Localization and in-Vivo Characterization of Thapsia garganica CYP76AE2 Indicates a Role in Thapsigargin Biosynthesis

Chemical constituents from Notopterygium incisum and their anti-neuroinflammatory activity

Phytochemical research on an extract of Notopterygium incisum yielded fifteen compounds (1-15), including four previously undescribed compounds (10-13). The structures of the unreported compounds were elucidated by spectroscopic and spectrometric data analysis such as 1D and 2D NMR, IR and HR-ESI-MS. Compounds 1-5 and 10-14 were isolated from N. incisum for the first time. 7S⁎,8R⁎-Phenethyl-(7-methoxy-8-isoeugenol)-ferulate (10), 7S⁎,8R⁎-p-hydroxyphenethyl-(7-methoxy-8-isoeugenol)-ferulate (11), 7S⁎,8R⁎-benzyl-(7-methoxy-8-isoeugenol)-ferulate (12) and p-hydroxyphenethyl-(4-benzoy-3-methoxy)-cinnamate (13) are the undescribed ferulic acid derivatives. Additionly, the anti-neuroinflammatory effects of compounds were evaluated in lipopolysaccharide (LPS)-induced BV2 cells. The pharmacological results showed that 6β,10β-epoxy-4α-hydroxy-guaiane (6), teuclatriol (7) and 7S⁎,8R⁎-p-hydroxyphenethyl-(7-methoxy-8-isoeugenol)-ferulate (11) inhibited the production and expression of nitric oxide (NO) in the LPS-induced BV2 cells in a concentration-dependent manner. Acorusnol (4), teucladiol (9), 7S⁎,8R⁎-benzyl-(7-methoxy-8-isoeugenol)-ferulate (12) and p-hydroxyphenethyl-(4-benzoy-3-methoxy)-cinnamate (13) only inhibited the release of NO at concentration of 20 μM. Moreover, 7S⁎,8R⁎-p-hydroxyphenethyl-(7-methoxy-8-isoeugenol)-ferulate (11) reduced the level of tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6) in LPS-stimulated BV2 cells. The results demonstrated 7S⁎,8R⁎-p-hydroxyphenethyl-(7-methoxy-8-isoeugenol)-ferulate (11) could be a potential anti-neuroinflammatory agent and is worthy of further study.

Plant Engineering to Enable Platforms for Sustainable Bioproduction of Terpenoids

Terpenoids represent the most diverse class of natural products, with a broad spectrum of industrial relevance including applications in green solvents, flavors and fragrances, nutraceuticals, colorants, and therapeutics. They are typically challenging to extract from their natural sources, where they occur in small amounts and mixtures of related but unwanted byproducts. Formal chemical synthesis, where established, is reliant on petrochemistry. Hence, there is great interest in developing sustainable solutions to assemble biosynthetic pathways in engineered host organisms. Metabolic engineering for chemical production has largely focused on microbial hosts, yet plants offer a sustainable production platform. In addition to containing the precursor pathways that generate the terpenoid building blocks as well as the cell structures and compartments required, or tractable localization for the enzymes involved, plants may provide a low input system to produce these chemicals using carbon dioxide and sunlight only. There have been significant recent advancements in the discovery of pathways to terpenoids of interest as well as strategies to boost yields in host plants. While part of the phytochemical field is focusing on the discovery of biosynthetic pathways, this review will focus on advancements using the pathway toolbox and toward engineering plants for the production of terpenoids. We will highlight strategies currently used to produce target products, optimization of known pathways to improve yields, compartmentalization of pathways within cells, and genetic tools developed to facilitate complex engineering of biosynthetic pathways. These advancements in Synthetic Biology are bringing engineered plant systems closer to commercially relevant hosts for the bioproduction of terpenoids.

Spatially resolved detection of small molecules from press-dried plant tissue using MALDI imaging

Premise

Matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) is a chemical imaging method that can visualize spatial distributions of particular molecules. Plant tissue imaging has so far mostly used cryosectioning, which can be impractical for the preparation of large-area imaging samples, such as full flower petals. Imaging unsectioned plant tissue presents its own difficulties in extracting metabolites to the surface due to the waxy cuticle.

Methods

We address this by using established delipidation techniques combined with a solvent vapor extraction prior to applying the matrix with many low-concentration sprays.

Results

Using this procedure, we imaged tissue from three different plant species (two flowers and one carnivorous plant leaf). Material factorization analysis of the resulting data reveals a wide range of plant-specific small molecules with varying degrees of localization to specific portions of the tissue samples, while facilitating detection and removal of signal from background sources.

Conclusions

This work demonstrates applicability of MALDI-MSI to press-dried plant samples without freezing or cryosectioning, setting the stage for spatially resolved molecule identification. Increased mass resolution and inclusion of tandem mass spectrometry are necessary next steps to allow more specific and reliable compound identification.

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.

An engineered extraplastidial pathway for carotenoid biofortification of leaves

Carotenoids are lipophilic plastidial isoprenoids highly valued as nutrients and natural pigments. A correct balance of chlorophylls and carotenoids is required for photosynthesis and therefore highly regulated, making carotenoid enrichment of green tissues challenging. Here we show that leaf carotenoid levels can be boosted through engineering their biosynthesis outside the chloroplast. Transient expression experiments in Nicotiana benthamiana leaves indicated that high extraplastidial production of carotenoids requires an enhanced supply of their isoprenoid precursors in the cytosol, which was achieved using a deregulated form of the main rate-determining enzyme of the mevalonic acid (MVA) pathway. Constructs encoding bacterial enzymes were used to convert these MVA-derived precursors into carotenoid biosynthetic intermediates that do not normally accumulate in leaves, such as phytoene and lycopene. Cytosolic versions of these enzymes produced extraplastidial carotenoids at levels similar to those of total endogenous (i.e. chloroplast) carotenoids. Strategies to enhance the development of endomembrane structures and lipid bodies as potential extraplastidial carotenoid storage systems were not successful to further increase carotenoid contents. Phytoene was found to be more bioaccessible when accumulated outside plastids, whereas lycopene formed cytosolic crystalloids very similar to those found in the chromoplasts of ripe tomatoes. This extraplastidial production of phytoene and lycopene led to an increased antioxidant capacity of leaves. Finally, we demonstrate that our system can be adapted for the biofortification of leafy vegetables such as lettuce.

From Plant to Patient: Thapsigargin, a Tool for Understanding Natural Product Chemistry, Total Syntheses, Biosynthesis, Taxonomy, ATPases, Cell Death, and Drug Development

Thapsigargin, the first representative of the hexaoxygenated guaianolides, was isolated 40 years ago in order to understand the skin-irritant principles of the resin of the umbelliferous plant Thapsia garganica. The pronounced cytotoxicity of thapsigargin is caused by highly selective inhibition of the intracellular sarco-endoplasmic Ca²⁺-ATPase (SERCA) situated on the membrane of the endo- or sarcoplasmic reticulum. Thapsigargin is selective to the SERCA pump and to a minor extent the secretory pathway Ca²⁺/Mn²⁺ ATPase (SPCA) pump. Thapsigargin has become a tool for investigation of the importance of SERCA in intracellular calcium homeostasis. In addition, complex formation of thapsigargin with SERCA has enabled crystallization and structure determination of calcium-free states by X-ray crystallography. These results led to descriptions of the mechanism of action and kinetic properties of SERCA and other ATPases. Inhibition of SERCA depletes Ca²⁺ from the sarco- and endoplasmic reticulum provoking the unfolded protein response, and thereby has enabled new studies on the mechanism of cell death. Development of protocols for selective transformation of thapsigargin disclosed the chemistry and facilitated total synthesis of the molecule. Conversion of trilobolide into thapsigargin offered an economically feasible sustainable source of thapsigargin, which enables a future drug production. Principles for prodrug development were used by conjugating a payload derived from thapsigargin with a hydrophilic peptide selectively cleaved by proteases in the tumor. Mipsagargin was developed in order to obtain a drug for treatment of cancer diseases characterized by the presence of prostate specific membrane antigen (PSMA) in the neovascular tissue of the tumors. Even though mipsagargin showed interesting clinical effects the results did not encourage funding and consequently the attempt to register the drug has been abandoned. In spite of this disappointing fact, the research performed to develop the drug has resulted in important scientific discoveries concerning the chemistry, biosynthesis and biochemistry of sesquiterpene lactones, the mechanism of action of ATPases including SERCA, mechanisms for cell death caused by the unfolded protein response, and the use of prodrugs for cancer-targeting cytotoxins. The presence of toxins in only some species belonging to Thapsia also led to a major revision of the taxonomy of the genus.

Exploring evolutionary theories of plant defence investment using field populations of the deadly carrot

BACKGROUND AND AIMS

There are a number of disparate models predicting variation in plant chemical defences between species, and within a single species over space and time. These can give conflicting predictions. Here we review a number of these theories, before assessing their power to predict the spatial-temporal variation of thapsigargins between and within populations of the deadly carrot (Thapsia garganica). By utilizing multiple models simultaneously (optimum defence theory, growth rate hypothesis, growth-differentiation balance hypothesis, intra-specific framework and resource exchange model of plant defence), we will highlight gaps in their predictions and evaluate the performance of each.

METHODS

Thapsigargins are potent anti-herbivore compounds that occur in limited richness across the different plant tissues of T. garganica, and therefore represent an ideal system for exploring these models. Thapsia garganica plants were collected from six locations on the island of Ibiza, Spain, and the thapsigargins quantified within reproductive, vegetative and below-ground tissues. The effects of sampling time, location, mammalian herbivory, soil nutrition and changing root-associated fungal communities on the concentrations of thapsigargins within these in situ observations were analysed, and the results were compared with our model predictions.

KEY RESULTS

The models performed well in predicting the general defence strategy of T. garganica and the above-ground distribution of thapsigargins, but failed to predict the considerable proportion of defences found below ground. Models predicting variation over environmental gradients gave conflicting and less specific predictions, with intraspecific variation remaining less understood.

CONCLUSION

Here we found that multiple models predicting the general defence strategy of plant species could likely be integrated into a single model, while also finding a clear need to better incorporate below-ground defences into models of plant chemical defences. We found that constitutive and induced thapsigargins differed in their regulation, and suggest that models predicting intraspecific defences should consider them separately. Finally, we suggest that in situ studies be supplemented with experiments in controlled environments to identify specific environmental parameters that regulate variation in defences within species.

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.

Syntheses of Complex Terpenes from Simple Polyprenyl Precursors

From structure elucidation and biogenesis to synthetic methodology and total synthesis, terpene natural products have profoundly influenced the development of organic chemistry. Moreover, their myriad functional attributes range from fragrance to pharmaceuticals and have had great societal impact. Ruzicka's formulation of the "biogenetic isoprene rule," a Nobel Prize winning discovery now over 80 years old, allowed for identification of higher order terpene (aka "isoprenoid") structures from simple five-carbon isoprene fragments. Notably, the isoprene rule still holds pedagogical value to students of organic chemistry today. Our laboratory has completed syntheses of over two dozen terpene and meroterpene structures to date, and the isoprene rule has served as a key pattern recognition tool for our synthetic planning purposes. At the strategic level, great opportunity exists in finding unique and synthetically simplifying ways to connect the formal C₅ isoprene fragments embedded in terpenes. Biomimetic cationic polyene cyclizations represent the earliest incarnation of this idea, which has facilitated expedient routes to certain terpene polycycle classes. Nonetheless, a large swath of terpene chemical space remains inaccessible using this approach.In this Account, we describe strategic insight into our endeavors in terpene synthesis published over the last five years. We show how biosynthetic understanding, combined with a desire to utilize abundant and inexpensive [C₅]_n building blocks, has led to efficient, abiotic syntheses of multiple complex terpenes with disparate ring systems. Informed by nature, but unconstrained by its processes, our synthetic assembly exploits chemical reactivity across diverse reaction types-including radical, anionic, pericyclic, and metal-mediated transformations.First, we detail an eight-step synthesis of the cembrane diterpene chatancin from dihydrofarnesal using a bioinspired-but not -mimetic-cycloaddition. Next, we describe the assembly of the antimalarial cardamom peroxide using a polyoxygenation cascade to fuse multiple units of molecular oxygen onto a dimeric skeleton. This three-to-four-step synthesis arises from (-)-myrtenal, an inexpensive pinene oxidation product. We then show how a radical cyclization cascade can forge the hallmark cyclooctane ring system of the complex sesterterpene 6-epi-ophiobolin N from two simple polyprenyl precursors, (-)-linalool and farnesol. To access the related, more complex metabolite 6-epi-ophiobolin A, we exploited the plasticity of our synthetic route and found that use of geraniol (C₁₀) rather than farnesol (C₁₅) gave us the flexibility needed to address the additional oxidation found in this congener. Following this work, we describe two strategies to access several guaianolide sesquiterpenes. Retrosynthetic disconnection to monoterpenes, carvone or (-)-linalool, coupled with a powerful allylation strategy allowed us to address guaianolides with disparate stereochemical motifs. Finally, we examine a semisynthetic approach to the illicium sesquiterpenes from the abundant 15-carbon feedstock terpene (+)-cedrol using an abiotic ring shift and multiple C-H oxidation reactions inspired by a postulated biosynthesis of this natural product class.

Comparative functional genomics analysis of cytochrome P450 gene superfamily in wheat and maize

BACKGROUND

The cytochrome P450s (CYP450s) as the largest enzyme family of plant metabolism participate in various physiological processes, whereas no study has demonstrated interest in comprehensive comparison of the genes in wheat and maize. Genome-wide survey, characterization and comparison of wheat and maize CYP450 gene superfamily are useful for genetic manipulation of the Gramineae crops.

RESULTS

In total, 1285 and 263 full-length CYP450s were identified in wheat and maize, respectively. According to standard nomenclature, wheat CYP450s (TaCYP450s) were categorized into 45 families, while maize CYP450s (ZmCYP450s) into 43 families. A comprehensive analysis of wheat and maize CYP450s, involved in functional domains, conserved motifs, phylogeny, gene structures, chromosome locations and duplicated events was performed. The result showed that each family/subfamily in both species exhibited characteristic features, suggesting their phylogenetic relationship and the potential divergence in their functions. Functional divergence analysis at the amino acid level of representative clans CYP51, CYP74 and CYP97 in wheat, maize and rice identified some critical amino acid sites that are responsible for functional divergence of a gene family. Expression profiles of Ta-, ZmCYP450s were investigated using RNA-seq data, which contribute to infer the potential functions of the genes during development and stress responses. We found in both species CYP450s had preferential expression in specific tissues, and many tissue-specific genes were identified. Under water-deficit condition, 82 and 39 significantly differentially expressed CYP450s were respectively detected in wheat and maize. These genes may have some roles in protecting plants against drought damage. Thereinto, fourteen CYP450s were selected to validate their expression level through qRT-PCR. To further elucidating molecular mechanisms of CYP450 action, gene co-expression network was constructed. In total, 477 TaCYP450s were distributed in 22 co-expression modules, and some co-expressed genes that likely take part in the same biochemical pathway were identified. For instance, the expression of TaCYP74A98_4D was highly correlated with TaLOX9, TaLOX36, TaLOX39, TaLOX44 and TaOPR8, and all of them may be involved in jasmonate (JA) biosynthesis. TaCYP73A201_3A showed coexpression with TaPAL1.25, TaCCoAOMT1.2, TaCOMT.1, TaCCR1.6 and TaLAC5, which probably act in the wheat stem and/or root lignin synthesis pathway.

CONCLUSION

Our study first established systematic information about evolutionary relationship, expression pattern and function characterization of CYP450s in wheat and maize.

Mining genes associated with furanocoumarin biosynthesis in an endangered medicinal plant, Glehnia littoralis

The endangered medicinal plant Glehnia littoralis is one of the important natural source of furanocoumarin, which has been used as mucolytic, antitussive, antitumour and antibacterial. However, the genetic information of furanocoumarin biosynthesis in G. littoralis is scarce at present. The objective of this study was to mine the putative candidate genes involved in the biosynthesis pathwayof furanocoumarin and provide references for gene identification, and functional genomics of G. littoralis. We carried out the transcriptome analysis of leaves and roots in G. littoralis, which provided a dataset for gene mining. Psoralen, imperatorin and isoimperatorin were detected in G. littoralis by high performance liquid chromatography analysis. Candidate key genes were mined based on the annotations and local BLAST with homologous sequences using BioEdit software. The relative expression of genes was analysed using quantitative real-time polymerase chain reaction. Further, the CYP450 genes were mined using phylogenetic analyses using MEGA 6.0 software. Atotal of 156,949 unigenes were generated, of which 9021 were differentially-expressed between leaves and roots. A total of 82 unigenes encoding eight enzymes in furanocoumarin biosynthetic pathway were first obtained. Seven genes that encoded key enzymes in the downstream furanocoumarin biosynthetic pathway and expressed more in roots than leaves were screened. Twenty-six candidate CYP450 unigenes expressed abundantly in roots and were chiefly concentrated in CYP71, CYP85 and CYP72 clans. Finally, we filtered 102 differentially expressed transcription factors (TFs) unigenes. The transcriptome of G. littoralis was characterized which would help to elucidate the furanocoumarin biosynthetic pathway in G. littoralis and provide an invaluable resource for further study of furanocoumarin.

Medically Useful Plant Terpenoids: Biosynthesis, Occurrence, and Mechanism of Action

Specialized plant terpenoids have found fortuitous uses in medicine due to their evolutionary and biochemical selection for biological activity in animals. However, these highly functionalized natural products are produced through complex biosynthetic pathways for which we have a complete understanding in only a few cases. Here we review some of the most effective and promising plant terpenoids that are currently used in medicine and medical research and provide updates on their biosynthesis, natural occurrence, and mechanism of action in the body. This includes pharmacologically useful plastidic terpenoids such as p-menthane monoterpenoids, cannabinoids, paclitaxel (taxol®), and ingenol mebutate which are derived from the 2-C-methyl-d-erythritol-4-phosphate (MEP) pathway, as well as cytosolic terpenoids such as thapsigargin and artemisinin produced through the mevalonate (MVA) pathway. We further provide a review of the MEP and MVA precursor pathways which supply the carbon skeletons for the downstream transformations yielding these medically significant natural products.

Allylative Approaches to the Synthesis of Complex Guaianolide Sesquiterpenes from Apiaceae and Asteraceae

With hundreds of unique members isolated to date, guaianolide lactones represent a particularly prolific class of terpene natural products. Given their extensive documented therapeutic properties and fascinating chemical structures, these metabolites have captivated the synthetic chemistry community for many decades. As a result of divergent biosynthetic pathways, which produce a wide array of stereochemical and oxidative permutations, a unifying synthetic pathway to this broad family of natural products is challenging. Herein we document the evolution of a chiral-pool-based synthetic program aimed at accessing an assortment of guaianolides, particularly those from the plant family Apiaceae as well as Asteraceae, members of which possess distinct chemical substructures and necessitate deviating synthetic platforms. An initial route employing the linear monoterpene linalool generated a lower oxidation state guaianolide but was not compatible with the majority of family members. A double-allylation disconnection using a carvone-derived fragment was then developed to access first an Asteraceae-type guaianolide and then various Apiaceae congeners. Finally, using these findings in conjunction with a tandem polyoxygenation cascade, we developed a pathway to highly oxygenated nortrilobolide. A variety of interesting observations in metal-mediated aldehyde allylation and alkene polyoxygenation are reported and discussed.

Ti(II) and Rh(I) Complexes as Reagents toward a Thapsigargin Core

A novel approach toward the [5-7]fused bicyclic core of thapsigargin, a subnanomolar inhibitor of the endo/sarcoplasmic calcium ATPase (SERCA), is presented. The synthetic route includes an original Ti(II)-mediated hydroxy-directed reductive coupling of an enantiomerically enriched propargylic alcohol and an intramolecular Rh(I)-catalyzed cyclocarbonylation reaction as key steps. Interestingly, through the first experiments of titanocene-mediated reductive cyclization of a 1,8-enyne, a seven-membered cycle was isolated as a unique product with a total diastereoselectivity.

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.

UPLC and ESI-MS analysis of metabolites of Rauvolfia tetraphylla L. and their spatial localization using desorption electrospray ionization (DESI) mass spectrometric imaging

Rauvolfia tetraphylla L. (family Apocynaceae), often referred to as the wild snakeroot plant, is an important medicinal plant and produces a number of indole alkaloids in its seeds and roots. The plant is often used as a substitute for Ravuolfia serpentine (L.) Benth. ex Kurz known commonly as the Indian snakeroot plant or sarphagandha in the preparation of Ayurvedic formulations for a range of diseases including hypertension. In this study, we examine the spatial localization of the various indole alkaloids in developing fruits and plants of R. tetraphylla using desorption electrospray ionization mass spectrometry imaging (DESI-MSI). A semi-quantitative analysis of the various indole alkaloids was performed using UPLC-ESI/MS. DESI-MS images showed that the distribution of ajmalcine, yohimbine, demethyl serpentine and mitoridine are largely localized in the fruit coat while that for ajmaline is restricted to mesocarp of the fruit. At a whole plant level, the ESI-MS intensities of many of the ions were highest in the roots and lesser in the shoot region. Within the root tissue, except sarpagine and ajmalcine, all other indole alkaloids occurred in the epidermal and cortex tissues. In leaves, only serpentine, ajmalcine, reserpiline and yohimbine were present. Serpentine was restricted to the petiolar region of leaves. Principal component analysis based on the presence of the indole alkaloids, clearly separated the four tissues (stem, leaves, root and fruits) into distinct clusters. In summary, the DESI-MSI results indicated a clear tissue localization of the various indole alkaloids, in fruits, leaves and roots of R. tetraphylla. While it is not clear of how such localization is attained, we discuss the possible pathways of indole alkaloid biosynthesis and translocation during fruit and seedling development in R. tetraphylla. We also briefly discuss the functional significance of the spatial patterns in distribution of metabolites.

Quantification and Localization of Formylated Phloroglucinol Compounds (FPCs) in Eucalyptus Species

The Eucalyptus genus is a hyper-diverse group of long-lived trees from the Myrtaceae family, consisting of more than 700 species. Eucalyptus are widely distributed across their native Australian landscape and are the most widely planted hardwood forest trees in the world. The ecological and economic success of Eucalyptus trees is due, in part, to their ability to produce a plethora of specialized metabolites, which moderate abiotic and biotic interactions. Formylated phloroglucinol compounds (FPCs) are an important class of specialized metabolites in the Myrtaceae family, particularly abundant in Eucalyptus. FPCs are mono- to tetra-formylated phloroglucinol based derivatives, often with an attached terpene moiety. These compounds provide chemical defense against herbivory and display various bioactivities of pharmaceutical relevance. Despite their ecological and economic importance, and continued improvements into analytical techniques, FPCs have proved challenging to study. Here we present a simple and reliable method for FPCs extraction, identification and quantification by UHPLC-DAD-ESI-Q-TOF-MS/MS. The method was applied to leaf, flower bud, and flower samples of nine different eucalypt species, using a small amount of plant material. Authentic analytical standards were used to provide high resolution mass spectra and fragmentation patterns. A robust method provides opportunities for future investigations into the identification and quantification of FPCs in complex biological samples with high confidence. Furthermore, we present for the first time the tissue-based localization of FPCs in stem, leaf, and flower bud of Eucalyptus species measured by mass spectrometry imaging, providing important information for biosynthetic pathway discovery studies and for understanding the role of those compounds in planta.

Biosynthesis of tovarol and other sesquiterpenoids in Thapsia laciniata Rouy

The genus Thapsia produces a wide variety of sesquiterpenoids. The Mediterranean plant Thapsia laciniata Rouy is known to have a product profile that differs from several other species in the genus. Thus, the biosynthesis of sesquiterpenoids in Thapsia laciniata Rouy was investigated. Here we describe three terpene synthases, TlTPS820, TlTPS509 and TlTPS18983. TlTPS18983 is a multi-product enzyme with farnesene as the major product, while TlTPS509 produces guaiol and bulnesol along with other major and several minor unknown products. TlTPS820 is orthologous to TgTPS2 from Thapsia garganica L. and is an epikunzeaol synthase. TgCYP76AE2 from Thapsia garganica performs a triple hydroxylation of epikunzeaol at C-12 to make dihydrocostunolide. It was therefore investigated if the cytochrome P450, TlCYP76AE4 was able to use epikunzeaol as a substrate. It was found that TlCYP76AE4 hydroxylates epikunzeaol at C-8 to yield tovarol instead of dihydrocostunolide.

Interrogation of spatial metabolome of Ginkgo biloba with high-resolution matrix-assisted laser desorption/ionization and laser desorption/ionization mass spectrometry imaging

Ginkgo biloba is one of the oldest extant seed plants and has a number of unique properties and uses. Numerous efforts have characterized metabolites within the ginkgo plant and their corresponding biosynthesis pathways, but spatio-chemical information on ginkgo metabolites is lacking. Mass spectrometry (MS) imaging was used to interrogate the spatio-chemical localization of metabolites with matrix-assisted laser desorption/ionization and laser desorption/ionization Fourier-transform ion cyclotron resonance MS across the ginkgo leaf. Flavonoids, particularly unexpected and rare flavonoid cyclodimers, were detected predominately from leaf epidermis; ginkgolic acids and cardanols were observed exclusively in the secretory cavities. A non-uniform distribution of flavonoids observed between the upper and lower leaf epidermis was verified by liquid chromatography-MS analyses. Other metabolites, such as saccharides, phospholipids, and chlorophylls, occurred mainly in mesophyll cells. Furthermore, organ- and tissue-specific distributions of ginkgolides were revealed in the ginkgo root, young stem, and leaf. The acquired ion images provide important information regarding biosynthesis, transportation, and accumulation of metabolites throughout the ginkgo plant and should help us to understand the physiological roles of several plant secondary metabolites.

Use of a temporary immersion bioreactor system for the sustainable production of thapsigargin in shoot cultures of Thapsia garganica

BACKGROUND

Thapsigargin and nortrilobolide are sesquiterpene lactones found in the Mediterranean plant Thapsia garganica L. Thapsigargin is a potent inhibitor of the sarco/endoplasmic reticulum calcium ATPase pump, inducing apoptosis in mammalian cells. This mechanism has been used to develop a thapsigargin-based cancer drug first by GenSpera and later Inspyr Therapeutics (Westlake Village, California). However, a stable production of thapsigargin is not established.

RESULTS

In vitro regeneration from leaf explants, shoot multiplication and rooting of T. garganica was obtained along with the production of thapsigargins in temporary immersion bioreactors (TIBs). Thapsigargin production was enhanced using reduced nutrient supply in combination with methyl jasmonate elicitation treatments. Shoots grown in vitro were able to produce 0.34% and 2.1% dry weight of thapsigargin and nortrilobolide, respectively, while leaves and stems of wild T. garganica plants contain only between 0.1 and 0.5% of thapsigargin and below detectable levels of nortrilobolide. In addition, a real-time reverse transcription PCR (qRT-PCR) study was performed to study the regulatory role of the biosynthetic genes HMG-CoA reductase (HMGR), farnesyl diphosphate synthase (FPPS), epikunzeaol synthase (TgTPS2) and the cytochrome P450 (TgCYP76AE2) of stem, leaf and callus tissues. Nadi staining showed that the thapsigargins are located in secretory ducts within these tissues.

CONCLUSIONS

Shoot regeneration, rooting and biomass growth from leaf explants of T. garganica were achieved, together with a high yield in vitro production of thapsigargin in TIBs.

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.

De novo synthesis of the sedative valerenic acid in Saccharomyces cerevisiae

Valeriana officinalis (Valerian) root extracts have been used by European and Asian cultures for millennia for their anxiolytic and sedative properties. However, the efficacy of these extracts suffers from variable yields and composition, making these extracts a prime candidate for microbial production. Recently, valerenic acid, a C15 sesquiterpenoid, was identified as the active compound that modulates the GABA_A channel. Although the first committed step, valerena-4,7(11)-diene synthase, has been identified and described, the complete valerenic acid biosynthetic pathway remains to be elucidated. Sequence homology and tissue-specific expression profiles of V. officinalis putative P450s led to the discovery of a V. officinalis valerena-4,7(11)-diene oxidase, VoCYP71DJ1, which required coexpression with a V. officinalis alcohol dehydrogenase and aldehyde dehydrogenase to complete valerenic acid biosynthesis in yeast. Further, we demonstrated the stable integration of all pathway enzymes in yeast, resulting in the production of 140 mg/L of valerena-4,7(11)-diene and 4 mg/L of valerenic acid in milliliter plates. These findings showcase Saccharomyces cerevisiae's potential as an expression platform for facilitating multiply-oxidized medicinal terpenoid pathway discovery, possibly paving the way for scale up and FDA approval of valerenic acid and other active compounds from plant-derived herbal medicines.

Mass Spectrometry Imaging (MSI) for Plant Metabolomics

Mass spectrometry imaging (MSI) is a developing technique to measure the spatiotemporal distribution of many biomolecules in tissues. Over the preceding decade MSI has been adopted by plant biologists and applied in a broad range of areas including: primary metabolism, natural products, plant defense, plant responses to abiotic and biotic stress, plant lipids, and the developing field of spatial metabolomics. This methods chapter covers preparation of plant tissues for matrix-assisted laser desorption ionization (MALDI)-MSI, including sample embedding and freezing, sectioning, mounting, and matrix deposition using both sublimation and spray deposition prior to MSI analysis.

High-mass-resolution MALDI mass spectrometry imaging reveals detailed spatial distribution of metabolites and lipids in roots of barley seedlings in response to salinity stress

INTRODUCTION

Mass spectrometry imaging (MSI) is a technology that enables the visualization of the spatial distribution of hundreds to thousands of metabolites in the same tissue section simultaneously. Roots are below-ground plant organs that anchor plants to the soil, take up water and nutrients, and sense and respond to external stresses. Physiological responses to salinity are multifaceted and have predominantly been studied using whole plant tissues that cannot resolve plant salinity responses spatially.

OBJECTIVES

This study aimed to use a comprehensive approach to study the spatial distribution and profiles of metabolites, and to quantify the changes in the elemental content in young developing barley seminal roots before and after salinity stress.

METHODS

Here, we used a combination of liquid chromatography-mass spectrometry (LC-MS), inductively coupled plasma mass spectrometry (ICP-MS), and matrix-assisted laser desorption/ionization (MALDI-MSI) platforms to profile and analyze the spatial distribution of ions, metabolites and lipids across three anatomically different barley root zones before and after a short-term salinity stress (150 mM NaCl).

RESULTS

We localized, visualized and discriminated compounds in fine detail along longitudinal root sections and compared ion, metabolite, and lipid composition before and after salt stress. Large changes in the phosphatidylcholine (PC) profiles were observed as a response to salt stress with PC 34:n showing an overall reduction in salt treated roots. ICP-MS analysis quantified changes in the elemental content of roots with increases of Na⁺ and decreases of K⁺ content.

CONCLUSION

Our results established the suitability of combining three mass spectrometry platforms to analyze and map ionic and metabolic responses to salinity stress in plant roots and to elucidate tolerance mechanisms in response to abiotic stress, such as salinity stress.

Discovery of a non-stereoselective cytochrome P450 catalyzing either 8α- or 8β-hydroxylation of germacrene A acid from the Chinese medicinal plant, Inula hupehensis

Sesquiterpene lactones (STLs) are C15 terpenoid natural products with α-methylene γ-lactone moiety. A large proportion of STLs in Asteraceae species is derived from the central precursor germacrene A acid (GAA). Formation of the lactone rings depends on the regio-(C6 or C8) and stereoselective (α- or β-)hydroxylations of GAA, producing STLs with four distinct stereo-configurations (12,6α-, 12,6β-, 12,8α-, and 12,8β-olide derivatives of GAA) in nature. Curiously, two configurations of STLs (C12,8α and C12,8β) are simultaneously present in the Chinese medicinal plant, Inula hupehensis. However, how these related yet distinct STL stereo-isomers are co-synthesized in I. hupehensis remains unknown. Here, we describe the functional identification of the I. hupehensis cytochrome P450 (CYP71BL6) that can catalyze the hydroxylation of GAA in either 8α- or 8β-configuration, resulting in the synthesis of both 8α- and 8β-hydroxyl GAAs. Of these two products, only 8α-hydroxyl GAA spontaneously lactonizes to the C12,8α-STL while the 8β-hydroxyl GAA remains stable without lactonization. Chemical structures of the C12,8α-STL, named inunolide, and 8β-hydroxyl GAA were fully elucidated by nuclear magnetic resonance analysis and mass spectrometry. The CYP71BL6 displays 63-66% amino acid identity to the previously reported CYP71BL1/2 catalyzing GAA 6α- or 8β-hydroxylation, indicating CYP71BL6 shares the same evolutionary lineage with other stereoselective cytochrome P450s, but catalyzes hydroxylation in a non-stereoselective manner. We observed that the CYP71BL6 transcript abundance correlates closely to the accumulation of C12,8-STLs in I. hupehensis. The identification of CYP71BL6 provides an insight into the biosynthesis of STLs in Asteraceae.

A Concise, Efficient and Scalable Total Synthesis of Thapsigargin and Nortrilobolide from (R)-(-)-Carvone

A concise, efficient and scalable synthesis of thapsigargin and nortrilobolide from commercially available (R)-(-)-carvone was developed. Our synthetic strategy is inspired by nature's carbon-carbon bond formation sequence, which facilitates the construction of a highly functionalized sesquiterpene lactone skeleton in five steps via an enantioselective ketone alkylation and a diastereoselective pinacol cyclization. We envision that this strategy will permit the construction of other members of the family, structural analogs and provide a practical synthetic route to these important bioactive agents. In addition, we anticipate that the prodrug Mipsagargin, which is currently in late-stage clinical trials for the treatment of cancer, will also be accessible via this strategy. Hence, the limited availability from natural sources, coupled with an estimated demand of one metric ton per annum for the prodrug, provides a compelling mandate to develop practical total syntheses of these agents.