Saponin Biosynthesis Research Articles

A tandem array of UDP-glycosyltransferases from the UGT73C subfamily glycosylate sapogenins, forming a spectrum of mono- and bisdesmosidic saponins.

This study identifies six UGT73Cs all able to glucosylate sapogenins at positions 3 and/or 28 which demonstrates that B. vulgaris has a much richer arsenal of UGTs involved in saponin biosynthesis than initially anticipated. The wild cruciferous plant Barbarea vulgaris is resistant to some insects due to accumulation of two monodesmosidic triterpenoid saponins, oleanolic acid 3-O-β-cellobioside and hederagenin 3-O-β-cellobioside. Insect resistance depends on the structure of the sapogenin aglycone and the glycosylation pattern. The B. vulgaris saponin profile is complex with at least 49 saponin-like metabolites, derived from eight sapogenins and including up to five monosaccharide units. Two B. vulgaris UDP-glycosyltransferases, UGT73C11 and UGT73C13, O-glucosylate sapogenins at positions 3 and 28, forming mainly 3-O-β-D-glucosides. The aim of this study was to identify UGTs responsible for the diverse saponin oligoglycoside moieties observed in B. vulgaris. Twenty UGT genes from the insect resistant genotype were selected and heterologously expressed in Nicotiana benthamiana and/or Escherichia coli. The extracts were screened for their ability to glycosylate sapogenins (oleanolic acid, hederagenin), the hormone 24-epibrassinolide and sapogenin monoglucosides (hederagenin and oleanolic acid 3-O-β-D-glucosides). Six UGTs from the UGT73C subfamily were able to glucosylate both sapogenins and both monoglucosides at positions 3 and/or 28. Some UGTs formed bisdesmosidic saponins efficiently. At least four UGT73C genes were localized in a tandem array with UGT73C11 and possibly UGT73C13. This organization most likely reflects duplication events followed by sub- and neofunctionalization. Indeed, signs of positive selection on several amino acid sites were identified and modelled to be localized on the UGT protein surface. This tandem array is proposed to initiate higher order bisdesmosidic glycosylation of B. vulgaris saponins, leading to the recently discovered saponin structural diversity, however, not directly to known cellobiosidic saponins.

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

[This corrects the article DOI: 10.1371/journal.pone.0181784.].

Expression of saponin biosynthesis related genes in different tissues of Panax quinquefolius

The relationship between saponin content of Panax quinquefolius in different parts of the organization and expression of ginsenoside biosynthesis related gene was obtained by the correlation analysis between saponin content and gene expression. The 14 tissue parts of P. quinquefolius were studied, six saponins in P. quinquefolius. Samples (ginsenoside Rg₁, Re, Rb₁, Rc, Rb₂ and Rd), group saponins and total saponins were determined by high performance liquid chromatography and vanillin-sulfuric acid colorimetric method. Simultaneously, the expression levels of 7 ginsenoside biosynthesis related genes (SQS, OSC, DS, β-AS, SQE, P450 and FPS) in different tissues of P. quinquefolius were determined by Real-time fluorescence quantitative PCR. Although 7 kinds of ginsenoside biosynthesis related enzyme gene in the P. quinquefolius involved in ginsenoside synthesis, the expression of β-AS and P450 genes had no significant effect on the content of monosodium saponins, grouping saponins and total saponins, FPS, SQS, OSC, DS and SQE had significant or extremely significant on the contents of single saponins Re, Rg1, Rb1, Rd, group saponin PPD and PPT, total saponin TMS and total saponin TS (P<0.05 or P<0.01). The biosynthesis of partial saponins, grouping saponins and total saponins in P. quinquefolius was affected by the interaction of multiple enzyme genes in the saponin synthesis pathway, the content of saponins in different tissues of P. quinquefolius was determined by the differences in the expression of key enzymes in the biosynthetic pathway. Therefore, this study further clarified that FPS, SQS, OSC, DS and SQE was the key enzyme to control the synthesis of saponins in P. quinquefolius by correlation analysis, the biosynthesis of ginsenosides in P. quinquefolius was regulated by these five kind of enzymes in cluster co-expression of interaction mode.

Enhanced biosynthesis of saponins by coronatine in cell suspension culture of Kalopanax septemlobus.

Kalopanax septemlobus is a medicinal woody species of the family Araliaceae, and the pharmaceutical properties of saponins obtained from K. septemlobus suggest that K. septemlobus has the potential to be a crude drug and dietary health supplement. In this study, we established cell suspension culture of K. septemlobus to develop a sustainable source of natura-ceuticals. Friable calli were used for establishing cell suspension culture. The maximum amount of total saponins (1.56mg/60ml suspension) was obtained during the 15th day of incubation, whereas the maximum capacity of saponin production was reached after day 6 (0.42μg/mg of fresh weight). The total saponin production in the cell suspension of K. septemlobus was significantly increased by coronatine (COR) at 160% at a dose of 1μM compared with the mock-treated control, whereas methyl jasmonate treated cells exhibited less increase in total saponin level as compared to the COR-treated cells. In addition, the elicitation of COR strongly induced the expression of beta-amyrin synthase, thus resulting in the accumulation of oleanolic acid (2.369±0.98μg/mg of extract), a precursor for oleanane-type triterpene saponins. These results indicate that COR is an efficient elicitor for inducing phytochemicals in cell suspension culture and that it provides the possibility for producing saponins of K. septemlobus using cell suspension culture.

Cloning and Expression Analysis of a Squalene Epoxidase Gene from Ginkgo biloba

Squalene epoxidase is a key enzyme involved in triterpene saponins biosynthetic pathways in plants. In this study, the SE gene was isolated from Ginkgo biloba by RT-PCR, which designated as GbSE, GenBank accession number: KY751713. The length of GbSE gene is 1646 bp, contains an open reading frame of 1617 bp encoding 538 amino acids. The theoretical molecular weight and pI of GbSE protein was 58.3 kDa and 8.35, respectively. Sequence multiple alignment found that GbSE protein had high homology with SE proteins from other plants, including several highly conservative motifs and amino acids. Phylogenetic tree analysis showed that Ginkgo biloba SE belonged to the gymnospermous group, and was closely related to the SE protein of Picea sitchensis. A study of gene expression analysis indicated that the GbSE gene was highly expressed in stems, low expressed in fruits. The cDNA sequence of GbSE gene was cloned from G. biloba and the expression level of this gene was analyzed. This work can provide some theoretical basis for the research on the molecular mechanism of triterpenoid saponin biosynthesis.

Identification, characterization and expression analysis of genes involved in steroidal saponin biosynthesis in Dracaena cambodiana.

Dracaena cambodiana is a traditional medicinal plant used for producing dragon's blood. The plants and dragon's blood of D. cambodiana contain a rich variety of steroidal saponins. However, little is known about steroidal saponin biosynthesis and its regulation in D. cambodiana. Here, 122 genes encoding enzymes involved in steroidal saponin biosynthesis were identified based on transcriptome data, with 29 of them containing complete open reading frames (ORF). Transcript expression analysis revealed that several genes related to steroidal saponin biosynthesis showed distinct tissue-specific expression patterns; the expression levels of genes encoding the key enzymes involved in the biosynthesis and early modification of steroidal saponins were significantly down-regulated in the stems in response to the inducer of dragon's blood, exhibiting positive correlations with the content of steroidal saponins. These results provide insights on the steroidal saponins biosynthetic pathway and mechanisms underlying induced formation of dragon's blood in D. cambodiana.

Functional characterization of the β-amyrin synthase gene involved in platycoside biosynthesis in Platycodon grandiflorum

Platycodon grandiflorum A. DC. contains a variety of triterpene saponins induced from the base structure of oleanane-type in roots. The β-amyrin synthase (bAS) plays an important role in controlling the biosynthesis of triterpene saponins and we obtained the full-length cDNA of the gene, which is named PlgOSC1 in P. grandiflorum. To identify the PlgOSC1 products, we expressed it in heterologous yeast cells. Results of GC-MS analysis showed a unique peak that was consistent with that of the authentic β-amyrin standard with exactly the same retention time. Furthermore, the pattern of MS fragments of the peak was also exactly the same as that of the β-amyrin. In qRT-PCR analysis of four organs of P. grandiflorum, no difference in PlgOSC1 expression level was observed; therefore, platycosides derived from β-amyrin might be present in all P. grandiflorum tissues. In conclusion, PlgOSC1 encodes a bAS enzyme that catalyzes the first committed step of platycosides biosynthesis in P. grandiflorum.

Small RNA profiling for identification of miRNAs involved in regulation of saponins biosynthesis in Chlorophytum borivilianum

BackgroundMicroRNAs act as molecular regulator of cell signaling, plant growth and development, and regulate various primary and secondary plant metabolic processes. In the present study, deep sequencing of small RNAs was carried out to identify known and novel miRNAs from pharmaceutically important plant, Chlorophytum borivilianum.ResultsTotal 442 known miRNAs and 5 novel miRNAs were identified from young leaf small RNA library. Experimental validation with stem loop RT-PCR confirmed the in silico identification. Based on transcriptome data of root and leaf of C. borivilianum, Oryza sativa, and Arabidopsis thaliana target gene prediction was done using psRNAtarget and mirRanda. BLAST2GO helped in localization of predicted targets and KEGG (Kyoto Encyclopedia for Genes and Genomes) pathway analysis concluded that miR9662, miR894, miR172, and miR166 might be involved in regulating saponin biosynthetic pathway. The correlation between miRNA and its target gene was further validated by RT-qPCR analysis.ConclusionThis study provides first elaborated glimpse of miRNA pool of C. borivilianum, which can help to understand the miRNA dependent regulation of saponin biosynthesis and to design further metabolic engineering experiment to enhance their contents in the plant.

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.

Cytochrome P450 CYP716A254 catalyzes the formation of oleanolic acid from β-amyrin during oleanane-type triterpenoid saponins biosynthesis in Anemone flaccida

Anemone flaccida Fr. Shmidt (Ranunculaceae), known as ‘Di Wu’ in China, is a perennial herb which has long been used to treat arthritis. The rhizome of A. flaccida contains pharmacologically active components i.e. oleanane-type triterpenoid saponins. Oleanolic acid is natural triterpenoid in plants with diverse biological activities. The biosynthesis of oleanolic acid involves cyclization of 2,3-oxidosqualene to the oleanane-type triterpenoid skeleton, followed by a series of oxidation reactions catalyzed by cytochrome P450 monooxygenase (CYP450). Previously, we identified four possible cytochrome P450 genes belonging to CYP716A subfamily from the transcriptome of A. flaccida. In this study, we identified one of those genes “CYP716A254” encoding a cytochrome P450 monooxygenase from A. flaccida that catalyzes the conversion of the β-amyrin into oleanolic acid. The heterologous expression of CYP716A254 in yeast resulted in oxidation of β-amyrin at the C-18 position to oleanolic acid production. These results provide an important basis for further studies of oleanane-type triterpenoid saponins synthesis in A. flaccida.

Two Cycloartenol Synthases for Phytosterol Biosynthesis in Polygala tenuifolia Willd.

Oxidosqualene cyclases (OSCs) are enzymes that play a key role in control of the biosynthesis of phytosterols and triterpene saponins. In order to uncover OSC genes from Polygala tenuifolia seedlings induced by methyl jasmonate (MeJA), RNA-sequencing analysis was performed using the Illumina sequencing platform. A total of 148,488,632 high-quality reads from two samples (control and the MeJA treated) were generated. We screened genes related to phytosterol and triterpene saponin biosynthesis and analyzed the transcriptional changes of differentially expressed unigene (DEUG) values calculated by fragments per kilobase million (FPKM). In our datasets, two full-length cDNAs of putative OSC genes, PtCAS1, and PtCAS2, were found, in addition to the PtBS (β-amyrin synthase) gene reported in our previous studies and the two cycloartenol synthase genes of P. tenuifolia. All genes were isolated and characterized in yeast cells. The functional expression of the two PtCAS genes in yeast cells showed that the genes all produce a cycloartenol as the sole product. When qRT-PCR analysis from different tissues was performed, the expressions of PtCAS1 and PtCAS2 were highest in flowers and roots, respectively. After MeJA treatment, the transcripts of PtCAS1 and PtCAS2 genes increased by 1.5- and 2-fold, respectively. Given these results, we discuss the potential roles of the two PtCAS genes in relation to triterpenoid biosynthesis.

Changes in fenugreek transcriptome induced by methyl jasmonate and steroid precursors revealed by RNA-Seq

Changes in fenugreek transcriptome related to enhanced production of steroids were induced by methyl jasmonate, cholesterol and squalene, and recorded using RNA-seq. A total of 112,850 unigenes were obtained after de novo assembling of next generation sequencing data, and used for functional annotations. In steroidal saponins pathway, transcripts involved in mevalonate, terpenoid backbone and plant sterol synthesis were annotated. Overexpression of several transcripts from phytosterol biosynthesis pathway was confirmed by quantitative RT-PCR. In diosgenin biosynthesis pathway, fatty acid ω-hydroxylase (CYP86A2) and steroid 22-alpha-hydroxylase (CYP90B1) genes were annotated in all induced transcriptomes. Moreover, direct sequencing confirmed increased levels of CYP90B1, unspecific monooxygenase and 26-hydroxylase genes in plants with elevated level of diosgenin. New unigenes corresponding to enzymes involved in biosynthesis of diosgenin from cycloartenol via cholesterol were obtained and the role of CYP72A family in steroidal saponin biosynthesis was proposed. Additional support for biosynthetic pathway from cycloartenol to diosgenin was provided.

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.

De novo leaf and root transcriptome analysis to identify putative genes involved in triterpenoid saponins biosynthesis in Hedera helix L.

Hedera helix L. is an important traditional medicinal plant in Europe. The main active components are triterpenoid saponins, but none of the potential enzymes involved in triterpenoid saponins biosynthesis have been discovered and annotated. Here is reported the first study of global transcriptome analyses using the Illumina HiSeq™ 2500 platform for H. helix. In total, over 24 million clean reads were produced and 96,333 unigenes were assembled, with an average length of 1385 nt; more than 79,085 unigenes had at least one significant match to an existing gene model. Differentially Expressed Gene analysis identified 6,222 and 7,012 unigenes which were expressed either higher or lower in leaf samples when compared with roots. After functional annotation and classification, two pathways and 410 unigenes related to triterpenoid saponins biosynthesis were discovered. The accuracy of these de novo sequences was validated by RT-qPCR analysis and a RACE clone. These data will enrich our knowledge of triterpenoid saponin biosynthesis and provide a theoretical foundation for molecular research on H. helix.

Molecular Cloning and Characterization of PnbHLH1 Transcription Factor in Panax notoginseng.

Panax notoginseng has been extensively used as a traditional Chinese medicine. In the current study, molecular cloning and characterization of PnbHLH1 transcription factor were explored in Panax notoginseng. The full length of the PnbHLH1 gene obtained by splicing was 1430 bp, encoding 321 amino acids. Prokaryotic expression vector pET-28a-PnbHLH1 was constructed and transferred into the BL21 prokaryotic expression strain. An electrophoretic mobility shift assay of PnbHLH1 protein binding to E-box cis-acting elements verified that PnbHLH1 belonged to the bHLH class transcription factor which could interact with the promoter region of the E-box core sequence. The expression levels of key genes involved in the biosynthesis of triterpenoid saponins in PnbHLH1 transgenic cells were higher than those in the wild cells. Similarly, the total saponin contents were increased in the PnbHLH1 transgenic cell lines compared with the wild cell lines. Such results suggest that the PnbHLH1 transcription factor is a positive regulator in the biosynthesis of triterpenoid saponins in Panax notoginseng.

Comparative transcriptome analysis of shoot and root tissue of Bacopa monnieri identifies potential genes related to triterpenoid saponin biosynthesis

BackgroundBacopa monnieri commonly known as Brahmi is utilized in Ayurveda to improve memory and many other human health benefits. Bacosides enriched standardized extract of Bacopa monnieri is being marketed as a memory enhancing agent. In spite of its well known pharmacological properties it is not much studied in terms of transcripts involved in biosynthetic pathway and its regulation that controls the secondary metabolic pathway in this plant. The aim of this study was to identify the potential transcripts and provide a framework of identified transcripts involved in bacosides production through transcriptome assembly.ResultsWe performed comparative transcriptome analysis of shoot and root tissue of Bacopa monnieri in two independent biological replicate and obtained 22.48 million and 22.0 million high quality processed reads in shoot and root respectively. After de novo assembly and quantitative assessment total 26,412 genes got annotated in root and 18,500 genes annotated in shoot sample. Quality of raw reads was determined by using SeqQC-V2.2. Assembled sequences were annotated using BLASTX against public database such as NR or UniProt. Searching against the KEGG pathway database indicated that 37,918 unigenes from root and 35,130 unigenes from shoot were mapped to 133 KEGG pathways. Based on the DGE data we found that most of the transcript related to CYP450s and UDP-glucosyltransferases were specifically upregulated in shoot tissue as compared to root tissue. Finally, we have selected 43 transcripts related to secondary metabolism including transcription factor families which are differentially expressed in shoot and root tissues were validated by qRT-PCR and their expression level were monitored after MeJA treatment and wounding for 1, 3 and 5 h.ConclusionsThis study not only represents the first de novo transcriptome analysis of Bacopa monnieri but also provides information about the identification, expression and differential tissues specific distribution of transcripts related to triterpenoid sapogenin which is one of the most important pharmacologically active secondary metabolite present in Bacopa monnieri. The identified transcripts in this study will establish a foundation for future studies related to carrying out the metabolic engineering for increasing the bacosides biosynthesis and its regulation for human health benefits.

De novo assembly and annotation of the Zhe-Maidong (Ophiopogon japonicus (L.f.) Ker-Gawl) transcriptome in different growth stages

Zhe-Maidong (Ophiopogon japonicus (L.f.) Ker-Gawl) is a traditional medicinal herb in the family Liliaceae that has significant pharmacological effects on immunity and cardiovascular disease. In this study, three different growth stages of Zhe-Maidong were investigated using RNA-seq, and a total of 16.4 Gb of raw data was obtained. After filtering and assembling, 96,738 unigenes with an average length of 605.3 bp were ultimately generated. A total of 77,300 unigenes were annotated using information from five databases, including the NT, NR, SwissProt, Kyoto Encyclopedia of Genes and Genomes (KEGG) and Gene Ontology (GO) databases. Additionally, the mechanisms of flavonoid, saponin and polysaccharide biosynthesis and of accumulation at different stages of tuber development were also characterized. From the first to third years, the contents of flavonoids, saponins and polysaccharides all increased, whereas the expression levels of related genes decreased in the flavonoid and saponin pathways and first increased and then decreased in the polysaccharide pathway. The results of this study provide the most comprehensive expressed sequence resource for Zhe-Maidong and will expand the available O. japonicus gene library and facilitate further genome-wide research and analyses of this species.

Cytochrome P450 Monooxygenase CYP716A141 is a Unique b-Amyrin C-16b Oxidase Involved in Triterpenoid Saponin Biosynthesis in Platycodon grandiflorus.

The roots of Platycodon grandiflorus are widely used as a crude drug. The active components include a variety of triterpenoid saponins. Recent studies have revealed that Cyt P450 monooxygenases (P450s) function as triterpene oxidases in triterpenoid saponin biosynthesis in many plant species. However, there have been no reports regarding triterpene oxidases in P. grandiflorus. In this study, we performed transcriptome analysis of three different P. grandiflorus tissues (roots, leaves and petals) using RNA sequencing (RNA-Seq) technology. We cloned six P450 genes that were highly expressed in roots, and classified them as belonging to the CYP716A, CYP716D and CYP72A subfamilies. We heterologously expressed these P450s in an engineered yeast strain that produces β-amyrin, one of the most common triterpenes in plants. Two of the CYP716A subfamily P450s catalyzed oxidation reactions of the β-amyrin skeleton. One of these P450s, CYP716A140v2, catalyzed a three-step oxidation reaction at C-28 on β-amyrin to produce oleanolic acid, a reaction performed by CYP716A subfamily P450s in a variety of plant species. The other P450, CYP716A141, catalyzed the hydroxylation of β-amyrin at C-16β. This reaction is unique among triterpene oxidases isolated to date. These results enhance our knowledge of functional variation among CYP716A subfamily enzymes involved in triterpenoid biosynthesis, and provide novel molecular tools for use in synthetic biology to produce triterpenoid saponins with pre-defined structures.

Molecular cloning of an ester-forming triterpenoid: UDP-glucose 28-O-glucosyltransferase involved in saponin biosynthesis from the medicinal plant Centella asiatica

Triterpene saponins include bioactive compounds with structures consisting of triterpene aglycones (sapogenins) and one or more sugar moieties linked through acetal or ester glycosidic linkages at one or more sites. Centella asiatica (L.) Urban is a medicinal plant that contains bioactive ursane-type saponins, such as madecassoside and asiaticoside. In this work, glucosylation of triterpenoids in C. asiatica was investigated starting with plant extracts. An enzyme capable of glucosylating asiatic and madecassic acids was partially purified. Proteomics methods and cDNA sequence data were employed as tools to obtain a full-length cDNA clone encoding a glucosyltransferase. The recombinant gene product, UGT73AD1, was functionally expressed in Escherichia coli and purified by immobilized metal-affinity chromatography. Purified recombinant UGT73AD1 was found to have a narrow specificity, glucosylating asiatic and madecassic acids at the C28 carboxyl. mRNA accumulated in all tissues tested (leaves, stems, roots and flowers), with highest expression in leaves. Thus, UGT73AD1 was identified as a triterpenoid carboxylic acid: UDP-glucose 28-O-glucosyltransferase that appears to be involved in saponin biosynthesis in C. asiatica.

Transcriptomic Comparison Reveals Candidate Genes for Triterpenoid Biosynthesis in Two Closely Related Ilex Species.

Native to Southern China, Ilex pubescens and Ilex asprella are frequently used in traditional Chinese medicine. Both of them produce a large variety of ursane-type triterpenoid saponins, which have been demonstrated to have different pharmacological effects. However, little is known about their biosynthesis. In this study, transcriptomic analysis of I. pubescens and comparison with its closely related specie I. asprella were carried out to identify potential genes involved in triterpenoid saponin biosynthesis. Through RNA sequencing (RNA-seq) and de novo transcriptome assembly of I. pubescens, a total of 68,688 UniGene clusters are obtained, of which 32,184 (46.86%) are successfully annotated by comparison with the sequences in major public databases (NCBI, Swiss-Prot, and KEGG). It includes 128 UniGenes related to triterpenoid backbone biosynthesis, 11 OSCs (oxidosqualene cyclases), 233 CYPs (cytochrome P450), and 269 UGTs (UDP-glycosyltransferases). By homology-based blast and phylogenetic analysis with well-characterized genes involved in triterpenoid saponin biosynthesis, 5 OSCs, 14 CYPs, and 1 UGT are further proposed as the most promising candidate genes. Transcriptomic comparison between two Ilex species using blastp and OrthoMCL method reveals high sequence similarity. All OSCs and UGTs as well as most CYPs are classified as orthologous genes, while only 5 CYPs in I. pubescens and 3 CYPs in I. asprella are species-specific. One of OSC candidates, named as IpAS1, was successfully cloned and expressed in Saccharomyces cerevisiae INVSc1. Analysis of the yeast extract by gas chromatography (GC) and gas chromatography–mass spectrometry (GC-MS) shows IpAS1 is a mixed amyrin synthase, producing α-amyrin and β-amyrin at ratio of 5:1, which is similar to its ortholog IaAS1 from I. asprella. This study is the first exploration to profile the transcriptome of I. pubescens, the generated data and gene models will facilitate further molecular studies on the physiology and metabolism in this plant. By comparative transcriptomic analysis, a series of candidate genes involved in the biosynthetic pathway of triterpenoid saponins are identified, providing new insight into their biosynthesis at transcriptome level.