Developmental regulation of monoterpene biosynthesis in the glandular trichomes of peppermint

Chemical composition of the essential oil and botanical study of the flowers of Mentha suaveolens

CONTEXT

Herbal medicines play a paramount role in the treatment of wide range of diseases, so there is a growing need for their quality control and standardization. Traditionally, histological and morphological inspections have been the usual methods to authenticate herbs intended for medicinal applications. Mentha suaveolens Ehrh. (Lamiaceae) is native to Africa Temperate Asia and Europe and it's cultivated in Egypt.

OBJECTIVE

The macro- and micromorphology of the flowers of M. suaveolens Ehrh. cultivated in Egypt were studied to find the diagnostic characters of this species. In addition, the chemical composition of the essential oil of the flowers was also studied to define the chemotype of the plant.

MATERIALS AND METHODS

Photographs of macro- and micromorphology were taken using Casio and Leica DFC500 digital cameras, respectively. In addition, the essential oil was prepared by hydrodistillation followed by gas chromatographic/mass spectrometric (GC/MS) analysis for identification of its components.

RESULTS

The macro- and micromorphological characteristics of M. suaveolens were determined. The yield of the essential oil obtained by hydrodistillation from M. suaveolens flowers was 1.7% calculated on dry weight basis. GC/MS analysis of the oil resulted in identification of 29 components, which amounted to 99.77% of the total oil composition. The major component was carvone (50.59%) followed by limonene (31.25%).

DISCUSSION AND CONCLUSION

The results obtained herein revealed for the macro, micromorphological and chemical composition characteristics of the flowers. The results of GC/MS analysis of the essential oil supported that M. suaveolens cultivated in Egypt could be categorized as carvone-rich chemotype since this compound pertained its high relative percentile.

Differences in shoot and root terpenoid profiles and plant responses to fertilisation in Tanacetum vulgare

Intraspecific chemical diversity is a common phenomenon especially found in shoots of essential oil-accumulating plant species. Abiotic factors can influence the concentration of essential oils, but the effects are inconsistent and little is known in how far these may vary within an individual and within species between chemotypes. Tanacetum vulgare L. occurs in various chemotypes that differ in the composition of mono- and sesquiterpenoids in their shoot tissues. We investigated how far shoot chemotype grouping is mirrored in root terpenoid profiles. Furthermore, we studied whether different fertilisation amounts influence the plant growth and morphological traits as well as the constitutive terpenoid concentration of leaves and roots of three chemotypes, trans-carvyl acetate, β-thujone, and camphor, to different degrees. Shoot terpenoids were dominated by monoterpenoids, while the roots contained mainly sesquiterpenoids. The clear grouping in three chemotypes based on leaf chemistry was weakly mirrored in the root terpenoid composition. Furthermore, the leaf C/N ratio and the stem height differed between chemotypes. All plants responded to increased nutrient availability with increased total biomass and specific leaf area but decreased C/N and root/shoot ratios. Leaf terpenoid concentrations decreased with increasing fertiliser supply, independent of chemotype. In contrast to the leaves, the terpenoid concentrations of the roots were unaffected by fertilisation. Our results demonstrate that aboveground and belowground organs within a species can be under different selection pressures.

Characterization of two geraniol synthases from Valeriana officinalis and Lippia dulcis: similar activity but difference in subcellular localization

Two geraniol synthases (GES), from Valeriana officinalis (VoGES) and Lippia dulcis (LdGES), were isolated and were shown to have geraniol biosynthetic activity with Km values of 32 µM and 51 µM for GPP, respectively, upon expression in Escherichia coli. The in planta enzymatic activity and sub-cellular localization of VoGES and LdGES were characterized in stable transformed tobacco and using transient expression in Nicotiana benthamiana. Transgenic tobacco expressing VoGES or LdGES accumulate geraniol, oxidized geraniol compounds like geranial, geranic acid and hexose conjugates of these compounds to similar levels. Geraniol emission of leaves was lower than that of flowers, which could be related to higher levels of competing geraniol-conjugating activities in leaves. GFP-fusions of the two GES proteins show that VoGES resides (as expected) predominantly in the plastids, while LdGES import into to the plastid is clearly impaired compared to that of VoGES, resulting in both cytosolic and plastidic localization. Geraniol production by VoGES and LdGES in N. benthamiana was nonetheless very similar. Expression of a truncated version of VoGES or LdGES (cytosolic targeting) resulted in the accumulation of 30% less geraniol glycosides than with the plastid targeted VoGES and LdGES, suggesting that the substrate geranyl diphosphate is readily available, both in the plastids as well as in the cytosol. The potential role of GES in the engineering of the TIA pathway in heterologous hosts is discussed.

Biosynthesis of sesquiterpene lactones in pyrethrum (Tanacetum cinerariifolium)

The daisy-like flowers of pyrethrum (Tanacetum cinerariifolium) are used to extract pyrethrins, a botanical insecticide with a long history of safe and effective use. Pyrethrum flowers also contain other potential defense compounds, particularly sesquiterpene lactones (STLs), which represent problematic allergenic residues in the extracts that are removed by the pyrethrum industry. The STLs are stored in glandular trichomes present on the pyrethrum achenes, and have been shown to be active against herbivores, micro-organisms and in the below-ground competition with other plants. Despite these reported bioactivities and industrial significance, the biosynthetic origin of pyrethrum sesquiterpene lactones remains unknown. In the present study, we show that germacratrien-12-oic acid is most likely the central precursor for all sesquiterpene lactones present in pyrethrum. The formation of the lactone ring depends on the regio- (C6 or C8) and stereo-selective (α or β) hydroxylation of germacratrien-12-oic acid. Candidate genes implicated in three committed steps leading from farnesyl diphosphate to STL and other oxygenated derivatives of germacratrien-12-oic acid were retrieved from a pyrethrum trichome EST library, cloned, and characterized in yeast and in planta. The diversity and distribution of sesquiterpene lactones in different tissues and the correlation with the expression of these genes are shown and discussed.

Effect of gibberellic acid and calliterpenone on plant growth attributes, trichomes, essential oil biosynthesis and pathway gene expression in differential manner in Mentha arvensis L

Extensive research is going on throughout the world to find out new molecules from natural sources to be used as plant growth promoter. Mentha arvensis L. is the main source of menthol rich essential oil used commercially in various food, pharmaceutical and other preparations. Experiments were conducted on field grown plants for understanding the effect of calliterpenone (CA), a stereo-isomer of abbeokutone, in comparison to gibberellic acid (GA3) on growth attributes, trichomes, essential oil biosynthesis and expression of some oil biosynthetic pathway genes. The exogenous application of CA (1 μM, 10 μM and 100 μM) was found to be better in improving plant biomass and stolon yield, leaf area, branching and leaf stem ratio than with counterpart GA3 at the same concentrations. CA treated plants showed higher glandular trichome number, density and diameter and also correlated with enhanced oil biogenetic capacity as revealed by feeding labeled (14)C-sucrose for 72 h to excised shoots. Semi-quantitative PCR analysis of key pathway genes revealed differential up regulation under CA treatments. Transcript level of menthol dehydrogenase/menthone reductase was found highly up regulated in CA treated plants with increased content of menthone and menthol in oil. These findings demonstrate that CA positively regulated the yields by enhanced branching and higher density of trichomes resulting into higher accumulation of essential oil. The results suggest CA as a novel plant derived diterpenoid with growth promoting action and opens up new possibilities for improving the crop yields and essential oil biosynthesis in qualitative and quantitative manner.

Biosynthesis, function and metabolic engineering of plant volatile organic compounds

Plants synthesize an amazing diversity of volatile organic compounds (VOCs) that facilitate interactions with their environment, from attracting pollinators and seed dispersers to protecting themselves from pathogens, parasites and herbivores. Recent progress in -omics technologies resulted in the isolation of genes encoding enzymes responsible for the biosynthesis of many volatiles and contributed to our understanding of regulatory mechanisms involved in VOC formation. In this review, we largely focus on the biosynthesis and regulation of plant volatiles, the involvement of floral volatiles in plant reproduction as well as their contribution to plant biodiversity and applications in agriculture via crop-pollinator interactions. In addition, metabolic engineering approaches for both the improvement of plant defense and pollinator attraction are discussed in light of methodological constraints and ecological complications that limit the transition of crops with modified volatile profiles from research laboratories to real-world implementation.

Animal plant warfare and secondary metabolite evolution

Abstract

The long-lasting discussion, why plants produce secondary metabolites, which are pharmacologically and toxicologically active towards mammals traces back to the eminent role of medicinal plants in the millennia-old history of manhood. In recent years, the concept of an animal plant warfare emerged, which focused on the co-evolution between plants and herbivores. As a reaction to herbivory, plants developed mechanical defenses such as thorns and hard shells, which paved the way for adapted animal physiques. Plants evolved further defense systems by producing chemicals that exert toxic effects on the animals that ingest them. As a result of this selective pressure, animals developed special enzymes, e.g. cytochrome P450 monooxigenases (CYP450) that metabolize xenobiotic phytochemicals. As a next step in the evolutionary competition between plants and animals, plants evolved to produce non-toxic pro-drugs, which become toxic only after ingestion by animals through metabolization by enzymes such as CYP450. Because these sequestered evolutionary developments call to mind an arms race, the term animal plant warfare has been coined. The evolutionary competition between plants and animals may help to better understand the modes of action of medicinal plants and to foster the efficient and safe use of phytotherapy nowadays.

Graphical abstract

Terpenoid biosynthesis in trichomes--current status and future opportunities

Glandular trichomes are anatomical structures specialized for the synthesis of secreted natural products. In this review we focus on the description of glands that accumulate terpenoid essential oils and oleoresins. We also provide an in-depth account of the current knowledge about the biosynthesis of terpenoids and secretion mechanisms in the highly specialized secretory cells of glandular trichomes, and highlight the implications for metabolic engineering efforts.

A set of regioselective O-methyltransferases gives rise to the complex pattern of methoxylated flavones in sweet basil

Polymethoxylated flavonoids occur in a number of plant families, including the Lamiaceae. To date, the metabolic pathways giving rise to the diversity of these compounds have not been studied. Analysis of our expressed sequence tag database for four sweet basil (Ocimum basilicum) lines afforded identification of candidate flavonoid O-methyltransferase genes. Recombinant proteins displayed distinct substrate preferences and product specificities that can account for all detected 7-/6-/4'-methylated, 8-unsubstituted flavones. Their biochemical specialization revealed only certain metabolic routes to be highly favorable and therefore likely in vivo. Flavonoid O-methyltransferases catalyzing 4'- and 6-O-methylations shared high identity (approximately 90%), indicating that subtle sequence changes led to functional differentiation. Structure homology modeling suggested the involvement of several amino acid residues in defining the proteins' stringent regioselectivities. The roles of these individual residues were confirmed by site-directed mutagenesis, revealing two discrete mechanisms as a basis for the switch between 6- and 4'-O-methylation of two different substrates. These findings delineate major pathways in a large segment of the flavone metabolic network and provide a foundation for its further elucidation.

Cloning, functional characterization and genomic organization of 1,8-cineole synthases from Lavandula

Several members of the genus Lavandula produce valuable essential oils (EOs) that are primarily constituted of the low molecular weight isoprenoids, particularly monoterpenes. We isolated over 8,000 ESTs from the glandular trichomes of L. x intermedia flowers (where bulk of the EO is synthesized) to facilitate the discovery of genes that control the biosynthesis of EO constituents. The expression profile of these ESTs in L. x intermedia and its parents L. angustifolia and L. latifolia was established using microarrays. The resulting data highlighted a differentially expressed, previously uncharacterized cDNA with strong homology to known 1,8-cineole synthase (CINS) genes. The ORF, excluding the transit peptide, of this cDNA was expressed in E. coli, purified by Ni-NTA agarose affinity chromatography and functionally characterized in vitro. The ca. 63 kDa bacterially produced recombinant protein, designated L. x intermedia CINS (LiCINS), converted geranyl diphosphate (the linear monoterpene precursor) primarily to 1,8-cineole with K ( m ) and k ( cat ) values of 5.75 μM and 8.8 × 10(-3) s(-1), respectively. The genomic DNA of CINS in the studied Lavandula species had identical exon-intron architecture and coding sequences, except for a single polymorphic nucleotide in the L. angustifolia ortholog which did not alter protein function. Additional nucleotide variations restricted to L. angustifolia introns were also observed, suggesting that LiCINS was most likely inherited from L. latifolia. The LiCINS mRNA levels paralleled the 1,8-cineole content in mature flowers of the three lavender species, and in developmental stages of L. x intermedia inflorescence indicating that the production of 1,8 cineole in Lavandula is most likely controlled through transcriptional regulation of LiCINS.

Immunocytochemical localization of short-chain family reductases involved in menthol biosynthesis in peppermint

Biosynthesis of the p-menthane monoterpenes in peppermint occurs in the secretory cells of the peltate glandular trichomes and results in the accumulation of primarily menthone and menthol. cDNAs and recombinant enzymes are well characterized for eight of the nine enzymatic steps leading from the 5-carbon precursors to menthol, and subcellular localization of several key enzymes suggests a complex network of substrate and product movement is required during oil biosynthesis. In addition, studies concerning the regulation of oil biosynthesis have demonstrated a temporal partition of the pathway into an early, biosynthetic program that results in the accumulation of menthone and a later, oil maturation program that leads to menthone reduction and concomitant menthol accumulation. The menthone reductase responsible for the ultimate pathway reduction step, menthone-menthol reductase (MMR), has been characterized and found to share significant sequence similarity with its counterpart reductase, a menthone-neomenthol reductase, which catalyzes a minor enzymatic reaction associated with oil maturation. Further, the menthone reductases share significant sequence similarity with the temporally separate and mechanistically different isopiperitenone reductase (IPR). Here we present immunocytochemical localizations for these reductases using a polyclonal antibody raised against menthone-menthol reductase. The polyclonal antibody used for this study showed little specificity between these three reductases, but by using it for immunostaining of tissues of different ages we were able to provisionally separate staining of an early biosynthetic enzyme, IPR, found in young, immature leaves from that of the oil maturation enzyme, MMR, found in older, mature leaves. Both reductases were localized to the cytoplasm and nucleoplasm of the secretory cells of peltate glandular trichomes, and were absent from all other cell types examined.

Glandular trichomes: what comes after expressed sequence tags?

Glandular trichomes cover the surface of many plant species. They exhibit tremendous diversity, be it in their shape or the compounds they secrete. This diversity is expressed between species but also within species or even individual plants. The industrial uses of some trichome secretions and their potential as a defense barrier, for example against arthropod pests, has spurred research into the biosynthesis pathways that lead to these specialized metabolites. Because complete biosynthesis pathways take place in the secretory cells, the establishment of trichome-specific expressed sequence tag libraries has greatly accelerated their elucidation. Glandular trichomes also have an important metabolic capacity and may be considered as true cell factories. To fully exploit the potential of glandular trichomes as breeding or engineering objects, several research areas will have to be further investigated, such as development, patterning, metabolic fluxes and transcription regulation. The purpose of this review is to provide an update on the methods and technologies which have been used to investigate glandular trichomes and to propose new avenues of research to deepen our understanding of these specialized structures.

Seasonal influence on gene expression of monoterpene synthases in Salvia officinalis (Lamiaceae)

Garden sage (Salvia officinalis L., Lamiaceae) is one of the most important medicinal and aromatic plants and possesses antioxidant, antimicrobial, spasmolytic, astringent, antihidrotic and specific sensorial properties. The essential oil of the plant, formed mainly in very young leaves, is in part responsible for these activities. It is mainly composed of the monoterpenes 1,8-cineole, α- and β-thujone and camphor synthesized by the 1,8-cineole synthase, the (+)-sabinene synthase and the (+)-bornyl diphosphate synthase, respectively, and is produced and stored in epidermal glands. In this study, the seasonal influence on the formation of the main monoterpenes in young, still expanding leaves of field-grown sage plants was studied in two cultivars at the level of mRNA expression, analyzed by qRT-PCR, and at the level of end-products, analyzed by gas chromatography. All monoterpene synthases and monoterpenes were significantly influenced by cultivar and season. 1,8-Cineole synthase and its end product 1,8-cineole remained constant until August and then decreased slightly. The thujones increased steadily during the vegetative period. The transcript level of their corresponding terpene synthase, however, showed its maximum in the middle of the vegetative period and declined afterwards. Camphor remained constant until August and then declined, exactly correlated with the mRNA level of the corresponding terpene synthase. In summary, terpene synthase mRNA expression and respective end product levels were concordant in the case of 1,8-cineole (r=0.51 and 0.67 for the two cultivars, respectively; p<0.05) and camphor (r=0.75 and 0.82; p<0.05) indicating basically transcriptional control, but discordant for α-/β-thujone (r=-0.05 and 0.42; p=0.87 and 0.13, respectively).

Improving peppermint essential oil yield and composition by metabolic engineering

Peppermint (Mentha × piperita L.) was transformed with various gene constructs to evaluate the utility of metabolic engineering for improving essential oil yield and composition. Oil yield increases were achieved by overexpressing genes involved in the supply of precursors through the 2C-methyl-D-erythritol 4-phosphate (MEP) pathway. Two-gene combinations to enhance both oil yield and composition in a single transgenic line were assessed as well. The most promising results were obtained by transforming plants expressing an antisense version of (+)-menthofuran synthase, which is critical for adjusting the levels of specific undesirable oil constituents, with a construct for the overexpression of the MEP pathway gene 1-deoxy-D-xylulose 5-phosphate reductoisomerase (up to 61% oil yield increase over wild-type controls with low levels of the undesirable side-product (+)-menthofuran and its intermediate (+)-pulegone). Elite transgenic lines were advanced to multiyear field trials, which demonstrated consistent oil yield increases of up to 78% over wild-type controls and desirable effects on oil composition under commercial growth conditions. The transgenic expression of a gene encoding (+)-limonene synthase was used to accumulate elevated levels of (+)-limonene, which allows oil derived from transgenic plants to be recognized during the processing of commercial formulations containing peppermint oil. Our study illustrates the utility of metabolic engineering for the sustainable agricultural production of high quality essential oils at a competitive cost.

Biosynthesis and localization of parthenolide in glandular trichomes of feverfew (Tanacetum parthenium L. Schulz Bip.)

Feverfew (Tanacetum parthenium) is a perennial medicinal herb and is a rich source of sesquiterpene lactones. Parthenolide is the main sesquiterpene lactone in feverfew and has attracted attention because of its medicinal potential for treatment of migraine and cancer. In the present work the parthenolide content in different tissues and developmental stages of feverfew was analyzed to study the timing and localization of parthenolide biosynthesis. The strongest accumulating tissue was subsequently used to isolate sesquiterpene synthases with the goal to isolate the gene encoding the first dedicated step in parthenolide biosynthesis. This led to the isolation and charachterization of a germacrene A synthase (TpGAS) and an (E)-β-caryophyllene synthase (TpCarS). Transcript level patterns of both sesquiterpene synthases were analyzed in different tissues and glandular trichomes. Although TpGAS was expressed in all aerial tissues, the highest expression was observed in tissues that contain high concentrations of parthenolide and in flowers the highest expression was observed in the biosynthetically most active stages of flower development. The high expression of TpGAS in glandular trichomes which also contain the highest concentration of parthenolide, suggests that glandular trichomes are the secretory tissues where parthenolide biosynthesis and accumulation occur.

Variation in essential oil composition within individual leaves of sweet basil (Ocimum basilicum L.) is more affected by leaf position than by leaf age

The aroma in sweet basil is a factor affecting the commercial value of the crop. In previous studies leaf age was considered to be a factor that influences the composition of essential oil (EO). In this study it was hypothesized that a single observation of the EO content in leaves from different positions on the main stem (young vs old) could predict the developmental changes in the plant during its life cycle. Plants harvested at week 16 demonstrated an exponential increase (R(2) = 0.92) in EO concentration in leaves on the main stem and lateral shoots, indicating higher EO concentrations in younger than in older leaves. Eugenol and methyleugenol predominated (28-77%) in the extract. Eugenol levels were higher in younger leaves (∼53%), and methyl-eugenol levels predominated in older leaves (∼68%). Linalool was lower in mature leaves than in younger leaves. This suggested that eugenol converted into methyleugenol and linalool decreased as leaf mature. However, in weekly monitored plants, the levels of these compounds in the EO had limited variation in the maturing leaf regardless of its position on the stem. This proposed that the EO composition in an individual leaf is mostly affected by the leaf position on the stem and not by its maturation process. Because leaf position is related to plant development, it is probable that the plant's physiological age at the time of leaf formation from the primordial tissue is the factor affecting the EO composition. It was concluded that interpretation of scientific observations should be carried out with caution and that hypotheses should be tested utilizing multifaceted approaches.

Cloning and functional characterization of β-phellandrene synthase from Lavandula angustifolia

En route to building genomics resources for Lavandula, we have obtained over 14,000 ESTs for leaves and flowers of L. angustifolia, a major essential oil crop, and identified a number of previously uncharacterized terpene synthase (TPS) genes. Here we report the cloning, expression in E. coli, and functional characterization of β-phellandrene synthase, LaβPHLS. The ORF--excluding the transit peptide--for this gene encoded a 62.3 kDa protein that contained all conserved motifs present in plant TPSs. Expression in bacteria resulted in the production of a soluble protein that was purified by Ni-NTA agarose affinity chromatography. While the recombinant LaβPHLS did not utilize FPP as a substrate, it converted GPP (the preferred substrate) and NPP into β-phellandrene as the major product, with K (m) and k (cat) of 6.55 μM and 1.75 × 10(-2) s(-1), respectively, for GPP. The LaβPHLS transcripts were highly abundant in young leaves where β-phellandrene is produced, but were barely detectable in flowers and older leaves, where β-phellandrene is not synthesized in significant quantities. This data indicate that β-phellandrene biosynthesis is transcriptionally and developmentally regulated. We also cloned and expressed in E. coli a second TPS-like protein, LaTPS-I, that lacks an internal stretch of 73 amino acids, including the signature DDxxD divalent metal binding motif, compared to other plant TPSs. The recombinant LaTPS-I did not produce detectable products in vitro when assayed with GPP, NPP or FPP as substrates. The lack of activity is most likely due to the absence of catalytically important amino acid residues within the missing region.

Chrysolina herbacea modulates terpenoid biosynthesis of Mentha aquatica L

Interactions between herbivorous insects and plants storing terpenoids are poorly understood. This study describes the ability of Chrysolina herbacea to use volatiles emitted by undamaged Mentha aquatica plants as attractants and the plant's response to herbivory, which involves the production of deterrent molecules. Emitted plant volatiles were analyzed by GC-MS. The insect's response to plant volatiles was tested by Y-tube olfactometer bioassays. Total RNA was extracted from control plants, mechanically damaged leaves, and leaves damaged by herbivores. The terpenoid quantitative gene expressions (qPCR) were then assayed. Upon herbivory, M. aquatica synthesizes and emits (+)-menthofuran, which acts as a deterrent to C. herbacea. Herbivory was found to up-regulate the expression of genes involved in terpenoid biosynthesis. The increased emission of (+)-menthofuran was correlated with the upregulation of (+)-menthofuran synthase.

UV-B modulates the interplay between terpenoids and flavonoids in peppermint (Mentha x piperita L.)

Modulation of secondary metabolites by UV-B involves changes in gene expression, enzyme activity and accumulation of defence metabolites. After exposing peppermint (Mentha x piperita L.) plants grown in field (FP) and in a growth chamber (GCP) to UV-B irradiation, we analysed by qRT-PCR the expression of genes involved in terpenoid biosynthesis and encoding: 1-deoxy-D-xylulose-5-phosphate synthase (Dxs), 2-C-methyl-D-erythritol-2,4-cyclodiphosphate synthase (Mds), isopentenyl diphosphate isomerase (Ippi), geranyl diphosphate synthase (Gpps), (-)-limonene synthase (Ls), (-)-limonene-3-hydroxylase (L3oh), (+)-pulegone reductase (Pr), (-)-menthone reductase (Mr), (+)-menthofuran synthase (Mfs), farnesyl diphosphate synthase (Fpps) and a putative sesquiterpene synthase (S-TPS). GCP always showed a higher terpenoid content with respect to FP. We found that in both FP and GCP, most of these genes were regulated by the UV-B treatment. The amount of most of the essential oil components, which were analysed by gas chromatography-mass spectrometry (GC-MS), was not correlated to gene expression. The total phenol composition was found to be always increased after UV-B irradiation; however, FP always showed a higher phenol content with respect to GCP. Liquid chromatography-mass spectrometry (LC-ESI-MS/MS) analyses revealed the presence of UV-B absorbing flavonoids such as eriocitrin, hesperidin, and kaempferol 7-O-rutinoside whose content significantly increased in UV-B irradiated FP, when compared to GCP. The results of this work show that UV-B irradiation differentially modulates the expression of genes involved in peppermint essential oil biogenesis and the content of UV-B absorbing flavonoids. Plants grown in field were better adapted to increasing UV-B irradiation than plants cultivated in growth chambers. The interplay between terpenoid and phenylpropanoid metabolism is also discussed.

A copal-8-ol diphosphate synthase from the angiosperm Cistus creticus subsp. creticus is a putative key enzyme for the formation of pharmacologically active, oxygen-containing labdane-type diterpenes

The resin of Cistus creticus subsp. creticus, a plant native to Crete, is rich in labdane-type diterpenes with significant antimicrobial and cytotoxic activities. The full-length cDNA of a putative diterpene synthase was isolated from a C. creticus trichome cDNA library. The deduced amino acid sequence of this protein is highly similar (59%-70% identical) to type B diterpene synthases from other angiosperm species that catalyze a protonation-initiated cyclization. The affinity-purified recombinant Escherichia coli-expressed protein used geranylgeranyl diphosphate as substrate and catalyzed the formation of copal-8-ol diphosphate. This diterpene synthase, therefore, was named CcCLS (for C. creticus copal-8-ol diphosphate synthase). Copal-8-ol diphosphate is likely to be an intermediate in the biosynthesis of the oxygen-containing labdane-type diterpenes that are abundant in the resin of this plant. RNA gel-blot analysis revealed that CcCLS is preferentially expressed in the trichomes, with higher transcript levels found in glands on young leaves than on fully expanded leaves, while CcCLS transcript levels increased after mechanical wounding. Chemical analyses revealed that labdane-type diterpene production followed a similar pattern, with higher concentrations in trichomes of young leaves and increased accumulation upon wounding.

Ultraviolet-B and photosynthetically active radiation interactively affect yield and pattern of monoterpenes in leaves of peppermint (Mentha x piperita L.)

Solar radiation is a key environmental signal in regulation of plant secondary metabolism. Since metabolic responses to light and ultraviolet (UV) radiation exposure are known to depend on the ratio of spectral ranges (e.g., UV-B/PAR), we examined effects of different UV-B radiation (280-315 nm) and photosynthetically active radiation (PAR, 400-700 nm) levels and ratios on yield and pattern of monoterpenoid essential oil of peppermint. Experiments were performed in exposure chambers, technically equipped for realistic simulation of natural climate and radiation. The experimental design comprised four irradiation regimes created by the combination of two PAR levels including or excluding UV-B radiation. During flowering, the highest essential oil yield was achieved at high PAR (1150 micromol m(-2) s(-1)) and approximate ambient UV-B radiation (0.6 W m(-2)). Regarding the monoterpene pattern, low PAR (550 micromol m(-2) s(-1)) and the absence of UV-B radiation led to reduced menthol and increased menthone contents and thereby to a substantial decrease in oil quality. Essential oil yield could not be correlated with density or diameter of peltate glandular trichomes, the epidermal structures specialized on biosynthesis, and the accumulation of monoterpenes. The present results lead to the conclusion that production of high quality oils (fulfilling the requirements of the Pharmacopoeia Europaea) requires high levels of natural sunlight. In protected cultivation, the use of UV-B transmitting covering materials is therefore highly recommended.

Stress and developmental responses of terpenoid biosynthetic genes in Cistus creticus subsp. creticus

Plants, and specially species adapted in non-friendly environments, produce secondary metabolites that help them to cope with biotic or abiotic stresses. These metabolites could be of great pharmaceutical interest because several of those show cytotoxic, antibacterial or antioxidant activities. Leaves' trichomes of Cistus creticus ssp. creticus, a Mediterranean xerophytic shrub, excrete a resin rich in several labdane-type diterpenes with verified in vitro and in vivo cytotoxic and cytostatic activity against human cancer cell lines. Bearing in mind the properties and possible future exploitation of these natural products, it seemed interesting to study their biosynthesis and its regulation, initially at the molecular level. For this purpose, genes encoding enzymes participating in the early steps of the terpenoids biosynthetic pathways were isolated and their gene expression patterns were investigated in different organs and in response to various stresses and defence signals. The genes studied were the CcHMGR from the mevalonate pathway, CcDXS and CcDXR from the methylerythritol 4-phosphate pathway and the two geranylgeranyl diphosphate synthases (CcGGDPS1 and 2) previously characterized from this species. The present work indicates that the leaf trichomes are very active biosynthetically as far as it concerns terpenoids biosynthesis, and the terpenoid production from this tissue seems to be transcriptionally regulated. Moreover, the CcHMGR and CcDXS genes (the rate-limiting steps of the isoprenoids' pathways) showed an increase during mechanical wounding and application of defence signals (like meJA and SA), which is possible to reflect an increased need of the plant tissues for the corresponding metabolites.

Lavender inflorescence: a model to study regulation of terpenes synthesis

We analysed VOC composition of complete inflorescences and single flowers of lavender during the flowering period. Our analyses, focused on the 20 most abundant terpenes, showed that three groups of components could be separated according to their patterns of variation during inflorescence ontogeny. These three groups were associated with three developmental stages: flower in bud, flower in bloom and faded flower. The expression of two terpene synthases (TPS) was followed using qPCR during inflorescence ontogeny. A comparison of these chemical and molecular analyses suggested that VOC production in lavender spike is mainly regulated at the transcriptional level. These results highlighted that lavender could be a model plant for future investigations on terpene biosynthesis and regulation, and could be used to explore the functions of terpene metabolites.

Differential accumulation of volatile terpene and terpene synthase mRNAs during lavender (Lavandula angustifolia and L. x intermedia) inflorescence development

Despite the commercial importance of Lavandula angustifolia Mill. and L. x intermedia Emeric ex Loisel floral essential oils (EOs), no information is currently available on potential changes in individual volatile organic compound (VOC) content during inflorescence development. Calyces were found to be the main sites of VOC accumulation. The 20 most abundant VOCs could be separated into three sub-groups according to their patterns of change in concentration The three groups of VOCs sequentially dominated the global scent bouquet of inflorescences, the transition between the first and second groups occurring around the opening of the first flower of the inflorescence and the one between the second and third groups at the start of seed set. Changes in calyx VOC accumulation were linked to the developmental stage of individual flowers. Leaves accumulated a smaller number of VOCs which were a subset of those seen in preflowering inflorescences. Their nature and content remained constant during the growing season. Quantitative real time polymerase chain reaction assessments of the expression of two terpene synthase (TPS) genes, LaLIMS and LaLINS, revealed similar trends between their patterns of expression and those of their VOC products. Molecular and chemical analyses suggest that changes in TPS expression occur during lavender inflorescence development and lead to changes in EO composition. Both molecular data and terpene analysis support the findings that changes in biosynthesis of terpene occurred during inflorescence development.

Structure of a heterotetrameric geranyl pyrophosphate synthase from mint (Mentha piperita) reveals intersubunit regulation

Terpenes (isoprenoids), derived from isoprenyl pyrophosphates, are versatile natural compounds that act as metabolism mediators, plant volatiles, and ecological communicators. Divergent evolution of homomeric prenyltransferases (PTSs) has allowed PTSs to optimize their active-site pockets to achieve catalytic fidelity and diversity. Little is known about heteromeric PTSs, particularly the mechanisms regulating formation of specific products. Here, we report the crystal structure of the (LSU . SSU)(2)-type (LSU/SSU = large/small subunit) heterotetrameric geranyl pyrophosphate synthase (GPPS) from mint (Mentha piperita). The LSU and SSU of mint GPPS are responsible for catalysis and regulation, respectively, and this SSU lacks the essential catalytic amino acid residues found in LSU and other PTSs. Whereas no activity was detected for individually expressed LSU or SSU, the intact (LSU . SSU)(2) tetramer produced not only C(10)-GPP at the beginning of the reaction but also C(20)-GGPP (geranylgeranyl pyrophosphate) at longer reaction times. The activity for synthesizing C(10)-GPP and C(20)-GGPP, but not C(15)-farnesyl pyrophosphate, reflects a conserved active-site structure of the LSU and the closely related mustard (Sinapis alba) homodimeric GGPPS. Furthermore, using a genetic complementation system, we showed that no C(20)-GGPP is produced by the mint GPPS in vivo. Presumably through protein-protein interactions, the SSU remodels the active-site cavity of LSU for synthesizing C(10)-GPP, the precursor of volatile C(10)-monoterpenes.

Phenylpropanoid biosynthesis in leaves and glandular trichomes of basil (Ocimum basilicum L.)

Basil (Ocimum basilicum L.) essential oil phenylpropenes are synthesized and accumulate in peltate glandular trichomes and their content and composition depend on plant developmental stage. Studies on gene expression and enzymatic activity indicate that the phenylpropene biosynthetic genes are developmentally regulated. In this study, the methylchavicol accumulation in basil leaves and the enzyme activities and gene expression of both chavicol O-methyltransferase (CVOMT) and eugenol O-methyltransferase (EOMT) were investigated in all leaves at four plant developmental stages. Methylchavicol accumulation decreased over time as leaves matured. There was a significant correlation between methylchavicol accumulation and CVOMT (r(2) = 0.88) enzyme activity, suggesting that the levels of biosynthetic enzymes control the essential oil content. CVOMT and EOMT transcript expression levels, which decreased with leaf age, followed the same pattern in both whole leaves and isolated glandular trichomes, providing evidence that CVOMT transcript levels are developmentally regulated in basil glandular trichomes themselves and that differences in CVOMT expression observed in whole leaves are not solely the result of differences in glandular trichome density.

Essential Oil Biosynthesis and Regulation in the Genus Cymbopogon

Essential oils distilled from Cymbopogon species are of immense commercial value as flavors and fragrances in the perfumery, cosmetics, soaps, and detergents and in pharmaceutical industries. Two major constituents of the essential oil, geraniol and citral, due to their specific rose and lemon like aromas are widely used as flavors, fragrances and cosmetics. Citral is also used for the synthesis of vitamin A and ionones (for example, β-ionone, methyl ionone). Moreover, Cymbopogon essential oils and constituents possess many useful biological activities including cytotoxic, antiinflammatory and antioxidant. Despite the immense commercial and biological significance of the Cymbopogon essential oils, little is known about their biosynthesis and regulatory mechanisms. So far it is known that essential oils are biosynthesized via the classical acetate-MVA route and existence of a newly discovered MEP pathway in Cymbopogon remains as a topic for investigation. The aim of the present review is to discuss the biosynthesis and regulation of essential oils in the genus Cymbopogon with given emphasis to two elite members, lemongrass (C. flexuosus Nees ex Steud) and palmarosa (C. martinii Roxb.). This article highlights the work done so far towards understanding of essential oil biosynthesis and regulation in the genus Cymbopogon. Also, based on our experiences with Cymbopogon species, we would like to propose C. flexuosus as a model system for the study of essential oil metabolism beyond the much studied plant family Lamiaceae.

Factors Affecting Chemical Variability of Essential Oils: A Review of Recent Developments

This review, covering mainly papers of the last decade, focuses on recent findings on the different factors affecting the chemical composition of essential oils, such as exogenous and endogenous factors. The endogenous factors are related to anatomical and physiological characteristics of the plants and to the biosynthetic pathways of the volatiles, which might change in either the different tissues of the plants or in different seasons, but also could be influenced by DNA adaptation. The exogenous factors, over a long period, might affect some of the genes responsible for volatiles formation. Those factors lead to ecotypes or chemotypes in the same plant species. In the last few years chemotaxonomy has been widely used to classify plants with essential oils characterized by intra-specific chemical polymorphism. It could be evidenced that chemotypes are frequently genotypes and recently the application of the Random Amplified Polymorphic DNA (RAPD) markers, coupled with powerful statistical methods, appeared to be useful in discriminating the different genotypes. The data presented led to the suggestion that further chemotaxonomic studies should be the result of the analysis of morphological traits combined both with chemical and molecular markers.

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.

Expression of spearmint limonene synthase in transgenic spike lavender results in an altered monoterpene composition in developing leaves

We generated transgenic spike lavender (Lavandula latifolia) plants constitutively expressing the limonene synthase (LS) gene from spearmint (Mentha spicata), encoding the LS enzyme that catalyzes the synthesis of limonene from geranyl diphosphate. Overexpression of the LS transgene did not consistently affect monoterpene profile in pooled leaves or flowers from transgenic T(0) plants. Analyses from cohorts of leaves sampled at different developmental stages showed that essential oil accumulation in transgenic and control plants was higher in developing than in mature leaves. Furthermore, developing leaves of transgenic plants contained increased limonene contents (more than 450% increase compared to controls) that correlated with the highest transcript accumulation of the LS gene. The levels of other monoterpene pathway components were also significantly altered. T(0) transgenic plants were grown for 2 years, self-pollinated, and the T(1) seeds obtained. The increased limonene phenotype was maintained in the progenies that inherited the LS transgene.

Isolation and functional analysis of two Cistus creticus cDNAs encoding geranylgeranyl diphosphate synthase

Cistus creticus ssp. creticus is an indigenous shrub of the Mediterranean area. The glandular trichomes covering its leaf surfaces secrete a resin called "ladanum", which among others contains a number of specific labdane-type diterpenes that exhibit antibacterial and antifungal action as well as in vitro and in vivo cytotoxic and cytostatic activity against human cancer cell lines. In view of the properties and possible future exploitation of these metabolites, it was deemed necessary to study the geranylgeranyl diphosphate synthase enzyme (GGDPS, EC 2.5.1.30), a short chain prenyltransferase responsible for the synthesis of the precursor molecule of all diterpenes. In this work, we present the cloning, functional characterisation and expression profile at the gene and protein levels of two differentially expressed C. creticus full-length cDNAs, CcGGDPS1 and CcGGDPS2. Heterologous yeast cell expression system showed that these cDNAs exhibited GGDPS enzyme activity. Gene and protein expression analyses suggest that this enzyme is developmentally and tissue-regulated showing maximum expression in trichomes and smallest leaves (0.5-1.0cm). This work is the first attempt to study the terpenoid biosynthesis at the molecular level in C. creticus ssp. creticus.

Biosynthesis of plant-derived flavor compounds

Plants have the capacity to synthesize, accumulate and emit volatiles that may act as aroma and flavor molecules due to interactions with human receptors. These low-molecular-weight substances derived from the fatty acid, amino acid and carbohydrate pools constitute a heterogenous group of molecules with saturated and unsaturated, straight-chain, branched-chain and cyclic structures bearing various functional groups (e.g. alcohols, aldehydes, ketones, esters and ethers) and also nitrogen and sulfur. They are commercially important for the food, pharmaceutical, agricultural and chemical industries as flavorants, drugs, pesticides and industrial feedstocks. Due to the low abundance of the volatiles in their plant sources, many of the natural products had been replaced by their synthetic analogues by the end of the last century. However, the foreseeable shortage of the crude oil that is the source for many of the artificial flavors and fragrances has prompted recent interest in understanding the formation of these compounds and engineering their biosynthesis. Although many of the volatile constituents of flavors and aromas have been identified, many of the enzymes and genes involved in their biosynthesis are still not known. However, modification of flavor by genetic engineering is dependent on the knowledge and availability of genes that encode enzymes of key reactions that influence or divert the biosynthetic pathways of plant-derived volatiles. Major progress has resulted from the use of molecular and biochemical techniques, and a large number of genes encoding enzymes of volatile biosynthesis have recently been reported.

Harnessing plant trichome biochemistry for the production of useful compounds

Plant trichomes come in a variety of shapes, sizes and cellular composition. Some types, commonly called glandular trichomes, produce large amounts of specialized (secondary) metabolites of diverse classes. Trichomes are implicated in a variety of adaptive processes, including defense against herbivores and micro-organisms as well as in ion homeostasis. Because trichomes protrude from the epidermis and can often be easily separated from it and harvested, the mRNAs, proteins and small molecules that they contain are unusually accessible to analysis. This property makes them excellent experimental systems for identification of the enzymes and pathways responsible for the synthesis of the specialized metabolites found in these structures and sometimes elsewhere in the plant. We review the literature on the biochemistry of trichomes and consider the attributes that might make them highly useful targets for plant metabolic engineering.

Increased terpenoid accumulation in cotton (Gossypium hirsutum) foliage is a general wound response

The subepidermal pigment glands of cotton accumulate a variety of terpenoid products, including monoterpenes, sesquiterpenes, and terpenoid aldehydes that can act as feeding deterrents against a number of insect herbivore species. We compared the effect of herbivory by Spodoptera littoralis caterpillars, mechanical damage by a fabric pattern wheel, and the application of jasmonic acid on levels of the major representatives of the three structural classes of terpenoids in the leaf foliage of 4-week-old Gossypium hirsutum plants. Terpenoid levels increased successively from control to mechanical damage, herbivory, and jasmonic acid treatments, with E-beta-ocimene and heliocide H1 and H4 showing the highest increases, up to 15-fold. Herbivory or mechanical damage to older leaves led to terpenoid increases in younger leaves. Leaf-by-leaf analysis of terpenes and gland density revealed that higher levels of terpenoids were achieved by two mechanisms: (1) increased filling of existing glands with terpenoids and (2) the production of additional glands, which were found to be dependent on damage intensity. As the relative response of individual terpenoids did not differ substantially among herbivore, mechanical damage, and jasmonic acid treatments, the induction of terpenoids in cotton foliage appears to represent a non-specific wound response mediated by jasmonic acid.

A systems biology approach identifies the biochemical mechanisms regulating monoterpenoid essential oil composition in peppermint

The integration of mathematical modeling and experimental testing is emerging as a powerful approach for improving our understanding of the regulation of metabolic pathways. In this study, we report on the development of a kinetic mathematical model that accurately simulates the developmental patterns of monoterpenoid essential oil accumulation in peppermint (Mentha x piperita). This model was then used to evaluate the biochemical processes underlying experimentally determined changes in the monoterpene pathway under low ambient-light intensities, which led to an accumulation of the branchpoint intermediate (+)-pulegone and the side product (+)-menthofuran. Our simulations indicated that the environmentally regulated changes in monoterpene profiles could only be explained when, in addition to effects on biosynthetic enzyme activities, as yet unidentified inhibitory effects of (+)-menthofuran on the branchpoint enzyme pulegone reductase (PR) were assumed. Subsequent in vitro analyses with recombinant protein confirmed that (+)-menthofuran acts as a weak competitive inhibitor of PR (K(i) = 300 muM). To evaluate whether the intracellular concentration of (+)-menthofuran was high enough for PR inhibition in vivo, we isolated essential oil-synthesizing secretory cells from peppermint leaves and subjected them to steam distillations. When peppermint plants were grown under low-light conditions, (+)-menthofuran was selectively retained in secretory cells and accumulated to very high levels (up to 20 mM), whereas under regular growth conditions, (+)-menthofuran levels remained very low (<400 muM). These results illustrate the utility of iterative cycles of mathematical modeling and experimental testing to elucidate the mechanisms controlling flux through metabolic pathways.

Population variability in Thymus glabrescens Willd. from Serbia: Morphology, anatomy and essential oil composition

In five indigenous populations of Thymus glabrescens Willd. collected in the region of Banat (Serbia), the variability on leaf morphological traits, leaf and stem anatomy, and composition of the essential oil was studied. The major component in the studied populations was either thymol or ?-terpinene. Distinct differentiation of populations with respect to chemical composition of essential oils might be related to spatial distribution of the studied populations. No correlations between morphology, anatomy, and essential oil yield and composition were determined. Both capitate and peltate glandular trichomes were found on calyces, whereas the latter were noticed on the abaxial and adaxial leaf surface.

A molecular perspective on terpene variation in Australian Myrtaceae

The terpenoid-dominated essential oils in Australian Myrtaceae mediate many ecological interactions and are important industrially. Of all the significant essential oil-producing families, Myrtaceae is the only one for which there is no molecular information on terpene biosynthesis. Here we summarise available knowledge on terpene biosynthesis and its relevance to the Myrtaceae to provide a foundation for ecological and genetic studies of chemical diversity. There are several steps in the terpene biosynthesis pathway that have potential for influencing the oil yield, profile and composition of leaf oils in Myrtaceae. The biochemical steps that influence oil yield in Myrtaceae probably occur in the steps of the pathway leading up to the synthesis of the terpene backbone. Qualitative differences in oil profiles are more likely to be due to variation in terpene synthases and terpene-modifying enzymes. Most of the information on molecular variation in terpene biosynthesis is based on the analysis of artificially derived mutants but Australian Myrtaceae can provide examples of the same mechanisms in an ecological context.

(-)-Menthol biosynthesis and molecular genetics

(-)-Menthol is the most familiar of the monoterpenes as both a pure natural product and as the principal and characteristic constituent of the essential oil of peppermint (Mentha x piperita). In this paper, we review the biosynthesis and molecular genetics of (-)-menthol production in peppermint. In Mentha species, essential oil biosynthesis and storage is restricted to the peltate glandular trichomes (oil glands) on the aerial surfaces of the plant. A mechanical method for the isolation of metabolically functional oil glands, has provided a system for precursor feeding studies to elucidate pathway steps, as well as a highly enriched source of the relevant biosynthetic enzymes and of their corresponding transcripts with which cDNA libraries have been constructed to permit cloning and characterization of key structural genes. The biosynthesis of (-)-menthol from primary metabolism requires eight enzymatic steps, and involves the formation and subsequent cyclization of the universal monoterpene precursor geranyl diphosphate to the parent olefin (-)-(4S)-limonene as the first committed reaction of the sequence. Following hydroxylation at C3, a series of four redox transformations and an isomerization occur in a general "allylic oxidation-conjugate reduction" scheme that installs three chiral centers on the substituted cyclohexanoid ring to yield (-)-(1R, 3R, 4S)-menthol. The properties of each enzyme and gene of menthol biosynthesis are described, as are their probable evolutionary origins in primary metabolism. The organization of menthol biosynthesis is complex in involving four subcellular compartments, and regulation of the pathway appears to reside largely at the level of gene expression. Genetic engineering to up-regulate a flux-limiting step and down-regulate a side route reaction has led to improvement in the composition and yield of peppermint oil.

Signal transduction and metabolic flux of β-thujaplicin and monoterpene biosynthesis in elicited Cupressus lusitanica cell cultures

beta-Thujaplicin is an antimicrobial tropolone derived from geranyl pyrophosphate(GPP) and monoterpene intermediate. Yeast elicitor-treated Cupressus lusitanica cell cultures accumulate high levels of beta-thujaplicin at early stages and other monoterpenes at later stages post-elicitation. The different regulation of beta-thujaplicin and monoterpene biosynthesis and signal transduction directing metabolic flux to beta-thujaplicin firstly and then shifting metabolic flow from beta-thujaplicin to other monoterpene biosynthesis were investigated. The earlier rapid induction of beta-thujaplicin accumulation and a later stimulation of monoterpene biosynthesis by yeast elicitor are in well agreement with elicitor-induced changes in activity of three monoterpene biosynthetic enzymes including isopentenyl pyrophosphate isomerase, GPP synthase, and monoterpene synthase. Yeast elicitor induces an earlier and stronger beta-thujaplicin production and monoterpene biosynthetic enzyme activity than methyl jasmonate (MeJA) does. Profiling all monoterpenes produced by C. lusitanica cell cultures under different conditions reveals that beta-thujaplicin biosynthesis parallels with other monoterpenes and competes for common precursor pools. Yet beta-thujaplicin is produced pre-dominantly at early stage of elicitation whereas other monoterpenes are mainly accumulated at late stage while beta-thujaplicin is metabolized. It is suggested that yeast elicitor-treated C. lusitanica cells preferentially accumulate beta-thujaplicin as a primary defense and other monoterpenes as a secondary defense. Inhibitor treatments suggest that immediate production of beta-thujaplicin post-elicitation largely depends on pre-existing enzymes and translation of pre-existing transcripts as well as recruitment of precursor pools from both the cytosol and plastids. The later beta-thujaplicin and other monoterpene accumulation strictly depends on active transcription and translation. Induction of beta-thujaplicin production and activation of monoterpene biosynthetic enzymes by elicitor involves similar signaling pathways, which may activate early beta-thujaplicin production and later monoterpene biosynthesis and induce a metabolic flux shift from beta-thujaplicin to monoterpene accumulation.

Genetic engineering of peppermint for improved essential oil composition and yield

The biochemistry, organization, and regulation of essential oil metabolism in the epidermal oil glands of peppermint have been defined, and most of the genes encoding enzymes of the eight-step pathway to the principal monoterpene component (-)-menthol have been isolated. Using these tools for pathway engineering, two genes and two expression strategies have been employed to create transgenic peppermint plants with improved oil composition and yield. These experiments, along with related studies on other pathway genes, have led to a systematic, stepwise approach for the creation of a 'super' peppermint.

Microarray expression profiling and functional characterization of AtTPS genes: duplicated Arabidopsis thaliana sesquiterpene synthase genes At4g13280 and At4g13300 encode root-specific and wound-inducible (Z)-gamma-bisabolene synthases

The Arabidopsis thaliana genome contains at least 32 terpenoid synthase (AtTPS) genes [Aubourg et al., Mol. Genet. Genom. 267 (2002) 730] a few of which have recently been characterized. Based on hierarchical cluster analysis of AtTPS gene expression, measured by microarray profiling and validated with published expression data, we identified two groups of predominantly root expressed AtTPS genes containing five members with previously unknown biochemical functions (At4g13280, At4g13300, At5g48110, At1g33750, and At3g29410). Among the root expressed AtTPS genes, a pair of tandem-organized genes, At4g13280 (AtTPS12) and At4g13300 (AtTPS13), shares 91% predicted amino acid identity indicating recent gene duplication. Bacterial expression of cDNAs and enzyme assays showed that both At4g13280 and At4g13300 encode sesquiterpene synthases catalyzing the conversion of farnesyl diphosphate to (Z)-gamma-bisabolene and the additional minor products E-nerolidol and alpha-bisabolol. Expression of beta-glucuronidase (GUS) reporter gene fused to upstream genomic regions of At4g13280 or At4g13300 showed constitutive promoter activities in the cortex and sub-epidermal layers of Arabidopsis roots. In addition, highly localized promoter activities were found in leaf hydathodes and flower stigmata. Mechanical wounding of Arabidopsis leaves induced local expression of At4g13280 and At4g13300. The functional characterization of At4g13280 gene product AtTPS12 and At4g13230 gene product AtTPS13 as (Z)-gamma-bisabolene synthases, together with the recent characterization of two flower-specific AtTPS [At5g23960 and At5g44630; Tholl et al., Plant J. 42 (2005) 757], concludes the biochemical functional annotation of all four predicted Arabidopsis sesquiterpene synthase genes. Our data suggest biological functions for At4g13280 and At4g13300 in the rhizosphere with additional roles in aerial plant tissues.