is catalyzed by an inducible glutathione transferase (GSTs) from the larval gut. Purified GST fractions from the gut of Egyptian cotton leafworm (Spodoptera littoralis) and cotton bollworm (Helicoverpa armigera) both exhibited strong OPDA isomerization activity, most likely via transient formation of a glutathione-OPDA conjugate. Out of 16 cytosolic GST proteins cloned from the gut of cotton bollworm larvae and expressed in E. coli, only one catalyzed the OPDA isomerization. The biological function of the double bond shift might be seen in an inactivation of cis-OPDA, similar to the inactivation of prostaglandin A1 to prostaglandin B1 in mammalian tissue. The enzymatic isomerization is particularly widespread among generalist herbivores that have to cope with various amounts of cis-OPDA in their spectrum of host plants.

Selective binding of glutathione conjugates of fatty acid derivatives by plant glutathione transferases

Proteomic studies with Arabidopsis thaliana have revealed that the plant-specific Tau (U) class glutathione transferases (GSTs) are selectively retained by S-hexylglutathione affinity supports. Overexpression of members of the Arabidopsis GST superfamily in Escherichia coli showed that 25 of the complement of 28 GSTUs caused the aberrant accumulation of acylated glutathione thioesters in vivo, a perturbation that was not observed with other GST classes. Each GSTU caused a specific group of fatty acyl derivatives to accumulate, which varied in chain length (C(6) to C(18)), additional oxygen content (0 or 1), and desaturation (0 or 1). Thioesters bound tightly to recombinant GSTs (K(d) approximately 1 microm), explaining their accumulation. Transient expression of GSTUs in Nicotiana benthamiana followed by recovery by Strep-tag affinity chromatography allowed the respective plant ligands to be extracted and characterized. Again, each GST showed a distinct profile of recovered metabolites, notably glutathionylated oxophytodienoic acid and related oxygenated fatty acids. Similarly, the expression of the major Tau protein GSTU19 in the endogenous host Arabidopsis led to the selective binding of the glutathionylated oxophytodienoic acid-glutathione conjugate, with the enzyme able to catalyze the conjugation reaction. Additional ligands identified in planta included other fatty acid derivatives including divinyl ethers and glutathionylated chlorogenic acid. The strong and specific retention of various oxygenated fatty acids by each GSTU and the conservation in binding observed in the different hosts suggest that these proteins have selective roles in binding and conjugating these unstable metabolites in vivo.

Plant oxylipins: plant responses to 12-oxo-phytodienoic acid are governed by its specific structural and functional properties

One of the most challenging questions in modern plant science is how plants regulate their morphological and developmental adaptation in response to changes in their biotic and abiotic environment. A comprehensive elucidation of the underlying mechanisms will help shed light on the extremely efficient strategies of plants in terms of survival and propagation. In recent years, a number of environmental stress conditions have been described as being mediated by signaling molecules of the oxylipin family. In this context, jasmonic acid, its biosynthetic precursor, 12-oxo-phytodienoic acid (OPDA), and also reactive electrophilic species such as phytoprostanes play pivotal roles. Although our understanding of jasmonic acid-dependent processes and jasmonic acid signal-transduction cascades has made considerable progress in recent years, knowledge of the regulation and mode of action of OPDA-dependent plant responses is just emerging. This minireview focuses on recent work concerned with the elucidation of OPDA-specific processes in plants. In this context, aspects such as the differential recruitment of OPDA, either by de novo biosynthesis or by release from cyclo-oxylipin-galactolipids, and the conjugation of free OPDA are discussed.

A novel patatin-like protein from cotton plant, GhPat1, is co-expressed with GhLox1 during Xanthomonas campestris-mediated hypersensitive cell death

In cotton plant, Xanthomonas-induced hypersensitive response (HR) is accompanied by a lipid peroxidation process involving a 9-lipoxygenase (LOX), GhLox1. Initiation of this oxidative metabolism implies the release of the LOX substrates, or polyunsaturated fatty acids. Since patatin-like proteins (PLPs) are likely candidates for mediating the latter step, we searched for genes encoding such enzymes, identified and cloned one of them that we named GhPat1. Biochemical and molecular studies showed that GhPat1 expression was up-regulated during the incompatible interaction, prior to the onset of the corresponding galactolipase activity and cell death symptoms in tissues. Protein sequence analysis and modelling also revealed that GhPat1 catalytic amino acids and fold were conserved across plant PLPs. Based on these results and our previous work (Jalloul et al. in Plant J 32:1-12, 2002), a role for GhPat1, in synergy with GhLox1, during HR-specific lipid peroxidation is discussed.

Changes in the initial phase of lipid peroxidation induced by elicitor from Phytophthora infestans in Solanum species

The initial phase of the lipid peroxidation process in leaves of Solanum nigrum var. gigantea, Solanum tuberosum cv Bzura and clone H-8105, which represent non-host resistance, field resistance and susceptibility, respectively, against Phytophthora infestans, was investigated. Based on quantitative and qualitative high-performance liquid chromatography (HPLC) analyses of free and esterified fatty acid hydroperoxides (FAHs), we characterized the lipid peroxidation process induced by the pathogen-derived elicitor, culture filtrate (CF), in leaves of the studied genotypes. In all plants, FAHs generated due to 13-lipoxygenase (LOX) action dominated over those from the non-enzymatic pathway. The FAHs derived from 9-LOX activity were found only in CF-treated leaves of the non-host resistant S. nigrum. However, experiments in vitro and in planta with exogenous linoleic acid (LA) as a substrate for LOX revealed high constitutive activity of 9-LOX in all genotypes, which increased in response to CF treatment. The time course changes in polyunsaturated fatty acid (PUFA) pools in the total lipid fractions as well as the degree of their oxidation suggested that CF-induced PUFA peroxidation was enhanced mostly in S. nigrum, less so in Bzura and least in the susceptible clone H-8105. The obtained results are discussed in light of the overall biochemical cell status of plants in the studied interactions.

General detoxification and stress responses are mediated by oxidized lipids through TGA transcription factors in Arabidopsis

12-oxo-phytodienoic acid and several phytoprostanes are cyclopentenone oxylipins that are formed via the enzymatic jasmonate pathway and a nonenzymatic, free radical-catalyzed pathway, respectively. Both types of cyclopentenone oxylipins induce the expression of genes related to detoxification, stress responses, and secondary metabolism, a profile clearly distinct from that of the cyclopentanone jasmonic acid. Microarray analyses revealed that 60% of the induction by phytoprostanes and 30% of the induction by 12-oxo-phytodienoic acid was dependent on the TGA transcription factors TGA2, TGA5, and TGA6. Moreover, treatment with phytoprostanes and 12-oxo-phytodienoic acid inhibited cell division and root growth, a property also shared by jasmonic acid. Besides being potent signals, cyclopentenones and other lipid peroxidation products are reactive electrophiles that can covalently bind to and damage proteins. To this end, we show that at least two of the induced detoxification enzymes efficiently metabolize cyclopentenones in vitro. Accumulation of two of these metabolites was detectable during Pseudomonas infection. The cyclopentenone oxylipin gene induction profile resembles the defense response induced by a variety of lipophilic xenobiotics. Hence, oxidized lipids may activate chemosensory mechanisms of a general broad-spectrum detoxification network involving TGA transcription factors.

The Arabidopsis her1 mutant implicates GABA in E-2-hexenal responsiveness

When wounded or attacked by herbivores or pathogens, plants produce a blend of six-carbon alcohols, aldehydes and esters, known as C6-volatiles. Undamaged plants, when exposed to C6-volatiles, respond by inducing defense-related genes and secondary metabolites, suggesting that C6-volatiles can act as signaling molecules regulating plant defense responses. However, to date, the molecular mechanisms by which plants perceive and respond to these volatiles are unknown. To elucidate such mechanisms, we decided to isolate Arabidopsis thaliana mutants in which responses to C6-volatiles were altered. We observed that treatment of Arabidopsis seedlings with the C6-volatile E-2-hexenal inhibits root elongation. Among C6-volatiles this response is specific to E-2-hexenal, and is not dependent on ethylene, jasmonic and salicylic acid. Using this bioassay, we isolated 18 E-2-hexenal-response (her) mutants that showed sustained root growth after E-2-hexenal treatment. Here, we focused on the molecular characterization of one of these mutants, her1. Microarray and map-based cloning revealed that her1 encodes a gamma-amino butyric acid transaminase (GABA-TP), an enzyme that degrades GABA. As a consequence of the mutation, her1 plants accumulate high GABA levels in all their organs. Based on the observation that E-2-hexenal treatment induces GABA accumulation, and that high GABA levels confer resistance to E-2-hexenal, we propose a role for GABA in mediating E-2-hexenal responses.

Chlortetracycline detoxification in maize via induction of glutathione S-transferases after antibiotic exposure

Soil contamination with nonmetabolized antibiotics is an emerging environmental concern, especially on agricultural croplands that receive animal manure as fertilizer. In this study, phytotoxicity of chlortetracycline (CTC) antibiotics on pinto beans (Phaseolus vulgaris) and maize (Zea mays) was investigated under controlled conditions. When grown in CTC-treated soil, a significant increase in the activities of the plant stress proteins glutathione S-transferases (GST) and peroxidases (POX) were observed in maize plants, but not in pinto beans. In vitro conjugation reactions demonstrated that the induced GST in maize catalyzed the conjugation of glutathione (GSH) with CTC, producing stable conjugates that were structurally characterized using liquid chromatography/mass spectrometry. The antibiotic-induced GST produced CTC-glutathione conjugate at relative concentrations 2-fold higher than that produced by constitutively expressed GST extracted from untreated maize. On the other hand, GST extracted from pinto beans (both treated and untreated) did not efficiently catalyze glutathione conjugation with CTC. These results suggest that maize is able to detoxify chlortetracycline via the glutathione pathway, whereas pinto beans cannot. This may explain the observed stunted growth of pinto beans after antibiotic treatment. This study demonstrates the importance of plant uptake in determining the fate of antibiotics in soil and their potential phytotoxicity to susceptible plants.

Rapid Enzymatic Isomerization of 12-Oxophytodienoic Acid in the Gut of Lepidopteran Larvae

In response to feeding larvae of the Mediterranean climbing cutworm (Spodoptera littoralis), leaves of the lima bean (Phaseolus lunatus) produce fatty acid-derived signaling compounds (oxylipins). The major products are the phytohormones jasmonic acid and its biosynthetic precursor 12-oxophytodienoic acid (OPDA), along with 13-hydroxy-12-oxooctadeca-9,15-dienoic acid, 9-hydroxy-12-oxooctadeca-10,15-dienoic acid (alpha- and gamma-ketol), as well as unsaturated aldehydes. Oxylipin production is highest at the feeding zone of the insect and decreases with distance from the damaged area. Accordingly, the feeding insect experiences high local concentrations of oxylipins, which are taken up into the alimentary canal and are finally excreted with the feces. In contrast to most other oxylipins, OPDA was not detectable in the insect's gut; instead the structurally related tetrahydrodicranenone B (iso-OPDA) was identified. Feeding experiments with deuterium-labeled OPDA proved that the isomerization is catalyzed by an enzyme from the insect's gut tissue. The phenomenon appears to be widespread among Lepidopteran larvae.

Characterization of glutathione conjugates of chloroacetanilide pesticides using ultra-performance liquid chromatography/quadrupole time-of-flight mass spectrometry and liquid chromatography/ion trap mass spectrometry

Glutathione S-transferases (GSTs) isolated from maize were used to catalyze the conjugation of glutathione (GSH) with chloroacetanilide herbicides, producing stable conjugates that were structurally characterized using ultra-performance liquid chromatography/quadrupole time-of-flight mass spectrometry (UPLC/QqToF-MS) and liquid chromatography/ion trap mass spectrometry (LC/IT-MS). Enzyme-mediated dechlorination of alachlor, metolachlor, and propachlor resulted during GSH conjugation as revealed by the mass spectra of the conjugates, which was confirmed by the loss of the chlorine isotopic signature and from high accurate mass measurements. Several fragmentation patterns in the mass spectra of the chloroacetanilide-GSH conjugates can be used to verify the identities of the enzyme reaction products, such as characteristic ions corresponding to the neutral loss of glutamic acid residue (129 Da) and water (18 Da) observed in the product ion spectrum. For the first time, data are presented showing detection of chloroacetanilides that are conjugated with two GSH molecules, in addition to the known single GSH conjugates.

Determination of enzyme kinetics and glutathione conjugates of chlortetracycline and chloroacetanilides using liquid chromatography–mass spectrometry

Glutathione S-transferases (GSTs) isolated from chlortetracycline (CTC)-treated maize catalyzed the conjugation of glutathione (GSH) with CTC, producing stable conjugates that were structurally characterized using liquid chromatography-ion-trap mass spectrometry (LC-IT-MS). Enzyme-mediated dechlorination of CTC resulted during GSH conjugation as revealed by the mass spectra of the CTC-GSH conjugate, which was characterized by the loss of the chlorine isotopic signature, and shorter chromatographic retention time relative to the chlorinated parent compound. Several fragmentation patterns in the mass spectrum of the CTC-GSH conjugate can be used to verify the identity of the enzyme reaction products. The expected molecular ion [M + H](+) of the CTC-GSH conjugate (m/z 751) with chlorine removal was not observed in the positive electrospray ionization. Instead, a base peak of m/z 677, corresponding to the loss of glycine (MW = 75 Da), was observed. When m/z 677 was subjected to further fragmentation, characteristic peaks corresponding to the loss of glutamic acid (m/z = 129) and water (m/z 18) were observed in the MS/MS spectrum. The catalytic activity of the CTC-induced GST towards dechlorination of chloroacetanilide herbicides (alachlor, metolachlor and propachlor), which are known to be detoxified in plants via the glutathione pathway, was also evaluated in vitro. Glutathione conjugates of chloroacetanilides also showed the losses of m/z 129 and m/z 18 that are characteristic of GSH conjugates when characterized by LC-IT-MS. Interestingly, the sensitivity of LC-IT-MS made it possible, for the first time, to detect chloroacetanilides that are conjugated with two GSH molecules, in addition to the known single GSH conjugates. This research demonstrates a more sensitive and specific method of measuring enzyme reaction products using LC-IT-MS.

Adducts of oxylipin electrophiles to glutathione reflect a 13 specificity of the downstream lipoxygenase pathway in the tobacco hypersensitive response

The response to reactive electrophile species (RES) is now considered as part of the plant response to pathogen and insect attacks. Thanks to a previously established high-performance liquid chromatography tandem mass spectrometry methodology, we have investigated the production of oxylipin RES adducts to glutathione (GSH) during the hypersensitive response (HR) of plants. We have observed that RES conjugation to GSH in tobacco (Nicotiana tabacum) leaves is facile and nonspecific. In cryptogein-elicited tobacco leaves, we show that the oxylipin RES adducts to GSH are produced in correlation with GSH consumption, increase in glutathione S-transferase activity, and the appearance of the cell death symptoms. In this model, the adducts arise mainly from the downstream 13 lipoxygenase (LOX) metabolism, although the induced 9 LOX pathway leads massively to the accumulation of upstream metabolites. The main adducts were obtained from 2-hexenal and 12-oxo-phytodienoic acid. They accumulate transiently as 1-hexanol-3-GSH, a reduced adduct, and 12-oxo-phytodienoic acid-GSH, respectively. RES conjugation does not initiate cell death but explains part of the GSH depletion that accompanies HR cell death. The nature of these GSH conjugates shows the key role played by the 13 LOX pathway in RES signaling in the tobacco HR.