The activity of HYDROPEROXIDE LYASE 1 regulates accumulation of galactolipids containing 12-oxo-phytodienoic acid in Arabidopsis

Metabolite modification in oxidative stress responses: A case study of two defense hormones

Studies of the Arabidopsis cat2 mutant lacking the major leaf isoform of catalase have allowed the potential impact of intracellular H₂O₂ on plant function to be studied. Here, we report a robust analysis of modified gene expression associated with key families involved in metabolite modification in cat2. Through a combined transcriptomic and metabolomic analysis focused on the salicylic acid (SA) and jasmonic acid (JA) pathways, we report key features of the metabolic signatures linked to oxidative stress-induced signaling via these defence hormones and discuss the enzymes that are likely to be involved in determining these features. We provide evidence that specific UDP-glycosyl transferases contribute to the glucosylation of SA that accumulates as a result of oxidative stress in cat2. Glycosides of dihydroxybenzoic acids that accumulate alongside SA in cat2 are identified and, based on the expression of candidate genes, likely routes for their production are discussed. We also report that enhanced intracellular H₂O₂ triggers induction of genes encoding different enzymes that can metabolize JA. Integrated analysis of metabolite and transcript profiles suggests that a gene network involving specific hydrolases, hydroxylases, and sulfotransferases functions to limit accumulation of the most active jasmonates during oxidative stress.

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

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

Role of jasmonic acid in plants: the molecular point of view

Recent updates in JA biosynthesis, signaling pathways and the crosstalk between JA and others phytohormones in relation with plant responses to different stresses. In plants, the roles of phytohormone jasmonic acid (JA), amino acid conjugate (e.g., JA-Ile) and their derivative emerged in last decades as crucial signaling compounds implicated in stress defense and development in plants. JA has raised a great interest, and the number of researches on JA has increased rapidly highlighting the importance of this phytohormone in plant life. First, JA was considered as a stress hormone implicated in plant response to biotic stress (pathogens and herbivores) which confers resistance to biotrophic and hemibiotrophic pathogens contrarily to salicylic acid (SA) which is implicated in plant response to necrotrophic pathogens. JA is also implicated in plant responses to abiotic stress (such as soil salinity, wounding and UV). Moreover, some researchers have recently revealed that JA controls several physiological processes like root growth, growth of reproductive organs and, finally, plant senescence. JA is also involved in the biosynthesis of various metabolites (e.g., phytoalexins and terpenoids). In plants, JA signaling pathways are well studied in few plants essentially Arabidopsis thaliana, Nicotiana benthamiana, and Oryza sativa L. confirming the crucial role of this hormone in plants. In this review, we highlight the last foundlings about JA biosynthesis, JA signaling pathways and its implication in plant maturation and response to environmental constraints.

Jasmonates: biosynthesis, perception and signal transduction

Jasmonates (JAs) are physiologically important molecules involved in a wide range of plant responses from growth, flowering, senescence to defence against abiotic and biotic stress. They are rapidly synthesised from α-linolenic acid (ALA; C18:3 ∆9,12,15) by a process of oxidation, cyclisation and acyl chain shortening involving co-operation between the chloroplast and peroxisome. The active form of JA is the isoleucine conjugate, JA-isoleucine (JA-Ile), which is synthesised in the cytoplasm. Other active metabolites of JA include the airborne signalling molecules, methyl JA (Me-JA) and cis-jasmone (CJ), which act as inter-plant signalling molecules activating defensive genes encoding proteins and secondary compounds such as anthocyanins and alkaloids. One of the key defensive metabolites in many plants is a protease inhibitor that inactivates the protein digestive capabilities of insects, thereby, reducing their growth. The receptor for JA-Ile is a ubiquitin ligase termed as SCFCoi1 that targets the repressor protein JA Zim domain (JAZ) for degradation in the 26S proteasome. Removal of JAZ allows other transcription factors (TFs) to activate the JA response. The levels of JA-Ile are controlled through catabolism by hydroxylating enzymes of the cytochrome P450 (CYP) family. The JAZ proteins act as metabolic hubs and play key roles in cross-talk with other phytohormone signalling pathways in co-ordinating genome-wide responses. Specific subsets of JAZ proteins are involved in regulating different response outcomes such as growth inhibition versus biotic stress responses. Understanding the molecular circuits that control plant responses to pests and pathogens is a necessary pre-requisite to engineering plants with enhanced resilience to biotic challenges for improved agricultural yields.

Genetic controls of short- and long-term stomatal CO2 responses in Arabidopsis thaliana

BACKGROUND AND AIMS

The stomatal conductance (gs) of most plant species decreases in response to elevated atmospheric CO2 concentration. This response could have a significant impact on plant water use in a future climate. However, the regulation of the CO2-induced stomatal closure response is not fully understood. Moreover, the potential genetic links between short-term (within minutes to hours) and long-term (within weeks to months) responses of gs to increased atmospheric CO2 have not been explored.

METHODS

We used Arabidopsis thaliana recombinant inbred lines originating from accessions Col-0 (strong CO2 response) and C24 (weak CO2 response) to study short- and long-term controls of gs. Quantitative trait locus (QTL) mapping was used to identify loci controlling short- and long-term gs responses to elevated CO2, as well as other stomata-related traits.

KEY RESULTS

Short- and long-term stomatal responses to elevated CO2 were significantly correlated. Both short- and long-term responses were associated with a QTL at the end of chromosome 2. The location of this QTL was confirmed using near-isogenic lines and it was fine-mapped to a 410-kb region. The QTL did not correspond to any known gene involved in stomatal closure and had no effect on the responsiveness to abscisic acid. Additionally, we identified numerous other loci associated with stomatal regulation.

CONCLUSIONS

We identified and confirmed the effect of a strong QTL corresponding to a yet unknown regulator of stomatal closure in response to elevated CO2 concentration. The correlation between short- and long-term stomatal CO2 responses and the genetic link between these traits highlight the importance of understanding guard cell CO2 signalling to predict and manipulate plant water use in a world with increasing atmospheric CO2 concentration. This study demonstrates the power of using natural variation to unravel the genetic regulation of complex traits.

Waterlogging tolerance rendered by oxylipin-mediated metabolic reprogramming in Arabidopsis

Environmental stresses induce production of oxylipins synthesized by the two main biosynthetic branches, allene oxide synthase (AOS) and hydroperoxide lyase (HPL). Here, we investigate how waterlogging-mediated alteration of AOS- and HPL-derived metabolic profile results in modulation of central metabolism and ultimately enhanced tolerance to this environmental stress in Arabidopsis thaliana. Waterlogging leads to increased levels of AOS- and HPL-derived metabolites, and studies of genotypes lacking either one or both branches further support the key function of these oxylipins in waterlogging tolerance. Targeted quantitative metabolic profiling revealed oxylipin-dependent alterations in selected primary metabolites, and glycolytic and citric acid cycle intermediates, as well as a prominent shift in sucrose cleavage, hexose activation, the methionine salvage pathway, shikimate pathway, antioxidant system, and energy metabolism in genotypes differing in the presence of one or both functional branches of the oxylipin biosynthesis pathway. Interestingly, despite some distinct metabolic alterations caused specifically by individual branches, overexpression of HPL partially or fully alleviates the majority of altered metabolic profiles observed in AOS-depleted lines. Collectively, these data identify the key role of AOS- and HPL-derived oxylipins in altering central metabolism, and further provide a metabolic platform targeted at identification of gene candidates for enhancing plant tolerance to waterlogging.

Substrate channeling in oxylipin biosynthesis through a protein complex in the plastid envelope of Arabidopsis thaliana

Oxygenated membrane fatty acid derivatives termed oxylipins play important roles in plant defense against biotic and abiotic cues. Plants challenged by insect pests, for example, synthesize a blend of different defense compounds that include volatile aldehydes and jasmonic acid (JA), among others. Because all oxylipins are derived from the same pathway, we investigated how their synthesis might be regulated, focusing on two closely related atypical cytochrome P450 enzymes designated CYP74A and CYP74B, respectively, allene oxide synthase (AOS) and hydroperoxide lyase (HPL). These enzymes compete for the same substrate but give rise to different products: the final product of the AOS branch of the oxylipin pathway is JA, while those of the HPL branch comprise volatile aldehydes and alcohols. AOS and HPL are plastid envelope enzymes in Arabidopsis thaliana but accumulate at different locations. Biochemical experiments identified AOS as a constituent of complexes also containing lipoxygenase 2 (LOX2) and allene oxide cyclase (AOC), which catalyze consecutive steps in JA precursor biosynthesis, while excluding the concurrent HPL reaction. Based on published X-ray data, the structure of this complex was modelled and amino acids involved in catalysis and subunit interactions predicted. Genetic studies identified the microRNA 319-regulated clade of TCP (TEOSINTE BRANCHED/CYCLOIDEA/PCF) transcription factor genes and CORONATINE INSENSITIVE 1 (COI1) as controlling JA production through the LOX2-AOS-AOC2 complex. Together, our results define a molecular branch point in oxylipin biosynthesis that allows fine-tuning of the plant's defense machinery in response to biotic and abiotic stimuli.

Detection and identification of complex oxylipins in meadow buttercup (Ranunculus acris) leaves

Screening of linolipins, i.e. galactolipids containing esterified residues of divinyl ether oxylipins, in the leaves of several higher plants revealed the presence of these complex oxylipins in the meadow buttercup leaves. The rapid accumulation of linolipins occurred in the injured leaves of meadow buttercup, while intact leaves possessed no linolipins. These oxylipins were isolated from the injured leaves, separated and purified by HPLC. The structural analyses of linolipins by UV, mass-spectroscopy and NMR spectroscopy resulted in the identification of eight molecular species. Three of them were identical to linolipins B-D found earlier in the leaves of flax (Linum usitatissimum L.). Other molecular species were identified as 1-O-(ω5Z)-etherolenoyl-2-O-dinor-(ω5Z)-etherolenoyl-3-O-β-D-galactopyranosyl-sn-glycerol, 1-O-(ω5Z)-etherolenoyl-2-O-(7Z,10Z,13Z)-hexadecatrienoyl-3-O-β-D-galactopyranosyl-sn-glycerol, 1-O-(ω5Z)-etherolenoyl-2-O-(7Z,10Z)-hexadecadienoyl-3-O-β-D-galactopyranosyl-sn-glycerol, 1-O-(ω5Z)-etherolenoyl-2-O-α-linolenoyl-3-O-β-D-galactopyranosyl-sn-glycerol, and 1-O-(ω5Z)-etherolenoyl-2-O-palmitoyl-3-O-(α-galactopyranosyl-1-6-β-D-galactopyranosyl)-sn-glycerol. The trivial names "linolipins E, F, G, H and I," respectively, have been ascribed to these novel complex oxylipins.

Jasmonates: News on Occurrence, Biosynthesis, Metabolism and Action of an Ancient Group of Signaling Compounds

: Jasmonic acid (JA) and its related derivatives are ubiquitously occurring compounds of land plants acting in numerous stress responses and development. Recent studies on evolution of JA and other oxylipins indicated conserved biosynthesis. JA formation is initiated by oxygenation of α-linolenic acid (α-LeA, 18:3) or 16:3 fatty acid of chloroplast membranes leading to 12-oxo-phytodienoic acid (OPDA) as intermediate compound, but in Marchantiapolymorpha and Physcomitrellapatens, OPDA and some of its derivatives are final products active in a conserved signaling pathway. JA formation and its metabolic conversion take place in chloroplasts, peroxisomes and cytosol, respectively. Metabolites of JA are formed in 12 different pathways leading to active, inactive and partially active compounds. The isoleucine conjugate of JA (JA-Ile) is the ligand of the receptor component COI1 in vascular plants, whereas in the bryophyte M. polymorpha COI1 perceives an OPDA derivative indicating its functionally conserved activity. JA-induced gene expressions in the numerous biotic and abiotic stress responses and development are initiated in a well-studied complex regulation by homeostasis of transcription factors functioning as repressors and activators.

Gene Expression Patterns in Roots of Camelina sativa With Enhanced Salinity Tolerance Arising From Inoculation of Soil With Plant Growth Promoting Bacteria Producing 1-Aminocyclopropane-1-Carboxylate Deaminase or Expression the Corresponding acdS Gene

Camelina sativa treated with plant growth-promoting bacteria (PGPB) producing 1-aminocyclopropane-1-carboxylate deaminase (acdS) or transgenic lines expressing acdS exhibit increased salinity tolerance. AcdS reduces the level of stress ethylene to below the point where it is inhibitory to plant growth. The study determined that several mechanisms appear to be responsible for the increased salinity tolerance and that the effect of acdS on gene expression patterns in C. sativa roots during salt stress is a function of how it is delivered. Growth in soil treated with the PGPB (Pseudomonas migulae 8R6) mostly affected ethylene- and abscisic acid-dependent signaling in a positive way, while expression of acdS in transgenic lines under the control of the broadly active CaMV 35S promoter or the root-specific rolD promoter affected auxin, jasmonic acid and brassinosteroid signaling and/biosynthesis. The expression of genes involved in minor carbohydrate metabolism were also up-regulated, mainly in roots of lines expressing acdS. Expression of acdS also affected the expression of genes involved in modulating the level of reactive oxygen species (ROS) to prevent cellular damage, while permitting ROS-dependent signal transduction. Though the root is not a photosynthetic tissue, acdS had a positive effect on the expression of genes involved in photosynthesis.

The Oxylipin Pathways: Biochemistry and Function

Plant oxylipins form a constantly growing group of signaling molecules that comprise oxygenated fatty acids and metabolites derived therefrom. In the last decade, the understanding of biosynthesis, metabolism, and action of oxylipins, especially jasmonates, has dramatically improved. Additional mechanistic insights into the action of enzymes and insights into signaling pathways have been deepened for jasmonates. For other oxylipins, such as the hydroxy fatty acids, individual signaling properties and cross talk between different oxylipins or even with additional phytohormones have recently been described. This review summarizes recent understanding of the biosynthesis, regulation, and function of oxylipins.

The hydroperoxide lyase branch of the oxylipin pathway protects against photoinhibition of photosynthesis

This study describes a new role for hydroperoxide lyase branch of oxylipin biosynthesis pathway in protecting photosynthetic apparatus under high light conditions. Lipid-derived signaling molecules, oxylipins, produced by a multi-branch pathway are central in regulation of a wide range of functions. The two most known branches, allene oxide synthase (AOS) and 13-hydroperoxide lyase (HPL) pathways, are best recognized as producers of defense compounds against biotic challenges. In the present work, we examine the role of these two oxylipin branches in plant tolerance to the abiotic stress, namely excessive light. Towards this goal, we have analyzed variable chlorophyll fluorescence parameters of intact leaves of Arabidopsis thaliana genotypes with altered oxylipin profile, followed by examining the impact of exogenous application of selected oxylipins on functional activity of photosynthetic apparatus in intact leaves and isolated thylakoid membranes. Our findings unequivocally bridge the function of oxylipins to photosynthetic processes. Specifically, HPL overexpressing lines display enhanced adaptability in response to high light treatment as evidenced by lower rate constant of photosystem 2 (PS2) photoinhibition and higher rate constant of PS2 recovery after photoinhibition. In addition, exogenous application of linolenic acid, 13-hydroperoxy linolenic acid, 12-oxophytodienoic acid, and methyl jasmonate individually, suppresses photochemical activity of PS2 in intact plants and isolated thylakoid membranes, while application of HPL-branch metabolites-does not. Collectively these data implicate function of HPL branch of oxylipin biosynthesis pathway in guarding PS2 under high light conditions, potentially exerted through tight regulation of free linolenic acid and 13-hydroperoxy linolenic acid levels, as well as competition with production of metabolites by AOS-branch of the oxylipin pathway.

Jasmonates: biosynthesis, metabolism, and signaling by proteins activating and repressing transcription

The lipid-derived phytohormone jasmonate (JA) regulates plant growth, development, secondary metabolism, defense against insect attack and pathogen infection, and tolerance to abiotic stresses such as wounding, UV light, salt, and drought. JA was first identified in 1962, and since the 1980s many studies have analyzed the physiological functions, biosynthesis, distribution, metabolism, perception, signaling, and crosstalk of JA, greatly expanding our knowledge of the hormone's action. In response to fluctuating environmental cues and transient endogenous signals, the occurrence of multilayered organization of biosynthesis and inactivation of JA, and activation and repression of the COI1-JAZ-based perception and signaling contributes to the fine-tuning of JA responses. This review describes the JA biosynthetic enzymes in terms of gene families, enzymatic activity, location and regulation, substrate specificity and products, the metabolic pathways in converting JA to activate or inactivate compounds, JA signaling in perception, and the co-existence of signaling activators and repressors.