Shikonin: A geranyl diphosphate-derived plant hemiterpenoid formed via the mevalonate pathway

Review on plant terpenoid emissions worldwide and in China

Biogenic volatile organic compounds (BVOCs), particularly terpenoids, can significantly drive the formation of ozone (O₃) and secondary organic aerosols (SOA) in the atmosphere, as well as directly or indirectly affect global climate change. Understanding their emission mechanisms and the current progress in emission measurements and estimations are essential for the accurate determination of emission characteristics, as well as for evaluating their roles in atmospheric chemistry and climate change. This review summarizes the mechanisms of terpenoid synthesis and release, biotic and abiotic factors affecting their emissions, development of emission observation techniques, and emission estimations from hundreds of published papers. We provide a review of the main observations and estimations in China, which contributes a significant proportion to the total global BVOC emissions. The review suggests the need for further research on the comprehensive effects of environmental factors on terpenoid emissions, especially soil moisture and nitrogen content, which should be quantified in emission models to improve the accuracy of estimation. In China, it is necessary to conduct more accurate measurements for local plants in different regions using the dynamic enclosure technique to establish an accurate local emission rate database for dominant tree species. This will help improve the accuracy of both national and global emission inventories. This review provides a comprehensive understanding of terpenoid emissions as well as prospects for detailed research to accurately describe terpenoid emission characteristics worldwide and in China.

CYP82AR Subfamily Proteins Catalyze C-1' Hydroxylations of Deoxyshikonin in the Biosynthesis of Shikonin and Alkannin

Shikonin and S-enantiomer alkannin are important naphthoquinone derivatives present in many Boraginaceae species. We report that cytochrome P450 monooxygenases (CYPs) from a new CYP82AR subfamily catalyzed hydroxylations of deoxyshikonin at C-1' position of isoprenoid side chain. Two homologues were discovered from each species of the four Boraginaceae plants. One CYP preferred converting deoxyshikonin into shikonin, and the other stereoselectively hydroxylated deoxyshikonin into alkannin. The discovery might be a general feature of shikonin/alkannin-producing Boraginaceae plants.

Potential role of two cytochrome P450s obtained from Lithospermum erythrorhizon in catalyzing the oxidation of geranylhydroquinone during Shikonin biosynthesis

Shikonin is a natural naphthoquinone derivative that specifically occurs in boraginaceous plants, and the major active ingredient of the medicinal plant Lithospermum erythrorhizon. Previously, a cytochrome P450 oxygenase (CYP) CYP76B74 catalyzing 3″-hydroxylation of geranylhydroquinone (GHQ) - a key intermediate of shikonin biosynthesis, was identified from cultured cells of Arnebia euchroma. However, the enzymes catalyzing oxidation of the geranyl side-chain of GHQ from L. erythrorhizon remain unknown. In this study, we performed transcriptome analysis of different tissues (red roots and green leaves/stems) from L. erythrorhizon using RNA sequencing technology. Highly expressed CYP genes found in the roots were then heterologously expressed in Saccharomyces cerevisiae and functionally screened with GHQ as the substrate. As the result, two CYPs of CYP76B subfamily catalyzing the oxidation of GHQ were characterized. CYP76B100 catalyzed the hydroxylation of the geranyl side-chain of GHQ at the C-3″ position to form 3″-hydroxyl geranylhydroquinone (GHQ-3″-OH). The enzyme CYP76B101 carried out oxidation reaction of GHQ at the C-3″ position to produce a 3″-carboxylic acid derivative of GHQ (GHQ-3″-COOH) as well as GHQ-3″-OH. This enzyme-catalyzed oxidation reaction with GHQ as the substrate is reported for the first time. This study implicates CYP76B100 and CYP76B101 as having a potential role in shikonin biosynthesis in L. erythrorhizon.

Metabolic and transcriptional analyses in response to potent inhibitors establish MEP pathway as major route for camptothecin biosynthesis in Nothapodytes nimmoniana (Graham) Mabb

BACKGROUND

Nothapodytes nimmoniana, a plant of pivotal medicinal significance is a source of potent anticancer monoterpene indole alkaloid (MIA) camptothecin (CPT). This compound owes its potency due to topoisomerase-I inhibitory activity. However, biosynthetic and regulatory aspects of CPT biosynthesis so far remain elusive. Production of CPT is also constrained due to unavailability of suitable in vitro experimental system. Contextually, there are two routes for the biosynthesis of MIAs: the mevalonate (MVA) pathway operating in cytosol and the methylerythritol phosphate (MEP) pathway in the plastids. Determination of relative precursor flux through either of these pathways may provide a new vista for manipulating the enhanced CPT production.

RESULTS

In present study, specific enzyme inhibitors of MVA (lovastatin) and MEP pathways (fosmidomycin) were used to perturb the metabolic flux in N. nimmoniana. Interaction of both these pathways was investigated at transcriptional level by using qRT-PCR and at metabolite level by evaluating secologanin, tryptamine and CPT contents. In fosmidomycin treated plants, highly significant reduction was observed in both secologanin and CPT accumulation in the range 40-57% and 64-71.5% respectively, while 4.61-7.69% increase was observed in tryptamine content as compared to control. Lovastatin treatment showed reduction in CPT (7-11%) and secologanin (7.5%) accumulation while tryptamine registered slight increase (3.84%) in comparison to control. These inhibitor mediated changes were reflected at transcriptional level via altering expression levels of deoxy-xylulose-5-phosphate reductoisomerase (DXR) and hydroxymethylglutaryl-CoA reductase (HMG). Further, mRNA expression of four more genes downstream to DXR and HMG of MEP and MVA pathways respectively were also investigated. Expression analysis also included secologanin synthase (SLS) and strictosidine synthase (STR) of seco-iridoid pathway. Present investigation also entailed development of an efficient in vitro multiplication system as a precursor to pathway flux studies. Further, a robust Agrobacterium-mediated transformed hairy root protocol was also developed for its amenability for up-scaling as a future prospect.

CONCLUSIONS

Metabolic and transcriptional changes reveal differential efficacy of cytosolic and plastidial inhibitors in context to pathway flux perturbations on seco-iridoid end-product camptothecin. MEP pathway plausibly is the major precursor contributor towards CPT production. These empirical findings allude towards developing suitable biotechnological interventions for enhanced CPT production.

Shikonin derivatives for cancer prevention and therapy

Phytochemicals gained considerable interest during the past years as source to develop new treatment options for chemoprevention and cancer therapy. Motivated by the fact that a majority of established anticancer drugs are derived in one way or another from natural resources, we focused on shikonin, a naphthoquinone with high potentials to be further developed as preventive or therapeutic drug to fight cancer. Shikonin is the major chemical component of Lithospermum erythrorhizon (Purple Cromwell) roots. Traditionally, the root extract has been applied to cure dermatitis, burns, and wounds. Over the past three decades, the anti-inflammatory and anticancer effects of root extracts, isolated shikonin as well as semi-synthetic and synthetic derivatives and nanoformulations have been described. In vitro and in vivo experiments were conducted to understand the effect of shikonin at cellular and molecular levels. Preliminary clinical trials indicate the potential of shikonin for translation into clinical oncology. Shikonin exerts additive and synergistic interactions in combination with established chemotherapeutics, immunotherapeutic approaches, radiotherapy and other treatment modalities, which further underscores the potential of this phytochemical to be integrated into standard treatment regimens.

Characterization of Shikonin Derivative Secretion in Lithospermum erythrorhizon Hairy Roots as a Model of Lipid-Soluble Metabolite Secretion from Plants

Shikonin derivatives are specialized lipophilic metabolites, secreted in abundant amounts from the root epidermal cells of Lithospermum erythrorhizon. Because they have anti-microbial activities, these compounds, which are derivatives of red naphthoquinone, are thought to serve as a chemical barrier for plant roots. The mechanism by which they are secreted from cells is, however, largely unknown. The shikonin production system in L. erythrorhizon is an excellent model for studying the mechanism by which lipophilic compounds are secreted from plant cells, because of the abundant amounts of these compounds produced by L. erythrorhizon, the 0 to 100% inducibility of their production, the light-specific inhibition of production, and the visibility of these products as red pigments. To date, many factors regulating shikonin biosynthesis have been identified, but no mechanism that regulates shikonin secretion without inhibiting biosynthesis has been detected. This study showed that inhibitors of membrane traffic strongly inhibit shikonin secretion without inhibiting shikonin production, suggesting that the secretion of shikonin derivatives into the apoplast utilizes pathways common to the ADP-ribosylation factor/guanine nucleotide exchange factor (ARF/GEF) system and actin filament polymerization, at least in part. These findings provide clues about the machinery involved in secreting lipid-soluble metabolites from cells.

Biosynthesis and molecular actions of specialized 1,4-naphthoquinone natural products produced by horticultural plants

The 1,4-naphthoquinones (1,4-NQs) are a diverse group of natural products found in every kingdom of life. Plants, including many horticultural species, collectively synthesize hundreds of specialized 1,4-NQs with ecological roles in plant-plant (allelopathy), plant-insect and plant-microbe interactions. Numerous horticultural plants producing 1,4-NQs have also served as sources of traditional medicines for hundreds of years. As a result, horticultural species have been at the forefront of many basic studies conducted to understand the metabolism and function of specialized plant 1,4-NQs. Several 1,4-NQ natural products derived from horticultural plants have also emerged as promising scaffolds for developing new drugs. In this review, the current understanding of the core metabolic pathways leading to plant 1,4-NQs is provided with additional emphasis on downstream natural products originating from horticultural species. An overview on the biochemical mechanisms of action, both from an ecological and pharmacological perspective, of 1,4-NQs derived from horticultural plants is also provided. In addition, future directions for improving basic knowledge about plant 1,4-NQ metabolism are discussed.

Biotechnological approaches to the production of shikonins: a critical review with recent updates

Shikonins are commercially important secondary compounds, known for array of biological activities such as antimicrobial, insecticidal, antitumor, antioxidants, etc. These compounds are usually colored and therefore have application in food, textiles and cosmetics. Shikonin and its derivatives, which are commercially most important of the naphthoquinone pigments, are distributed among members of the family Boraginaceae. These include different species of Lithospermum, Arnebia, Alkanna, Anchusa, Echium and Onosma. The growing demand for plant-based natural products has made this group of compounds one of the enthralling targets for their in vitro production. The aim of this review is to highlight the recent progress in production of shikonins by various biotechnological means. Different methods of increasing the levels of shikonins in plant cells such as selection of cell lines, optimization of culture conditions, elicitation, in situ product removal, genetic transformation and metabolic engineering are discussed. The experience of different researchers working worldwide on this aspect is also considered. Further, to meet market demand, the needs for continuous and reliable production systems, as well as future prospects, are included.

Cytosolic monoterpene biosynthesis is supported by plastid-generated geranyl diphosphate substrate in transgenic tomato fruits

Geranyl diphosphate (GPP), the precursor of most monoterpenes, is synthesized in plastids from dimethylallyl diphosphate and isopentenyl diphosphate by GPP synthases (GPPSs). In heterodimeric GPPSs, a non-catalytic small subunit (GPPS-SSU) interacts with a catalytic large subunit, such as geranylgeranyl diphosphate synthase, and determines its product specificity. Here, snapdragon (Antirrhinum majus) GPPS-SSU was over-expressed in tomato fruits under the control of the fruit ripening-specific polygalacturonase promoter to divert the metabolic flux from carotenoid formation towards GPP and monoterpene biosynthesis. Transgenic tomato fruits produced monoterpenes, including geraniol, geranial, neral, citronellol and citronellal, while exhibiting reduced carotenoid content. Co-expression of the Ocimum basilicum geraniol synthase (GES) gene with snapdragon GPPS-SSU led to a more than threefold increase in monoterpene formation in tomato fruits relative to the parental GES line, indicating that the produced GPP can be used by plastidic monoterpene synthases. Co-expression of snapdragon GPPS-SSU with the O. basilicum α-zingiberene synthase (ZIS) gene encoding a cytosolic terpene synthase that has been shown to possess both sesqui- and monoterpene synthase activities resulted in increased levels of ZIS-derived monoterpene products compared to fruits expressing ZIS alone. These results suggest that re-direction of the metabolic flux towards GPP in plastids also increases the cytosolic pool of GPP available for monoterpene synthesis in this compartment via GPP export from plastids.

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.

Biosynthetic origin of 2-geranyl-1,4-naphthoquinone and its related anthraquinone in a Sesamum indicum hairy root culture

In order to clarify the biosynthetic origin of 2-geranyl-1,4-naphthoquinone and its biogenetically related anthraquinone, which are possible intermediates of anthrasesamones, [1-(13)C]glucose was administered to a hairy root culture of Sesamum indicum. The labeling patterns of these quinone derivatives indicated that the naphthoquinone ring and geranyl side-chain of geranylnaphthoquinone were respectively biosynthesized through the shikimate and methylerythritol phosphate pathways, and that these quinone derivatives have the same biosynthetic origin.

Biogenesis, molecular regulation and function of plant isoprenoids

Isoprenoids represent the oldest class of known low molecular-mass natural products synthesized by plants. Their biogenesis in plastids, mitochondria and the endoplasmic reticulum-cytosol proceed invariably from the C5 building blocks, isopentenyl diphosphate and/or dimethylallyl diphosphate according to complex and reiterated mechanisms. Compounds derived from the pathway exhibit a diverse spectrum of biological functions. This review centers on advances obtained in the field based on combined use of biochemical, molecular biology and genetic approaches. The function and evolutionary implications of this metabolism are discussed in relation with seminal informations gathered from distantly but related organisms.

Biosynthesis of benzofuran derivatives in root cultures of Tagetes patula via phenylalanine and 1-deoxy-D-xylulose 5-phosphate

Root cultures of Tagetes patula L. cv. Carmen were grown with a mixture of unlabeled glucose and [U-(13)C(6)]glucose or [1-(13)C(1)]glucose as carbon source. Isoeuparin and (-)-4-hydroxytremetone were isolated by solvent extraction of the cultured tissue, purified by chromatography and analysed by (1)H and (13)C NMR spectroscopy. Amino acids obtained by hydrolysis of protein from the same experiments were used for the reconstruction of the labelling patterns in central metabolic intermediates. These labelling patterns were used for the prediction of isotopolog compositions in the benzofuranone derivatives via different hypothetical pathways. Comparison with the experimentally observed isotopolog distributions showed that the benzenoid ring and the acetoxy group are exclusively or predominantly (>98%) derived from phenylalanine and not from acetyl-CoA via a polyketide-type biosynthesis. The isopropylidene side chain and two carbon atoms of the furan and dihydrofuran moiety, respectively, originate from an isoprenoid building block obtained exclusively or predominantly (>98%) via the deoxyxylulose phosphate pathway. The exomethylene atom of the isopropylidene side chain is biosynthetically equivalent to the (Z)-methyl group of dimethylallyl diphosphate. The data indicate that isoeuparin and (-)-4-hydroxytremetone are assembled from 4-hydroxyacetophenone and dimethylallyl diphosphate via prenyl-substituted 4-hydroxyacetophenone and dihydrobenzofurans as intermediates.

Cytochrome P450 Enzymes in Biosyntheses of Some Plant Secondary Metabolites

Secologanin, a secoiridoid glucoside, is a pivotal terpenoid intermediate in the biosynthesis of biologically active monoterpenoid indole alkaloids such as reserpine, ajmaline, and vinblastine which are biosynthesized via strictosidine, an alkaloidal glucoside, formed from secologanin and tryptamine. In secologanin biosynthesis, the oxidative cleavage process of loganin to secologanin and the hydroxylation of 7-deoxyloganin to loganin have remained unknown enzymologically and mechanistically. Cornoside is a unique glucoside with 4-hydroxy-2,5-cyclohexadien-1-one (benzoquinol) ring and is widespread in families such as Cornaceae, Oleaceae, and Scrophulariaceae but its biosynthesis, especially the oxidative process, remain to be investigated. Shikonin is a red naphthazarin pigment derived from the roots of Lithospermum erythorhizon and produced biotechnologically by cell cultures. Its biosynthesis including the production regulation mechanism has been investigated in detail. However, the naphthazarin ring formation process, probably starting with the hydroxylation of the side chain of geranylhydroquinone, a key intermediate at the late stage of shikonin biosynthesis, remained unknown. In the present review, cytochrome P450 monooxygenases involved in the biosyntheses of three structurally and biosynthetically interesting compounds, secologanin, cornoside, and shikonin, a described together with the results of previous and recent biosynthetic studies. The biosyntheses of related compounds are also discussed.

Biosynthesis of camptothecin. In silico and in vivo tracer study from [1-13C]glucose

Camptothecin derivatives are clinically used antitumor alkaloids that belong to monoterpenoid indole alkaloids. In this study, we investigated the biosynthetic pathway of camptothecin from [1-13C]glucose (Glc) by in silico and in vivo studies. The in silico study measured the incorporation of Glc into alkaloids using the Atomic Reconstruction of Metabolism software and predicted the labeling patterns of successive metabolites from [1-13C]Glc. The in vivo study followed incorporation of [1-13C]Glc into camptothecin with hairy roots of Ophiorrhiza pumila by 13C nuclear magnetic resonance spectroscopy. The 13C-labeling pattern of camptothecin isolated from the hairy roots clearly showed that the monoterpene-secologanin moiety was synthesized via the 2C-methyl-D-erythritol 4-phosphate pathway, not via the mevalonate pathway. This conclusion was supported by differential inhibition of camptothecin accumulation by the pathway-specific inhibitors (fosmidomycin and lovastatin). The quinoline moiety from tryptophan was also labeled as predicted by the Atomic Reconstruction of Metabolism program via the shikimate pathway. These results indicate that camptothecin is formed by the combination of the 2C-methyl-D-erythritol 4-phosphate pathway and the shikimate pathway. This study provides the innovative example for how a computer-aided comprehensive metabolic analysis will refine the experimental design to obtain more precise biological information.

Anthrasesamones from roots of Sesamum indicum

Three anthraquinones, named anthrasesamones A, B and C, were isolated from the roots of Sesamum indicum, and their respective structures were determined to be 1-hydroxy-2-(4-methylpent-3-enyl)anthraquinone, 1,4-dihydroxy-2-(4-methylpent-3-enyl)anthraquinone and 2-chloro-1,4-dihydroxy-3-(4-methylpent-3-enyl)anthraquinone on the basis of spectroscopic evidence. Two known anthraquinones were also isolated for the first time from S. indicum roots and characterized as 2-(4-methylpent-3-enyl)anthraquinone and (E)-2-(4-methylpenta-1,3-dienyl)anthraquinone. Anthrasesamone C is a rare chlorinated anthraquinone in higher plants.

High level expression of chorismate pyruvate-lyase (UbiC) and HMG-CoA reductase in hairy root cultures of Lithospermum erythrorhizon

Shikonin, a red naphthoquinone pigment, is produced by cell cultures of Lithospermum erythrorhizon (Boraginaceae). It is biosynthetically derived from two key precursors, 4-hydroxybenzoate (4HB) and geranyldiphosphate (GPP). The bacterial ubiC gene, encoding chorismate pyruvate-lyase (CPL) which converts chorismate to 4-hydroxybenzoate, was expressed in L. erythrorhizon under the control of the strong (ocs)(3)mas-promoter. This introduced an efficient biosynthetic pathway to 4HB, i.e. a one-step reaction from chorismate, in addition to the endogeneous multi-step phenylpropanoid pathway. Feeding experiments with [1,7-(13)C(2)]shikimic acid showed that in the most active transgenic line, 73% of 4HB was synthesized via the genetically introduced pathway. However, there was no correlation between CPL activity and 4HB glucoside or shikonin accumulation in the transgenic lines. HMG-CoA reductase (HMGR) is involved in the biosynthesis of GPP in L. erythrorhizon. Two forms of HMGR1 of Arabidopsis thaliana were expressed in Lithospermum under control of the (ocs)(3)mas promoter. Only moderate increases in enzyme activity were obtained with the complete enzyme, but high activity was achieved using the soluble cytosolic domain of HMGR1. Shikonin accumulation remained unchanged even upon high expression of soluble HMGR.

8-dimethylallylnaringenin 2'-hydroxylase, the crucial cytochrome P450 mono-oxygenase for lavandulylated flavanone formation in Sophora flavescens cultured cells

8-dimethylallylnaringenin (8-DMAN) 2'-hydroxylase, which is indispensable for the formation of a lavandulylated flavanone, sophoraflavanone G, was detected in cell suspension cultures of Sophora flavescens. The enzyme catalyzes the 2'-hydroxylation of 8-DMAN to leachianone G, and is tightly bound to the membrane. It required NADPH and molecular oxygen as cofactors, and was inhibited by several cytochrome P450 inhibitors such as carbon monoxide and cytochrome c, indicating that the reaction is mediated by a cytochrome P450 monooxygenase. The optimum pH of 8-DMAN 2'-hydroxylase was 8.5, and the enzyme hydroxylated only 8-DMAN. Apparent Km values for 8-DMAN and NADPH of the enzyme were 55 and 34 microM, respectively.

A novel dark-inducible protein, LeDI-2, and its involvement in root-specific secondary metabolism in Lithospermum erythrorhizon

Lithospermum erythrorhizon produces red naphthoquinone pigments that are shikonin derivatives. They are accumulated exclusively in the roots of this plant. The biosynthesis of shikonin is strongly inhibited by light, even though other environmental conditions are optimized. Thus, L. erythrorhizon dark-inducible genes (LeDIs) were isolated to investigate the regulatory mechanism of shikonin biosynthesis. LeDI-2, showing the strict dark-specific expression, was further characterized by use of cell suspension cultures and hairy root cultures as model systems. Its mRNA accumulation showed a similar pattern with that of shikonin. In the intact plants LeDI-2 expression was observed solely in the root, and the longitudinal distribution of its mRNA was also in accordance to that of shikonin. LeDI-2 encoded a very hydrophobic polypeptide of 114 amino acids that shared significant similarities with some root-specific polypeptides such as ZRP3 (maize) and RcC3 (rice). Reduction of LeDI-2 expression by its antisense DNA in hairy roots of L. erythrorhizon decreased the shikonin accumulation, whereas other biosynthetic enzymes, e.g. p-hydroxybenzoic acid:geranyltransferase, which catalyzed a critical biosynthetic step, showed similar activity as the wild-type clone. This is the first report of the gene that is involved in production of secondary metabolites without affecting biosynthetic enzyme activities.

Cloning and expression of cDNAs encoding two enzymes of the MEP pathway in Catharanthus roseus

Two periwinkle cDNAs (crdxr and crmecs) encoding enzymes of the non-mevalonate terpenoid pathway were characterized using reverse transcription-PCR strategy based on the design of degenerated oligonucleotides. The deduced amino acid sequence of crdxr is homologue to 1-deoxy-D-xylulose 5-phosphate reductoisomerases. Crmecs represents the first plant cDNA encoding a protein similar to the 2C-methyl-D-erythritol 2,4-cyclodiphosphate synthase from Escherichia coli. Expression of crdxr and crmecs genes was up-regulated in periwinkle cells producing monoterpenoid indole alkaloids. Involvement of the 2C-methyl-D-erythritol 4-phosphate pathway in alkaloid biosynthesis is discussed.

Molecular cloning of geranyl diphosphate synthase and compartmentation of monoterpene synthesis in plant cells

The nature of isoprenoids synthesized in plants is primarily determined by the specificity of prenyltransferases. Several of these enzymes have been characterized at the molecular level. The compartmentation and molecular regulation of geranyl diphosphate (GPP), the carbon skeleton that is the backbone of myriad monoterpene constituents involved in plant defence, allelopathic interactions and pollination, is poorly understood. We describe here the cloning and functional expression of a GPP synthase (GPPS) from Arabidopsis thaliana. Immunohistological analyses of diverse non-secretory and secretory plant tissues reveal that GPPS and its congeners, monoterpene synthase, deoxy-xylulose phosphate synthase and geranylgeranyl diphosphate synthase, are equally compartmentalized and distributed in non-green plastids as well in chloroplasts of photosynthetic cells. This argues that monoterpene synthesis is not solely restricted to specialized secretory structures but can also occur in photosynthetic parenchyma. These data provide new information as to how monoterpene biosynthesis is compartmentalized and induced de novo in response to biotic and abiotic stress in diverse plants.

Genetic engineering on shikonin biosynthesis: expression of the bacterial ubiA gene in Lithospermum erythrorhizon

The naphthoquinone pigment shikonin from Lithospermum erythrorhizon Sieb. et Zucc. (Boraginaceae) was the first plant secondary metabolite produced in industrial scale from plant cell cultures. We have now manipulated the biosynthetic pathway leading to shikonin in L. erythrorhizon by introduction of the bacterial gene ubiA. This gene of Escherichia coli encodes 4-hydroxybenzoate-3-polyprenyltransferase, a membrane-bound enzyme that catalyzes a key step in ubiquinone biosynthesis. Using geranyl diphosphate (GPP) as substrate, it is able to catalyze the formation of 3-geranyl-4-hydroxybenzoate (GBA), a principal step of shikonin biosynthesis. The prokaryotic ubiA gene was fused to two signal sequences for targeting of the resulting peptide to the endoplasmic reticulum (ER). Constructs with different constitutive promoters were introduced into L. erythrorhizon using Agrobacterium rhizogenes-mediated transformation. In the resulting hairy root lines, high UbiA enzyme activities could be observed, reaching 133 pkat mg(-1). Expression of ubiA resulted in an accumulation of GBA in an amount exceeding that of the control culture by a factor of 50. However, the ubiA-transformed lines showed only a marginal (average 22%) increase of shikonin production in comparison to the control lines, and there was no significant correlation of UbiA enzyme activity and shikonin accumulation. This suggests that overexpression of ubiA alone is not sufficient to increase shikonin formation, and that further enzymes are involved in the regulation of this pathway.

The deoxyxylulose phosphate pathway of terpenoid biosynthesis in plants and microorganisms

Recent studies have uncovered the existence of an alternative, non-mevalonate pathway for the formation of isopentenyl pyrophosphate and dimethylallyl pyrophosphate, the two building blocks of terpene biosynthesis.