Elucidation and in planta reconstitution of the parthenolide biosynthetic pathway

Trends in parthenolide research over the past two decades: A bibliometric analysis

Parthenolide (PTL) is a new compound extracted from traditional Chinese medicine. In recent years, it has been proven to play an undeniable role in tumors, autoimmune diseases, and inflammatory diseases. Similarly, an increasing number of experiments have also confirmed the biological mechanism of PTL in these diseases. In order to better understand the development trend and potential hot spots of PTL in cancer and other diseases, we conducted a detailed bibliometric analysis. The purpose of presenting this bibliometric analysis was to highlight and inform researchers of the important research directions, co-occurrence relationships and research status in this field. Publications related to PTL research from 2002 to 2022 were extracted on the web of science core collection (WoSCC) platform. CiteSpace, VOSviewers and R package "bibliometrix" were applied to build relevant network diagrams. The bibliometric analysis was presented in terms of performance analysis (including publication statistics, top publishing countries, top publishing institutions, publishing journals and co-cited journals, authors and co-cited authors, co-cited references statistics, citation bursts statistics, keyword statistics and trend topic statistics) and science mapping (including citations by country, citations by institution, citations by journal, citations by author, co-citation analysis, and keyword co-occurrence). The detailed discussion of the results explained the focus and latest trends from the bibliometric analysis. Finally, the current status and shortcomings of the research field on PTLwere clearly pointed out for reference by scholars.

Identification and characterization of CYP71 subclade cytochrome P450 enzymes involved in the biosynthesis of bitterness compounds in Cichorium intybus

Industrial chicory (Cichorium intybus var. sativum) and witloof (C. intybus var. foliosum) are crops with an important economic value, mainly cultivated for inulin production and as a leafy vegetable, respectively. Both crops are rich in nutritionally relevant specialized metabolites with beneficial effects for human health. However, their bitter taste, caused by the sesquiterpene lactones (SLs) produced in leaves and taproot, limits wider applications in the food industry. Changing the bitterness would thus create new opportunities with a great economic impact. Known genes encoding enzymes involved in the SL biosynthetic pathway are GERMACRENE A SYNTHASE (GAS), GERMACRENE A OXIDASE (GAO), COSTUNOLIDE SYNTHASE (COS) and KAUNIOLIDE SYNTHASE (KLS). In this study, we integrated genome and transcriptome mining to further unravel SL biosynthesis. We found that C. intybus SL biosynthesis is controlled by the phytohormone methyl jasmonate (MeJA). Gene family annotation and MeJA inducibility enabled the pinpointing of candidate genes related with the SL biosynthetic pathway. We specifically focused on members of subclade CYP71 of the cytochrome P450 family. We verified the biochemical activity of 14 C. intybus CYP71 enzymes transiently produced in Nicotiana benthamiana and identified several functional paralogs for each of the GAO, COS and KLS genes, pointing to redundancy in and robustness of the SL biosynthetic pathway. Gene functionality was further analyzed using CRISPR/Cas9 genome editing in C. intybus. Metabolite profiling of mutant C. intybus lines demonstrated a successful reduction in SL metabolite production. Together, this study increases our insights into the C. intybus SL biosynthetic pathway and paves the way for the engineering of C. intybus bitterness.

Plant terpene specialized metabolism: complex networks or simple linear pathways?

From the perspectives of pathway evolution, discovery and engineering of plant specialized metabolism, the nature of the biosynthetic routes represents a critical aspect. Classical models depict biosynthesis typically from an end-point angle and as linear, for example, connecting central and specialized metabolism. As the number of functionally elucidated routes increased, the enzymatic foundation of complex plant chemistries became increasingly well understood. The perception of linear pathway models has been severely challenged. With a focus on plant terpenoid specialized metabolism, we review here illustrative examples supporting that plants have evolved complex networks driving chemical diversification. The completion of several diterpene, sesquiterpene and monoterpene routes shows complex formation of scaffolds and their subsequent functionalization. These networks show that branch points, including multiple sub-routes, mean that metabolic grids are the rule rather than the exception. This concept presents significant implications for biotechnological production.

Current Status and De Novo Synthesis of Anti-Tumor Alkaloids in Nicotiana

Alkaloids are the most diversified nitrogen-containing secondary metabolites, having antioxidant and antimicrobial properties, and are extensively used in pharmaceuticals to treat different types of cancer. Nicotiana serves as a reservoir of anti-cancer alkaloids and is also used as a model plant for the de novo synthesis of various anti-cancer molecules through genetic engineering. Up to 4% of the total dry weight of Nicotiana was found to be composed of alkaloids, where nicotine, nornicotine, anatabine, and anabasine are reported as the dominant alkaloids. Additionally, among the alkaloids present in Nicotiana, β-carboline (Harmane and Norharmane) and Kynurenines are found to show anti-tumor effects, especially in the cases of colon and breast cancers. Creating new or shunting of existing biosynthesis pathways in different species of Nicotiana resulted in de novo or increased synthesis of different anti-tumor molecules or their derivatives or precursors including Taxadiane (~22.5 µg/g), Artemisinin (~120 μg/g), Parthenolide (~2.05 ng/g), Costunolide (~60 ng/g), Etoposide (~1 mg/g), Crocin (~400 µg/g), Catharanthine (~60 ng/g), Tabersonine (~10 ng/g), Strictosidine (~0.23 mg/g), etc. Enriching the precursor pool, especially Dimethylallyl Diphosphate (DMAPP), down-regulating other bi-product pathways, compartmentalization or metabolic shunting, or organelle-specific reconstitution of the precursor pool, might trigger the enhanced accumulation of the targeted anti-cancer alkaloid in Nicotiana.

Lactucin Synthase Inactivation Boosts the Accumulation of Anti-inflammatory 8-Deoxylactucin and Its Derivatives in Chicory (Cichorium intybus L.)

For several sesquiterpene lactones (STLs) found in Asteraceae plants, very interesting biomedical activities have been demonstrated. Chicory roots accumulate the guaianolide STLs 8-deoxylactucin, lactucin, and lactucopicrin predominantly in oxalated forms in the latex. In this work, a supercritical fluid extract fraction of chicory STLs containing 8-deoxylactucin and 11β,13-dihydro-8-deoxylactucin was shown to have anti-inflammatory activity in an inflamed intestinal mucosa model. To increase the accumulation of these two compounds in chicory taproots, the lactucin synthase that takes 8-deoxylactucin as the substrate for the regiospecific hydroxylation to generate lactucin needs to be inactivated. Three candidate cytochrome P450 enzymes of the CYP71 clan were identified in chicory. Their targeted inactivation using the CRISPR/Cas9 approach identified CYP71DD33 to have lactucin synthase activity. The analysis of the terpene profile of the taproots of plants with edits in CYP71DD33 revealed a nearly complete elimination of the endogenous chicory STLs lactucin and lactucopicrin and their corresponding oxalates. Indeed, in the same lines, the interruption of biosynthesis resulted in a strong increase of 8-deoxylactucin and its derivatives. The enzyme activity of CYP71DD33 to convert 8-deoxylactucin to lactucin was additionally demonstrated in vitro using yeast microsome assays. The identified chicory lactucin synthase gene is predominantly expressed in the chicory latex, indicating that the late steps in the STL biosynthesis take place in the latex. This study contributes to further elucidation of the STL pathway in chicory and shows that root chicory can be positioned as a crop from which different health products can be extracted.

Identification and functional characterization of three cytochrome P450 genes for the abietane diterpenoid biosynthesis in Isodon lophanthoides

We identify two ferruginol synthases and a 11-hydroxyferruginol synthase from a traditional Chinese medicinal herb Isodon lophanthoides and propose their involvement in two independent abietane diterpenoids biosynthetic pathways. Isodon lophanthoides is a traditional Chinese medicinal herb rich in highly oxidized abietane-type diterpenoids. These compounds exhibit a wide range of pharmaceutical activities, yet the biosynthesis is barely known. Here, we describe the screening and functional characterization of P450s that oxidize the abietane skeleton abietatriene. We mainly focused on CYP76 family and identified 12 CYP76AHs by mining the RNA-seq data of I. lophanthoides. Among the 12 CYP76AHs, 6 exhibited similar transcriptional expression features as upstream diterpene synthases, including root or leaf-preferential expression pattern and highly MeJA inducibility. These six P450s were considered as first-tier candidates and functionally characterized in yeast and plant cells. In yeast assays showed that both CYP76AH42 and CYP76AH43 were ferruginol synthases hydroxylating the C12 position of abietatriene, whereas CYP76AH46 was characterized as a 11-hydroxyferruginol synthase which catalyzes two successive oxidations at C12 and C11 of abietatriene. Heterologous expression of three CYP76AHs in Nicotiana benthamiana resulted in the formation of ferruginol. qPCR analysis showed CYP76AH42 and CYP76AH43 were mainly expressed in the root, which was consistent with the distribution of ferruginol in the root periderms. CYP76AH46 was primarily expressed in the leaves where barely ferruginol or 11-hydroxyferruginol was detected. In addition to distinct organ-specific expression pattern, three CYP76AHs exhibited different genomic structures (w or w/o introns), low protein sequence identities (51-63%) and were placed in separate subclades in the phylogenetic tree. These results suggest that the identified CYP76AHs may be involved in at least two independent abietane biosynthetic pathways in the aerial and underground parts of I. lophanthoides.

Biosynthesis of anticancer phytochemical compounds and their chemistry

Cancer is a severe health issue, and cancer cases are rising yearly. New anticancer drugs have been developed as our understanding of the molecular mechanisms behind diverse solid tumors, and metastatic malignancies have increased. Plant-derived phytochemical compounds target different oncogenes, tumor suppressor genes, protein channels, immune cells, protein channels, and pumps, which have attracted much attention for treating cancer in preclinical studies. Despite the anticancer capabilities of these phytochemical compounds, systemic toxicity, medication resistance, and limited absorption remain more significant obstacles in clinical trials. Therefore, drug combinations of new phytochemical compounds, phytonanomedicine, semi-synthetic, and synthetic analogs should be considered to supplement the existing cancer therapies. It is also crucial to consider different strategies for increased production of phytochemical bioactive substances. The primary goal of this review is to highlight several bioactive anticancer phytochemical compounds found in plants, preclinical research, their synthetic and semi-synthetic analogs, and clinical trials. Additionally, biotechnological and metabolic engineering strategies are explored to enhance the production of bioactive phytochemical compounds. Ligands and their interactions with their putative targets are also explored through molecular docking studies. Therefore, emphasis is given to gathering comprehensive data regarding modern biotechnology, metabolic engineering, molecular biology, and in silico tools.

Individual lipid transfer proteins from Tanacetum parthenium show different specificity for extracellular accumulation of sesquiterpenes

A highly specialized function for individual LTPs for different products from the same terpenoid biosynthesis pathway is described and the function of an LTP GPI anchor is studied. Sequiterpenes produced in glandular trichomes of the medicinal plant Tanacetum parthenium (feverfew) accumulate in the subcuticular extracellular space. Transport of these compounds over the plasma membrane is presumably by specialized membrane transporters, but it is still not clear how these hydrophobic compounds are subsequently transported over the hydrophilic cell wall. Here we identified eight so-called non-specific Lipid transfer proteins (nsLTPs) genes that are expressed in feverfew trichomes. A putative function of these eight nsLTPs in transport of the lipophilic sesquiterpene lactones produced in feverfew trichomes, was tested in an in-planta transport assay using transient expression in Nicotiana benthamiana. Of eight feverfew nsLTP candidate genes analyzed, two (TpLTP1 and TpLTP2) can specifically improve extracellular accumulation of the sesquiterpene costunolide, while one nsLTP (TpLTP3) shows high specificity towards export of parthenolide. The specificity of the nsLTPs was also tested in an assay that test for the exclusion capacity of the nsLTP for influx of extracellular substrates. In such assay, TpLTP3 was identified as most effective in blocking influx of both costunolide and parthenolide, when these substrates are infiltrated into the apoplast. The TpLTP3 is special in having a GPI-anchor domain, which is essential for the export activity of TpLTP3. However, addition of the TpLTP3 GPI-anchor domain to TpLTP1 resulted in loss of TpLTP1 export activity. These novel export and exclusion assays thus provide new means to test functionality of plant nsLTPs.

An Integrative Transcriptional Network Revealed Spatial Molecular Interplay Underlying Alantolactone and Inulin Biosynthesis in Inula racemosa Hook f

Inula racemosa Hook. f. (Pushkarmula), a perennial Himalayan herb known for its aromatic and phytopharmaceutical attributes, is not yet explored at genomic/transcriptomic scale. In this study, efforts were made to unveil the global transcriptional atlas underlying organ-specific specialized metabolite biosynthesis by integrating RNA-Seq analysis of 433 million sequenced reads with the phytochemical analysis of leaf, stem, and root tissues. Overall, 7242 of 83,772 assembled nonredundant unigenes were identified exhibiting spatial expression in leaf (3761), root (2748), and stem (733). Subsequently, integration of the predicted transcriptional interactome network of 2541 unigenes (71,841 edges) with gene ontology and KEGG pathway enrichment analysis revealed isoprenoid, terpenoid, diterpenoid, and gibberellin biosynthesis with antimicrobial activities in root tissue. Interestingly, the root-specific expression of germacrene-mediated alantolactone biosynthesis (GAS, GAO, G8H, IPP, DMAP, and KAO) and antimicrobial activities (BZR1, DEFL, LTP) well-supported with both quantitative expression profiling and phytochemical accumulation of alantolactones (726.08 μg/10 mg) and isoalantolactones (988.59 μg/10 mg), which suggests “roots” as the site of alantolactone biosynthesis. A significant interaction of leaf-specific carbohydrate metabolism with root-specific inulin biosynthesis indicates source (leaf) to sink (root) regulation of inulin. Our findings comprehensively demonstrate the source-sink transcriptional regulation of alantolactone and inulin biosynthesis, which can be further extended for upscaling the targeted specialized metabolites. Nevertheless, the genomic resource created in this study can also be utilized for development of genome-wide functionally relevant molecular markers to expedite the breeding strategies for genetic improvement of I. racemosa.

Reconstitution of monoterpene indole alkaloid biosynthesis in genome engineered Nicotiana benthamiana

Monoterpene indole alkaloids (MIAs) are a diverse class of plant natural products that include a number of medicinally important compounds. We set out to reconstitute the pathway for strictosidine, a key intermediate of all MIAs, from central metabolism in Nicotiana benthamiana. A disadvantage of this host is that its rich background metabolism results in the derivatization of some heterologously produced molecules. Here we use transcriptomic analysis to identify glycosyltransferases that are upregulated in response to biosynthetic intermediates and produce plant lines with targeted mutations in the genes encoding them. Expression of the early MIA pathway in these lines produces a more favorable product profile. Strictosidine biosynthesis was successfully reconstituted, with the best yields obtained by the co-expression of 14 enzymes, of which a major latex protein-like enzyme (MLPL) from Nepeta (catmint) is critical for improving flux through the iridoid pathway. The removal of endogenous glycosyltransferases does not impact the yields of strictosidine, highlighting that the metabolic flux of the pathway enzymes to a stable biosynthetic intermediate minimizes the need to engineer the endogenous metabolism of the host. The production of strictosidine in planta expands the range of MIA products amenable to biological synthesis.

The biosynthesis of strictosidine, a key intermediate of monoterpene indole alkaloids, was successfully reconstructed in Nicotiana benthamiana, demonstrating the potential of Nicotiana benthamiana as a bioproduction chassis for small molecules.

Production of Plant Sesquiterpene Lactone Parthenolide in the Yeast Cell Factory

Parthenolide, a kind of sesquiterpene lactone, is the direct precursor for the promising anti-glioblastoma drug ACT001. Compared with traditional parthenolide source from plant extraction, de novo biosynthesis of parthenolide in microorganisms has the potential to make a sustainable supply. Herein, an integrated strategy was designed with P450 source screening, nicotinamide adenine dinucleotide phosphate (NADPH) supply, and endoplasmic reticulum (ER) size rewiring to manipulate three P450s regarded as the bottleneck for parthenolide production. Germacrene A oxidase from Cichorium intybus, costunolide synthase from Lactuca sativa, and parthenolide synthase from Tanacetum parthenium have the best efficiency, resulting in a parthenolide titer of 2.19 mg/L, which was first achieved in yeast. The parthenolide titer was further increased by 300% with NADPH supplementation and ER expanding stepwise. Finally, the highest titers of 31.0 mg/L parthenolide and 648.5 mg/L costunolide in microbes were achieved in 2.0 L fed-batch fermentation. This study not only provides an alternative microbial platform for producing sesquiterpene lactones in a sustainable way but also highlights a general strategy for manipulating multiple plant-derived P450s in microbes.

Cloning and Functional Characterization of Two Germacrene A Oxidases Isolated from Xanthium sibiricum

Sesquiterpene lactones (STLs) from the cocklebur Xanthium sibiricum exhibit significant anti-tumor activity. Although germacrene A oxidase (GAO), which catalyzes the production of Germacrene A acid (GAA) from germacrene A, an important precursor of germacrene-type STLs, has been reported, the remaining GAOs corresponding to various STLs’ biosynthesis pathways remain unidentified. In this study, 68,199 unigenes were studied in a de novo transcriptome assembly of X. sibiricum fruits. By comparison with previously published GAO sequences, two candidate X. sibiricum GAO gene sequences, XsGAO1 (1467 bp) and XsGAO2 (1527 bp), were identified, cloned, and predicted to encode 488 and 508 amino acids, respectively. Their protein structure, motifs, sequence similarity, and phylogenetic position were similar to those of other GAO proteins. They were most strongly expressed in fruits, according to a quantitative real-time polymerase chain reaction (qRT-PCR), and both XsGAO proteins were localized in the mitochondria of tobacco leaf epidermal cells. The two XsGAO genes were cloned into the expression vector for eukaryotic expression in Saccharomyces cerevisiae, and the enzyme reaction products were detected by gas chromatography–mass spectrometry (GC-MS) and liquid chromatography–mass spectrometry (LC-MS) methods. The results indicated that both XsGAO1 and XsGAO2 catalyzed the two-step conversion of germacrene A (GA) to GAA, meaning they are unlike classical GAO enzymes, which catalyze a three-step conversion of GA to GAA. This cloning and functional study of two GAO genes from X. sibiricum provides a useful basis for further elucidation of the STL biosynthesis pathway in X. sibiricum.

CRISPR/Cas9 targeted inactivation of the kauniolide synthase in chicory results in accumulation of costunolide and its conjugates in taproots

Chicory taproots accumulate sesquiterpene lactones lactucin, lactucopicrin, and 8-deoxylactucin, predominantly in their oxalated forms. The biosynthetic pathway for chicory sesquiterpene lactones has only partly been elucidated; the enzymes that convert farnesyl pyrophosphate to costunolide have been described. The next biosynthetic step of the conversion of costunolide to the tricyclic structure, guaianolide kauniolide, has so far not been elucidated in chicory. In this work three putative kauniolide synthase genes were identified in chicory named CiKLS1, CiKLS2, and CiKLS3. Their activity to convert costunolide to kauniolide was demonstrated in vitro using yeast microsome assays. Next, introduction of CRISPR/Cas9 reagents into chicory protoplasts was used to inactivate multiple chicory KLS genes and several chicory lines were successfully regenerated. The inactivation of the kauniolide synthase genes in chicory by the CRISPR/Cas9 approach resulted in interruption of the sesquiterpene lactone biosynthesis in chicory leaves and taproots. In chicory taproots, but not in leaves, accumulation of costunolide and its conjugates was observed to high levels, namely 1.5 mg/g FW. These results confirmed that all three genes contribute to STL accumulation, albeit to different extent. These observations demonstrate that three genes oriented in tandem on the chicory genome encode kauniolide synthases that initiate the conversion of costunolide toward the sesquiterpene lactones in chicory.

Combinatorial transient gene expression strategies to enhance terpenoid production in plants

INTRODUCTION

The monoterpenoid linalool and sesquiterpenoid costunolide are ubiquitous plant components that have been economically exploited for their respective essential oils and pharmaceutical benefits. In general, monoterpenes and sesquiterpenes are produced by the plastid 2-C-methyl-D-erythritol 4-phosphate (MEP) and cytosolic mevalonate (MVA) pathways, respectively. Herein, we investigated the individual and combinatorial potential of MEP and MVA pathway genes in increasing linalool and costunolide production in Nicotiana benthamiana.

METHODS

First, six genes from the MEP (1-deoxy-D-xylulose-5-phosphate synthase, 1-deoxy-D-xylulose 5-phosphate reductoisomerase, 4-diphosphocytidyl-2-C-methyl-D-erythritol kinase, geranyl pyrophosphate synthase, and linalool synthase) and MVA (acetoacetyl-CoA-thiolase, hydroxy-3-methylglutaryl-CoA reductase, farnesyl pyrophosphate synthase, germacrene A synthase, germacrene A oxidase, and costunolide synthase) pathways were separately cloned into the modular cloning (MoClo) golden gateway cassette. Second, the cassettes were transformed individually or in combination into the leaves of N. benthamiana by agroinfiltration.

RESULTS AND DISCUSSION

Five days post infiltration (DPI), all selected genes were transiently 5- to 94-fold overexpressed. Quantification using gas chromatography-Q-orbitrap-mass spectrometry (GC-Q-Orbitrap-MS) determined that the individual and combinatorial expression of MEP genes increased linalool production up to 50-90ng.mg^-1 fresh leaf weight. Likewise, MVA genes increased costunolide production up to 70-90ng.mg^-1 fresh leaf weight. Our findings highlight that the transient expression of MEP and MVA pathway genes (individually or in combination) enhances linalool and costunolide production in plants.

Metabolic Perturbation and Synthetic Biology Strategies for Plant Terpenoid Production-An Updated Overview

Terpenoids represent one of the high-value groups of specialized metabolites with vast structural diversity. They exhibit versatile human benefits and have been successfully exploited in several sectors of day-to-day life applications, including cosmetics, foods, and pharmaceuticals. Historically, the potential use of terpenoids is challenging, and highly hampered by their bioavailability in their natural sources. Significant progress has been made in recent years to overcome such challenges by advancing the heterologous production platforms of hosts and metabolic engineering technologies. Herein, we summarize the latest developments associated with analytical platforms, metabolic engineering, and synthetic biology, with a focus on two terpenoid classes: monoterpenoids and sesquiterpenoids. Accumulated data showed that subcellular localization of both the precursor pool and the introduced enzymes were the crucial factors for increasing the production of targeted terpenoids in plants. We believe this timely review provides a glimpse of current state-of-the-art techniques/methodologies related to terpenoid engineering that would facilitate further improvements in terpenoids research.

Recent advances and new insights in biosynthesis of dendrobine and sesquiterpenes

Sesquiterpenes are one of the most diverse groups of secondary metabolites that have mainly been observed in terpenoids. It is a natural terpene containing 15 carbon atoms in the molecule and three isoprene units with chain, ring, and other skeleton structures. Sesquiterpenes have been shown to display multiple biological activities such as anti-inflammatory, anti-feedant, anti-microbial, anti-tumor, anti-malarial, and immunomodulatory properties; therefore, their therapeutic effects are essential. In order to overcome the problem of low-yielding sesquiterpene content in natural plants, regulating their biosynthetic pathways has become the focus of many researchers. In plant and microbial systems, many genetic engineering strategies have been used to elucidate biosynthetic pathways and high-level production of sesquiterpenes. Here, we will introduce the research progress and prospects of the biosynthesis of artemisinin, costunolide, parthenolide, and dendrobine. Furthermore, we explore the biosynthesis of dendrobine by evaluating whether the biosynthetic strategies of these sesquiterpene compounds can be applied to the formation of dendrobine and its intermediate compounds. KEY POINTS: • The development of synthetic biology has promoted the study of terpenoid metabolism and provided an engineering platform for the production of high-value terpenoid products. • Some possible intermediate compounds of dendrobine were screened out and the possible pathway of dendrobine biosynthesis was speculated. • The possible methods of dendrobine biosynthesis were explored and speculated.

Plant-Derived Natural Products in Cancer Research: Extraction, Mechanism of Action, and Drug Formulation

Cancer is one of the main causes of death globally and considered as a major challenge for the public health system. The high toxicity and the lack of selectivity of conventional anticancer therapies make the search for alternative treatments a priority. In this review, we describe the main plant-derived natural products used as anticancer agents. Natural sources, extraction methods, anticancer mechanisms, clinical studies, and pharmaceutical formulation are discussed in this review. Studies covered by this review should provide a solid foundation for researchers and physicians to enhance basic and clinical research on developing alternative anticancer therapies.

CYP71BL9, the missing link in costunolide synthesis of sunflower

Despite intensive research in recent years, the biosynthetic route to costunolide in sunflower so far remained obscured. Additional P450 sequences from public sunflower transcriptomic database were screened to search for candidate enzymes which are able to introduce the 6α-hydroxy-group required for the esterification with the carboxy group of germacarane A acid, the final step in costunolide formation. CYP71BL9, a new P450 enzyme from sunflower was shown to catalyze this hydroxylation, hence being identified as HaCOS. Phylogentically, HaCOS is closer related to HaG8H than to any other known costunolide synthase in Asteraceae.The enzyme was successfully employed to reconstruct the sunflower biosynthesis of costunolide in transformed tobacco. Contrary, in yeast, only minor amounts of sesquiterpene lactone was produced, while 5-hydroxyfarnesylic acid was formed instead. HaCOS in combination with HaG8H produced 8β-hydroxycostunolide (eupatolide) in transformed plants, thus indicating that sunflower possesses two independent modes of eupatolide synthesis via HaCOS and via HaES. The lack of HaCOS expression and of costunolide in trichomes suggests that the enzyme triggers the costunolied synthesis of the inner tissues of sunflower and might be linked to growth regulation processes.

Metabonomic study of the intervention effects of Parthenolide on anti-thyroid cancer activity

Thyroid cancer is the most common endocrine malignant tumor in the world, and its incidence is increasing. Although the mortality rate of thyroid cancer is low, its persistence/recurrence rate is high. In addition, some patients with thyroid cancer fail to respond to radiation. Therefore, it is urgent need to develop a novel treatment for thyroid cancer. Parthenolide (PTL), a traditional Chinese medicine Tanacetum parthenium extract, has shown encouraging effects in anti-tumor, anti-inflammatory and anti-malaria. However, it is unclear whether PTL has an anti-thyroid cancer effect and its possible mechanism of action. In the recent years, metabonomics has been widely used in tumors research to explore the pharmacological mechanism of drugs, but few studies used metabonomics to investigate the pharmacological effects of PTL in thyroid tumors. In order to comprehensively reveal the mechanism and effects of PTL on anti-thyroid tumors, metabonomics combined cell biological research methods were conducted. The results showed that PTL promote apoptosis of thyroid cancer cells (TPC-1) in a concentration-dependent manner. The metabolic differences between the PTL group and the control group were compared by metabonomics, and 31 potential metabolites were identified. These metabolites were mainly involved in the tricarboxylic acid cycle, amino acid metabolism, choline metabolism and lipid metabolism. These results implied that PTL may inhibit the proliferation and development of thyroid carcinoma by accelerating oxidation emergency response, inhibiting adenosine triphosphate (ATP) synthesis and metabolic imbalance. The results of this study revealed that PTL can be an effective and potential drug for the treatment of thyroid cancer.

Beyond natural: synthetic expansions of botanical form and function

Powered by developments that enabled genome-scale investigations, systems biology emerged as a field aiming to understand how phenotypes emerge from network functions. These advances fuelled a new engineering discipline focussed on synthetic reconstructions of complex biological systems with the goal of predictable rational design and control. Initially, progress in the nascent field of synthetic biology was slow due to the ad hoc nature of molecular biology methods such as cloning. The application of engineering principles such as standardisation, together with several key technical advances, enabled a revolution in the speed and accuracy of genetic manipulation. Combined with mathematical and statistical modelling, this has improved the predictability of engineering biological systems of which nonlinearity and stochasticity are intrinsic features leading to remarkable achievements in biotechnology as well as novel insights into biological function. In the past decade, there has been slow but steady progress in establishing foundations for synthetic biology in plant systems. Recently, this has enabled model-informed rational design to be successfully applied to the engineering of plant gene regulation and metabolism. Synthetic biology is now poised to transform the potential of plant biotechnology. However, reaching full potential will require conscious adjustments to the skillsets and mind sets of plant scientists.

Traps and Pitfalls-Unspecific Reactions in Metabolic Engineering of Sesquiterpenoid Pathways

The characterization of plant enzymes by expression in prokaryotic and eukaryotic (yeast and plants) heterologous hosts has widely been used in recent decades to elucidate metabolic pathways in plant secondary metabolism. Yeast and plant systems provide the cellular environment of a eukaryotic cell and the subcellular compartmentalization necessary to facilitate enzyme function. The expression of candidate genes in these cell systems and the identification of the resulting products guide the way for the identification of enzymes with new functions. However, in many cases, the detected compounds are not the direct enzyme products but are caused by unspecific subsequent reactions. Even if the mechanisms for these unspecific reactions are in many cases widely reported, there is a lack of overview of potential reactions that may occur to provide a guideline for researchers working on the characterization of new enzymes. Here, an across-the-board summary of rearrangement reactions of sesquiterpenes in metabolic pathway engineering is presented. The different kinds of unspecific reactions as well as their chemical and cellular background are explained and strategies how to spot and how to avoid these unspecific reactions are given. Also, a systematic approach of classification of unspecific reactions is introduced. It is hoped that this mini-review will stimulate a discussion on how to systematically classify unspecific reactions in metabolic engineering and to expand this approach to other classes of plant secondary metabolites.

Advances in chemistry and bioactivity of parthenolide

(−)-Parthenolide is a germacrane sesquiterpene lactone, available in ample amounts from the traditional medical plant feverfew (Tanacetum parthenium).

CYP76 Oxidation Network of Abietane Diterpenes in Lamiaceae Reconstituted in Yeast

Rosemary and sage species from Lamiaceae contain high amounts of structurally related but diverse abietane diterpenes. A number of substances from this compound family have potential pharmacological activities and are used in the food and cosmetic industry. This has raised interest in their biosynthesis. Investigations in Rosmarinus officinalis and some sage species have uncovered two main groups of cytochrome P450 oxygenases that are involved in the oxidation of the precursor abietatriene. CYP76AHs produce ferruginol and 11-hydroxyferruginol, while CYP76AKs catalyze oxidations at the C20 position. Using a modular Golden-Gate-compatible assembly system for yeast expression, these enzymes were systematically tested either alone or in combination. A total of 14 abietane diterpenes could be detected, 8 of which have not been reported thus far. We demonstrate here that yeast is a valid system for engineering and reconstituting the abietane diterpene network, allowing for the discovery of novel compounds with potential bioactivity.

Metabolomics responses of Bambusa pervariabilis × Dendrocalamopsis grandis varieties to Biotic (pathogenic fungus) stress

Bambusa pervariabilis × Dendrocalamopsis grandis blight, caused by Arthrinium phaeospermum, is one of the most common and serious diseases in bamboo and occurs in the newly born twigs. Bamboo has suffered large dead areas, including more than 3000 hm², which greatly threatens the process of returning farmlands to forests and the construction of ecological barriers. To identify differential metabolites and metabolic pathways associated with B. pervariabilis × D. grandis to A. phaeospermum, ultra-performance liquid chromatography (UPLC) and quadrupole-time of flight (Q-TOF) Mass Spectrometry (MS) combined with a data-dependent acquisition method was used to analyse the entire sample spectrum. In total, 13223 positive ion peaks and 10616 negative ion peaks were extracted. OPLS-DA and several other analyses were performed using the original data. The OPLS-DA models showed good quality and had strong predictive power, indicating clear trends in the analyses of the treatment and control groups. Clustering and KEGG pathway analyses were used to screen the differential metabolites in the treatment and control groups from the three B. pervariabilis × D. grandis varieties and reflected their metabolic responses induced by A. phaeospermum infection. The results showed that the three B. pervariabilis × D. grandis varieties mode showed significant changes in the following six resistance-related metabolites after A. phaeospermum invasion in positive and negative ion modes: proline, glutamine, dictamnine, apigenin 7-O-neohesperidoside, glutamate, and cis-Aconitate. The following four main metabolic pathways are involved: Arginine and proline metabolism, Glyoxylate and dicarboxylate metabolism, Biosynthesis of alkaloids derived from shikimate pathway, and Flavone and flavonol biosynthesis. This study lays a foundation for the later detection of differential metabolites and metabolic pathways for targeting, and provides a theoretical basis for disease-resistant breeding and the control of B. pervariabilis × D. grandis blight.

Characterization of CYP71AX36 from Sunflower (Helianthus annuus L., Asteraceae)

Sesquiterpene lactones (STL) are a subclass of isoprenoids with many known bioactivities frequently found in the Asteraceae family. In recent years, remarkable progress has been made regarding the biochemistry of STL, and today the biosynthetic pathway of the core backbones of many STLs has been elucidated. Consequently, the focus has shifted to the discovery of the decorating enzymes that can modify the core skeleton with functional hydroxy groups. Using in vivo pathway reconstruction assays in heterologous organisms such as Saccharomyces cerevisiae and Nicotiana benthamiana, we have analyzed several cytochrome P450 enzyme genes of the CYP71AX subfamily from Helianthus annuus clustered in close proximity to one another on the sunflower genome. We show that one member of this subfamily, CYP71AX36, can catalyze the conversion of costunolide to 14-hydroxycostunolide. The catalytic activity of CYP71AX36 may be of use for the chemoenzymatic production of antileukemic 14-hydroxycostunolide derivatives and other STLs of pharmaceutical interest. We also describe the full 2D-NMR assignment of 14-hydroxycostunolide and provide all 13C chemical shifts of the carbon skeleton for the first time.

The cytochrome P450 CYP72A552 is key to production of hederagenin-based saponins that mediate plant defense against herbivores

Plants continuously evolve new defense compounds. One class of such compounds is triterpenoid saponins. A few species in the Barbarea genus produce saponins as the only ones in the large crucifer family. However, the molecular mechanism behind saponin biosynthesis and their role in plant defense remains unclear. We used pathway reconstitution in planta, enzymatic production of saponins in vitro, insect feeding assays, and bioinformatics to identify a missing gene involved in saponin biosynthesis and saponin-based herbivore defense. A tandem repeat of eight CYP72A cytochromes P450 colocalise with a quantitative trait locus (QTL) for saponin accumulation and flea beetle resistance in Barbarea vulgaris. We found that CYP72A552 oxidises oleanolic acid at position C-23 to hederagenin. In vitro-produced hederagenin monoglucosides reduced larval feeding by up to 90% and caused 75% larval mortality of the major crucifer pest diamondback moth and the tobacco hornworm. Sequence analysis indicated that CYP72A552 evolved through gene duplication and has been under strong selection pressure. In conclusion, CYP72A552 has evolved to catalyse the formation of hederagenin-based saponins that mediate plant defense against herbivores. Our study highlights the evolution of chemical novelties by gene duplication and selection for enzyme innovations, and the importance of chemical modification in plant defense evolution.

Functional characterization of the cytochrome P450 monooxygenase CYP71AU87 indicates a role in marrubiin biosynthesis in the medicinal plant Marrubium vulgare

BACKGROUND

Horehound (Marrubium vulgare) is a medicinal plant whose signature bioactive compounds, marrubiin and related furanoid diterpenoid lactones, have potential applications for the treatment of cardiovascular diseases and type II diabetes. Lack of scalable plant cultivation and the complex metabolite profile of M. vulgare limit access to marrubiin via extraction from plant biomass. Knowledge of the marrubiin-biosynthetic enzymes can enable the development of metabolic engineering platforms for marrubiin production. We previously identified two diterpene synthases, MvCPS1 and MvELS, that act sequentially to form 9,13-epoxy-labd-14-ene. Conversion of 9,13-epoxy-labd-14-ene by cytochrome P450 monooxygenase (P450) enzymes can be hypothesized to facilitate key functional modification reactions in the formation of marrubiin and related compounds.

RESULTS

Mining a M. vulgare leaf transcriptome database identified 95 full-length P450 candidates. Cloning and functional analysis of select P450 candidates showing high transcript abundance revealed a member of the CYP71 family, CYP71AU87, that catalyzed the hydroxylation of 9,13-epoxy-labd-14-ene to yield two isomeric products, 9,13-epoxy labd-14-ene-18-ol and 9,13-epoxy labd-14-ene-19-ol, as verified by GC-MS and NMR analysis. Additional transient Nicotiana benthamiana co-expression assays of CYP71AU87 with different diterpene synthase pairs suggested that CYP71AU87 is specific to the sequential MvCPS1 and MvELS product 9,13-epoxy-labd-14-ene. Although the P450 products were not detectable in planta, high levels of CYP71AU87 gene expression in marrubiin-accumulating tissues supported a role in the formation of marrubiin and related diterpenoids in M. vulgare.

CONCLUSIONS

In a sequential reaction with the diterpene synthase pair MvCPS1 and MvELS, CYP71AU87 forms the isomeric products 9,13-epoxy labd-14-ene-18/19-ol as probable intermediates in marrubiin biosynthesis. Although its metabolic relevance in planta will necessitate further genetic studies, identification of the CYP71AU87 catalytic activity expands our knowledge of the functional landscape of plant P450 enzymes involved in specialized diterpenoid metabolism and can provide a resource for the formulation of marrubiin and related bioactive natural products.

Substrate promiscuity of enzymes from the sesquiterpene biosynthetic pathways from Artemisia annua and Tanacetum parthenium allows for novel combinatorial sesquiterpene production

The therapeutic properties of complex terpenes often depend on the stereochemistry of their functional groups. However, stereospecific chemical synthesis of terpenes is challenging. To overcome this challenge, metabolic engineering can be employed using enzymes with suitable stereospecific catalytic activity. Here we used a combinatorial metabolic engineering approach to explore the stereospecific modification activity of the Artemisia annua artemisinic aldehyde ∆11(13) double bond reductase2 (AaDBR2) on products of the feverfew sesquiterpene biosynthesis pathway (GAS, GAO, COS and PTS). This allowed us to produce dihydrocostunolide and dihydroparthenolide. For dihydroparthenolide we demonstrate that the preferred order of biosynthesis of dihydroparthenolide is by reduction of the exocyclic methylene of parthenolide, rather than through C4-C5 epoxidation of dihydrocostunolide. Moreover, we demonstrate a promiscuous activity of feverfew CYP71CB1 on dihydrocostunolide and dihydroparthenolide for the production of 3β-hydroxy-dihydrocostunolide and 3β-hydroxy-dihydroparthenolide, respectively. Combined, these results offer new opportunities for engineering novel sesquiterpene lactones with potentially improved medicinal value.

Non-volatile natural products in plant glandular trichomes: chemistry, biological activities and biosynthesis

The investigation methods, chemistry, bioactivities, and biosynthesis of non-volatile natural products involving 489 compounds in plant glandular trichomes are reviewed.

Comparative transcriptomics between high and low rubber producing Taraxacum kok-saghyz R. plants

Background

Taraxacum kok-saghyz R. (Tks) is a promising alternative species to Hevea brasiliensis for production of high quality natural rubber (NR). A comparative transcriptome analysis of plants with differential production of NR will contribute to elucidate which genes are involved in the synthesis, regulation and accumulation of this natural polymer and could help to develop Tks into a rubber crop.

Results

We measured rubber content in the latex of 90 individual Tks plants from 9 accessions, observing a high degree of variability. We carried out de novo root transcriptome sequencing, assembly, annotation and comparison of gene expression of plants with the lower (LR plants) and the higher rubber content (HR plants). The transcriptome analysis also included one plant that did not expel latex, in principle depleted of latex transcripts. Moreover, the transcription of some genes well known to play a major role in rubber biosynthesis, was probed by qRT-PCR. Our analysis showed a high modulation of genes involved in the synthesis of NR between LR and HR plants, and evidenced that genes involved in sesquiterpenoids, monoterpenoids and phenylpropanoid biosynthesis are upregulated in LR plants.

Conclusions

Our results show that a higher amount of rubber in the latex in HR plants is positively correlated with high expression levels of a number of genes directly involved in rubber synthesis showing that NR production is highly controlled at transcriptional level. On the other hand, lower amounts of rubber in LR plants is related with higher expression of genes involved in the synthesis of other secondary metabolites that, we hypothesize, may compete towards NR biosynthesis. This dataset represents a fundamental genomic resource for the study of Tks and the comprehension of the synthesis of NR and other biochemically and pharmacologically relevant compounds in the Taraxacum genus.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-5287-4) contains supplementary material, which is available to authorized users.

Kauniolide synthase is a P450 with unusual hydroxylation and cyclization-elimination activity

Guaianolides are an important class of sesquiterpene lactones with unique biological and pharmaceutical properties. They have been postulated to be derived from germacranolides, but for years no progress has been made in the elucidation of their biosynthesis that requires an unknown cyclization mechanism. Here we demonstrate the isolation and characterization of a cytochrome P450 from feverfew (Tanacetum parthenium), kauniolide synthase. Kauniolide synthase catalyses the formation of the guaianolide kauniolide from the germacranolide substrate costunolide. Unlike most cytochrome P450s, kauniolide synthase combines stereoselective hydroxylation of costunolide at the C3 position, with water elimination, cyclization and regioselective deprotonation. This unique mechanism of action is supported by in silico modelling and docking experiments. The full kauniolide biosynthesis pathway is reconstructed in the heterologous hosts Nicotiana benthamiana and yeast, paving the way for biotechnological production of guaianolide-type sesquiterpene lactones.

Engineering terpenoid production through transient expression in Nicotiana benthamiana

Terpenoids are the most structurally diverse class of plant natural products with a huge range of commercial and medical applications. Exploiting this enormous potential has historically been hindered due to low levels of these compounds in their natural sources, making isolation difficult, while their structural complexity frequently makes synthetic chemistry approaches uneconomical. Engineering terpenoid biosynthesis in heterologous host production platforms provides a means to overcome these obstacles. In particular, plant-based production systems are attractive as they provide the compartmentalisation and cofactors necessary for the transfer of functional pathways from other plants. Nicotiana benthamiana, a wild relative of tobacco, has become increasingly popular as a heterologous expression platform for reconstituting plant natural product pathways, because it is amenable to Agrobacterium-mediated transient expression, a scalable and highly flexible process that enables rapid expression of genes and enzymes from other plant species. Here, we review recent work describing terpene production in N. benthamiana. We examine various strategies taken to engineer this host for increased production of the target metabolite. We also look at how transient expression can be utilised for rapid generation of molecular diversity, including new-to-nature products. Finally, we highlight current issues surrounding this expression platform and discuss the future directions and developments which will be needed to fully realise the potential of this system.

Biosynthesis of Eupatolide-A Metabolic Route for Sesquiterpene Lactone Formation Involving the P450 Enzyme CYP71DD6

Sesquiterpene lactones are a class of natural compounds well-known for their bioactivity and are characteristic for the Asteraceae family. Most sesquiterpene lactones are considered derivatives of germacrene A acid (GAA). GAA can be stereospecifically hydroxylated by the cytochrome P450 enzymes (CYP) Lactuca sativa costunolide synthase CYP71BL2 (LsCOS) and Helianthus annuus GAA 8β-hydroxylase CYP71BL1 (HaG8H) at C6 (in α-orientation) or C8 (in β-orientation), respectively. Spontaneous subsequent lactonization of the resulting 6α-hydroxy-GAA leads to costunolide, whereas 8β-hydroxy-GAA has not yet been reported to cyclize to a sesquiterpene lactone. Sunflower and related species of the Heliantheae tribe contain sesquiterpene lactones mainly derived from inunolide (7,8-cis lactone) and eupatolide (8β-hydroxy-costunolide) precursors. However, the mechanism of 7,8-cis lactonization in general, and the 6,7-trans lactone formation in the sunflower tribe, remain elusive. Here, we show that, in plant cells, heterologous expression of CYP71BL1 leads to the formation of inunolide. Using a phylogenetic analysis of enzymes from the CYP71 family involved in sesquiterpenoid metabolism, we identified the CYP71DD6 gene, which was able to catalyze the 6,7-trans lactonization in sunflowers, using as a substrate 8β-hydroxy-GAA. Consequently, CYP71DD6 resulted in the synthesis of eupatolide, thus called HaES ( Helianthus annuus eupatolide synthase). Thus, our study shows the entry point for the biosynthesis of two distinct types of sesquiterpene lactones in sunflowers: the 6,7-trans lactones derived from eupatolide and the 7,8-cis lactones derived from inunolide. The implications for tissue-specific localization, based on expression studies, are discussed.

Metabolic responses of Eucalyptus species to different temperature regimes

Species and hybrids of Eucalyptus are the world's most widely planted hardwood trees. They are cultivated across a wide range of latitudes and therefore environmental conditions. In this context, comprehensive metabolomics approaches have been used to assess how different temperature regimes may affect the metabolism of three species of Eucalyptus, E. dunnii, E. grandis and E. pellita. Young plants were grown for 53 d in the greenhouse and then transferred to growth chambers at 10°C, 20°C or 30°C for another 7 d. In all three species the leaf chlorophyll content was positively correlated to temperature, and in E. pellita the highest temperature also resulted in a significant increase in stem biomass. Comprehensive metabolomics was performed using untargeted gas chromatography mass spectrometry (GC-MS) and liquid chromatography (LC)-MS. This approach enabled the comparison of the relative abundance of 88 polar primary metabolites from GC-MS and 625 semi-polar secondary metabolites from LC-MS. Using principal components analysis, a major effect of temperature was observed in each species which was larger than that resulting from the genetic background. Compounds mostly affected by temperature treatment were subsequently selected using partial least squares discriminant analysis and were further identified. These putative annotations indicated that soluble sugars and several polyphenols, including tannins, triterpenes and alkaloids were mostly influenced.

Synthetic biology strategies toward heterologous phytochemical production

This review summarizes the recent progress in heterologous phytochemical biosynthetic pathway reconstitution in plant, bacteria, and yeast, with a focus on the synthetic biology strategies applied in these engineering efforts.

Stable Production of the Antimalarial Drug Artemisinin in the Moss Physcomitrella patens

Malaria is a real and constant danger to nearly half of the world's population of 7.4 billion people. In 2015, 212 million cases were reported along with 429,000 estimated deaths. The World Health Organization recommends artemisinin-based combinatorial therapies, and the artemisinin for this purpose is mainly isolated from the plant Artemisia annua. However, the plant supply of artemisinin is irregular, leading to fluctuation in prices. Here, we report the development of a simple, sustainable, and scalable production platform of artemisinin. The five genes involved in artemisinin biosynthesis were engineered into the moss Physcomitrella patens via direct in vivo assembly of multiple DNA fragments. In vivo biosynthesis of artemisinin was obtained without further modifications. A high initial production of 0.21 mg/g dry weight artemisinin was observed after only 3 days of cultivation. Our study shows that P. patens can be a sustainable and efficient production platform of artemisinin that without further modifications allow for industrial-scale production. A stable supply of artemisinin will lower the price of artemisinin-based treatments, hence become more affordable to the lower income communities most affected by malaria; an important step toward containment of this deadly disease threatening millions every year.

Magnesium and manganese interactively modulate parthenolide accumulation and the antioxidant defense system in the leaves of Tanacetum parthenium

A balanced nutrient supply is a critical factor affecting accumulation of terpenoids in plants, yet data related to the interactive effects of two essential nutrients for the biosynthesis of sesquiterpenes are scarce. Here, the interactional effects between magnesium (Mg) and manganese (Mn) on plant growth, oxidative status, parthenolide accumulation and expression of key genes involved in parthenolide biosynthesis including 1-deoxy-d-xylulose-5-phosphate reductoisomerase (DXR), hydroxy-2-methyl-2-(E)-butenyl-4-diphosphate reductase (HDR), 3-hydroxy-3-methylglutarylcoenzyme A reductase (HMGR), germacrene A synthase (GAS), germacrene A oxidase (GAO), costunolide synthase (COS) and parthenolide synthase (PTS) in the leaves of feverfew plants grown at different Mn and Mn levels were assessed. Plant growth and leaf pigment concentrations were associated with the amount of applied Mg but could be modified by the Mn level. Deprivation and the addition of both Mg and Mn induce oxidative stress. Mg supply also alleviated the adverse effects of Mn excess on plant growth and oxidative status. In addition, parthenolide biosynthesis decreased under deprivation of Mg or Mn, but the addition of Mn up to 50μM under 2mM Mg supply considerably increased its accumulation. The parthenolide accumulation trend might reflect the up-regulation of terpenoid-related genes and enzyme activities as well as the oxidative status of feverfew leaves. Our data suggest a profound effect of the combined supply of Mg and Mn on parthenolide biosynthesis through the activation of terpene synthases, which concomitantly modulate by oxidative status.

Extending the biosynthetic repertoires of cyanobacteria and chloroplasts

Chloroplasts in plants and algae and photosynthetic microorganisms such as cyanobacteria are emerging hosts for sustainable production of valuable biochemicals, using only inorganic nutrients, water, CO2 and light as inputs. In the past decade, many bioengineering efforts have focused on metabolic engineering and synthetic biology in the chloroplast or in cyanobacteria for the production of fuels, chemicals and complex, high-value bioactive molecules. Biosynthesis of all these compounds can be performed in photosynthetic organelles/organisms by heterologous expression of the appropriate pathways, but this requires optimization of carbon flux and reducing power, and a thorough understanding of regulatory pathways. Secretion or storage of the compounds produced can be exploited for the isolation or confinement of the desired compounds. In this review, we explore the use of chloroplasts and cyanobacteria as biosynthetic compartments and hosts, and we estimate the levels of production to be expected from photosynthetic hosts in light of the fraction of electrons and carbon that can potentially be diverted from photosynthesis. The supply of reducing power, in the form of electrons derived from the photosynthetic light reactions, appears to be non-limiting, but redirection of the fixed carbon via precursor molecules presents a challenge. We also discuss the available synthetic biology tools and the need to expand the molecular toolbox to facilitate cellular reprogramming for increased production yields in both cyanobacteria and chloroplasts.

Comparative Transcriptome Analysis Identifies Putative Genes Involved in the Biosynthesis of Xanthanolides in Xanthium strumarium L

Xanthium strumarium L. is a traditional Chinese herb belonging to the Asteraceae family. The major bioactive components of this plant are sesquiterpene lactones (STLs), which include the xanthanolides. To date, the biogenesis of xanthanolides, especially their downstream pathway, remains largely unknown. In X. strumarium, xanthanolides primarily accumulate in its glandular trichomes. To identify putative gene candidates involved in the biosynthesis of xanthanolides, three X. strumarium transcriptomes, which were derived from the young leaves of two different cultivars and the purified glandular trichomes from one of the cultivars, were constructed in this study. In total, 157 million clean reads were generated and assembled into 91,861 unigenes, of which 59,858 unigenes were successfully annotated. All the genes coding for known enzymes in the upstream pathway to the biosynthesis of xanthanolides were present in the X. strumarium transcriptomes. From a comparative analysis of the X. strumarium transcriptomes, this study identified a number of gene candidates that are putatively involved in the downstream pathway to the synthesis of xanthanolides, such as four unigenes encoding CYP71 P450s, 50 unigenes for dehydrogenases, and 27 genes for acetyltransferases. The possible functions of these four CYP71 candidates are extensively discussed. In addition, 116 transcription factors that are highly expressed in X. strumarium glandular trichomes were also identified. Their possible regulatory roles in the biosynthesis of STLs are discussed. The global transcriptomic data for X. strumarium should provide a valuable resource for further research into the biosynthesis of xanthanolides.

Parthenolide accumulation and expression of genes related to parthenolide biosynthesis affected by exogenous application of methyl jasmonate and salicylic acid in Tanacetum parthenium

Up-regulation of germacrene A synthase and down-regulation of parthenolide hydroxylase genes play key role in parthenolide accumulation of feverfew plants treated with methyl jasmonate and salicylic acid. Parthenolide is an important sesquiterpene lactone due to its anti-migraine and anti-cancer properties. Parthenolide amount was quantified by high-performance liquid chromatography after foliar application of methyl jasmonate (100 µM) or salicylic acid (1.0 mM) on feverfew leaves in time course experiment (3-96 h). Results indicate that exogenous application of methyl jasmonate or salicylic acid activated parthenolide biosynthesis. Parthenolide content reached its highest amount at 24 h after methyl jasmonate or salicylic acid treatments, which were 3.1- and 1.96-fold higher than control plants, respectively. Parthenolide transiently increased due to methyl jasmonate or salicylic acid treatments until 24 h, but did not show significant difference compared with control plants at 48 and 96 h time points in both treatments. Also, the transcript levels of early pathway (upstream) genes of terpene biosynthesis including 3-hydroxy-3-methylglutaryl-coenzyme A reductase, 1-deoxy-D-xylulose-5-phosphate reductoisomerase and hydroxy-2-methyl-2-(E)-butenyl 4-diphosphate reductase and the biosynthetic genes of parthenolide including germacrene A synthase, germacrene A oxidase, costunolide synthase and parthenolide synthase were increased by methyl jasmonate and salicylic acid treatments, but with different intensity. The transcriptional levels of these genes were higher in methyl jasmonate-treated plants than salicylic acid-treated plants. Parthenolide content measurements along with expression pattern analysis of the aforementioned genes and parthenolide hydroxylase as side branch gene of parthenolide suggest that the expression patterns of early pathway genes were not directly consistent with parthenolide accumulation pattern; hence, parthenolide accumulation is probably further modulated by the expression of its biosynthetic genes, especially germacrene A synthase and also its side branch gene, parthenolide hydroxylase.

Transcriptional transitions in Nicotiana benthamiana leaves upon induction of oil synthesis by WRINKLED1 homologs from diverse species and tissues

BACKGROUND

Carbon accumulation and remobilization are essential mechanisms in plants to ensure energy transfer between plant tissues with different functions or metabolic needs and to support new generations. Knowledge about the regulation of carbon allocation into oil (triacylglycerol) in plant storage tissue can be of great economic and environmental importance for developing new high-yielding oil crops. Here, the effect on global gene expression as well as on physiological changes in leaves transiently expressing five homologs of the transcription factor WRINKLED1 (WRI1) originating from diverse species and tissues; Arabidopsis thaliana and potato (Solanum tuberosum) seed embryo, poplar (Populus trichocarpa) stem cambium, oat (Avena sativa) grain endosperm, and nutsedge (Cyperus esculentus) tuber parenchyma, were studied by agroinfiltration in Nicotiana benthamiana.

RESULTS

All WRI1 homologs induced oil accumulation when expressed in leaf tissue. Transcriptome sequencing revealed that all homologs induced the same general patterns with a drastic shift in gene expression profiles of leaves from that of a typical source tissue to a source-limited sink-like tissue: Transcripts encoding enzymes for plastid uptake and metabolism of phosphoenolpyruvate, fatty acid and oil biosynthesis were up-regulated, as were also transcripts encoding starch degradation. Transcripts encoding enzymes in photosynthesis and starch synthesis were instead down-regulated. Moreover, transcripts representing fatty acid degradation were up-regulated indicating that fatty acids might be degraded to feed the increased need to channel carbons into fatty acid synthesis creating a futile cycle. RT-qPCR analysis of leaves expressing Arabidopsis WRI1 showed the temporal trends of transcripts selected as 'markers' for key metabolic pathways one to five days after agroinfiltration. Chlorophyll fluorescence measurements of leaves expressing Arabidopsis WRI1 showed a significant decrease in photosynthesis, even though effect on starch content could not be observed.

CONCLUSIONS

This data gives for the first time a general view on the transcriptional transitions in leaf tissue upon induction of oil synthesis by WRI1. This yields important information about what effects WRI1 may exert on global gene expression during seed and embryo development. The results suggest why high oil content in leaf tissue cannot be achieved by solely transcriptional activation by WRI1, which can be essential knowledge in the development of new high-yielding oil crops.

A library of synthetic transcription activator-like effector-activated promoters for coordinated orthogonal gene expression in plants

A library of synthetic promoters containing the binding site of a single designer transcription activator-like effector (dTALE) was constructed. The promoters contain a constant sequence, consisting of an 18-base long dTALE-binding site and a TATA box, flanked by degenerate sequences of 49 bases downstream and 19 bases upstream. Forty-three of these promoters were sequenced and tested in transient assays in Nicotiana benthamiana using a GUS reporter gene. The strength of expression of the promoters ranged from around 5% to almost 100% of the viral 35S promoter activity. We then demonstrated the utility of these promoters for metabolic engineering by transiently expressing three genes for the production of a plant diterpenoid in N. benthamiana. The simplicity of the promoter structure shows great promise for the development of genetic circuits, with wide potential applications in plant synthetic biology and metabolic engineering.

Heterologous biosynthesis of costunolide in Escherichia coli and yield improvement

Costunolide, the main bioactive compound of the medicinal plant, Radix Aucklandiae, is a sesquiterpene lactone (SL) and has a broad range of biological activities. It is also a precursor of many biologically-active SLs and is a branching point in the biosynthesis of SLs. Here we have reconstituted the costunolide biosynthetic pathway in Escherichia coli by co-expression of three genes (GAS, GAO, LsCOS) involved in costunolide biosynthesis and eight genes involved in converting acetyl-CoA into farnesyl diphosphate from mevalonate pathway. Costunolide production was then detected. By screening and optimization of cultured medium and inducing temperature, costunolide yield was up to 100 mg l(-1) in E. coli.

Wax esters of different compositions produced via engineering of leaf chloroplast metabolism in Nicotiana benthamiana

In a future bio-based economy, renewable sources for lipid compounds at attractive cost are needed for applications where today petrochemical derivatives are dominating. Wax esters and fatty alcohols provide diverse industrial uses, such as in lubricant and surfactant production. In this study, chloroplast metabolism was engineered to divert intermediates from de novo fatty acid biosynthesis to wax ester synthesis. To accomplish this, chloroplast targeted fatty acyl reductases (FAR) and wax ester synthases (WS) were transiently expressed in Nicotiana benthamiana leaves. Wax esters of different qualities and quantities were produced providing insights to the properties and interaction of the individual enzymes used. In particular, a phytyl ester synthase was found to be a premium candidate for medium chain wax ester synthesis. Catalytic activities of FAR and WS were also expressed as a fusion protein and determined functionally equivalent to the expression of individual enzymes for wax ester synthesis in chloroplasts.