The ancient CYP716 family is a major contributor to the diversification of eudicot triterpenoid biosynthesis

Transcriptome Profiling of Two Ornamental and Medicinal Papaver Herbs

The Papaver spp. (Papaver rhoeas (Corn poppy) and Papaver nudicaule (Iceland poppy)) genera are ornamental and medicinal plants that are used for the isolation of alkaloid drugs. In this study, we generated 700 Mb of transcriptome sequences with the PacBio platform. They were assembled into 120,926 contigs, and 1185 (82.2%) of the benchmarking universal single-copy orthologs (BUSCO) core genes were completely present in our assembled transcriptome. Furthermore, using 128 Gb of Illumina sequences, the transcript expression was assessed at three stages of Papaver plant development (30, 60, and 90 days), from which we identified 137 differentially expressed transcripts. Furthermore, three co-occurrence heat maps are generated from 51 different plant genomes along with the Papaver transcriptome, i.e., secondary metabolite biosynthesis, isoquinoline alkaloid biosynthesis (BIA) pathway, and cytochrome. Sixty-nine transcripts in the BIA pathway along with 22 different alkaloids (quantified with LC-QTOF-MS/MS) were mapped into the BIA KEGG map (map00950). Finally, we identified 39 full-length cytochrome transcripts and compared them with other genomes. Collectively, this transcriptome data, along with the expression and quantitative metabolite profiles, provides an initial recording of secondary metabolites and their expression related to Papaver plant development. Moreover, these profiles could help to further detail the functional characterization of the various secondary metabolite biosynthesis and Papaver plant development associated problems.

Co-expression of squalene epoxidases with triterpene cyclases boosts production of triterpenoids in plants and yeast

Triterpene cyclases catalyze the first committed step in triterpene biosynthesis, by forming mono- to pentacyclic backbone structures from oxygenated C30 isoprenoid precursors. Squalene epoxidase precedes this cyclization by providing the oxygenated and activated substrate for triterpene biosynthesis. Three squalene epoxidases from Cucurbita pepo (CpSEs) were isolated and shown to have evolved under purifying selection with signs of sites under positive selection in their N- and C-termini. They all localize to the Endoplasmic Reticulum (ER) and produce 2,3-oxidosqualene and 2,3:22,23-dioxidosqualene when expressed in a yeast erg1 (squalene epoxidase) erg7 (lanosterol synthase) double mutant. Co-expression of the CpSEs with four different triterpene cyclases, either transiently in Nicotiana benthamiana or constitutively in yeast, showed that CpSEs boost triterpene production. CpSE2 was the best performing in this regard, which could reflect either increased substrate production or superior channeling of the substrate to the triterpene cyclases. Fluorescence Lifetime Imaging Microscopy (FLIM) analysis with C. pepo cucurbitadienol synthase (CpCPQ) revealed a specific interaction with CpSE2 but not with the other CpSEs. When CpSE2 was transformed into C. pepo hairy root lines, cucurbitacin E production was increased two folds compared to empty vector control lines. This study provides new insight into the importance of SEs in triterpene biosynthesis, suggesting that they may facilitate substrate channeling, and demonstrates that SE overexpression is a new tool for increasing triterpene production in plants and yeast.

Homopiperazine-rhodamine B adducts of triterpenoic acids are strong mitocans

Parent pentacyclic triterpenoic acids such as ursolic-, oleanolic, glycyrrhetinic, betulinic and boswellic acid were converted into their acetylated piperazinyl amides that were coupled with rhodamine B. SRB assays to evaluate their cytotoxicity showed all of these triterpene-homopiperazinyl-rhodamine adducts 16-20 being highly cytotoxic for a panel of human tumor cell lines even in nanomolar concentrations while being significantly less cytotoxic for non-malignant cells. Interestingly enough, these compounds were even more cytotoxic than previously prepared piperazinyl analogs, thus making the homopiperazinyl spacer a very interesting scaffold for the development of biologically active compounds. Extra staining experiments showed that the cytostatic effect of compounds 18 and 20 onto A2780 cancer cells is due to their ability to act as a mitocan.

Comparative analysis of CYP716A subfamily enzymes for the heterologous production of C-28 oxidized triterpenoids in transgenic yeast

Several enzymes of the CYP716A subfamily have been reported to be involved in triterpenoid biosynthesis. Members of this subfamily oxidize various positions along the triterpenoid backbone and the majority of them catalyze a three-step oxidation at the C-28 position. Interestingly, C-28 oxidation is a common feature in oleanolic acid, ursolic acid, and betulinic acid, which are widely distributed in plants and exhibit important biological activities. In this work, three additional CYP716A enzymes isolated from olive, sugar beet, and coffee, were characterized as multifunctional C-28 oxidases. Semi-quantitative comparisons of in vivo catalytic activity were made against the previously characterized enzymes CYP716A12, CYP716A15, and CYP716A52v2. When heterologously expressed in yeast, the isolated enzymes differed in both catalytic activity and substrate specificity. This study indicates that the screening of enzymes from different plants could be a useful means of identifying enzymes with enhanced catalytic activity and desired substrate specificity. Furthermore, we show that "naturally-evolved" enzymes can be useful in the heterologous production of pharmacologically and industrially important triterpenoids.

A Novel Multifunctional C-23 Oxidase, CYP714E19, is Involved in Asiaticoside Biosynthesis

Centella asiatica is widely used as a medicinal plant due to accumulation of the ursane-type triterpene saponins asiaticoside and madecassoside. The molecular structure of both compounds suggests that they are biosynthesized from α-amyrin via three hydroxylations, and the respective Cyt P450-dependent monooxygenases (P450 enzymes) oxidizing the C-28 and C-2α positions have been reported. However, a third enzyme hydroxylating C-23 remained elusive. We previously identified 40,064 unique sequences in the transcriptome of C. asiatica elicited by methyl jasmonate, and among them we have now found 149 unigenes encoding putative P450 enzymes. In this set, 23 full-length cDNAs were recognized, 13 of which belonged to P450 subfamilies previously implicated in secondary metabolism. Four of these genes were highly expressed in response to jasmonate treatment, especially in leaves, in accordance with the accumulation patterns of asiaticoside. The functions of these candidate genes were tested using heterologous expression in yeast cells. Gas chromatography-mass spectrometry (GC-MS) analysis revealed that yeast expressing only the oxidosqualene synthase CaDDS produced the asiaticoside precursor α-amyrin (along with its isomer β-amyrin), while yeast co-expressing CaDDS and CYP716A83 also contained ursolic acid along with oleanolic acid. This P450 enzyme thus acts as a multifunctional triterpenoid C-28 oxidase converting amyrins into corresponding triterpenoid acids. Finally, yeast strains co-expressing CaDDS, CYP716A83 and CYP714E19 produced hederagenin and 23-hydroxyursolic acid, showing that CYP714E19 is a multifunctional triterpenoid oxidase catalyzing the C-23 hydroxylation of oleanolic acid and ursolic acid. Overall, our results demonstrate that CaDDS, CYP716A83 and CYP714E19 are C. asiatica enzymes catalyzing consecutive steps in asiaticoside biosynthesis.

An engineered combinatorial module of transcription factors boosts production of monoterpenoid indole alkaloids in Catharanthus roseus

To fend off microbial pathogens and herbivores, plants have evolved a wide range of defense strategies such as physical barriers, or the production of anti-digestive proteins or bioactive specialized metabolites. Accumulation of the latter compounds is often regulated by transcriptional activation of the biosynthesis pathway genes by the phytohormone jasmonate-isoleucine. Here, we used our recently developed flower petal transformation method in the medicinal plant Catharanthus roseus to shed light on the complex regulatory mechanisms steering the jasmonate-modulated biosynthesis of monoterpenoid indole alkaloids (MIAs), to which the anti-cancer compounds vinblastine and vincristine belong. By combinatorial overexpression of the transcriptional activators BIS1, ORCA3 and MYC2a, we provide an unprecedented insight into the modular transcriptional control of MIA biosynthesis. Furthermore, we show that the expression of an engineered de-repressed MYC2a triggers a tremendous reprogramming of the MIA pathway, finally leading to massively increased accumulation of at least 23 MIAs. The current study unveils an innovative approach for future metabolic engineering efforts for the production of valuable bioactive plant compounds in non-model plants.

Triterpene Messages from the EU-FP7 Project TriForC

TriForC is an innovative EU-funded collaborative project that has established an integrative pipeline for the exploitation of plant triterpenes for commercialization in agriculture and pharmacology. We discuss the main outcomes of TriForC and reflect on its potential long-term impact and on the importance of EU projects for science, industry, and society.

Transcriptomic Analysis of Kalopanax septemlobus and Characterization of KsBAS, CYP716A94 and CYP72A397 Genes Involved in Hederagenin Saponin Biosynthesis

Kalopanax septemlobus, commonly named the castor aralia tree, is a highly valued woody medicinal tree belonging to the family Araliaceae. Kalopanax septemlobus contains approximately 15 triterpenoid saponins primarily constituted of hederagenin aglycones. Hederagenin is a representative precursor for hemolytic saponin in plants. In the present study, transcriptome analysis was performed to discover genes involved in hederagenin saponin biosynthesis in K. septemlobus. De novo assembly generated 82,698 unique sequences, including 17,747 contigs and 64,951 singletons, following 454 pyrosequencing. Oxidosqualene cyclases (OSCs) are enzymes that catalyze the formation of diverse triterpene skeletons from 2,3-oxidosqualene. Heterologous expression of an OSC sequence in yeast revealed that KsBAS is a β-amyrin synthase gene. Cytochrome P450 genes (CYPs) make up a supergene family in the plant genome and play a key role in the biosynthesis of sapogenin aglycones. In total, 95 contigs and 110 singletons annotated as CYPs were obtained by sequencing the K. septemlobus transcriptome. By heterologous expression in yeast, we found that CYP716A94 was β-amyrin 28-oxidase involved in oleanolic acid production from β-amyrin, and CYP72A397 was oleanolic acid 23-hydroxylase involved in hederagenin production from oleanolic acid. Engineered yeast co-expressing KsBAS, CYP716A94 and CYP72A397 produced hederagenin. Kalopanax septemlobus CYP72A397 is a novel CYP enzyme that synthesizes hederagenin aglycone from oleanolic acid as a single product. In conclusion, we characterized three genes participating in sequential steps for hederagenin biosynthesis from β-amyrin, which are likely to play a major role in hederagenin saponin biosynthesis in K. septemlobus.

Boosting 11-oxo-β-amyrin and glycyrrhetinic acid synthesis in Saccharomyces cerevisiae via pairing novel oxidation and reduction system from legume plants

Glycyrrhetinic acid (GA) and its precursor, 11-oxo-β-amyrin, are typical triterpenoids found in the roots of licorice, a traditional Chinese medicinal herb that exhibits diverse functions and physiological effects. In this study, we developed a novel and highly efficient pathway for the synthesis of GA and 11-oxo-β-amyrin in Saccharomyces cerevisiae by introducing efficient cytochrome P450s (CYP450s: Uni25647 and CYP72A63) and pairing their reduction systems from legume plants through transcriptome and genome-wide screening and identification. By increasing the copy number of Uni25647 and pairing cytochrome P450 reductases (CPRs) from various plant sources, the titers of 11-oxo-β-amyrin and GA were increased to 108.1 ± 4.6mg/L and 18.9 ± 2.0mg/L, which were nearly 1422-fold and 946.5-fold higher, respectively, compared with previously reported data. To the best of our knowledge, these are the highest titers reported for GA and 11-oxo-β-amyrin from S. cerevisiae, indicating an encouraging and promising approach for obtaining increased GA and its related triterpenoids without destroying the licorice plant or the soil ecosystem.

RNA-sequencing-based transcriptome and biochemical analyses of steroidal saponin pathway in a complete set of Allium fistulosum-A. cepa monosomic addition lines

The genus Allium is a rich source of steroidal saponins, and its medicinal properties have been attributed to these bioactive compounds. The saponin compounds with diverse structures play a pivotal role in Allium’s defense mechanism. Despite numerous studies on the occurrence and chemical structure of steroidal saponins, their biosynthetic pathway in Allium species is poorly understood. The monosomic addition lines (MALs) of the Japanese bunching onion (A. fistulosum, FF) with an extra chromosome from the shallot (A. cepa Aggregatum group, AA) are powerful genetic resources that enable us to understand many physiological traits of Allium. In the present study, we were able to isolate and identify Alliospiroside A saponin compound in A. fistulosum with extra chromosome 2A from shallot (FF2A) and its role in the defense mechanism against Fusarium pathogens. Furthermore, to gain molecular insight into the Allium saponin biosynthesis pathway, high-throughput RNA-Seq of the root, bulb, and leaf of AA, MALs, and FF was carried out using Illumina's HiSeq 2500 platform. An open access Allium Transcript Database (Allium TDB, http://alliumtdb.kazusa.or.jp) was generated based on RNA-Seq data. The resulting assembled transcripts were functionally annotated, revealing 50 unigenes involved in saponin biosynthesis. Differential gene expression (DGE) analyses of AA and MALs as compared with FF (as a control) revealed a strong up-regulation of the saponin downstream pathway, including cytochrome P450, glycosyltransferase, and beta-glucosidase in chromosome 2A. An understanding of the saponin compounds and biosynthesis-related genes would facilitate the development of plants with unique saponin content and, subsequently, improved disease resistance.

A translational synthetic biology platform for rapid access to gram-scale quantities of novel drug-like molecules

Plants are an excellent source of drug leads. However availability is limited by access to source species, low abundance and recalcitrance to chemical synthesis. Although plant genomics is yielding a wealth of genes for natural product biosynthesis, the translation of this genetic information into small molecules for evaluation as drug leads represents a major bottleneck. For example, the yeast platform for artemisinic acid production is estimated to have taken >150 person years to develop. Here we demonstrate the power of plant transient transfection technology for rapid, scalable biosynthesis and isolation of triterpenes, one of the largest and most structurally diverse families of plant natural products. Using pathway engineering and improved agro-infiltration methodology we are able to generate gram-scale quantities of purified triterpene in just a few weeks. In contrast to heterologous expression in microbes, this system does not depend on re-engineering of the host. We next exploit agro-infection for quick and easy combinatorial biosynthesis without the need for generation of multi-gene constructs, so affording an easy entrée to suites of molecules, some new-to-nature, that are recalcitrant to chemical synthesis. We use this platform to purify a suite of bespoke triterpene analogs and demonstrate differences in anti-proliferative and anti-inflammatory activity in bioassays, providing proof of concept of this system for accessing and evaluating medicinally important bioactives. Together with new genome mining algorithms for plant pathway discovery and advances in plant synthetic biology, this advance provides new routes to synthesize and access previously inaccessible natural products and analogs and has the potential to reinvigorate drug discovery pipelines.

Triterpene Structural Diversification by Plant Cytochrome P450 Enzymes

Cytochrome P450 monooxygenases (P450s) represent the largest enzyme family of the plant metabolism. Plants typically devote about 1% of the protein-coding genes for the P450s to execute primary metabolism and also to perform species-specific specialized functions including metabolism of the triterpenes, isoprene-derived 30-carbon compounds. Triterpenes constitute a large and structurally diverse class of natural products with various industrial and pharmaceutical applications. P450-catalyzed structural modification is crucial for the diversification and functionalization of the triterpene scaffolds. In recent times, a remarkable progress has been made in understanding the function of the P450s in plant triterpene metabolism. So far, ∼80 P450s are assigned biochemical functions related to the plant triterpene metabolism. The members of the subfamilies CYP51G, CYP85A, CYP90B-D, CYP710A, CYP724B, and CYP734A are generally conserved across the plant kingdom to take part in plant primary metabolism related to the biosynthesis of essential sterols and steroid hormones. However, the members of the subfamilies CYP51H, CYP71A,D, CYP72A, CYP81Q, CYP87D, CYP88D,L, CYP93E, CYP705A, CYP708A, and CYP716A,C,E,S,U,Y are required for the metabolism of the specialized triterpenes that might perform species-specific functions including chemical defense toward specialized pathogens. Moreover, a recent advancement in high-throughput sequencing of the transcriptomes and genomes has resulted in identification of a large number of candidate P450s from diverse plant species. Assigning biochemical functions to these P450s will be of interest to extend our knowledge on triterpene metabolism in diverse plant species and also for the sustainable production of valuable phytochemicals.