Metabolic profiling of oxylipins upon salicylate treatment in barley leaves--preferential induction of the reductase pathway by salicylate(1)

Ketols Emerge as Potent Oxylipin Signals Regulating Diverse Physiological Processes in Plants

Plants produce an array of oxylipins implicated in defense responses against various stresses, with about 600 oxylipins identified in plants to date. Most known oxylipins are the products of lipoxygenase (LOX)-mediated oxygenation of polyunsaturated fatty acids. One of the most well-characterized oxylipins produced by plants is the hormone jasmonic acid (JA); however, the function of the vast majority of oxylipins remains a mystery. One of the lesser-studied groups of oxylipins is comprised of ketols produced by the sequential action of LOX, allene oxide synthase (AOS), followed by non-enzymatic hydrolysis. For decades, ketols were mostly considered mere by-products of JA biosynthesis. Recent accumulating evidence suggests that ketols exhibit hormone-like signaling activities in the regulation of diverse physiological processes, including flowering, germination, plant-symbiont interactions, and defense against biotic and abiotic stresses. To complement multiple reviews on jasmonate and overall oxylipin biology, this review focuses specifically on advancing our understanding of ketol biosynthesis, occurrence, and proposed functions in diverse physiological processes.

A 13-Lipoxygenase, GhLOX2, positively regulates cotton tolerance against Verticillium dahliae through JA-mediated pathway

Verticillium wilt is a major limiting factor for sustainable production of cotton but the mechanism of controlling this disease is still poorly understood. Lipoxygenase (LOX)-derived oxylipins have been implicated in defense responses against diverse pathogens; however there is limited information about the functional characterization of LOXs in response to Verticillium dahliae infection. In this study, we report the characterization of a cotton LOX gene, GhLOX2, which phylogenetically clustered into 13-LOX subfamily and is closely related to Arabidopsis LOX2 gene. GhLOX2 was predominantly expressed in leaves and strongly induced following V. dahliae inoculation and treatment of methyl jasmonate (MeJA). RNAi-mediated knock-down of GhLOX2 enhanced cotton susceptibility to V. dahliae and was coupled with suppression of jasmonic acid (JA)-related genes both after inoculation with the cotton defoliating strain V991 or MeJA treatment. Interestingly, lignin contents, transcripts of lignin synthesis genes and H₂O₂ contents were also decreased in GhLOX2-silenced plants. This study suggests that GhLOX2 is involved in defense responses against infection of V. dahliae in cotton and supports that JA is one of the major defense hormones against this pathogen.

Molecularization of Bitter Off-Taste Compounds in Pea-Protein Isolates (Pisum sativum L.)

Activity-guided fractionations, combined with taste dilution analyses (TDA), were performed to locate the key compounds contributing to the bitter off-taste of pea-protein isolates (Pisum sativum L.). Purification of the compounds perceived with the highest sensory impact, followed by 1D/2D-NMR, (LC-)MS/MS, LC-TOF-MS, and MSE experiments, led to the identification of 14 lipids and lipid oxidation products, namely, 9,10,13-trihydroxyoctadec-12-enoic acid, 9,12,13-trihydroxyoctadec-10-enoic acid, 9,10,11-trihydroxyoctadec-12-enoic, 11,12,13-trihydroxyoctadec-9-enoic acid, (10E,12E)-9-hydroxyoctadeca-10,12-dienoic acid, (9Z,11E)-13-hydroxyoctadeca-9,11-dienoic acid, (9E,11E)-13-hydroxyoctadeca-9,11-dienoic acid, 1-linoleoyl glycerol, α-linolenic acid, 2-hydroxypalmitic acid, 2-hydroxyoleic acid, linoleic acid, (9Z,11E)-13-oxooctadeca-9,11-dienoic acid, and octacosa-6,9,19,22-tetraen. Herein, we present the isolation, structure determination, and sensory activity of these molecules. Depending on their structure, the isolated compounds showed human bitter recognition thresholds between 0.06 and 0.99 mmol/L in water.

The expression of several pepper fatty acid desaturase genes is robustly activated in an incompatible pepper-tobamovirus interaction, but only weakly in a compatible interaction

The replication of positive strand RNA viruses in plant cells is markedly influenced by the desaturation status of fatty acid chains in lipids of intracellular plant membranes. At present, little is known about the role of lipid desaturation in the replication of tobamoviruses. Therefore, we investigated the expression of fatty acid desaturase (FAD) genes and the fatty acid composition of pepper leaves inoculated with two different tobamoviruses. Obuda pepper virus (ObPV) inoculation induced a hypersensitive reaction (incompatible interaction) while Pepper mild mottle virus (PMMoV) inoculation caused a systemic infection (compatible interaction). Changes in the expression of 16 FADs were monitored in pepper leaves following ObPV and PMMoV inoculations. ObPV inoculation rapidly and markedly upregulated seven Δ12-FADs that encode enzymes putatively located in the endoplasmic reticulum membrane. In contrast, PMMoV inoculation resulted in a weaker but rapid upregulation of two Δ12-FADs and a Δ15-FAD. The expression of genes encoding plastidial FADs was not influenced neither by ObPV nor by PMMoV. In accordance with gene expression results, a significant accumulation of linoleic acid was observed by gas chromatography-mass spectrometry in ObPV-, but not in PMMoV-inoculated leaves. ObPV inoculation led to a marked accumulation of H₂O₂ in the inoculated leaves. Therefore, the effect of H₂O₂ treatments on the expression of six tobamovirus-inducible FADs was also studied. The expression of these FADs was upregulated to different degrees by H₂O₂ that correlated with ObPV-inducibility of these FADs. These results underline the importance of further studies on the role of pepper FADs in pepper-tobamovirus interactions.

The glycosyltransferase UGT76E1 significantly contributes to 12-O-glucopyranosyl-jasmonic acid formation in wounded Arabidopsis thaliana leaves

Jasmonoyl-isoleucine (JA-Ile) is a phytohormone that orchestrates plant defenses in response to wounding, feeding insects, or necrotrophic pathogens. JA-Ile metabolism has been studied intensively, but its catabolism as a potentially important mechanism for the regulation of JA-Ile-mediated signaling is not well-understood. Especially the enzyme(s) responsible for specifically glycosylating 12-hydroxy-jasmonic acid (12-OH-JA) and thereby producing 12-O-glucopyranosyl-jasmonic acid (12-O-Glc-JA) is still elusive. Here, we used co-expression analyses of available Arabidopsis thaliana transcriptomic data, identifying four UDP-dependent glycosyltransferase (UGT) genes as wound-induced and 12-OH-JA-related, namely, UGT76E1, UGT76E2, UGT76E11, and UGT76E12 We heterologously expressed and purified the corresponding proteins to determine their individual substrate specificities and kinetic parameters. We then used an ex vivo metabolite-fingerprinting approach to investigate these proteins in conditions as close as possible to their natural environment, with an emphasis on greatly extending the range of potential substrates. As expected, we found that UGT76E1 and UGT76E2 are 12-OH-JA-UGTs, with UGT76E1 contributing a major in vivo UGT activity, as deduced from Arabidopsis mutants with abolished or increased UGT gene expression. In contrast, recombinant UGT76E11 acted on an unidentified compound and also glycosylated two other oxylipins, 11-hydroxy-7,9,13-hexadecatrienoic acid (11-HHT) and 13-hydroxy-9,11,15-octadecatrienoic acid (13-HOT), which were also accepted by recombinant UGT76E1, UGT76E2, and UGT76E12 enzymes. UGT76E12 glycosylated 12-OH-JA only to a low extent, but also accepted an artificial hydroxylated fatty acid and low amounts of kaempferol. In conclusion, our findings have elucidated the missing step in the wound-induced synthesis of 12-O-glucopyranosyl-jasmonic acid in A. thaliana.

Enhanced arsenic tolerance and secondary metabolism by modulation of gene expression and proteome profile in Artemisia annua L. after application of exogenous salicylic acid

This study was designed to investigate the effect of exogenous application of salicylic acid (SA) on proteins pattern and secondary metabolites in arsenic (As) stressed Artemisia annua. A. annua was treated by As 100 μM, SA 100 μM and combined treatment of SA 100 μM + As 100 μM upto 3 days. Significant accumulation of As was observed in roots than shoots at As 100 μM treatment. Under As treatment, oxidative stress was induced as indicated by increased TBARS content. Biomass, carotenoid, flavonoids were enhanced whereas total chlorophyll pigment was reduced under As treatment. Combined treatment of SA 100 μM + As 100 μM was more effective for increment of biomass, total chlorophyll content, and flavonoids as compared to As 100 μM treatment. Protein profiling revealed 20 differentially abundant proteins by 2-DE PAGE and MALDI-TOF-MS analysis. Identified proteins were related to photosynthesis, energy metabolism, transcriptional regulators, secondary metabolism, lipid metabolism, transport proteins and unknown/hypothetical proteins. All identified proteins were significantly increased in abundance under combined treatments of SA 100 μM + As 100 μM. The expression analysis of key genes involved in biosynthesis of lipid metabolism, signal molecule, transcriptional regulators, artemisinin biosynthetic genes, isoprenoids pathway, terpenes and flavonoids pathway were significantly upregulated under combined treatments of SA 100 μM + As 100 μM, suggesting a fine linkage in regulation of primary and secondary metabolism to modulate tolerance capacity and to improve phytoremediation property of A. annua against arsenic toxicity.

Root Exudates of Stressed Plants Stimulate and Attract Trichoderma Soil Fungi

Plant roots release complex mixtures of bioactive molecules, including compounds that affect the activity and modify the composition of the rhizosphere microbiome. In this work, we investigated the initial phase of the interaction between tomato and an effective biocontrol strain of Trichoderma harzianum (T22). We found that root exudates (RE), obtained from plants grown in a split-root system and exposed to various biotic and abiotic stress factors (wounding, salt, pathogen attack), were able to stimulate the growth and act as chemoattractants of the biocontrol fungus. On the other hand, some of the treatments did not result in an enhanced chemotropism on Fusarium oxysporum f. sp. lycopersici, indicating a mechanism that may be selective for nonpathogenic microbes. The involvement of peroxidases and oxylipins, both known to be released by roots in response to stress, was demonstrated by using RE fractions containing these molecules or their commercial purified analogs, testing the effect of an inhibitor, and characterizing the complex pattern of these metabolites released by tomato roots both locally and systemically.

Metabolomic analysis of two rice (Oryza sativa) varieties exposed to 2, 2', 4, 4'-tetrabromodiphenyl ether

Polybrominated diphenyl ethers (PBDEs) are toxic chemicals widely distributed in the environment, but few studies are available on their potential toxicity to rice at metabolic level. Therefore we exposed ten rice (Oryza sativa) varieties to 2,2',4,4'-tetrabromodiphenyl ether (BDE-47), a predominant congener of PBDEs, in hydroponic solutions with different concentrations. Two varieties that showed different biological effects to BDE-47, YY-9 and LJ-7, were screened as sensitive and tolerant varieties according to changes of morphological and physiological indicators. Metabolic research was then conducted using gas chromatography-mass spectrometry combined with diverse analyses. Results showed that LJ-7 was more active in metabolite profiles and adopted more effective antioxidant defense machinery to protect itself against oxidative damages induced by BDE-47 than YY-9. For LJ-7, the contents of 13 amino acids and 24 organic acids, especially l-glutamic acid, beta-alanine, glycolic acid and glyceric acid were up-regulated significantly which contributed to scavenging reactive oxygen species. In the treatment of 500 μg/L BDE-47, the contents of these four metabolites increased by 33.6-, 19.3-, 10.6- and 10.2-fold, respectively. The levels of most saccharides (such as d-glucose, lactulose, maltose, sucrose and d-cellobiose) also increased by 1.7-12.4 fold which promoted saccharide-related biosynthesis metabolism. Elevation of tricarboxylic acid cycle and glyoxylate and dicarboxylate metabolism enhanced energy-producing processes. Besides, the contents of secondary metabolites, chiefly polyols and glycosides increased significantly to act on defending oxidative stress induced by BDE-47. In contrast, the levels of most metabolites decreased significantly for YY-9, especially those of 13 amino acids (by 0.9%-67.1%) and 19 organic acids (by 7.8%-70.0%). The positive metabolic responses implied LJ-7 was tolerant to BDE-47, while the down-regulation of most metabolites indicated the susceptible nature of YY-9. Since metabolic change might affect the yield and quality of rice, this study can provide useful reference for rice cultivation in PBDEs-polluted areas.

Overexpression and Down-Regulation of Barley Lipoxygenase LOX2.2 Affects Jasmonate-Regulated Genes and Aphid Fecundity

Aphids are pests on many crops and depend on plant phloem sap as their food source. In an attempt to find factors improving plant resistance against aphids, we studied the effects of overexpression and down-regulation of the lipoxygenase gene LOX2.2 in barley (Hordeum vulgare L.) on the performance of two aphid species. A specialist, bird cherry-oat aphid (Rhopalosiphum padi L.) and a generalist, green peach aphid (Myzus persicae Sulzer) were studied. LOX2.2 overexpressing lines showed up-regulation of some other jasmonic acid (JA)-regulated genes, and antisense lines showed down-regulation of such genes. Overexpression or suppression of LOX2.2 did not affect aphid settling or the life span on the plants, but in short term fecundity tests, overexpressing plants supported lower aphid numbers and antisense plants higher aphid numbers. The amounts and composition of released volatile organic compounds did not differ between control and LOX2.2 overexpressing lines. Up-regulation of genes was similar for both aphid species. The results suggest that LOX2.2 plays a role in the activation of JA-mediated responses and indicates the involvement of LOX2.2 in basic defense responses.

Avocado fruit maturation and ripening: dynamics of aliphatic acetogenins and lipidomic profiles from mesocarp, idioblasts and seed

BACKGROUND

Avocado fruit contains aliphatic acetogenins (oft-acetylated, odd-chain fatty alcohols) with promising bioactivities for both medical and food industries. However, we have scarce knowledge about their metabolism. The present work aimed to study changes in acetogenin profiles from mesocarp, lipid-containing idioblasts, and seeds from 'Hass' cultivar during fruit development, germination, and three harvesting years. An untargeted LC-MS based lipidomic analysis was also conducted to profile the lipidome of avocado fruit in each tissue.

RESULTS

The targeted analysis showed that acetogenin profiles and contents remained unchanged in avocado mesocarp during maturation and postharvest ripening, germination, and different harvesting years. However, a shift in the acetogenin profile distribution, accompanied with a sharp increase in concentration, was observed in seed during early maturation. Untargeted lipidomics showed that this shift was accompanied with remodeling of glycerolipids: TAGs and DAGs decreased during fruit growing in seed. Remarkably, the majority of the lipidome in mature seed was composed by acetogenins; we suggest that this tissue is able to synthesize them independently from mesocarp. On the other hand, lipid-containing idioblasts accumulated almost the entire acetogenin pool measured in the whole mesocarp, while only having 4% of the total fatty acids. The lipidome of this cell type changed the most when the fruit was ripening after harvesting, TAGs decreased while odd-chain DAGs increased. Notably, idioblast lipidome was more diverse than that from mesocarp.

CONCLUSIONS

Evidence shown here suggests that idioblasts are the main site of acetogenin biosynthesis in avocado mesocarp. This work unveiled the prevalence of aliphatic acetogenins in the avocado fruit lipidome and evidenced TAGs as initial donors of the acetogenin backbones in its biosynthesis. It also sets evidence for acetogenins being included in future works aimed at characterizing the avocado seed, as they are a main component of their lipidome.

Excessive nitrogen application dampens antioxidant capacity and grain filling in wheat as revealed by metabolic and physiological analyses

In this study, field-grown wheat (Triticum aestivum L.) was treated with normal (Nn) and excessive (Ne) levels of fertilizer N. Results showed that Ne depressed the activity of superoxide dismutase and peroxidase and increased the accumulation of reactive oxygen species (ROS) and malondialdehyde. The normalized difference vegetation index (NDVI) was higher under Ne at anthesis and medium milk but similar at the early dough stage and significantly lower at the hard dough stage than that under Nn. The metabolomics analysis of the leaf responses to Ne during grain filling showed 99 metabolites that were different between Ne and Nn treatments, including phenolic and flavonoid compounds, amino acids, organic acids and lipids, which are primarily involved in ROS scavenging, N metabolism, heat stress adaptation and disease resistance. Organic carbon (C) and total N contents were affected by the Ne treatment, with lower C/N ratios developing after medium milk. Ultimately, grain yields decreased with Ne. Based on these data, compared with the normal N fertilizer treatment, we concluded that excessive N application decreased the ability to scavenge ROS, increased lipid peroxidation and caused significant metabolic changes disturbing N metabolism, secondary metabolism and lipid metabolism, which led to reduced grain filling in wheat.

Phosphatidic acid synthesis, octadecanoic pathway and fatty acids content as lipid markers of exogeneous salicylic acid-induced elicitation in wheat

Activators of plant defence responses against pathogens are a potential alternative to fungicides, and the well-known resistance inducer salicylic acid (SA) protects wheat (Triticum aestivum L.) against powdery mildew. The aim of our work was to investigate through biochemical and molecular approaches whether lipid metabolism alteration could be considered as a characteristic feature of induced resistance in wheat upon SA infiltration. Expression levels of lox, PI-PLC2 and ltp genes encoding for a lipoxygenase (LOX), a phospholipase C2 and a lipid-transfer protein, respectively, were investigated. Increase of phosphatidic acid (PA) content 48h after SA infiltration in wheat leaves, upregulation of PI-PLC2 gene expression and increased diacylglycerol content were recorded, indicating the involvement of the PLC pathway in the PA synthesis. The wheat octadecanoid pathway was shown to be highly responsive to SA infiltration through simultaneous increases in lox gene expression and LOX activity, as well as a reduction in the content of linolenic acid. Changes in several FA contents and increases of the ltp gene expression were also recorded during the latest hours after SA infiltration. The status of lipid metabolism, as well as the connections between its components as markers of SA-induced resistance in wheat, are discussed.

The plant immunity inducer pipecolic acid accumulates in the xylem sap and leaves of soybean seedlings following Fusarium virguliforme infection

The causal agent of the soybean sudden death syndrome (SDS), Fusarium virguliforme, remains in infected roots and secretes toxins to cause foliar SDS. In this study we investigated the xylem sap, roots, and leaves of F. virguliforme-infected and -uninfected soybean seedlings for any changes in a set of over 3,000 metabolites following pathogen infection by conducting GC/MS and LC/MS/MS, and detected 273 biochemicals. Levels of many intermediates of the TCA cycle were reduced suggesting suppression of this metabolic pathway by the pathogen. There was an increased accumulation of peroxidated lipids in leaves of F. virguliforme-infected plants suggesting possible involvement of free radicals and lipoxygenases in foliar SDS development. Levels of both isoflavone conjugates and isoflavonoid phytoalexins were decreased in infected roots suggesting degradation of these metabolites by the pathogen to promote root necrosis. The levels of the plant immunity inducer pipecolic acid (Pip) and the plant hormone salicylic acid (SA) were significantly increased in xylem sap (in case of Pip) and leaves (in case of both Pip and SA) of F. virguliforme-infected soybean plants compared to the control plants. This suggests a major signaling role of Pip in inducing host defense responses in above ground parts of the F. virguliforme-infected soybean. Increased accumulation of pipecolic acid in foliar tissues was associated with the induction of GmALD1, the soybean homolog of Arabidopsis ALD1. This metabolomics study generated several novel hypotheses for studying the mechanisms of SDS development in soybean.

Targeted lipidomic strategies for oxygenated metabolites of polyunsaturated fatty acids

Oxidation of polyunsaturated fatty acids (PUFA) through enzymatic or non-enzymatic free radical-mediated reactions can yield an array of lipid metabolites including eicosanoids, octadecanoids, docosanoids and related species. In mammals, these oxygenated PUFA mediators play prominent roles in the physiological and pathological regulation of many key biological processes in the cardiovascular, renal, reproductive and other systems including their pivotal contribution to inflammation. Mass spectrometry-based technology platforms have revolutionized our ability to analyze the complex mixture of lipid mediators found in biological samples, with increased numbers of metabolites that can be simultaneously quantified from a single sample in few analytical steps. The recent development of high-sensitivity and high-throughput analytical tools for lipid mediators affords a broader view of these oxygenated PUFA species, and facilitates research into their role in health and disease. In this review, we illustrate current analytical approaches for a high-throughput lipidomic analysis of eicosanoids and related mediators in biological samples. This article is part of a Special Issue entitled "Oxygenated metabolism of PUFA: analysis and biological relevance."

Comparative analysis of antiviral responses in Brachypodium distachyon and Setaria viridis reveals conserved and unique outcomes among C3 and C4 plant defenses

Viral diseases cause significant losses in global agricultural production, yet little is known about grass antiviral defense mechanisms. We previously reported on host immune responses triggered by Panicum mosaic virus (PMV) and its satellite virus (SPMV) in the model C3 grass Brachypodium distachyon. To aid comparative analyses of C3 and C4 grass antiviral defenses, here, we establish B. distachyon and Setaria viridis (a C4 grass) as compatible hosts for seven grass-infecting viruses, including PMV and SPMV, Brome mosaic virus, Barley stripe mosaic virus, Maize mild mottle virus, Sorghum yellow banding virus, Wheat streak mosaic virus (WSMV), and Foxtail mosaic virus (FoMV). Etiological and molecular characterization of the fourteen grass-virus pathosystems showed evidence for conserved crosstalk among salicylic acid (SA), jasmonic acid, and ethylene pathways in B. distachyon and S. viridis. Strikingly, expression of PHYTOALEXIN DEFICIENT4, an upstream modulator of SA signaling, was consistently suppressed during most virus infections in B. distachyon and S. viridis. Hierarchical clustering analyses further identified unique antiviral responses triggered by two morphologically similar viruses, FoMV and WSMV, and uncovered other host-dependent effects. Together, the results of this study establish B. distachyon and S. viridis as models for the analysis of plant-virus interactions and provide the first framework for conserved and unique features of C3 and C4 grass antiviral defenses.

Evolution of jasmonate and salicylate signal crosstalk

The evolution of land plants approximately 470 million years ago created a new adaptive zone for natural enemies (attackers) of plants. In response to attack, plants evolved highly effective, inducible defense systems. Two plant hormones modulating inducible defenses are salicylic acid (SA) and jasmonic acid (JA). Current thinking is that SA induces resistance against biotrophic pathogens and some phloem feeding insects and JA induces resistance against necrotrophic pathogens, some phloem feeding insects and chewing herbivores. Signaling crosstalk between SA and JA commonly manifests as a reciprocal antagonism and may be adaptive, but this remains speculative. We examine evidence for and against adaptive explanations for antagonistic crosstalk, trace its phylogenetic origins and provide a hypothesis-testing framework for future research on the adaptive significance of SA-JA crosstalk.

Lipoxygenase-mediated oxidation of polyunsaturated N-acylethanolamines in Arabidopsis

N-acylethanolamines (NAEs) are bioactive fatty acid derivatives that occur in all eukaryotes. In plants, NAEs have potent negative growth-regulating properties, and fatty acid amide hydrolase (FAAH)-mediated hydrolysis is a primary catabolic pathway that operates during seedling establishment to deplete these compounds. Alternatively, polyunsaturated (PU)-NAEs may serve as substrates for lipid oxidation. In Arabidopsis, PU-NAEs (NAE 18:2 and NAE 18:3) were the most abundant NAE species in seeds, and their levels were depleted during seedling growth even in FAAH tDNA knock-out plants. Therefore, we hypothesized that lipoxygenase (LOX) participated in the metabolism of PU-NAEs through the formation of NAE-oxylipins. Comprehensive chromatographic and mass spectrometric methods were developed to identify NAE hydroperoxides and -hydroxides. Recombinant Arabidopsis LOX enzymes expressed in Escherichia coli utilized NAE 18:2 and NAE 18:3 as substrates with AtLOX1 and AtLOX5 exhibiting 9-LOX activity and AtLOX2, AtLOX3, AtLOX4, and AtLOX6 showing predominantly 13-LOX activity. Feeding experiments with exogenous PU-NAEs showed they were converted to hydroxide metabolites indicating that indeed Arabidopsis seedlings had the capacity for LOX-mediated metabolism of PU-NAEs in planta. Detectable levels of endogenous NAE-oxylipin metabolites were identified in FAAH fatty acid amide hydrolase seedlings but not in wild-type or FAAH overexpressors, suggesting that NAE hydroxide pools normally do not accumulate unless flux through hydrolysis is substantially reduced. These data suggest that Arabidopsis LOXs indeed compete with FAAH to metabolize PU-NAEs during seedling establishment. Identification of endogenous amide-conjugated oxylipins suggests potential significance of these metabolites in vivo, and FAAH mutants may offer opportunities to address this in the future.

Nicotiana attenuata SIPK, WIPK, NPR1, and fatty acid-amino acid conjugates participate in the induction of jasmonic acid biosynthesis by affecting early enzymatic steps in the pathway

Wounding and herbivore attack elicit the rapid (within minutes) accumulation of jasmonic acid (JA) that results from the activation of previously synthesized biosynthetic enzymes. Recently, several regulatory factors that affect JA production have been identified; however, how these regulators affect JA biosynthesis remains at present unknown. Here we demonstrate that Nicotiana attenuata salicylate-induced protein kinase (SIPK), wound-induced protein kinase (WIPK), nonexpressor of PR-1 (NPR1), and the insect elicitor N-linolenoyl-glutamate [corrected] (18:3-Glu) participate in mechanisms affecting early enzymatic steps of the JA biosynthesis pathway. Plants silenced in the expression of SIPK and NPR1 were affected in the initial accumulation of 13-hydroperoxy-linolenic acid (13-OOH-18:3) after wounding and 18:3-Glu elicitation by mechanisms independent of changes in 13-lipoxygenase activity. Moreover, 18:3-Glu elicited an enhanced and rapid accumulation of 13-OOH-18:3 that depended partially on SIPK and NPR1 but was independent of increased 13-lipoxygenase activity. Together, the results suggested that substrate supply for JA production was altered by 18:3-Glu elicitation and SIPK- and NPR1-mediated mechanisms. Consistent with a regulation at the level of substrate supply, we demonstrated by virus-induced gene silencing that a wound-repressed plastidial glycerolipase (NaGLA1) plays an essential role in the induction of de novo JA biosynthesis. In contrast to SIPK and NPR1, mechanisms mediated by WIPK did not affect the production of 13-OOH-18:3 but were critical to control the conversion of this precursor into 12-oxo-phytodienoic acid. These differences could be partially accounted for by reduced allene oxide synthase activity in WIPK-silenced plants.

Changes in C12:0, C18:1, C18:2 and C20:2 fatty acid content in wheat treated with resistance inducers and infected by powdery mildew

This work presents a global investigation of total fatty acid (FA) content in wheat in relation to treatment with four inducers of resistance and to powdery mildew infection. Linolenic acid (C18:3), linoleic acid (C18:2) and palmitic acid (16:0) were the most abundant FAs in wheat leaves. We investigated the effect of the following inducers of resistance: Iodus40, heptanoyl salicylic acid (HSA), Milsana and trehalose on FA accumulation. Previous studies established that lipid metabolism is altered by these compounds, and we therefore aimed to characterise their impact at the FA level. During a time course experiment, content (quantitative analysis) and percentage (qualitative analysis) of FAs were compared in treated plants and in controls, as well as in plants inoculated with Blumeria graminis f. sp. tritici (i) and non-inoculated (ni) plants. No change in C18:3 content was observed. C18:1 in Iodus 40-treated (ni) plants showed a quantitative 1.2-fold increase. Lauric acid (C12:0) content quantitatively increased after Iodus 40 (2.8-fold), Milsana (4.8-fold) and trehalose (4.0-fold) treatment in (i) plants. However, eicosadienoic acid (C20:2) quantitatively decreased in (ni) plants after Iodus 40 (1.5-fold) and Milsana (2.3-fold) treatment. The amount of C18:2 increased (1.6-fold) after HSA treatment in (i) plants. All these variations in FA content were correlated with variations in the corresponding relative percentages. Our work provides the first evidence for alterations in C12:0, C18:1, C18:2 and C20:2 FA content caused by four resistance inducers. We also compared the amount and percentage of each FA in untreated (i) and (ni) plants. In (i) plants, eicosadienoic acid (C20:2) increased and C18:2 decreased slightly. The potential involvement of these FAs during induced resistance and infection is discussed.

Oxylipins produced by the 9-lipoxygenase pathway in Arabidopsis regulate lateral root development and defense responses through a specific signaling cascade

Arabidopsis thaliana seedling growth with pure oxylipins resulted in root waving, loss of root apical dominance, and decreased root elongation. 9-Hydroxyoctadecatrienoic acid (9-HOT) was a potent inducer of root waving. Studies with noxy2 (for nonresponding to oxylipins2), a new 9-HOT-insensitive mutant, and coronatine insensitive1-1 (jasmonate-insensitive) revealed at least three signaling cascades mediating the oxylipin actions. Treatment with 9-HOT resulted in a reduction in lateral roots and an increase in stage V primordia. Roots showed strong 9-lipoxygenase (9-LOX) activity, and root primordia expressed 9-LOX genes. These results, along with findings that noxy2 and mutants with defective 9-LOX activity showed increased numbers of lateral roots, suggest that 9-HOT, or a closely related 9-LOX product, is an endogenous modulator of lateral root formation. Histochemical and molecular analyses revealed that 9-HOT activated events common to development and defense responses. A subset of 9-HOT-responding root genes was also induced in leaves after 9-HOT treatment or pathogen inoculation. The results that noxy2 displayed altered root development, enhanced susceptibility to Pseudomonas, and reduced the activation of 9-HOT-responding genes are consistent with mechanistic links among these processes. The nature of the changes detected suggests that oxylipins from the 9-LOX pathway function in cell wall modifications required for lateral root development and pathogen arrest.

Peroxidase activity of selenoprotein GrdB of glycine reductase and stabilisation of its integrity by components of proprotein GrdE from Eubacterium acidaminophilum

The anaerobe Eubacterium acidaminophilum has been shown to contain an uncharacterized peroxidase, which may serve to protect the sensitive selenoproteins in that organism. We purified this peroxidase and found that it was identical with the substrate-specific "protein B"-complex of glycine reductase. The "protein B"-complex consists of the selenocysteine-containing GrdB subunit and two subunits, which derive from the GrdE proprotein. The specific peroxidase activity was 1.7 U (mg protein)(-1) with DTT and cumene hydroperoxide as substrates. Immunoprecipitation experiments revealed that GrdB was important for DTT- and NADH-dependent peroxidase activities in crude extracts, whereas the selenoperoxiredoxin PrxU could be depleted without affecting these peroxidase activities. GrdB could be heterologously produced in Escherichia coli with coexpression of selB and selC from E. acidaminophilum for selenocysteine insertion. Although GrdB was sensitive to proteolysis, some full-size protein was present which accounted for a peroxidase activity of about 0.5 U (mg protein)(-1) in these extracts. Mutation of the potentially redox-active UxxCxxC motif in GrdB resulted in still significant, but decreased activity. Heterologous GrdB was protected from degradation by full-length GrdE or by GrdE-domains. The GrdB-GrdE interaction was confirmed by copurification of GrdE with Strep-tagged GrdB. The data suggest that GrdE domains serve to stabilise GrdB.

Oxylipin profiling of the hypersensitive response in Arabidopsis thaliana. Formation of a novel oxo-phytodienoic acid-containing galactolipid, arabidopside E

Oxidation products of unsaturated fatty acids, collectively known as oxylipins, function as signaling molecules in plants during development, wounding, and insect and pathogen attack. Certain oxylipins are also known to have direct cytotoxic effects on pathogens. We used inducible expression of bacterial avirulence proteins in planta to study the involvement of oxylipins in race-specific defense against bacterial pathogens. We demonstrate that recognition of the Pseudomonas syringae avirulence protein AvrRpm1 induces 9- and 13-lipoxygenase-dependent oxylipin synthesis in Arabidopsis thaliana. The major oxylipins accumulated were jasmonic acid, 12-oxo-phytodienoic acid, and dinor-oxo-phytodienoic acid. The majority of the newly formed oxylipins (>90%) was found to be esterified to glycerolipids, whereby 12-oxo-phytodienoic acid and dinor-oxo-phytodienoic acid were found to be esterified to a novel galactolipid. The structure of the substance was determined as a monogalactosyldiacylglycerol containing two 12-oxo-phytodienoic acids and one dinor-oxo-phytodienoic acid acyl chain and was given the trivial name arabidopside E. This substance accumulated to surprisingly high levels, 7-8% of total lipid content, and was shown to inhibit growth of a bacterial pathogen in vitro. Arabidopside E was formed also after recognition of the avirulence protein AvrRpt2, suggesting that this could be a conserved feature of defense reactions against bacterial pathogens. In conclusion, the data presented suggest a role of enzymatically formed oxylipins, especially the octadecanoids and arabidopside E in race-specific resistance against bacterial pathogens.

Transgenic barley plants overexpressing a 13-lipoxygenase to modify oxylipin signature

Three chimeric gene constructs were designed comprising the full length cDNA of a lipoxygenase (LOX) from barley (LOX2:Hv:1) including its chloroplast targeting sequence (cTP) under control of either (1) CaMV35S- or (2) polyubiquitin-1-promoter, whereas the third plasmid contains 35S promoter and the cDNA without cTP. Transgenic barley plants overexpressing LOX2:Hv:1 were generated by biolistics of scutella from immature embryos. Transformation frequency for 35S::LOX with or without cTP was in a range known for barley particle bombardment, whereas for Ubi::cTP-LOX no transgenic plants were detected. In general, a high number of green plantlets selected on bialaphos became yellow and finally died either in vitro or after potting. All transgenic plants obtained were phenotypically indistinguishable from wild type plants and all of them set seeds. The corresponding protein (LOX-100) in transgenic T0 and T1 plants accumulated constitutively to similar levels as in the jasmonic acid methyl ester (JAME)-treated wild type plants. Moreover, LOX-100 was clearly detectable immunocytochemically within the chloroplasts of untreated T0 plants containing the LOX-100-cDNA with the chloroplast target sequence. In contrast, an exclusive localization of LOX-100 in the cytoplasm was detectable when the target sequence was removed. In comparison to sorbitol-treated wild type leaves, analysis of oxylipin profiles in T2 progenies showed higher levels of jasmonic acid (JA) for those lines that displayed elevated levels of LOX-100 in the chloroplasts and for those lines that harboured LOX-100 in the cytoplasm, respectively. The studies demonstrate for the first time the constitutive overexpression of a cDNA coding for a 13-LOX in a monocotyledonous species and indicate a link between the occurrence of LOX-100 and senescence.

Profiling of structurally labile oxylipins in plants by in situ derivatization with pentafluorobenzyl hydroxylamine

A GC-MS-based method for the simultaneous quantification of common oxylipins along with labile and highly reactive compounds based on in situ derivatization with pentafluorobenzyl hydroxylamine to the corresponding O-2,3,4,5,6-pentafluorobenzyl oximes (PFB oximes) is presented. The approach covers oxo derivatives such as jasmonic acid (JA), 12-oxophytodienoic acid (OPDA), certain phytoprostanes, unsaturated oxo-acids, oxo-hydroxy acids, and aldehyde fragments from the polar head of fatty acids. In the positive electron impact-MS mode, the PFB oximes display characteristic fragment ions that greatly facilitate the identification of oxylipins in complex matrices. In addition, the fluorinated derivatives allow a highly selective and low-background analysis by negative chemical ionization. Besides showing the general value of the method for the identification of a broad range of oxylipins (18 examples), we also demonstrate sensitivity, linearity, and reproducibility for the quantification of JA, OPDA, 11-oxo-9-undecenoic acid, and 13-oxo-9,11-tridecadienoic acid. The efficiency of the method is demonstrated by differential profiling of these four oxylipins in lima bean leaves after mechanical wounding and feeding by the herbivore Spodoptera littoralis. Caterpillar feeding induced several oxylipins, whereas after wounding only the level of JA increased. The rapid in situ derivatization prevents the isomerization of cis-JA to trans-JA. The resting level of JA in lima beans showed an isomer ratio of 80:20 for trans/cis-JA. After wounding, de novo synthesis of JA alters the ratio to 20:80 in favor of the cis isomer.

Duplicate maize 13-lipoxygenase genes are differentially regulated by circadian rhythm, cold stress, wounding, pathogen infection, and hormonal treatments

Most plant oxylipins, a large class of diverse oxygenated polyunsaturated fatty acids and their derivatives, are produced through the lipoxygenase (LOX) pathway. Recent progress in dicots has highlighted the biological roles of oxylipins in plant defence responses to pathogens and pests. By contrast, the physiological function of LOXs and their metabolites in monocots is poorly understood. Two maize LOXs, ZmLOX10 and ZmLOX11 that share >90% amino acid sequence identity but are localized on different chromosomes, were cloned and characterized. Phylogenetic analysis revealed that ZmLOX10 and ZmLOX11 cluster together with well-characterized plastidic type 2 linoleate 13-LOXs from diverse plant species. Regio-specificity analysis of recombinant ZmLOX10 protein overexpressed in Escherichia coli proved it to be a linoleate 13-LOX with a pH optimum at approximately pH 8.0. Both predicted proteins contain putative transit peptides for chloroplast import. ZmLOX10 was preferentially expressed in leaves and was induced in response to wounding, cold stress, defence-related hormones jasmonic acid (JA), salicylic acid (SA), and abscisic acid (ABA), and inoculation with an avirulent strain of Cochliobolus carbonum. These data suggested a role for this gene in maize adaptation to abiotic stresses and defence responses against pathogens and pests. ZmLOX11 was preferentially expressed in silks and was induced in leaves only by ABA, indicating its possible involvement in responses to osmotic stress. In leaves, mRNA accumulation of ZmLOX10 is strictly regulated by a circadian rhythm, with maximal expression coinciding temporally with the highest photosynthetic activity. This study reveals the evolutionary divergence of physiological roles for relatively recently duplicated genes. Possible physiological functions of these 13-LOXs are suggested.

Evaluation of the antimicrobial activities of plant oxylipins supports their involvement in defense against pathogens

Plant oxylipins are a large family of metabolites derived from polyunsaturated fatty acids. The characterization of mutants or transgenic plants affected in the biosynthesis or perception of oxylipins has recently emphasized the role of the so-called oxylipin pathway in plant defense against pests and pathogens. In this context, presumed functions of oxylipins include direct antimicrobial effect, stimulation of plant defense gene expression, and regulation of plant cell death. However, the precise contribution of individual oxylipins to plant defense remains essentially unknown. To get a better insight into the biological activities of oxylipins, in vitro growth inhibition assays were used to investigate the direct antimicrobial activities of 43 natural oxylipins against a set of 13 plant pathogenic microorganisms including bacteria, oomycetes, and fungi. This study showed unequivocally that most oxylipins are able to impair growth of some plant microbial pathogens, with only two out of 43 oxylipins being completely inactive against all the tested organisms, and 26 oxylipins showing inhibitory activity toward at least three different microbes. Six oxylipins strongly inhibited mycelial growth and spore germination of eukaryotic microbes, including compounds that had not previously been ascribed an antimicrobial activity, such as 13-keto-9(Z),11(E),15(Z)-octadecatrienoic acid and 12-oxo-10,15(Z)-phytodienoic acid. Interestingly, this first large-scale comparative assessment of the antimicrobial effects of oxylipins reveals that regulators of plant defense responses are also the most active oxylipins against eukaryotic microorganisms, suggesting that such oxylipins might contribute to plant defense through their effects both on the plant and on pathogens, possibly through related mechanisms.

Induced disease resistance and gene expression in cereals

Disease resistance strategies reduce chemical input into the environment and are therefore powerful approaches to sustainable agriculture. Induced resistance (IR) has emerged as a potential alternative, or a complementary strategy, for crop protection. IR signifies the control of pathogens and pests by prior activation of plant defence pathways. A molecular understanding of IR in cereals, including the most important global crops wheat and rice, has been largely missing. Evidence indicating that central elements of IR pathways are conserved among Di- and Monocotyledoneae has only recently been presented, although their regulation and interaction with other plant pathways may be quite divergent. We present here a synopsis of current molecular knowledge of cereal IR mechanisms.

Lipids, lipases, and lipid-modifying enzymes in plant disease resistance

Lipids and lipid metabolites influence pathogenesis and resistance mechanisms associated with plant-microbe interactions. Some microorganisms sense their presence on a host by perceiving plant surface waxes, whereas others produce toxins that target plant lipid metabolism. In contrast, plants have evolved to recognize microbial lipopolysaccharides (LPSs), sphingolipids, and lipid-binding proteins as elicitors of defense response. Recent studies have demonstrated that the plasma membrane provides a surface on which some plant resistance (R) proteins perceive pathogen-derived effectors and thus confer race-specific resistance. Plant cell membranes also serve as reservoirs from which biologically active lipids and precursors of oxidized lipids are released. Some of these oxylipins, for example jasmonic acid (JA), are important signal molecules in plant defense. Arabidopsis thaliana is an excellent model plant to elucidate the biosynthesis and metabolism of lipids and lipid metabolites, and the characterization of signaling mechanisms involved in the modulation of plant defense responses by phytolipids. This review focuses on recent studies that highlight the involvement of lipids and lipid metabolites, and enzymes involved in lipid metabolism and modification in plant disease resistance.

The use of vapor phase extraction in metabolic profiling of phytohormones and other metabolites

Through complex networks of signaling interactions, phytohormones regulate growth, development, reproduction and responses to biotic and abiotic stress. Comprehensive metabolomic approaches, seeking to quantify changes in vast numbers of plant metabolites, may ultimately clarify these complex signaling interactions and consequently explain pleiotropic effects on plant metabolism. Synergistic and antagonistic phytohormone signaling interactions, referred to as crosstalk, are often considered at the level of transduction without proper consideration of synthesis or accumulation of phytohormones because of the limitation and difficulty in quantifying numerous signals. Significant progress has recently been made in the expansion of metabolic profiling and analysis of multiple phytohormones [Birkemeyer et al. (J. Chromatogr. A, 2003, 993, 89); Chiwocha et al. (Plant J., 2003, 35, 405); Müller et al. (Planta, 2002, 216, 44); Schmelz et al. (Proc. Natl Acad. Sci. USA, 2003, 100, 10552)]. We recently presented a novel metabolic profiling approach to the analysis of acidic phytohormones and other metabolites based on a simplistic preparation scheme and analysis by chemical ionization-gas chromatography/mass spectrometry. We now provide a detailed description of this vapor phase extraction technique and use pathogen infection of Arabidopsis with Pseudomonas syringae DC3000 to illustrate metabolic changes in salicylic acid, cinnamic acid, jasmonic acid, indole-3-acetic acid, abscisic acid, unsaturated C(18) fatty acids, 12-oxo-phytodienoic acid, and phytotoxin coronatine. Directions for further method expansion are provided and include issues of recovery, derivatization, range of accessible analytes, optimization, reproducibility and future directions.

Stimulation of the lipoxygenase pathway is associated with systemic resistance induced in bean by a nonpathogenic Pseudomonas strain

Systemic defense reactions induced in bean by the non-pathogenic Pseudomonas putida BTP1 strain reduced disease caused by Botrytis cinerea. Phenylalanine ammonialyase activity and the level of endogenous free salicylic acid were compared in plant growth-promoting rhizobacteria-treated versus control plants, but no significant differences were detected. Furthermore, no enhanced fungitoxicity was detected in methanolic leaf extracts, suggesting that accumulation of bean phytoalexins was not part of the stimulated defense mechanisms. However, BTP1-inoculated plants showed increased levels of both linoleic and linolenic acids. On this basis, we further investigated whether the lipoxygenase pathway, leading to antifungal phytooxylipins, could have been stimulated. Two key enzymatic activities of this metabolic route, namely lipoxygenase and hydroperoxide lyase, were significantly stimulated during the first four days after challenging BTP1-treated plants with the pathogen. This was observed in parallel with a more rapid consumption of the respective substrates of these enzymes, as revealed by measurements of endogenous concentrations of linolenic acid and their hydroperoxide derivatives. Moreover, headspace-gas chromatography analyses showed significantly higher concentrations of the fungitoxic final product Z-3-hexenal in leaves from BTP1-inoculated beans as compared with control plants. Taken together, these results strongly suggest that the oxylipin pathway can be associated with enhanced disease resistance induced in bean plants by nonpathogenic rhizobacteria.

Inverse correlation between jasmonic acid and salicylic acid during early wound response in rice

This study presents a kinetic analysis of the response to wounding in rice plants. In particular, jasmonic acid, salicylic acid, and lipoxygenase activity were measured in leaves of wounded rice plants during the early tillering phase. The results show that endogenous jasmonic acid transiently increases to a maximum 30 min after wounding (jasmonic acid burst) and lipoxygenase activity increases after the jasmonic acid burst, but not after the second smaller peak of endogenous jasmonic acid 23 h after wounding. In contrast, endogenous salicylic acid decreases during the jasmonic acid burst, such that the kinetic profiles of jasmonic acid and salicylic acid are inversely correlated during the early response to wounding. It is proposed here that the increase in endogenous jasmonic acid and the decrease in endogenous salicylic acid may contribute for establishing the efficient negative cross-talk between jasmonic acid and salicylic acid signaling pathways during the early response to wounding in rice.

Constitutive overexpression of allene oxide cyclase in tomato (Lycopersicon esculentum cv. Lukullus) elevates levels of some jasmonates and octadecanoids in flower organs but not in leaves

The allene oxide cyclase (AOC), an enzyme in jasmonate biosynthesis, occurs in vascular bundles and ovules of tomato flowers which exhibit a tissue-specific oxylipin signature (Plant J. 24, 113-126, 2000). Constitutive overexpression of the AOC did not led to altered levels of jasmonates in leaves, but these levels increased upon wounding or other stresses suggesting regulation of jasmonate biosynthesis by substrate availability (Plant J. 33, 577-589, 2003). Here, we show dramatic changes in levels of jasmonic acid (JA), of 12-oxo-phytodienoic acid (OPDA), their methyl esters (JAME, OPDAME), and of dinor-OPDA in most flower organs upon constitutive overexpression of AOC. Beside a dominant occurrence of OPDAME and JA in most flower organs, the ratio among the various compounds was altered differentially in the organs of transgenic flowers, e.g. OPDAME increased up to 53-fold in stamen, and JA increased about 51-fold in buds and 7.5-fold in sepals. The increase in jasmonates and octadecanoids was accompanied by decreased levels of free lipid hydro(per)oxy compounds. Except for 16:2, the AOC overexpression led to a significant increase in free but not esterified polyunsaturated fatty acids in all flower organs. The data suggest different regulation of JA biosynthesis in leaves and flowers of tomato.

Lipid Peroxidation during the Hypersensitive Response in Potato in the Absence of 9-Lipoxygenases

Hypersensitive cell death is an important defense reaction of plants to pathogen infection and is accompanied by lipid peroxidation processes. These may occur non-enzymatically by the action of reactive oxygen species or may be catalyzed by enzymes such as alpha-dioxygenases, lipoxygenases, or peroxidases. Correlative data showing increases in 9-lipoxygenase products in hyper-sensitively reacting cells have so far suggested that a large part of lipid peroxidation is mediated by a specific set of 9-lipoxygenases. To address the significance of 9-lipoxygenases for this type of pathogen response in potato, RNA interference constructs of a specific pathogen-induced potato 9-lipoxygenase were transferred to potato plants. Significantly reduced 9-lipoxygenase transcript levels were observed in transgenic plants after pathogen treatment. In addition, 9-lipoxygenase activity was hardly detectable, and levels of 9-lipoxygenase-derived oxylipins were reduced up to 12-fold after pathogen infection. In contrast to wild type plants, high levels of non-enzymatically as well as 13-lipoxygenase-derived oxylipins were present in 9-lipoxygenase-deficient plants. From this we conclude that during the normal hypersensitive response in potato, lipid peroxidation may occur as a controlled and directed process that is facilitated by the action of a specific 9-lipoxygenase. If 9-lipoxygenase-mediated formation of hydroperoxides is repressed, autoxidative lipid peroxidation processes and 13-lipoxygenase-mediated oxylipins synthesis become prominent. The unaltered timing and extent of necrosis formation suggests that the origin of lipid hydroperoxides does not influence pathogen-induced cell death in potato.

Systemic acquired resistance in crop protection: from nature to a chemical approach

Plant natural resistance to potential parasites is regulated by two fundamental mechanisms: the "nonhost" and the "gene-for-gene" resistance, respectively. The latter is relevant when a cultivar resistant (R) gene product recognizes an avirulence gene product in the attacking pathogen and triggers an array of biochemical reactions that halt the pathogen around the site of attempted invasion. To cope with virulent pathogens, plants may benefit by some temporary immunity after a challenge triggering such an array of defense reactions, following a localized necrotizing infection as a possible consequence of a hypersensitive response (HR). This process, mediated by accumulation of endogenous salicylic acid (SA), is called systemic acquired resistance (SAR) and provides resistance, to a certain extent even against unrelated pathogens, such as viruses, bacteria, and fungi, for a relatively long-lasting period. SAR may be more potently activated in plants pretreated with chemical inducers, most of which appear to act as functional analogues of SA. This review summarizes the complex aspects of SAR as a way to prevent crop diseases by activating the plants' own natural defenses. The following outline is taken: (1) introduction through the historical insight of the phenomenon; (2) oxidative burst, which produces high levels of oxygen reactive species in a way similar to the inflammation state in animals and precedes the HR to the pathogen attack; (3) SAR as a coordinate action of several gene products leading to the expression of defenses well beyond the time and space limits of the HR; (4) jasmonic acid (JA) and ethylene as other endogenous factors mediating a different pathway of induced resistance; (5) pathogenesis related proteins (PR proteins) de novo synthesized as specific markers of SAR; (6) exogenous inducers of SAR, which include both synthetic chemicals and natural products; (7) the pathway of signal transduction between sensitization by inducers and PR expression, as inferred by mutageneses, a process that is still, to a large extent, not completely elucidated; (8) prospects and costs; (9) final remarks on the state-of-the-art of the topic reflecting the chemical view of the author, based on the more authoritative ones expressed by the authors of the reviewed papers.

Spatio-temporal expression of patatin-like lipid acyl hydrolases and accumulation of jasmonates in elicitor-treated tobacco leaves are not affected by endogenous levels of salicylic acid

We have previously isolated three tobacco genes (NtPat) encoding patatin-like proteins, getting rapidly induced during the hypersensitive response (HR) to tobacco mosaic virus, in advance to jasmonate accumulation. NtPAT enzymes are lipid acyl hydrolases that display high phospholipase A2 (PLA2) activity and may mobilize fatty acid precursors of oxylipins. Here, we performed a detailed study of NtPat gene regulation under various biotic and abiotic stresses. PLA2 activity was poorly induced in response to drought, wounding, reactive oxygen intermediates, salicylic acid (SA) or methyl-jasmonate (MJ) whereas the ethylene (ET) precursor, 1-aminocyclopropane-1-carboxylic acid (ACC), provoked a moderate induction. In contrast, PLA2 activity was strongly induced when ACC was combined with MJ, and in response to the bacterium Erwinia carotovora or to the fungus Botrytis cinerea, as well as to treatment with beta-megaspermin, a cell death-inducing protein elicitor. A simplified system based on the infiltration of beta-megaspermin into leaves was used to dissect the spatio-temporal activation of PLA2 activity with regards to the accumulation of jasmonates and to the influence of endogenous SA. NtPat-encoded PLA2 activity was rapidly induced in the infiltrated zone before the appearance of cell death and with some delay in the surrounding living cells. A massive accumulation of 12-oxo-phytodienoic and jasmonic acids occurred in the elicitor-infiltrated zone, but only low levels were detectable outside this area. A similar picture was found in SA-deficient plants, showing that in tobacco, accumulation of jasmonates is not affected by the concomitant HR-induced build-up of endogenous SA. Finally, ET-insensitive plants showed a weakened induction of PLA2 activity outside the elicitor-infiltrated tissue.

Jasmonates and octadecanoids: signals in plant stress responses and development

Plants are sessile organisms. Consequently they have to adapt constantly to fluctuations in the environment. Some of these changes involve essential factors such as nutrients, light, and water. Plants have evolved independent systems to sense nutrients such as phosphate and nitrogen. However, many of the environmental factors may reach levels which represent stress for the plant. The fluctuations can range between moderate and unfavorable, and the factors can be of biotic or abiotic origin. Among the biotic factors influencing plant life are pathogens and herbivores. In case of bacteria and fungi, symbiotic interactions such as nitrogen-fixating nodules and mycorrhiza, respectively, may be established. In case of insects, a tritrophic interaction of herbivores, carnivores, and plants may occur mutualistically or parasitically. Among the numerous abiotic factors are low temperature, frost, heat, high light conditions, ultraviolet light, darkness, oxidation stress, hypoxia, wind, touch, nutrient imbalance, salt stress, osmotic adjustment, water deficit, and desiccation. In the last decade jasmonates were recognized as being signals in plant responses to most of these biotic and abiotic factors. Signaling via jasmonates was found to occur intracellularly, intercellularly, and systemically as well as interorganismically. Jasmonates are a group of ubiquitously occurring plant growth regulators originally found as the major constituents in the etheric oil of jasmine, and were first suggested to play a role in senescence due to a strong senescence-promoting effect. Subsequently, numerous developmental processes were described in which jasmonates exhibited hormone-like properties. Recent knowledge is reviewed here on jasmonates and their precursors, the octadecanoids. After discussing occurrence and biosynthesis, emphasis is placed upon the signal transduction pathways in plant stress responses in which jasmonates act as a signal. Finally, examples are described on the role of jasmonates in developmental processes.

Lipid peroxidation in cotton: Xanthomonas interactions and the role of lipoxygenases during the hypersensitive reaction

Lipid peroxidation, often associated with hypersensitive cell death, may be initiated either by active oxygen species (AOS) or lipoxygenases (LOX). Here we report a detailed analysis of this oxidative process in both incompatible and compatible interactions between the cotton cultivar Reba B50 and Xanthomonas campestris pv. malvacearum (Xcm). The hypersensitive reaction (HR) was characterized by a massive production of polyunsaturated fatty acid (PUFA) hydroperoxides together with typical tissue dehydration. Among these, isomers peroxidized on carbon 9, largely predominant, were chiral, showing an excess in the S enantiomer. The HR process was accompanied by an increase in 9S-LOX activity and preceded by transcription of a LOX gene (GhKLox1). These results showed that: (i) AOS produced during the oxidative burst were not involved in PUFA peroxidation during HR; and (ii) as previously described in elicited leaves of tobacco, the massive enzymatic lipid peroxidation was closely associated with hypersensitive cell death. During disease development in this cotton cultivar, the 9-lipoxygenation of PUFAs was late, weak, preceded by a faint accumulation of GhKLox1 transcripts, and associated with chlorosis but not with necrosis. Consequently, the main difference between incompatible and compatible interactions was in the precocity and intensity of the oxidative process, rather than in its nature. These data provide the evidence for a correlation between lipid peroxidation and hypersensitive cell death induced by pathogens.

Jasmonate-induced lipid peroxidation in barley leaves initiated by distinct 13-LOX forms of chloroplasts

In addition to a previously characterized 13-lipoxygenase of 100 kDa encoded by LOX2:Hv:1 [Vörös et al., Eur. J. Biochem. 251 (1998), 36-44], two full-length cDNAs (LOX2:Hv:2, LOX2:Hv:3) were isolated from barley leaves (Hordeum vulgare cv. Salome) and characterized. Both of them encode 13-lipoxygenases with putative target sequences for chloroplast import. Immunogold labeling revealed preferential, if not exclusive, localization of lipoxygenase proteins in the stroma. The ultrastructure of the chloroplast was dramatically altered following methyl jasmonate treatment, indicated by a loss of thylakoid membranes, decreased number of stacks and appearance of numerous osmiophilic globuli. The three 13-lipoxygenases are differentially expressed during treatment with jasmonate, salicylate, glucose or sorbitol. Metabolite profiling of free linolenic acid and free linoleic acid, the substrates of lipoxygenases, in water floated or jasmonate-treated leaves revealed preferential accumulation of linolenic acid. Remarkable amounts of free 9- as well as 13-hydroperoxy linolenic acid were found. In addition, metabolites of these hydroperoxides, such as the hydroxy derivatives and the respective aldehydes, appeared following methyl jasmonate treatment. These findings were substantiated by metabolite profiling of isolated chloroplasts, and subfractions including the envelope, the stroma and the thylakoids, indicating a preferential occurrence of lipoxygenase-derived products in the stroma and in the envelope. These data revealed jasmonate-induced activation of the hydroperoxide lyase and reductase branch within the lipoxygenase pathway and suggest differential activity of the three 13-lipoxygenases under different stress conditions.

Oxylipin profiling in pathogen-infected potato leaves

Plants respond to pathogen attack with a multicomponent defense response. Synthesis of oxylipins via the lipoxygenase (LOX) pathway appears to be an important factor for establishment of resistance in a number of pathosystems. In potato cells, pathogen-derived elicitors preferentially stimulate the 9-LOX-dependent metabolism of polyunsaturated fatty acids (PUFAs). Here we show by oxylipin profiling that potato plants react to pathogen infection with increases in the amounts of the 9-LOX-derived 9,10,11- and 9,12,13-trihydroxy derivatives of linolenic acid (LnA), the divinyl ethers colnelenic acid (CnA) and colneleic acid (CA) as well as 9-hydroxy linolenic acid. Accumulation of these compounds is faster and more pronounced during the interaction of potato with the phytopathogenic bacterium Pseudomonas syringae pv. maculicola, which does not lead to disease, compared to the infection of potato with Phytophthora infestans, the causal agent of late blight disease. Jasmonic acid (JA), a 13-LOX-derived oxylipin, accumulates in potato leaves after infiltration with P. syringae pv. maculicola, but not after infection with P. infestans.

Characterization of a novel lipoxygenase-independent senescence mechanism in Alstroemeria peruviana floral tissue

The role of lipoxygenase (lox) in senescence of Alstroemeria peruviana flowers was investigated using a combination of in vitro assays and chemical profiling of the lipid oxidation products generated. Phospholipids and galactolipids were extensively degraded during senescence in both sepals and petals and the ratio of saturated/unsaturated fatty acids increased. Lox protein levels and enzymatic activity declined markedly after flower opening. Stereochemical analysis of lox products showed that 13-lox was the major activity present in both floral tissues and high levels of 13-keto fatty acids were also synthesized. Lipid hydroperoxides accumulated in sepals, but not in petals, and sepals also had a higher chlorophyll to carotenoid ratio that favors photooxidation of lipids. Loss of membrane semipermeability was coincident for both tissue types and was chronologically separated from lox activity that had declined by over 80% at the onset of electrolyte leakage. Thus, loss of membrane function was not related to lox activity or accumulation of lipid hydroperoxides per se and differs in these respects from other ethylene-insensitive floral tissues representing a novel pattern of flower senescence.

Impact of phyto-oxylipins in plant defense

Phyto-oxylipins are metabolites produced in plants by the oxidative transformation of unsaturated fatty acids via a series of diverging metabolic pathways. Biochemical dissection and genetic approaches have provided compelling evidence that these oxygenated derivatives actively participate in plant defense mechanisms. During the past decade, interest in this field was focused on the biosynthesis of jasmonic acid (one branch of C18 polyunsaturated fatty acid metabolism) and on its relationship to the other plant defense-signaling pathways. However, recently, antisense strategies have revealed that oxylipins other than jasmonates are probably also essential for the resistance of plants to pathogens.

The apoplastic oxidative burst in response to biotic stress in plants: a three-component system

The oxidative burst, the generation of reactive oxygen species (ROS) in response to microbial pathogen attack, is a ubiquitous early part of the resistance mechanisms of plant cells. It has also become apparent from the study of a number of plant-pathogen interactions and those modelled by elicitor treatment of cultured cells that there may be more than one mechanism operating. However, one mechanism may be dominant in any given species. NADPH oxidases have been implicated in a number of systems and have been cloned and characterized. However, the enzyme system which is the major source of ROS in French bean (Phaseolus vulgaris) cells treated with a cell wall elicitor from Colletotrichum lindemuthianum, appears to be dependent on an exocellular peroxidase. The second component, the extracellular alkalinization, occurs as a result of the Ca(2+) and proton influxes and the K(+) efflux common to most elicitation systems as one of the earliest responses. The third component, the actual reductant/substrate, has remained elusive. The low molecular weight compound composition of apoplastic fluid was compared before and after elicitation. The substrate only becomes available some min after elicitation and can be extracted, so that by comparing the profiles by LC-MS it has been possible to identify possible substrates. The mechanism has proved to be complex and may involve a number of low molecular weight components. Stimulation of H(2)O(2) production was observed with saturated fatty acids such as palmitate and stearate without concomitant oxylipin production. This biochemical evidence is supported by immunolocalization studies on papillae forming at bacterial infection sites that show the peroxidase isoform present at sites of H(2)O(2) production revealed by cerium chloride staining together with the cross-linked wall proteins and callose and callose synthase. The peroxidase has been cloned and expressed in Pichia pastoris and has been shown to catalyse the oxidation reaction with the same kinetics as the purified enzyme. Furthermore, Arabidopsis plants transformed heterologously using the French bean peroxidase in antisense orientation have proved to be highly susceptible to bacterial and fungal pathogens. Thus it is possible that Arabidopsis is another species with the potential to mount an apoplastic oxidative burst and these transformed plant lines may be useful to identify the peroxidase that is responsible.

Lipid mediators from pollen act as chemoattractants and activators of polymorphonuclear granulocytes

BACKGROUND

Under natural exposure conditions, pollen grains function as allergen carriers that release allergens from internal binding sites on contact with the aqueous phase of mucosa membranes. In addition, we recently demonstrated that pollen are a rich source of eicosanoid-like mediators, which are rapidly released on contact with the aqueous phase.

OBJECTIVE

The current study was designed to characterize the biochemical nature of pollen-derived lipid mediators in more detail and to delineate their biologic activity on polymorphonuclear granulocytes (PMNs).

METHODS

Aqueous and lipid extracts from Phleum pratense L and Betula alba L pollen were analyzed by means of HPLC. PMNs were exposed to aqueous extracts or lipid fractions from pollen or to HPLC-purified lipid mediators identified in pollen extracts. Effects on PMNs were tested with transwell migration, calcium mobilization, and surface expression of CD11b.

RESULTS

Aqueous pollen extracts (APEs) contained predominantly monohydroxylated products derived of linoleic acid and linolenic acid. In chemotaxis assays PMNs displayed significant migration to APEs. Lipid extracts from pollen and the HPLC fraction containing 13-hydroxy-octadecadienoic acid/hydroxy-linoleic acid and 13-hydroxy-octadecatrienoic acid/hydroxy-linolenic acid induced migratory responses, although to a lesser degree than the APEs. In addition, APE, as well as lipid, extracts induced PMN activation, as documented by means of calcium mobilization and upregulation of CD11b.

CONCLUSION

Pollen grains release mediators that recruit and activate PMNs in vitro. Similar mechanisms may be effective in vivo, suggesting that pollen-derived lipid mediators may act as adjuvants in the elicitation phase of allergic reactions.

Fatty acid-derived signals in plants

Plants synthesize many fatty acid derivatives, several of which play important regulatory roles. Jasmonates are the best characterized examples. Jasmonate-insensitive mutants and mutants with a constitutive jasmonate response have given us new insights into jasmonate signalling. The jasmonate biosynthesis mutant opr3 allowed the dissection of cyclopentanone and cyclopentenone signalling, thus defining specific roles for these molecules. Jasmonate signalling is a complex network of individual signals and recent findings on specific activities of methyl jasmonate and (Z)-jasmone add to this picture. In addition, there are keto, hydroxy and hydroperoxy fatty acids that might be involved in cell death and the expression of stress-related genes. Finally, there are bruchins and volicitin, signal molecules from insects that are perceived by plants in the picomole to femtomole range. They highlight the importance of fatty acid-derived molecules in interspecies communication and in plant defence.

The lipoxygenase pathway

Lipid peroxidation is common to all biological systems, both appearing in developmentally and environmentally regulated processes of plants. The hydroperoxy polyunsaturated fatty acids, synthesized by the action of various highly specialized forms of lipoxygenases, are substrates of at least seven different enzyme families. Signaling compounds such as jasmonates, antimicrobial and antifungal compounds such as leaf aldehydes or divinyl ethers, and a plant-specific blend of volatiles including leaf alcohols are among the numerous products. Cloning of many lipoxygenases and other key enzymes within the lipoxygenase pathway, as well as analyses by reverse genetic and metabolic profiling, revealed new reactions and the first hints of enzyme mechanisms, multiple functions, and regulation. These aspects are reviewed with respect to activation of this pathway as an initial step in the interaction of plants with pathogens, insects, or abiotic stress and at distinct stages of development.

Enzymatic and non-enzymatic lipid peroxidation in leaf development

Enzymatic and non-enzymatic lipid peroxidation has been implicated in programmed cell death, which is a major process of leaf senescence. To test this hypothesis we developed a high-performance liquid chromatography (HPLC) method for a simultaneous analysis of the major hydro(pero)xy polyenoic fatty acids. Quantities of lipid peroxidation products in leaves of different stages of development including natural senescence indicated a strong increase in the level of oxygenated polyenoic fatty acids (PUFAs) during the late stages of leaf senescence. Comprehensive structural elucidation of the oxygenation products by means of HPLC, gas chromatography/mass spectrometry and (1)H nuclear magnetic resonance suggested a non-enzymatic origin. However, in some cases a small share of specifically oxidized PUFAs was identified suggesting involvement of lipid peroxidizing enzymes. To inspect the possible role of enzymatic lipid peroxidation in leaf senescence, we analyzed the abundance of lipoxygenases (LOXs) in rosette leaves of Arabidopsis. LOXs and their product (9Z,11E,13S,15Z)-13-hydroperoxy-9,11,15-octadecatrienoic acid were exclusively detected in young green leaves. In contrast, in senescing leaves the specific LOX products were overlaid by large amounts of stereo-random lipid peroxidation products originating from non-enzymatic oxidation. These data indicate a limited contribution of LOXs to total lipid peroxidation, and a dominant role of non-enzymatic lipid peroxidation in late stages of leaf development.