Triterpene Saponin Biosynthesis Research Articles

Isolation and characterization of an oxidosqualene cyclase gene encoding a β-amyrin synthase involved in Polygala tenuifolia Willd. saponin biosynthesis

Expression of PtBS (Polygala tenuifolia β-amyrin synthase) led to the production of β-amyrin as sole product. Polygala tenuifolia Willdenow is a rich source of triterpene saponins, onjisaponins and polygalasaponins, used as herbal medicine to treat phlegms and for detumescence in traditional Asian healing. The Polygala saponins share the oleanane backbone structure and are, therefore, likely synthesized via β-amyrin as a common precursor. We hypothesized that, in analogy to diverse other plant species, this central intermediate should be formed by a β-amyrin synthase catalyzing the complex cyclization of oxidosqualene. This member of the oxidosqualene cyclase (OSC) family of enzymes is thus defining an important branch point between primary and secondary metabolisms, and playing a crucial role in the control of oleanane-type triterpene saponin biosynthesis. From P. tenuifolia roots, we isolated an OSC cDNA containing a reading frame of 2,289 bp nucleotides. The predicted protein of 763 amino acids (molecular weight 87.353 kDa) showed particularly high amino acid sequence identities to known β-amyrin synthases (85-87 %) and was, therefore, named PtBS. Expression of PtBS in the triterpenoid synthase-deficient yeast mutant GIL77 led to the production of β-amyrin as sole product. qRT-PCR analysis of various P. tenuifolia organs showed that PtBS transcript levels were highest in the roots, consistent with onjisaponin accumulation patterns. Therefore, we conclude that PtBS is the β-amyrin synthase enzyme catalyzing the first committed step in the biosynthesis of onjisaponins and polygalasaponins in P. tenuifolia.

The protein quality control system manages plant defence compound synthesis

Jasmonates are ubiquitous oxylipin-derived phytohormones that are essential in the regulation of many development, growth and defence processes. Across the plant kingdom, jasmonates act as elicitors of the production of bioactive secondary metabolites that serve in defence against attackers. Knowledge of the conserved jasmonate perception and early signalling machineries is increasing, but the downstream mechanisms that regulate defence metabolism remain largely unknown. Here we show that, in the legume Medicago truncatula, jasmonate recruits the endoplasmic-reticulum-associated degradation (ERAD) quality control system to manage the production of triterpene saponins, widespread bioactive compounds that share a biogenic origin with sterols. An ERAD-type RING membrane-anchor E3 ubiquitin ligase is co-expressed with saponin synthesis enzymes to control the activity of 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR), the rate-limiting enzyme in the supply of the ubiquitous terpene precursor isopentenyl diphosphate. Thus, unrestrained bioactive saponin accumulation is prevented and plant development and integrity secured. This control apparatus is equivalent to the ERAD system that regulates sterol synthesis in yeasts and mammals but that uses distinct E3 ubiquitin ligases, of the HMGR degradation 1 (HRD1) type, to direct destruction of HMGR. Hence, the general principles for the management of sterol and triterpene saponin biosynthesis are conserved across eukaryotes but can be controlled by divergent regulatory cues.

Expression profiling of the triterpene saponin biosynthesis genes FPS, SS, SE, and DS in the medicinal plant Panax notoginseng

Panax notoginseng (Burk) F. H. Chen, an economically significant medicinal plant with hemostatic and health tonic activities, has been used in Traditional Chinese Medicine (TCM) for more than 3000years. Triterpene saponins are responsible for most of the pharmacological activities of P. notoginseng. Here, we cloned five cDNA sequences encoding the key enzymes involved in triterpene saponin biosynthesis, namely, PnFPS, PnSS, PnSE1, PnSE2, and PnDS, and analyzed the conserved domains and phylogenetics of their corresponding proteins. Their organ-specific expression patterns in four-year-old P. notoginseng were detected by real-time PCR, showing that they were all most highly expressed in flowers. In addition, four of the genes, excluding PnSE2, were upregulated in leaves following stimulation with methyl jasmonate. This study is the first comprehensive analysis of the expression patterns of pivotal genes for triterpene saponin biosynthesis in P. notoginseng and provides a basis to further elucidate the molecular mechanism for the biosynthesis of these medically important compounds.

Analysis of the transcriptome of Panax notoginseng root uncovers putative triterpene saponin-biosynthetic genes and genetic markers

BackgroundPanax notoginseng (Burk) F.H. Chen is important medicinal plant of the Araliacease family. Triterpene saponins are the bioactive constituents in P. notoginseng. However, available genomic information regarding this plant is limited. Moreover, details of triterpene saponin biosynthesis in the Panax species are largely unknown.ResultsUsing the 454 pyrosequencing technology, a one-quarter GS FLX titanium run resulted in 188,185 reads with an average length of 410 bases for P. notoginseng root. These reads were processed and assembled by 454 GS De Novo Assembler software into 30,852 unique sequences. A total of 70.2% of unique sequences were annotated by Basic Local Alignment Search Tool (BLAST) similarity searches against public sequence databases. The Kyoto Encyclopedia of Genes and Genomes (KEGG) assignment discovered 41 unique sequences representing 11 genes involved in triterpene saponin backbone biosynthesis in the 454-EST dataset. In particular, the transcript encoding dammarenediol synthase (DS), which is the first committed enzyme in the biosynthetic pathway of major triterpene saponins, is highly expressed in the root of four-year-old P. notoginseng. It is worth emphasizing that the candidate cytochrome P450 (Pn02132 and Pn00158) and UDP-glycosyltransferase (Pn00082) gene most likely to be involved in hydroxylation or glycosylation of aglycones for triterpene saponin biosynthesis were discovered from 174 cytochrome P450s and 242 glycosyltransferases by phylogenetic analysis, respectively. Putative transcription factors were detected in 906 unique sequences, including Myb, homeobox, WRKY, basic helix-loop-helix (bHLH), and other family proteins. Additionally, a total of 2,772 simple sequence repeat (SSR) were identified from 2,361 unique sequences, of which, di-nucleotide motifs were the most abundant motif.ConclusionThis study is the first to present a large-scale EST dataset for P. notoginseng root acquired by next-generation sequencing (NGS) technology. The candidate genes involved in triterpene saponin biosynthesis, including the putative CYP450s and UGTs, were obtained in this study. Additionally, the identification of SSRs provided plenty of genetic makers for molecular breeding and genetics applications in this species. These data will provide information on gene discovery, transcriptional regulation and marker-assisted selection for P. notoginseng. The dataset establishes an important foundation for the study with the purpose of ensuring adequate drug resources for this species.

Metabolite Profiling of Triterpene Saponins in Medicago truncatula Hairy Roots by Liquid Chromatography Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

Triterpenes are one of the largest classes of plant natural products, with an enormous variety in structure and bioactivities. Here, triterpene saponins from hairy roots of the model legume Medicago truncatula were profiled with reversed-phase liquid chromatography coupled to negative-ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (LC ESI FT-ICR MS). Owing to the accuracy of the FT-ICR MS, reliable molecular formulas of the detected compounds could be predicted, which, together with the generated MS(n) spectra, allowed the tentative identification of 79 different saponins, of which 61 had not been detected previously in M. truncatula. Upon collision-induced dissociation of saponins that contain a uronic acid residue in the sugar chain, fragment ions resulting from cross-ring cleavages of the uronic acid residues were observed. The identified saponins are glycosides of 10 different sapogenins, of which three were not detected before in M. truncatula. Zanhic acid glycosides, which are prevalent in the aerial parts of M. truncatula, were absent in the hairy root extracts. This metabolite compendium will facilitate future functional genomic studies of triterpene saponin biosynthesis in M. truncatula.

Expression and Functional Characterization of Three Squalene Synthase Genes Associated with Saponin Biosynthesis in Panax ginseng

Squalene synthase (SQS) catalyzes the biosynthesis of squalene by condensing two molecules of farnesyl pyrophosphate (FPP), a key precursor in sterol and triterpene biosynthesis. Previously, we reported that PgSS1 overexpression results in the enhanced biosynthesis of both phytosterols and triterpene saponins in Panax ginseng. Here, cDNAs encoding two new SQS homologs (PgSS2 and PgSS3) from a P. ginseng expressed sequence tag (EST) library are described. Functional complementation analysis revealed that ectopic expression of PgSS1, PgSS2 and PgSS3 in the yeast erg9 mutant strain 2C1 lacking SQS activity restored ergosterol prototrophy. The recombinant mutant yeast produced squalene, squalene epoxide and ergosterol. PgSS1 (mRNA) was highly transcribed in all organs, whereas PgSS2 and PgSS3 (mRNAs) were only transcribed in specific organs. All three genes were activated positively by an elicitor (methyl jasmonate), but their transcriptional patterns were different. In situ hybridization analysis revealed that both PgSS1 and PgSS3 transcripts were preferentially accumulated near conducting tissue in the petiole. The PgSS1 and PgSS3 promoters were isolated, and the tissue- and organ-specific regulation of PgSS genes was examined. Transgenic ginseng was constructed by introducing PgSS1 and PgSS3 promoters fused to the β-glucuronidase (GUS) gene. GUS expression driven by the PgSS1 promoter was found in both roots and shoots, but PgSS3-driven GUS was only found in shoots. These results suggest that the three SQS genes are differently expressed and that all three SQS enzymes are involved in squalene production in P. ginseng.

Identification and characterization of glycosyltransferases involved in the biosynthesis of soyasaponin I in Glycine max

Triterpene saponins are a diverse group of compounds with a structure consisting of a triterpene aglycone and sugars. Identification of the sugar-transferase involved in triterpene saponin biosynthesis is difficult due to the structural complexity of triterpene saponin. Two glycosyltransferases from Glycine max, designated as GmSGT2 and GmSGT3, were identified and characterized. In vitro analysis revealed that GmSGT2 transfers a galactosyl group from UDP-galactose to soyasapogenol B monoglucuronide, and that GmSGT3 transfers a rhamnosyl group from UDP-rhamnose to soyasaponin III. These results suggest that soyasaponin I is biosynthesized from soyasapogenol B by successive sugar transfer reactions.

Genomic and coexpression analyses predict multiple genes involved in triterpene saponin biosynthesis in Medicago truncatula.

Saponins, an important group of bioactive plant natural products, are glycosides of triterpenoid or steroidal aglycones (sapogenins). Saponins possess many biological activities, including conferring potential health benefits for humans. However, most of the steps specific for the biosynthesis of triterpene saponins remain uncharacterized at the molecular level. Here, we use comprehensive gene expression clustering analysis to identify candidate genes involved in the elaboration, hydroxylation, and glycosylation of the triterpene skeleton in the model legume Medicago truncatula. Four candidate uridine diphosphate glycosyltransferases were expressed in Escherichia coli, one of which (UGT73F3) showed specificity for multiple sapogenins and was confirmed to glucosylate hederagenin at the C28 position. Genetic loss-of-function studies in M. truncatula confirmed the in vivo function of UGT73F3 in saponin biosynthesis. This report provides a basis for future studies to define genetically the roles of multiple cytochromes P450 and glycosyltransferases in triterpene saponin biosynthesis in Medicago.

Methyl jasmonate induced accumulation of kalopanaxsaponin I in Nigella sativa

Hydroponically cultivated Nigella sativa L. plants treated with methyl jasmonate (MeJA) showed a twelve-fold increase in levels of the monodesmosidic triterpene saponins α-hederin and kalopanaxsaponin I (KsI) in the leaves. We will demonstrate that these two saponins accounted for approximately 10% of the dry plant matter, of which 93% was KsI and 7% α-hederin. To address the molecular basis of saponin induction by MeJA, we cloned and characterized the β-amyrin synthase gene ( NsβAS1) encoding one of the key enzymes in triterpene saponin biosynthesis. As expected, NsβAS1 transcription was induced by MeJA and led to the production of β-amyrin when over-expressed in yeast.

Enhanced triterpene saponin biosynthesis and root nodulation in transgenic barrel medic (Medicago truncatula Gaertn.) expressing a novel β‐amyrin synthase (AsOXA1) gene

Triterpene saponins are a group of bioactive compounds abundant in the genus Medicago, and have been studied extensively for their biological and pharmacological properties. In this article, we evaluated the effects of the ectopic expression of AsOXA1 cDNA from Aster sedifolius on the production of triterpene saponins in barrel medic (Medicago truncatula Gaertn.). AsOXA1 cDNA encodes beta-amyrin synthase, a key enzyme involved in triterpene saponin biosynthesis. One of the four transgenic lines expressing AsOXA1 accumulated significantly larger amounts of some triterpenic compounds in leaf and root than did control plants. In particular, the leaf exhibited significantly higher levels of bayogenin, medicagenic acid and zanhic acid. The amounts of medicagenic acid and zanhic acid, which represent the core of the M. truncatula leaf saponins, were 1.7 and 2.1 times higher, respectively, than the amounts extracted from the control line. In root, the production of bayogenin, hederagenin, soyasapogenol E and 2beta-hydroxyoleanolic acid was increased significantly. The increase in the total amounts of triterpenic compounds observed in the leaves of transgenic lines correlated with the AsOXA1 expression level. Interestingly, the plants expressing AsOXA1 showed, under different growth conditions, improved nodulation when compared with the control line. Nodulation enhancement was also accompanied by a significant change in the soyasapogenol B content. Our results indicate that the ectopic expression of AsOXA1 in barrel medic leads to a greater accumulation of triterpene saponins and enhanced root nodulation.

Isolation and Characterization of Terpene Synthase Gene from Panax ginseng

Terpene synthase plays a key role in biosynthesis of triterpene saponins (ginsenosides) and is intermediate in the biosynthesis of a number of secondary metabolites. A terpene synthase (PgTPS) cDNA was isolated and characterized from the root of Panax ginseng c.A. Meyer. The deduced amino acid sequence of PgTPS showed a similarity with A. deliciosa (AAX16121) 61%, V. vinifera (AAS66357) 61%, L. hirsutum (AAG41891) 55%, M. truncatula (AAV36464) 52%. And the segment of a terpene synthase gene was amplified by reverse transcriptase-polymerase chain reaction (RTPCR). We studied expression of terpene synthase under stressful conditions like chilling, salt, UV, and heavy metal stress treatment. Expression of PgTPS was increased gradually after exposure to stresses such as chilling, salt, and UV illumination. But its transcription seems to be reduced by cadmium and copper treatment.

Ri-mediated Transformation of Glycyrrhiza uralensis with a Squalene Synthase Gene (GuSQS1) for Production of Glycyrrhizin

Root of Glycyrrhiza uralensis, one of the most important medicinal plants, containing bioactive triterpene saponins (glycyrrhizin). Squalene synthase (SQS) plays a regulatory role in the biosynthesis of triterpene saponins. In the present investigation, SQS coding sequence from G. uralensis was cloned by polymerase chain reaction (PCR) and a transgenic system was developed for G. uralensis through Agrobacterium rhizogenes-mediated transformation. The SQS gene placed under a CaMV 35S promoter was transferred into G. uralensis using A. rhizogenes strain ACCC10060. The transformed hairy roots were selected on Murashige and Skoog (1962)-containing phosphinothricin (PPT) and root lines were established. The integration of SQS gene was confirmed by PCR and Southern blot. Three transgenic root lines UP1, UP24, UP31 were obtained and their growth rates were detected. The result showed that transgenic root lines but UP1 line grew faster than control hairy roots; high-performance liquid chromatography (HPLC) analysis demonstrated the highest glycyrrhizin content of transgenic roots was 2.5 mg/g dry weight and was about 2.6 times higher than control hairy roots.

MedicCyc: a biochemical pathway database forMedicago truncatula

There is an imperative need to integrate functional genomics data to obtain a more comprehensive systems-biology view of the results. We believe that this is best achieved through the visualization of data within the biological context of metabolic pathways. Accordingly, metabolic pathway reconstruction was used to predict the metabolic composition for Medicago truncatula and these pathways were engineered to enable the correlated visualization of integrated functional genomics data. Metabolic pathway reconstruction was used to generate a pathway database for M. truncatula (MedicCyc), which currently features more than 250 pathways with related genes, enzymes and metabolites. MedicCyc was assembled from more than 225,000 M. truncatula ESTs (MtGI Release 8.0) and available genomic sequences using the Pathway Tools software and the MetaCyc database. The predicted pathways in MedicCyc were verified through comparison with other plant databases such as AraCyc and RiceCyc. The comparison with other plant databases provided crucial information concerning enzymes still missing from the ongoing, but currently incomplete M. truncatula genome sequencing project. MedicCyc was further manually curated to remove non-plant pathways, and Medicago-specific pathways including isoflavonoid, lignin and triterpene saponin biosynthesis were modified or added based upon available literature and in-house expertise. Additional metabolites identified in metabolic profiling experiments were also used for pathway predictions. Once the metabolic reconstruction was completed, MedicCyc was engineered to visualize M. truncatula functional genomics datasets within the biological context of metabolic pathways. freely accessible at http://www.noble.org/MedicCyc/

Genomics‐based selection and functional characterization of triterpene glycosyltransferases from the model legume Medicago truncatula

The biosynthesis of triterpene saponins is poorly characterized in spite of the importance of these glycosylated secondary metabolites for plant defense and animal health. The model legume Medicago truncatula synthesizes more than 30 different saponins based on at least five triterpene aglycones; soyasapogenols B and E, medicagenic acid, hederagenin and bayogenin. We have employed an inducible cell culture system, DNA array-based and in silico transcript profiling, and targeted metabolite profiling, to identify triterpene glycosyltransferases (GTs) from among the more than 300 GTs expressed in M. truncatula. Two uridine diphosphate glucosyltransferases were functionally characterized; UGT73K1 with specificity for hederagenin and soyasapogenols B and E, and UGT71G1 with specificity for medicagenic acid. The latter enzyme also glycosylated certain isoflavones and the flavonol quercetin with higher efficiency than triterpenes; however, integrated transcript and metabolite profiling supported a function for UGT71G1 in terpenoid but not (iso)flavonoid biosynthesis in the elicited cell cultures.

Enhanced triterpene and phytosterol biosynthesis in Panax ginseng overexpressing squalene synthase gene.

Roots of Panax ginseng, one of the most famous medicinal plants, contain various phytosterols and bioactive triterpene saponins (ginsenosides). In P. ginseng, phytosterols and triterpenes share the common biosynthetic intermediate, squalene. Here, we investigate the regulatory role of Panax ginseng squalene synthase (PgSS1) on the biosynthesis of phytosterols and triterpene saponins. PgSS1 transcripts are expressed ubiquitously in the various plant tissues, but higher in shoot apex and root. The transcript levels of PgSS1 increased markedly in the adventitious roots during 12- to 96-h period after metyl jasmonate (MeJA) treatment; MeJA treatment induced the activation of the transcripts of squalene epoxidase (SE), beta-amyrin synthase (bAS), but not cycloartenol synthase (CAS). Unlike MeJA treatment, overexpression of PgSS1 in adventitious roots of transgenic P. ginseng was followed by the up-regulation of all the downstream genes tested, such as SE, bAS, and CAS. The enhanced activity of PgSS1 enzyme resulted in remarkable increase of phytosterols as well as ginsenoside contents. These results demonstrate that PgSS1 is a key regulatory enzyme not only for phytosterol but also for triterpene biosynthesis and overexpressing of PgSS1 confers the hyperproduction of triterpene saponins to P. ginseng.

Up-regulation of soyasaponin biosynthesis by methyl jasmonate in cultured cells of Glycyrrhiza glabra.

Exogenously applied methyl jasmonate (MeJA) stimulated soyasaponin biosynthesis in cultured cells of Glycyrrhiza glabra (common licorice). mRNA level and enzyme activity of beta-amyrin synthase (bAS), an oxidosqualene cyclase (OSC) situated at the branching point for oleanane-type triterpene saponin biosynthesis, were up-regulated by MeJA, whereas those of cycloartenol synthase, an OSC involved in sterol biosynthesis, were relatively constant. Two mRNAs of squalene synthase (SQS), an enzyme common to both triterpene and sterol biosyntheses, were also up-regulated by MeJA. In addition, enzyme activity of UDP-glucuronic acid: soyasapogenol B glucuronosyltransferase, an enzyme situated at a later step of soyasaponin biosynthesis, was also up-regulated by MeJA. Accumulations of bAS and two SQS mRNAs were not transient but lasted for 7 d after exposure to MeJA, resulting in the high-level accumulation (more than 2% of dry weight cells) of soyasaponins in cultured licorice cells. In contrast, bAS and SQS mRNAs were coordinately down-regulated by yeast extract, and mRNA accumulation of polyketide reductase, an enzyme involved in 5-deoxyflavonoid biosynthesis in cultured licorice cells, was induced transiently by yeast extract and MeJA, respectively.

A genomics approach to the early stages of triterpene saponin biosynthesis in Medicago truncatula.

The saponins of the model legume Medicago truncatula are glycosides of at least five different triterpene aglycones: soyasapogenol B, soyasapogenol E, medicagenic acid, hederagenin and bayogenin. These aglycones are most likely derived from beta-amyrin, a product of the cyclization of 2,3-oxidosqualene. Mining M. truncatula EST data sets led to the identification of sequences putatively encoding three early enzymes of triterpene aglycone formation: squalene synthase (SS), squalene epoxidase (SE), and beta-amyrin synthase (beta-AS). SS was functionally characterized by expression in Escherichia coli, two forms of SE by complementation of the yeast erg1 mutant, and beta-AS by expression in yeast. Beta-amyrin was the sole product of the cyclization of squalene epoxide by the recombinant M. truncatulabeta-AS, as judged by GC-MS and NMR. Transcripts encoding beta-AS, SS and one form of SE were strongly and co-ordinately induced, associated with accumulation of triterpenes, upon exposure of M. truncatula cell suspension cultures to methyl jasmonate. Sterol composition remained unaffected by jasmonate treatment. Molecular verification of induction of the triterpene pathway in a cell culture system provides a new tool for saponin pathway gene discovery by DNA array-based approaches.