Classification of Terpenoids according to the Methylerythritolphosphate or the Mevalonate Pathway with Natural12C/13C Isotope Ratios: Dynamic Allocation of Resources in Induced Plants

A central role for polyprenol reductase in plant dolichol biosynthesis

Dolichol is an essential polyisoprenoid within the endoplasmic reticulum of all eukaryotes. It serves as a membrane bound anchor onto which N-glycans are assembled prior to being transferred to nascent polypeptides, many of which enter the secretory pathway. Historically, it has been posited that the accumulation of dolichol represents the 'rate-limiting' step in the evolutionary conserved process of N-glycosylation, which ultimately affects the efficacy of approximately one fifth of the entire eukaryotic proteome. Therefore, this study aimed to enhance dolichol accumulation by manipulating the enzymes involved in its biosynthesis using an established Nicotiana benthamiana platform. Co-expression of a Solanum lycopersicum (tomato) cis-prenyltransferase (CPT) and its cognate partner protein, CPT binding protein (CPTBP), that catalyze the antepenultimate step in dolichol biosynthesis led to a 400-fold increase in the levels of long-chain polyprenols but resulted in only modest increases in dolichol accumulation. However, when combined with a newly characterized tomato polyprenol reductase, dolichol biosynthesis was enhanced by approximately 20-fold. We provide further evidence that in the aquatic macrophyte, Lemna gibba, dolichol is derived exclusively from the mevalonic acid (MVA) pathway with little participation from the evolutionary co-adopted non-MVA pathway. Taken together these results indicate that to effectively enhance the in planta accumulation of dolichol, coordinated synthesis and reduction of polyprenol to dolichol, is strictly required.

Biosynthesis of Isoprene Units in Euphorbia lathyris Laticifers vs. Other Tissues: MVA and MEP Pathways, Compartmentation and Putative Endophytic Fungi Contribution

Euphorbia species are characterized by a net of laticifers producing large amounts of triterpenes. These hydrocarbon-like metabolites can be converted into fuel by the methods of the oil industry. Euphorbia lathyris is easily grown at an industrial scale. In an attempt to increase its triterpene production, the metabolic pathways leading to isoprenoid were investigated by incorporation of ¹³C labeled glucose and mevalonate and ²H labeled deoxyxylulose as well as by natural abundance isotope ratio GC-MS. Latex triterpenes are exclusively synthesized via the mevalonate (MVA) pathway: this may orient future search for improving the triterpene production in E. lathyris. Phytosterols and their precursors are mainly derived from MVA pathway with a slight contribution of the methylerythritol phosphate (MEP) pathway, whereas phytol is issued from MEP pathway with a minor contribution of the MVA pathway: this is in accordance with the metabolic cross-talk between cytosolic and plastidial compartments in plants. In addition, hopenol B behaved differently from the other latex triterpenes. Its ¹³C isotope abundance after incorporation of ¹³C labeled glucose and its natural abundance δ²H signature clearly differed from those of the other latex triterpenes indicating another metabolic origin and suggesting that it may be synthesized by an endophytic fungus.

Linking Terpene Synthases to Sesquiterpene Metabolism in Grapevine Flowers

Grapevine (Vitis vinifera L.) terpene synthases (VviTPS) are responsible for the biosynthesis of terpenic volatiles. Volatile profiling of nine commercial wine cultivars showed unique cultivar-specific variation in volatile terpenes emitted from grapevine flowers. The flower chemotypes of three divergent cultivars, Muscat of Alexandria, Sauvignon Blanc and Shiraz were subsequently investigated at two flower developmental stages (EL-18 and -26). The cultivars displayed unique flower sesquiterpene compositions that changed during flower organogenesis and the profiles were dominated by either (E)-β-farnesene, (E,E)-α-farnesene or (+)-valencene. In silico remapping of microarray probes to VviTPS gene models allowed for a meta-analysis of VviTPS expression patterns in the grape gene atlas to identify genes that could regulate terpene biosynthesis in flowers. Selected sesquiterpene synthase genes were isolated and functionally characterized in three cultivars. Genotypic differences that could be linked to the function of a targeted gene model resulted in the isolation of a novel and cultivar-specific single product sesquiterpene synthase from Muscat of Alexandria flowers (VvivMATPS10), synthesizing (E)-β-farnesene as its major volatile. Furthermore, we identified structural variations (SNPs, InDels and splice variations) in the characterized VviTPS genes that potentially impact enzyme function and/or volatile sesquiterpene production in a cultivar-specific manner.

Mechanistic studies of sesquiterpene cyclases based on their carbon isotope ratios at natural abundance

During the process of terpene biosynthesis, C-C bond breaking and forming steps are subjected to kinetic carbon isotope effects, leading to distinct carbon isotopic signatures of the products. Accordingly, carbon isotopic signatures could be used to reveal the 'biosynthetic history' of the produced terpenoids. Five known sesquiterpene cyclases, regulating three different pathways, representing simple to complex biosynthetic sequences, were heterologously expressed and used for in vitro assays with farnesyl diphosphate as substrate. Compound specific isotope ratio mass spectrometry measurements of the enzyme substrate farnesyl diphosphate (FDP) and the products of all the five cyclases were performed. The calculated δ¹³ C value for FDP, based on δ¹³ C values and relative amounts of the products, was identical with its measured δ¹³ C value, confirming the reliability of the approach and the precision of measurements. The different carbon isotope ratios of the products reflect the complexity of their structure and are correlated with the frequency of carbon-carbon bond forming and breaking steps on their individual biosynthetic pathways. Thus, the analysis of carbon isotopic signatures of terpenes at natural abundance can be used as a powerful tool in elucidation of associated biosynthetic mechanisms of terpene synthases and in future in vivo studies even without 'touching' the plant.

Multi-Substrate Terpene Synthases: Their Occurrence and Physiological Significance

Terpene synthases are responsible for synthesis of a large number of terpenes in plants using substrates provided by two distinct metabolic pathways, the mevalonate-dependent pathway that is located in cytosol and has been suggested to be responsible for synthesis of sesquiterpenes (C15), and 2-C-methyl-D-erythritol-4-phosphate pathway located in plastids and suggested to be responsible for the synthesis of hemi- (C5), mono- (C10), and diterpenes (C20). Recent advances in characterization of genes and enzymes responsible for substrate and end product biosynthesis as well as efforts in metabolic engineering have demonstrated existence of a number of multi-substrate terpene synthases. This review summarizes the progress in the characterization of such multi-substrate terpene synthases and suggests that the presence of multi-substrate use might have been significantly underestimated. Multi-substrate use could lead to important changes in terpene product profiles upon substrate profile changes under perturbation of metabolism in stressed plants as well as under certain developmental stages. We therefore argue that multi-substrate use can be significant under physiological conditions and can result in complicate modifications in terpene profiles.

Isoprenoid generating systems in plants - A handy toolbox how to assess contribution of the mevalonate and methylerythritol phosphate pathways to the biosynthetic process

Isoprenoids comprise an astonishingly diverse group of metabolites with numerous potential and actual applications in medicine, agriculture and the chemical industry. Generation of efficient platforms producing isoprenoids is a target of numerous laboratories. Such efforts are generally enhanced if the native biosynthetic routes can be identified, and if the regulatory mechanisms responsible for the biosynthesis of the compound(s) of interest can be determined. In this review a critical summary of the techniques applied to establish the contribution of the two alternative routes of isoprenoid production operating in plant cells, the mevalonate and methylerythritol pathways, with a focus on their co-operation (cross-talk) is presented. Special attention has been paid to methodological aspects of the referred studies, in order to give the reader a deeper understanding for the nuances of these powerful techniques. This review has been designed as an organized toolbox, which might offer the researchers comments useful both for project design and for interpretation of results obtained.

A Latex Metabolite Benefits Plant Fitness under Root Herbivore Attack

Plants produce large amounts of secondary metabolites in their shoots and roots and store them in specialized secretory structures. Although secondary metabolites and their secretory structures are commonly assumed to have a defensive function, evidence that they benefit plant fitness under herbivore attack is scarce, especially below ground. Here, we tested whether latex secondary metabolites produced by the common dandelion (Taraxacum officinale agg.) decrease the performance of its major native insect root herbivore, the larvae of the common cockchafer (Melolontha melolontha), and benefit plant vegetative and reproductive fitness under M. melolontha attack. Across 17 T. officinale genotypes screened by gas and liquid chromatography, latex concentrations of the sesquiterpene lactone taraxinic acid β-D-glucopyranosyl ester (TA-G) were negatively associated with M. melolontha larval growth. Adding purified TA-G to artificial diet at ecologically relevant concentrations reduced larval feeding. Silencing the germacrene A synthase ToGAS1, an enzyme that was identified to catalyze the first committed step of TA-G biosynthesis, resulted in a 90% reduction of TA-G levels and a pronounced increase in M. melolontha feeding. Transgenic, TA-G-deficient lines were preferred by M. melolontha and suffered three times more root biomass reduction than control lines. In a common garden experiment involving over 2,000 T. officinale individuals belonging to 17 different genotypes, high TA-G concentrations were associated with the maintenance of high vegetative and reproductive fitness under M. melolontha attack. Taken together, our study demonstrates that a latex secondary metabolite benefits plants under herbivore attack, a result that provides a mechanistic framework for root herbivore driven natural selection and evolution of plant defenses below ground.

Both methylerythritol phosphate and mevalonate pathways contribute to biosynthesis of each of the major isoprenoid classes in young cotton seedlings

In higher plants, both the methylerythritol phosphate (MEP) and mevalonate (MVA) pathways contribute to the biosynthesis of isoprenoids. However, despite a significant amount of research on the activity of these pathways under different conditions, the relative contribution of each to the biosynthesis of diverse isoprenoids remains unclear. In this work, we examined the formation of several classes of isoprenoids in cotton (Gossypium hirsutum L.). After feeding [5,5-(2)H2]-1-deoxy-D-xylulose ([5,5-(2)H2]DOX) and [2-(13)C]MVA to intact cotton seedlings hydroponically, incorporation into isoprenoids was analyzed by MS and NMR. The predominant pattern of incorporation followed the classical scheme in which C5 units from the MEP pathway were used to form monoterpenes (C10), phytol side chains (C20) and carotenoids (C40) while C5 units from the MVA pathway were used to form sesquiterpenes (C15), terpenoid aldehydes (C15 and C25) and steroids/triterpenoids (C30). However, both pathways contributed to all classes of terpenoids, sometimes substantially. For example, the MEP pathway provided up to 20% of the substrate for sterols and the MVA pathway provided as much as 50% of the substrate for phytol side chains and carotenoids. Incorporation of C5 units from the MEP pathway was highest in cotyledons, compared to true leaves, and not observed at all in the roots. Incorporation of C5 units from the MVA pathway was highest in the roots (into sterols) and more prominent in the first true leaves than in other above-ground organs. The relative accumulation of label in intermediates vs. end products of phytosterol metabolism confirmed previous identification of slow steps in this pathway.

Purification and gas chromatography-combustion-isotope ratio mass spectrometry of aroma compounds from green tea products and comparison to bulk analysis

A method for carbon isotope ratio (δ(13)C) analysis was developed for compound-specific isotope analysis of tea volatiles, and the values were compared with the δ(13)C value from bulk isotope analyses. The δ(13)C value of 2-phenylethanol liberated via enzymatic hydrolysis of the 2-phenylethyl β-primeveroside standard was examined first. Isotope fractionations for 2-phenylethyl β-primeveroside from preparative high-performance liquid chromatography (HPLC) were also analyzed. The enzymatic treatment and the preparative HPLC process did not cause carbon isotope fractionations, substantiating the strategies available for δ(13)C analysis of volatile compounds. On the basis of the gas chromatography-combustion-isotope ratio mass spectrometry data from 2-phenylethanol, it was possible to derive the conditions for enzyme treatment and preparative HPLC of the glycoconjugates of 2-phenylethanol, (Z)-3-hexenol, and benzyl alcohol isolated from green tea leaves. Larger variations in δ(13)C were found for individual volatile compounds compared with bulk analytical data from the leaves, indicating the potential to utilize this strategy in assigning the geographical origin of green tea.

Biosynthesis of sesquiterpenes in grape berry exocarp of Vitis vinifera L.: evidence for a transport of farnesyl diphosphate precursors from plastids to the cytosol

The participation of the mevalonic acid (MVA) and 1-deoxy-d-xylulose 5-phosphate/2-C-methyl-d-erythritol-4-phosphate (DOXP/MEP) pathways in sesquiterpene biosynthesis of grape berries was investigated. There is an increasing interest in this class of terpenoids, since the oxygenated sesquiterpene rotundone was identified as the peppery aroma impact compound in Australian Shiraz wines. To investigate precursor supply pathway utilization, in vivo feeding experiments were performed with the deuterium labeled, pathway specific, precursors [5,5-(2)H2]-1-deoxy-d-xylulose and [5,5-(2)H2]-mevalonic acid lactone. Head Space-Solid Phase Micro Extraction-Gas Chromatography-Mass Spectrometry (HS-SPME-GC-MS) analysis of the generated volatile metabolites demonstrated that de novo sesquiterpene biosynthesis is mainly located in the grape berry exocarp (skin), with no detectable activity in the mesocarp (flesh) of the Lemberger variety. Interestingly, precursors from both the (primarily) cytosolic MVA and plastidial DOXP/MEP pathways were incorporated into grape sesquiterpenes in the varieties Lemberger, Gewürztraminer and Syrah. Our labeling data provide evidence for a homogenous, cytosolic pool of precursors for sesquiterpene biosynthesis, indicating that a transport of precursors occurs mostly from plastids to the cytosol. The labeling patterns of the sesquiterpene germacrene D were in agreement with a cyclization mechanism analogous to that of a previously cloned enantioselective (R)-germacrene D synthase from Solidago canadensis. This observation was subsequently confirmed by enantioselective GC-MS analysis demonstrating the exclusive presence of (R)-germacrene D, and not the (S)-enantiomer, in grape berries.

Role of plant peroxisomes in protection against herbivores

Peroxisomes are subcellular organelles of vital importance. They are ubiquitous, have a single membrane and execute numerous metabolic reactions in plants. Plant peroxisomes are multifaceted and have diverse functions including, but not limited to, photomorphogenesis, lipid metabolism, photorespiration, nitrogen metabolism, detoxification and plant biotic interactions. Plants have evolved a variety of defence barriers against herbivory. These barriers are unique and loaded with various metabolites. Peroxisomes play an important role in cells, maintaining the compartmentation of certain specific reactions. They serve as a first line of defence, as peroxisomes generate primary signals such as reactive oxygen species (ROS) and reactive nitrogen species (RNS). Both ROS and RNS sense the invasion by herbivores and dramatically reshape the plant transcriptomes, proteomes, and metabolomes, so indicating the importance of signals generated by peroxisomes. Peroxisomes also store a plethora of important enzymes, which have a key role in producing defence molecules. Some of the main enzymes in the biosynthesis of isoprenoids are present in peroxisomes. These enzymes generate plant volatiles, which have numerous functions and important roles in plant-herbivore communication.Although disputed, the enzyme myrosinase has also been reported to be present in peroxisomes, and myrosinases are well known for their role in the mustard bomb, a powerful defence against herbivores. This chapter focuses on the diverse roles of peroxisomes in the generation of direct and indirect defenses against herbivores.

A raison d'être for two distinct pathways in the early steps of plant isoprenoid biosynthesis?

When compared to other organisms, plants are atypical with respect to isoprenoid biosynthesis: they utilize two distinct and separately compartmentalized pathways to build up isoprene units. The co-existence of these pathways in the cytosol and in plastids might permit the synthesis of many vital compounds, being essential for a sessile organism. While substrate exchange across membranes has been shown for a variety of plant species, lack of complementation of strong phenotypes, resulting from inactivation of either the cytosolic pathway (growth and development defects) or the plastidial pathway (pigment bleaching), seems to be surprising at first sight. Hundreds of isoprenoids have been analyzed to determine their biosynthetic origins. It can be concluded that in angiosperms, under standard growth conditions, C₂₀-phytyl moieties, C₃₀-triterpenes and C₄₀-carotenoids are made nearly exclusively within compartmentalized pathways, while mixed origins are widespread for other types of isoprenoid-derived molecules. It seems likely that this coexistence is essential for the interaction of plants with their environment. A major purpose of this review is to summarize such observations, especially within an ecological and functional context and with some emphasis on regulation. This latter aspect still requires more work and present conclusions are preliminary, although some general features seem to exist.

The isogene 1-deoxy-D-xylulose 5-phosphate synthase 2 controls isoprenoid profiles, precursor pathway allocation, and density of tomato trichomes

Plant isoprenoids are formed from precursors synthesized by the mevalonate (MVA) pathway in the cytosol or by the methyl-D-erythritol 4-phosphate (MEP) pathway in plastids. Although some exchange of precursors occurs, cytosolic sesquiterpenes are assumed to derive mainly from MVA, while plastidial monoterpenes are produced preferentially from MEP precursors. Additional complexity arises in the first step of the MEP pathway, which is typically catalyzed by two divergent 1-deoxy-D-xylulose 5-phosphate synthase isoforms (DXS1, DXS2). In tomato (Solanum lycopersicum), the SlDXS1 gene is ubiquitously expressed with highest levels during fruit ripening, whereas SlDXS2 transcripts are abundant in only few tissues, including young leaves, petals, and isolated trichomes. Specific down-regulation of SlDXS2 expression was performed by RNA interference in transgenic plants to investigate feedback mechanisms. SlDXS2 down-regulation led to a decrease in the monoterpene β-phellandrene and an increase in two sesquiterpenes in trichomes. Moreover, incorporation of MVA-derived precursors into residual monoterpenes and into sesquiterpenes was elevated as determined by comparison of ¹³C to ¹²C natural isotope ratios. A compensatory up-regulation of SlDXS1 was not observed. Down-regulated lines also exhibited increased trichome density and showed less damage by leaf-feeding Spodoptera littoralis caterpillars. The results reveal novel, non-redundant roles of DXS2 in modulating isoprenoid metabolism and a pronounced plasticity in isoprenoid precursor allocation.

A comparative genomics study of genetic products potentially encoding ladderane lipid biosynthesis

BACKGROUND

The fatty acids of anaerobic ammonium oxidizing (anammox) bacteria contain linearly concatenated cyclobutane moieties, so far unique to biology. These moieties are under high ring strain and are synthesised by a presently unknown biosynthetic pathway.

RESULTS

Gene clusters encoding enzymes of fatty acid biosynthesis in the anammox bacterium Kuenenia stuttgartiensis and 137 other organisms were analysed and compared in silico to gain further insight into the pathway of (ladderane) fatty acid biosynthesis. In K. stuttgartiensis four large gene clusters encode fatty acid biosynthesis. Next to the regular enzyme complex needed for fatty acid biosynthesis (FASII), the presence of four putative S-adenosyl-methionine (SAM) radical enzymes, two enzymes similar to phytoene desaturases and many divergent paralogues of beta-ketoacyl-ACP synthase (fabF) were unusual. Surprisingly, extensive synteny was observed with FASII gene clusters in the deltaproteobacterium Desulfotalea psychrophila. No ladderane lipids were detected in lipid extracts of this organism but we did find unusual polyunsaturated hydrocarbons (PUHC), not detected in K. stuttgartiensis.

CONCLUSION

We suggest that the unusual gene clusters of K. stuttgartiensis and D. psychrophila encode a novel pathway for anaerobic PUFA biosynthesis and that K. stuttgartiensis further processes PUFA into ladderane lipids, in similar fashion to the previously proposed route of ladderane lipid biosynthesis. However, the presence of divergent paralogues of fabF with radically different active site topologies may suggest an alternative pathway where ladderane moieties are synthesised externally and are recruited into the pathway of fatty acid biosynthesis.

Divergent regulation of terpenoid metabolism in the trichomes of wild and cultivated tomato species

The diversification of chemical production in glandular trichomes is important in the development of resistance against pathogens and pests in two species of tomato. We have used genetic and genomic approaches to uncover some of the biochemical and molecular mechanisms that underlie the divergence in trichome metabolism between the wild species Solanum habrochaites LA1777 and its cultivated relative, Solanum lycopersicum. LA1777 produces high amounts of insecticidal sesquiterpene carboxylic acids (SCAs), whereas cultivated tomatoes lack SCAs and are more susceptible to pests. We show that trichomes of the two species have nearly opposite terpenoid profiles, consisting mainly of monoterpenes and low levels of sesquiterpenes in S. lycopersicum and mainly of SCAs and very low monoterpene levels in LA1777. The accumulation patterns of these terpenoids are different during development, in contrast to the developmental expression profiles of terpenoid pathway genes, which are similar in the two species, but they do not correlate in either case with terpenoid accumulation. However, our data suggest that the accumulation of monoterpenes in S. lycopersicum and major sesquiterpenes in LA1777 are linked both genetically and biochemically. Metabolite analyses after targeted gene silencing, inhibitor treatments, and precursor feeding all show that sesquiterpene biosynthesis relies mainly on products from the plastidic 2-C-methyl-d-erythritol-4-phosphate pathway in LA1777 but less so in the cultivated species. Furthermore, two classes of sesquiterpenes produced by the wild species may be synthesized from distinct pools of precursors via cytosolic and plastidial cyclases. However, highly trichome-expressed sesquiterpene cyclase-like enzymes were ruled out as being involved in the production of major LA1777 sesquiterpenes.

The plastidial 2-C-methyl-D-erythritol 4-phosphate pathway provides the isoprenyl moiety for protein geranylgeranylation in tobacco BY-2 cells

Protein farnesylation and geranylgeranylation are important posttranslational modifications in eukaryotic cells. We visualized in transformed Nicotiana tabacum Bright Yellow-2 (BY-2) cells the geranylgeranylation and plasma membrane localization of GFP-BD-CVIL, which consists of green fluorescent protein (GFP) fused to the C-terminal polybasic domain (BD) and CVIL isoprenylation motif from the Oryza sativa calmodulin, CaM61. Treatment with fosmidomycin (Fos) or oxoclomazone (OC), inhibitors of the plastidial 2-C-methyl-d-erythritol 4-phosphate (MEP) pathway, caused mislocalization of the protein to the nucleus, whereas treatment with mevinolin, an inhibitor of the cytosolic mevalonate pathway, did not. The nuclear localization of GFP-BD-CVIL in the presence of MEP pathway inhibitors was completely reversed by all-trans-geranylgeraniol (GGol). Furthermore, 1-deoxy-d-xylulose (DX) reversed the effects of OC, but not Fos, consistent with the hypothesis that OC blocks 1-deoxy-d-xylulose 5-phosphate synthesis, whereas Fos inhibits its conversion to 2-C-methyl-d-erythritol 4-phosphate. By contrast, GGol and DX did not rescue the nuclear mislocalization of GFP-BD-CVIL in the presence of a protein geranylgeranyltransferase type 1 inhibitor. Thus, the MEP pathway has an essential role in geranylgeranyl diphosphate (GGPP) biosynthesis and protein geranylgeranylation in BY-2 cells. GFP-BD-CVIL is a versatile tool for identifying pharmaceuticals and herbicides that interfere either with GGPP biosynthesis or with protein geranylgeranylation.

From molecular fossils of bacterial hopanoids to the formation of isoprene units: discovery and elucidation of the methylerythritol phosphate pathway

Investigations on the biosynthesis of bacterial triterpenoids of the hopane series led to the unexpected discovery of an alternative mevalonate independent pathway for the formation of isoprene units. Methylerythritol phosphate, already presenting the C5 branched isoprene skeleton, is the key intermediate. This pathway was independently characterized in ginkgo embryos for the formation of diterpenoids. It is present in most bacteria and in the plastids of all organisms belonging to phototrophic phyla. The key steps of the discovery and elucidation of this metabolic route are presented in this review.

De novo biosynthesis versus sequestration: a network of transport systems supports in iridoid producing leaf beetle larvae both modes of defense

In the larval chrysomelines the de novo synthesis of monoterpenoids (iridoids) is believed to represent the ancestral state in the evolution of chemical defenses. Here we demonstrate that the iridoid producing larvae of Plagiodera versicolora and Phratora laticollis have the potential to sequester precursors from food. In nature, iridoids may even have a dual origin, namely plant-derived and de novo produced. The ability to sequester plant-derived precursors was proved by (i) (13)C-labelling of the terpenoids in the food plant, (ii) by larval feeding on leaves impregnated with analogs and labelled putative precursors for iridoid biosynthesis; and (iii) by injection of the precursors into the hemolymph followed by mass spectroscopic analysis of their distribution in the hemolymph, defensive secretion, and faeces. The experimental findings support a network of transport systems which allows a broader range of glucosides to enter and to leave the hemocoel, while only the appropriate precursor, 8-hydroxygeraniol-8-O-beta-d-glucoside, is channelled to the reservoir and processed to iridoids. The dual system of de novo biosynthesis and sequestration of phytogenic precursors may have favoured the larvae to shift from one host plant to another without losing their defense.

Significance of terpenoids in induced indirect plant defence against herbivorous arthropods

Many plants respond to herbivory by arthropods with an induced emission of volatiles such as green leaf volatiles and terpenoids. These herbivore-induced plant volatiles (HIPVs) can attract carnivores, for example, predators and parasitoids. We investigated the significance of terpenoids in attracting herbivores and carnivores in two tritrophic systems where we manipulated the terpenoid emission by treating the plants with fosmidomycin, which inhibits one of the terpenoid biosynthetic pathways and consequently terpenoid emission. In the 'lima bean' system, volatiles from spider-mite-infested fosmidomycin-treated plants were less attractive to the predatory mite Phytoseiulus persimilis than from infested control plants. In the 'cabbage' system, fosmidomycin treatment did not alter the attractiveness of Brussels sprouts to two Pieris butterflies for oviposition. The parasitoid Cotesia glomerata did not discriminate between the volatiles of fosmidomycin-treated and water-treated caterpillar-infested cabbage. Both P. persimilis and C. glomerata preferred volatiles from infested plants to uninfested ones when both were treated with fosmidomycin. Chemical analysis showed that terpenoid emission was inhibited more strongly in infested lima bean plants than in Brussels sprouts plants after fosmidomycin treatment. This study shows an important role of terpenoids in the indirect defence of lima bean, which is discussed relative to the role of other HIPVs.

Terpenoids in Grapes and Wines: Origin and Micrometabolism during the Vinification Process

Terpenoids, which are typical components of the essential oils of flowers and fruits, are also present as free and glycosylated conjugates amongst the secondary metabolites of wine grape varieties of Vitis vinifera. Hence, when these compounds are present in wine, they are considered to originate from the grapes and not from fermentation. However, the biosynthesis of monoterpenes by Saccharomyces cerevisiae in the absence of grape derived precursors was shown recently to be of de novo origin in wine yeast strains. The contribution of yeast and bacterial fermentation metabolites to the aromatic profile of wine is well documented. However, the biotechnological application of this knowledge is still rather limited and often contradictory. Redox conditions, size of inoculums, temperatures of fermentation, osmotic pressure and the medium nutritional content can profoundly affect the profile of yeast and bacterial metabolites produced or their biotransformation capacity in wine. Results obtained in the last decades in relation to microbial micrometabolism of aroma compounds measured with more sophisticated GC-MS methods are discussed in relation to the known terpenoid biosynthetic pathways and wine composition. Further development of metabolic footprinting techniques for the discrimination of wine quality must be one of the main challenges for wine biotechnologists in the near future.

Ursolic acid: an anti- and pro-inflammatory triterpenoid

There is growing interest in the elucidation of the biological functions of triterpenoids, ubiquitously distributed throughout the plant kingdom, some of which are used as anticancer and anti-inflammatory agents in Asian countries. Ursolic acid (UA), a natural pentacyclic triterpenoid carboxylic acid, is the major component of some traditional medicine herbs and is well known to possess a wide range of biological functions, such as antioxidative, anti-inflammation, and anticancer activities, that are able to counteract endogenous and exogenous biological stimuli. In contrast to these beneficial properties, some laboratory studies have recently revealed that the effects of UA on normal cells and tissues are occasionally pro-inflammatory. Thus, UA may be designated as a double-edged sword with both positive and negative effects, and further evaluations of the effects of UA on the biological status of target cells or tissues are necessary. This review summarizes previous and current information regarding UA, and provides new insights into the underlying molecular mechanisms of its activities.

VOC emissions of Grey poplar leaves as affected by salt stress and different N sources

Nitrogen nutrition and salt stress experiments were performed in a greenhouse with hydroponic-cultured, salt-sensitive Grey poplar (Populus x canescens) plants to study the combined influence of different N sources (either 1 mm NO(3) (-) or NH(4)(+)) and salt (up to 75 mm NaCl) on leaf gas exchange, isoprene biosynthesis and VOC emissions. Net assimilation and transpiration proved to be highly sensitive to salt stress and were reduced by approximately 90% at leaf sodium concentrations higher than 1,800 microg Na g dry weight (dw)(-1). In contrast, emissions of isoprene and oxygenated VOC (i.e. acetaldehyde, formaldehyde and acetone) were unaffected. There was no significant effect of combinations of salt stress and N source, and neither NO(3)(-) or NH(4)(+) influenced the salt stress response in the Grey poplar leaves. Also, transcript levels of 1-deoxy-d-xylulose 5-phosphate reductoisomerase (PcDXR) and isoprene synthase (PcISPS) did not respond to the different N sources and only responded slightly to salt application, although isoprene synthase (PcISPS) activity was negatively affected at least in one of two experiments, despite high isoprene emission rates. A significant salt effect was the strong reduction of leaf dimethylallyl diphosphate (DMADP) content, probably due to restricted availability of photosynthates for DMADP biosynthesis. Further consequences of reduced photosynthetic gas exchange and maintaining VOC emissions are a very high C loss, up to 50%, from VOC emissions related to net CO(2) uptake and a strong increase in leaf internal isoprene concentrations, with maximum mean values up to 6.6 microl x l(-1). Why poplar leaves maintain VOC biosynthesis and emission under salt stress conditions, despite impaired photosynthetic CO(2) fixation, is discussed.

Identification of quantitative trait loci influencing foliar concentrations of terpenes and formylated phloroglucinol compounds in Eucalyptus nitens

Leaves of eucalypt species contain a variety of plant secondary metabolites, including terpenoids and formylated phloroglucinol compounds (FPCs). Both terpene and FPC concentrations are quantitative traits that can show large variation within a population and have been shown to be heritable. The molecular genetic basis of this variation is currently unknown. Progeny from a field trial of a three-generation mapping pedigree of Eucalyptus nitens were assayed for terpenes and FPCs. Quantitative trait loci (QTL) analyses were conducted using a map constructed from 296 markers to locate regions of the genome influencing foliar concentrations of these plant secondary compounds. A large number of significant QTL for 14 traits were located across nine linkage groups, with significant clustering of QTL on linkage groups 7, 8 and 9. As expected, QTL for biosynthetically related compounds commonly colocated, but QTL for unrelated monterpenes and FPCs also mapped closely together. Colocation of these QTL with mapped candidate genes from the various biosynthetic pathways, and subsequent use of these genes in association mapping, will assist in determining the causes of variation in plant secondary metabolites in eucalypts.

Caterpillar herbivory and salivary enzymes decrease transcript levels of Medicago truncatula genes encoding early enzymes in terpenoid biosynthesis

In response to caterpillar herbivory, alfalfa and related plant species defend themselves through the induction of saponin and volatile terpenoid biosynthesis. Both these types of defensive compounds are derived from the metabolic intermediate, isopentenyl diphosphate (IPP). In plants, two distinct biosynthetic pathways can generate IPP; the cytosolic mevalonate pathway and the plastid-associated 2C-methyl erythritol 4-phosphate (MEP) pathway. In Medicago truncatula, transcript levels of key regulatory genes active in the early steps of these biosynthetic pathways were measured in response to larval herbivory by the beet army worm, Spodoptera exigua. Transcripts encoding enzymes at early steps of both terpenoid pathways were lower in caterpillar-damaged leaves. Higher degrees of herbivore damage accentuated the decrease in transcript levels; however, transcript amounts were not affected by insect larval stage. Insect larvae, manipulated to reduce labial gland salivary secretions, were used to examine the role of the salivary elicitors in modulating gene expression. Results suggest that an insect salivary factor, possibly glucose oxidase (GOX), may be involved in reduction of transcript levels following herbivory. Addition of GOX or hydrogen peroxide to mechanically wounded leaves confirm these findings. In comparison, transcript levels of a gene encoding a putative terpene synthase are induced in mechanically- or insect-damaged leaves. These data show that insect salivary factors can act to suppress transcript levels of genes involved in plant defense pathways. Findings also suggest that in response to stress such as insect herbivory, regulation occurs at the early steps of the MEP pathway.

Practical approaches to plant volatile analysis

Plants emit volatile organic compounds (VOCs) that play important roles in their interaction with the environment and have a major impact on atmospheric chemistry. The development of static and dynamic techniques for headspace collection of volatiles in combination with gas chromatography-mass spectrometry analysis has significantly improved our understanding of the biosynthesis and ecology of plant VOCs. Advances in automated analysis of VOCs have allowed the monitoring of fast changes in VOC emissions and facilitated in vivo studies of VOC biosynthesis. This review presents an overview of methods for the analysis of plant VOCs, including their advantages and disadvantages, with a focus on the latest technical developments. It provides guidance on how to select appropriate instrumentation and protocols for biochemical, physiological and ecologically relevant applications. These include headspace analyses of plant VOCs emitted by the whole organism, organs or enzymes as well as advanced on-line analysis methods for simultaneous measurements of VOC emissions with other physiological parameters.

The nonmevalonate pathway supports both monoterpene and sesquiterpene formation in snapdragon flowers

Terpenoids, the largest class of plant secondary metabolites, play essential roles in both plant and human life. In higher plants, the five-carbon building blocks of all terpenoids, isopentenyl diphosphate (IPP) and dimethylallyl diphosphate, are derived from two independent pathways localized in different cellular compartments. The methylerythritol phosphate (MEP or nonmevalonate) pathway, localized in the plastids, is thought to provide IPP and dimethylallyl diphosphate for hemiterpene, monoterpene, and diterpene biosynthesis, whereas the cytosol-localized mevalonate pathway provides C5 units for sesquiterpene biosynthesis. Stable isotope-labeled, pathway-specific precursors (1-deoxy-[5,5-2H2]-D-xylulose and [2,2-2H2]-mevalolactone) were supplied to cut snapdragon flowers, which emit both monoterpenes and the sesquiterpene, nerolidol. We show that only one of the two pathways, the plastid-localized MEP pathway, is active in the formation of volatile terpenes. The MEP pathway provides IPP precursors for both plastidial monoterpene and cytosolic sesquiterpene biosynthesis in the epidermis of snapdragon petals. The trafficking of IPP occurs unidirectionally from the plastids to cytosol. The MEP pathway operates in a rhythmic manner controlled by the circadian clock, which determines the rhythmicity of terpenoid emission.

Are iridoids in leaf beetle larvae synthesized de novo or derived from plant precursors? A methodological approach

Iridoids, belonging to a group of cyclopentanoid monoterpenoids, are secreted by many species of leaf beetles as a defense against predators. Using chemically modified precursors of iridoid biosynthesis, it has been shown that some leaf beetle larvae can synthesize these iridoids de novo as well as sequester plant-produced molecules. Stable isotope techniques can provide useful methods for studying terpenoid biosynthesis without disturbing the natural conditions much. Two terpenoid biosynthesis pathways (mevalonic acid (MVA) pathway and methylerythritol-4-phosphate (MEP) pathway) may lead to different delta13C signatures of the products. Our results from natural abundance 13C and 13C-labelled iridoid precursors in Gastrophysa viridula and Phaedon cochleariae suggested that the two leaf beetle species use only de novo synthesis of their defensive iridoids. We observed that the isotope signature of the leaf-beetle-produced iridoids (via the MVA pathway) resembled that of the MEP-derived monoterpenoids from plants. Owing to this close similarity in the natural 13C abundances in the plant and insect compounds, a determination of iridoid-origin in leaf beetle secretion may only be possible by use of isotopically labelled compounds.

A review of tobacco BY-2 cells as an excellent system to study the synthesis and function of sterols and other isoprenoids

In plants, two pathways are utilized for the synthesis of isopentenyl diphosphate (IPP), the universal precursor for isoprenoid biosynthesis. In this paper we review findings and observations made primarily with tobacco BY-2 cells (TBY-2), which have proven to be an excellent system in which to study the two biosynthetic pathways. A major advantage of these cells as an experimental system is their ability to readily take up specific inhibitors and stably- and/or radiolabeled precursors. This permits the functional elucidation of the role of isoprenoid end products and intermediates. Because TBY-2 cells undergo rapid cell division and can be synchronized within the cell cycle, they constitute a highly suitable test system for determination of those isoprenoids and intermediates that act as cell cycle inhibitors, thus giving an indication of which branches of the isoprenoid pathway are essential. Through chemical complementation; and use of precursors, intracellular compartmentation can be elucidated, as well as the extent to which the plastidial and cytosolic pathways contribute to the syntheses of specific groups of isoprenoids (e.g., sterols) via exchange of intermediates across membranes. These topics are discussed in the context of the pertinent literature.

Isoprenoid biosynthesis in the diatoms Rhizosolenia setigera (Brightwell) and Haslea ostrearia (Simonsen)

Isoprenoid biosynthesis in the widespread diatomaceous algae, Rhizosolenia setigera (Brightwell) and Haslea ostrearia (Simonsen), results not only in the production of diterpenoids, triterpenoids, and sterols but, unusually for diatoms, also in the production of sesterterpenoids. By using 13C and 2H isotopic labeling techniques followed by NMR and mass spectrometry, specific inhibition of mevalonate (MVA) and methylerythritol (MEP) pathways, and comparison with the natural 13C/12C isotope ratios of the lipids, the different biosynthetic pathways of the sesterterpenes and other isoprenoids have now been determined. Surprisingly, whereas the sesterterpenes (Delta(7(20))-haslenes) in R. setigera were made by the MVA pathway, as were the related triterpenoid rhizenes and desmosterol, in H. ostrearia the structurally similar Delta(6(17))-haslenes and the major sterol, 24-ethylcholest-5-en-3beta-ol, were instead biosynthesized by the MEP route. Phytol was biosynthesized in both diatoms by the MEP route. Subfractionation of R. setigera cells revealed that although phytol was located in the chloroplasts, the haslenes, rhizenes, and sterols were present in the cytoplasm. The observations described here for R. setigera and H. ostrearia show that terpenoid biosynthesis in diatoms is species-dependent and cannot simply be grouped according to structural type. Triterpenes appear to be made by the MVA route as in higher plants, whereas sesterterpenes and sterols can be made by either the MVA or MEP routes. In neither organism were the isoprenoids biosynthesized by leucine metabolism. Sesterterpene and triterpene biosynthesis in diatoms has not been investigated previously.

Cross-talk between the cytosolic mevalonate and the plastidial methylerythritol phosphate pathways in tobacco bright yellow-2 cells

In plants, two pathways are utilized for the synthesis of isopentenyl diphosphate, the universal precursor for isoprenoid biosynthesis. The key enzyme of the cytoplasmic mevalonic acid (MVA) pathway is 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR). Treatment of Tobacco Bright Yellow-2 (TBY-2) cells by the HMGR-specific inhibitor mevinolin led to growth reduction and induction of apparent HMGR activity, in parallel to an increase in protein representing two HMGR isozymes. Maximum induction was observed at 24 h. 1-Deoxy-d-xylulose (DX), the dephosphorylated first precursor of the plastidial 2-C-methyl-d-erythritol 4-phosphate (MEP) pathway, complemented growth inhibition by mevinolin in the low millimolar concentration range. Furthermore, DX partially re-established feedback repression of mevinolin-induced HMGR activity. Incorporation studies with [1,1,1,4-2H4]DX showed that sterols, normally derived from MVA, in the presence of mevinolin are synthesized via the MEP pathway. Fosmidomycin, an inhibitor of 1-deoxy-d-xylulose-5-phosphate reductoisomerase, the second enzyme of the MEP pathway, was utilized to study the reverse complementation. Growth inhibition by fosmidomycin of TBY-2 cells could be partially overcome by MVA. Chemical complementation was further substantiated by incorporation of [2-13C]MVA into plastoquinone, representative of plastidial isoprenoids. Best rates of incorporation of exogenous stably labeled precursors were observed in the presence of both inhibitors, thereby avoiding internal isotope dilution.

Natural abundance carbon isotope composition of isoprene reflects incomplete coupling between isoprene synthesis and photosynthetic carbon flow

Isoprene emission from leaves is dynamically coupled to photosynthesis through the use of primary and recent photosynthate in the chloroplast. However, natural abundance carbon isotope composition (delta(13)C) measurements in myrtle (Myrtus communis), buckthorn (Rhamnus alaternus), and velvet bean (Mucuna pruriens) showed that only 72% to 91% of the variations in the delta(13)C values of fixed carbon were reflected in the delta(13)C values of concurrently emitted isoprene. The results indicated that 9% to 28% carbon was contributed from alternative, slow turnover, carbon source(s). This contribution increased when photosynthesis was inhibited by CO(2)-free air. The observed variations in the delta(13)C of isoprene under ambient and CO(2)-free air were consistent with contributions to isoprene synthesis in the chloroplast from pyruvate associated with cytosolic Glc metabolism. Irrespective of alternative carbon source(s), isoprene was depleted in (13)C relative to mean photosynthetically fixed carbon by 4 per thousand to 11 per thousand. Variable (13)C discrimination, its increase by partially inhibiting isoprene synthesis with fosmidomicin, and the associated accumulation of pyruvate suggested that the main isotopic discrimination step was the deoxyxylulose-5-phosphate synthase reaction.