Phytochemistry (v.70, #13-14)

Jasmonates in stress responses and development by Bettina Hause; Claus Wasternack; Dieter Strack (1483-1484).

Methods for the analysis of oxylipins in plants by Cornelia Göbel; Ivo Feussner (1485-1503).
Methods for oxylipin analysis are reviewed with respect to different levels of complexity.Plant oxylipins comprise a highly diverse and complex class of molecules that are derived from lipid oxidation. The initial oxidation of unsaturated fatty acids may either occur by enzymatic or chemical reactions. A large variety of oxylipin classes are generated by an array of alternative reactions further converting hydroperoxy fatty acids. The structural diversity of oxylipins is further increased by their occurrence either as free fatty acid derivatives or as esters in complex lipids. Lipid peroxidation is common to all biological systems, appearing in developmentally regulated processes and as a response to environmental changes. The oxylipins formed may perform various biological roles; some of them have signaling functions. In order to elucidate the roles of oxylipins in a given biological context, comprehensive analytical assays are available for determining the oxylipin profiles of plant tissues. This review summarizes indirect methods to estimate the general peroxidation state of a sample and more sophisticated techniques for the identification, structure determination and quantification of oxylipins.
Keywords: Gas chromatography; Lipid peroxidation; Liquid chromatography; Mass spectrometry;

Lipoxygenases – Structure and reaction mechanism by Alexandra Andreou; Ivo Feussner (1504-1510).
Insights into the enzyme mechanism of lipoxygenases are reviewed with respect to models on their product specificity.Lipid oxidation is a common metabolic reaction in all biological systems, appearing in developmentally regulated processes and as response to abiotic and biotic stresses. Products derived from lipid oxidation processes are collectively named oxylipins. Initial lipid oxidation may either occur by chemical reactions or is derived from the action of enzymes. In plants this reaction is mainly catalyzed by lipoxygenase (LOXs) enzymes and during recent years analysis of different plant LOXs revealed insights into their enzyme mechanism. This review aims at giving an overview of concepts explaining the catalytic mechanism of LOXs as well as the different regio- and stereo-specificities of these enzymes.
Keywords: Dioxygenases; Fatty acid peroxidation; Lipid peroxidation; Octadecanoids; Stereo- and regio-specific dioxygenation; Substrate specificity;

Reactive electrophilic oxylipins: Pattern recognition and signalling by Martin J. Mueller; Susanne Berger (1511-1521).
Reactive electrophilic species are double-edged swords: physiological levels of these oxylipins may trigger cell defense and rescue mechanisms while high levels will kill the plant.Oxidized lipids in plants comprise a variety of reactive electrophiles that contain an α,β-unsaturated carbonyl group. While some of these compounds are formed enzymatically, many of them are formed by non-enzymatic pathways. In addition to their chemical reactivity/toxicity low levels of these compounds are also biologically active. Despite their structural diversity and biosynthetic origin, common biological activities such as induction of defense genes, activation of detoxification responses and growth inhibition have been documented. However, reactive electrophilic oxylipins are poorly defined as a class of compounds but have at least two properties in common, i.e., lipophilicity and thiol-reactivity. Thiol-reactivity is a property of reactive oxylipins (RES) shared by reactive oxygen and nitrogen species (ROS and RNS) and enables these agents to modify proteins in vivo. Thiol-modification is assumed to represent a key mechanism involved in signal transduction. A metaanalysis of proteomic studies reveals that RES oxylipins, ROS and RNS apparently chemically modify a similar set of highly sensitive proteins, virtually all of which are targets for thioredoxins. Moreover, most of these proteins are redox-regulated, i.e., posttranslational thiol-modification alters the activity or function of these proteins. On the transcriptome level, effects of RES oxylipins and ROS on gene induction substantially overlap but are clearly different. Besides electrophilicity other structural properties such as target affinity apparently determine target selectivity and biological activity. In this context, different signalling mechanisms and signal transduction components identified in plants and non-plant organisms as well as putative functions of RES oxylipins are discussed.
Keywords: Jasmonates; Phytoprostanes; Redox-regulation; Thiol-switch; Xenobiotic response;

The CYP74 enzymes using fatty acid hydroperoxides as substrate have mechanisms in close parallel with CYP5 and CYP8A in mammalian biology, and in many respects with conventional P450 monooxygenases.The existence of CYP5, CYP8A, and the CYP74 enzymes specialized for reaction with fatty acid peroxide substrates presents opportunities for a “different look” at the catalytic cycle of the cytochrome P450s. This review considers how the properties of the peroxide-metabolizing enzymes are distinctive, and how they tie in with those of the conventional monooxygenase enzymes. Some unusual reactions of each class have parallels in the other. As enzyme reactions and P450 structures emerge there will be possibilities for finding their special properties and edging this knowledge into the big picture.
Keywords: Allene oxide; Allene oxide synthase; Hydroperoxide lyase; Divinyl ether synthase; CYP74; Cytochrome P450; Fatty acid hydroperoxide; Thromboxane synthase; Prostacyclin synthase; Mechanism; Compound I; Compound II; Catalytic cycle;

Enzymes in jasmonate biosynthesis – Structure, function, regulation by Andreas Schaller; Annick Stintzi (1532-1538).
Within the jasmonate branch of oxylipin biosynthesis, there are three enzymes truly committed to the biosynthesis of jasmonic acid: allene oxide synthase, allene oxide cyclase and oxophytodienoate reductase. Crystal structures have recently been solved for these three enzymes providing insights into the biochemistry, the specificity, and the regulation of JA biosynthesis at unprecedented level of detail.Jasmonates are a growing class of lipid-derived signaling molecules with diverse functions ranging from the initiation of biotic and abiotic stress responses to the regulation of plant growth and development. Jasmonate biosynthesis originates from polyunsaturated fatty acids in chloroplast membranes. In a first lipoxygenase-catalyzed reaction molecular oxygen is introduced to yield their 13-hydroperoxy derivatives. These fatty acid hydroperoxides are converted by allene oxide synthase and allene oxide cyclase to 12-oxophytodienoic acid (OPDA) and dinor-OPDA, i.e. the first cyclic intermediates of the pathway. In the subsequent step, the characteristic cyclopentanone ring structure of jasmonates is established by OPDA reductase. Until recently, jasmonic acid has been viewed as the end product of the pathway and as the bioactive hormone. It becomes increasingly clear, however, that biological activity extends to and may even differ between the various jasmonic acid metabolites and conjugates as well as its biosynthetic precursors. It has also become clear that oxygenated fatty acids give rise to a vast variety of bioactive compounds including but not limited to jasmonates. Recent insights into the structure, function, and regulation of the enzymes involved in jasmonate biosynthesis help to explain how this variety is generated while specificity is maintained.
Keywords: Allene oxide cyclase; Allene oxide synthase; Crystal structure; CYP74; Jasmonate biosynthesis; Oxylipins; Oxophytodienoate reductase; Substrate specificity;

Mutants have helped to elucidate synthesis and signaling of jasmonoyl-isoleucine.Mutant analysis includes approaches that range from traditional screening of mutant populations (forward genetics), to identifying mutations in known genes (reverse genetics), to examining the effects of site-specific mutations that encode modified proteins. All these methodologies have been applied to study jasmonate synthesis and signaling, and their use has led to important discoveries. The fad3 fad7 fad8 mutant of Arabidopsis, and other mutants defective in jasmonate synthesis, revealed the roles of jasmonate in flower development and plant defense against necrotrophic fungal pathogens. The coi1 mutant identified the F-box protein that is now known to be the receptor for jasmonoyl-isoleucine, the active form of jasmonate hormone. Investigations of how JASMONATE-ZIM DOMAIN (JAZ) proteins bind to COI1 and facilitate jasmonate perception have relied on the jai3 mutant, on JAZΔJas constructs, and on site-specific mutations in the Jas and ZIM domains of these proteins.
Keywords: Arabidopsis; Jasmonate; Jasmonoyl-isoleucine; JAZ proteins; Mutant;

Top hits in contemporary JAZ: An update on jasmonate signaling by Hoo Sun Chung; Yajie Niu; John Browse; Gregg A. Howe (1547-1559).
Recent analysis of JAZ repressor proteins has provided insight into the nature of the jasmonate receptor, the chemical specificity of jasmonate perception, and post-transcriptional mechanisms to increase versatility in gene regulation by this lipid-derived hormone.The phytohormone jasmonate (JA) regulates a wide range of growth, developmental, and defense-related processes during the plant life cycle. Identification of the JAZ family of proteins that repress JA responses has facilitated rapid progress in understanding how this lipid-derived hormone controls gene expression. Recent analysis of JAZ proteins has provided insight into the nature of the JA receptor, the chemical specificity of signal perception, and cross-talk between JA and other hormone response pathways. Functional diversification of JAZ proteins by alternative splicing, together with the ability of JAZ proteins to homo- and heterodimerize, provide mechanisms to enhance combinatorial diversity and versatility in gene regulation by JA.
Keywords: Jasmonate; COI1; JAZ; Ubiquitin; Alternative splicing; Protein–protein interaction;

This review presents an overview of promoter sequences and transcription factors involved in jasmonate-responsive gene expression.Plants possess inducible defense systems to oppose attack by pathogens and herbivores. Jasmonates are important signaling molecules produced by plants which regulate in positive or negative crosstalk with ethylene subsets of genes involved in defense against necrotrophic microorganisms or herbivorous insects, respectively. This review presents an overview of promoter sequences and transcription factors involved in jasmonate-responsive gene expression with the most important components summarized here. Frequently occurring jasmonate-responsive promoter sequences are the GCC motif, which is commonly found in promoters activated synergistically by jasmonate and ethylene, and the G-box, which is commonly found in promoters activated by jasmonates and repressed by ethylene. Important transcription factors conferring jasmonate-responsive gene expression in Arabidopsis are ORA59 and AtMYC2. ORA59 interacts with the GCC motif and controls the expression of genes that are synergistically induced by jasmonates and ethylene, whereas AtMYC2 interacts with the G-box and related sequences, and controls genes activated by jasmonate alone. AtMYC2 can interact with JAZ proteins, which are hypothesized to act as repressors. The bioactive jasmonate (+)-7-iso-JA-l-Ile promotes the interaction between the ubiquitin ligase complex SCFCOI1 and JAZ proteins, resulting in their degradation by the 26S proteasome, thereby liberating AtMYC2 from repression according to the prevailing model. Literature up to 1 June 2009 was used for this review.
Keywords: Arabidopsis; AtMYC2; Catharanthus roseus; COI1; G-box; JAZ; Nicotine; ORA59; ORCA3; Promoter;

The wound hormone jasmonate by Abraham J.K. Koo; Gregg A. Howe (1571-1580).
Recent advances in understanding the molecular mechanism of jasmonate signaling provide a framework for understanding how plants recognize and respond to tissue injury.Plant tissues are highly vulnerable to injury by herbivores, pathogens, mechanical stress, and other environmental insults. Optimal plant fitness in the face of these threats relies on complex signal transduction networks that link damage-associated signals to appropriate changes in metabolism, growth, and development. Many of these wound-induced adaptive responses are triggered by de novo synthesis of the plant hormone jasmonate (JA). Recent studies provide evidence that JA mediates systemic wound responses through distinct cell autonomous and non-autonomous pathways. In both pathways, bioactive JAs are recognized by an F-box protein-based receptor system that couples hormone binding to ubiquitin-dependent degradation of transcriptional repressor proteins. These results provide a framework for understanding how plants recognize and respond to tissue injury.
Keywords: Wound response; Jasmonate; COI1; JAZ; Systemic signaling; Plant defense;

Jasmonate signaling in plant interactions with resistance-inducing beneficial microbes by Sjoerd Van der Ent; Saskia C.M. Van Wees; Corné M.J. Pieterse (1581-1588).
Jasmonates are important regulators of plant immune responses that are triggered by beneficial soil-borne microorganisms. In many cases ISR is associated with potentiated expression of jasmonate-responsive genes. Recent advances in research on induced systemic resistance (ISR) signaling suggests a model in which jasmonate-related transcription factors play a central role in establishing the primed state that is characteristic for ISR.Beneficial soil-borne microorganisms can induce an enhanced defensive capacity in above-ground plant parts that provides protection against a broad spectrum of microbial pathogens and even insect herbivores. The phytohormones jasmonic acid (JA) and ethylene emerged as important regulators of this induced systemic resistance (ISR). ISR triggered by plant growth-promoting rhizobacteria and fungi is often not associated with enhanced biosynthesis of these hormones, nor with massive changes in defense-related gene expression. Instead, ISR-expressing plants are primed for enhanced defense. Priming is characterized by a faster and stronger expression of cellular defense responses that become activated only upon pathogen or insect attack, resulting in an enhanced level of resistance to the invader encountered. Recent advances in induced defense signaling research revealed regulators of ISR and suggest a model in which (JA)-related transcription factors play a central role in establishing the primed state.
Keywords: Beneficial microorganisms; Plant growth-promoting rhizobacteria (PGPR) and fungi (PGPF); Induced systemic resistance (ISR); Defense signaling; Ethylene; Jasmonic acid; Microbe-associated molecular patterns (MAMPs); Priming; Transcription factors;

The arbuscular mycorrhiza (AM) association between plants and biotrophic fungi and the legume-rhizobia symbiosis (LRS) between legume plants and nitrogen-fixing bacteria are two mutualistic symbioses of critical importance in nature and sustainable agriculture. Phytohormones are involved in the regulation of both interactions. This review summarizes the current knowledge about the role of jasmonates in AM and LRS.Many plants are able to develop mutualistic interactions with arbuscular mycorrhizal fungi and/or nitrogen-fixing bacteria. Whereas the former is widely distributed among most of the land plants, the latter is restricted to species of ten plant families, including the legumes. The establishment of both associations is based on mutual recognition and a high degree of coordination at the morphological and physiological level. This requires the activity of a number of signals, including jasmonates. Here, recent knowledge on the putative roles of jasmonates in both mutualistic symbioses will be reviewed. Firstly, the action of jasmonates will be discussed in terms of the initial signal exchange between symbionts and in the resulting plant signaling cascade common for nodulation and mycorrhization. Secondly, the putative role of jasmonates in the autoregulation of the endosymbioses will be outlined. Finally, aspects of function of jasmonates in the fully established symbioses will be presented. Various processes will be discussed that are possibly mediated by jasmonates, including the redox status of nodules and the carbohydrate partitioning of mycorrhizal roots.
Keywords: Allene oxide cyclase; Arbuscular mycorrhiza; Autoregulation; Glomus sp.; Jasmonic acid; Legumes; Nitrogen fixing nodules; Signal transduction; Rhizobia;

Methyl jasmonate: A plant stress hormone as an anti-cancer drug by Sharon Cohen; Eliezer Flescher (1600-1609).
Methyl jasmonate is an anti-cancer agent. It detaches hexokinase from the mitochondrial protein VDAC, resulting in mitochondrial membrane depolarization, cytochrome c release, and finally apoptotic cell death.Jasmonates act as signal transduction intermediates when plants are subjected to environmental stresses such as UV radiation, osmotic shock and heat. In the past few years several groups have reported that jasmonates exhibit anti-cancer activity in vitro and in vivo and induce growth inhibition in cancer cells, while leaving the non-transformed cells intact. Recently, jasmonates were also discovered to have cytotoxic effects towards metastatic melanoma both in vitro and in vivo.Three mechanisms of action have been proposed to explain this anti-cancer activity. The bio-energetic mechanism – jasmonates induce severe ATP depletion in cancer cells via mitochondrial perturbation. Furthermore, methyl jasmonate (MJ) has the ability to detach hexokinase from the mitochondria. Second, jasmonates induce re-differentiation in human myeloid leukemia cells via mitogen-activated protein kinase (MAPK) activity and were found to act similar to the cytokinin isopentenyladenine (IPA). Third, jasmonates induce apoptosis in lung carcinoma cells via the generation of hydrogen peroxide, and pro-apoptotic proteins of the Bcl-2 family.Combination of MJ with the glycolysis inhibitor 2-deoxy-d-glucose (2DG) and with four conventional chemotherapeutic drugs resulted in super-additive cytotoxic effects on several types of cancer cells. Finally, jasmonates have the ability to induce death in spite of drug-resistance conferred by either p53 mutation or P-glycoprotein (P-gp) over-expression.In summary, the jasmonates are anti-cancer agents that exhibit selective cytotoxicity towards cancer cells, and thus present hope for the development of cancer therapeutics.
Keywords: Jasmonates; Methyl jasmonate; Cancer; Apoptosis; Cytotoxic effect; Mitochondria;