UDP-glucuronic acid:soyasapogenol glucuronosyltransferase involved in saponin biosynthesis in germinating soybean seeds

UDP-glucosyltransferase 71C4 controls the flux of phenylpropanoid metabolism to shape cotton seed development

Seeds play a crucial role in plant reproduction, making it essential to identify genes that affect seed development. In this study, we focused on UDP-glucosyltransferase 71C4 (UGT71C4) in cotton, a member of the glycosyltransferase family that shapes seed width and length, thereby influencing seed index and seed cotton yield. Overexpression of UGT71C4 results in seed enlargement owing to its glycosyltransferase activity on flavonoids, which redirects metabolic flux from lignin to flavonoid metabolism. This shift promotes cell proliferation in the ovule via accumulation of flavonoid glycosides, significantly enhancing seed cotton yield and increasing the seed index from 10.66 g to 11.91 g. By contrast, knockout of UGT71C4 leads to smaller seeds through activation of the lignin metabolism pathway and redirection of metabolic flux back to lignin synthesis. This redirection leads to increased ectopic lignin deposition in the ovule, inhibiting ovule growth and development, and alters yield components, increasing the lint percentage from 41.42% to 43.40% and reducing the seed index from 10.66 g to 8.60 g. Our research sheds new light on seed size development and reveals potential pathways for enhancing seed yield.

Applications of protein engineering in the microbial synthesis of plant triterpenoids

Triterpenoids are a class of natural products widely used in fields related to medicine and health due to their biological activities such as hepatoprotection, anti-inflammation, anti-viral, and anti-tumor. With the advancement in biotechnology, microorganisms have been used as cell factories to produce diverse natural products. Despite the significant progress that has been made in the construction of microbial cell factories for the heterogeneous biosynthesis of triterpenoids, the industrial production of triterpenoids employing microorganisms has been stymied due to the shortage of efficient enzymes as well as the low expression and low catalytic activity of heterologous proteins in microbes. Protein engineering has been demonstrated as an effective way for improving the specificity, catalytic activity, and stability of the enzyme, which can be employed to overcome these challenges. This review summarizes the current progress in the studies of Oxidosqualene cyclases (OSCs), cytochrome P450s (P450s), and UDP-glycosyltransferases (UGTs), the key enzymes in the triterpenoids synthetic pathway. The main obstacles restricting the efficient catalysis of these key enzymes are analyzed, the applications of protein engineering for the three key enzymes in the microbial synthesis of triterpenoids are systematically reviewed, and the challenges and prospects of protein engineering are also discussed.

Uncovering the multi-level response of Glycine max L. to the application of allelopathic biostimulant from Levisticum officinale Koch

The interest expressed by the agriculture in the category of innovative biostimulants is due to the intensive search for natural preparations. Our study is the first ever to report a complex approach to the use of allelopathic extracts from Levisticum officinale Koch. roots in soybean cultivation, includes analyses of morphological observations, and analyses of biochemical indicators. Hot method of aqueous extraction was applied. The extracts were administered via foliar application and soil treatment. Lovage extracts had high contents of polyphenolic compounds and rich micro- and macroelemental composition. The infusions did not contain gibberellic acid and indole-3-acetic acid but the abscisic acid and saccharose, glucose, and fructose were found. The extracts modified soybean plant physiology, as manifested by changes in biometric traits. Plants responded positively by increased yield. Seeds from the treated plants had higher contents of micro- and macroelements, as well as total concentrations of lipids (with a slight decrease in protein content). In addition, they featured changes in their amino acid profile and fatty acid composition. The application of allelopathic biostimulant caused increased concentrations of isoflavones and saponins. The natural biostimulants from Levisticum officinale may become a valuable tool in the sustainable agriculture.

Dynamics of Germination Behaviour, Protein Secondary Structure, Technofunctional Properties, Antinutrients, Antioxidant Capacity and Mineral Elements in Germinated Dhaincha

Research background

Dhaincha (Sesbania aculeata) is a forage legume primarily used for green manuring and animal feed. Good nutritional profile of dhaincha makes it a potential alternative legume in human nutrition. However, the presence of high amount of antinutrients poses a problem in its utilisation for food applications. The present investigation intends to germinate dhaincha seeds at different time-temperature regimes and to evaluate the process of germination to ascertain optimal conditions and improve its potential for utilisation.

Experimental approach

Dhaincha seeds were germinated at 24, 28 and 32 °C for 24, 48 and 72 h. Germination characteristics and germination loss, spectral characteristics, technofunctionality, antinutrients, bioactive constituents, antioxidant capacity and mineral element content of germinated dhaincha were evaluated. Optimal balance of technobiofunctionality of germinated dhaincha seeds was validated by principal component analysis.

Results and conclusions

Sprout length and germination loss increased with the higher germination temperature and prolonged germination time. Seeds showed similar germination rate at 28 and 32 °C and it was markedly higher than at 24 °C. Germination for 24 h resulted in mild conformational changes in the secondary structure of proteins, whereas germination for 48 and 72 h exhibited major conformational changes in the β-sheets, resulting in the improvement in the hydration and foaming properties. Progression of germination (72 h) caused the decrease of tannin (24.47%), phytic acid (16.38%) and saponin (24.58%) mass fractions, and of trypsin inhibitor (40.33%) and lectin activity (62.50%). Slight decrease of DPPH˙ (3.7%) and ABTS˙⁺ (18.5%) values was also observed, whereas total flavonoid content (36.14%) and metal chelating activity (26.76%) increased. Total phenolics, FRAP, and reducing power decreased after 24 h, followed by a gradual increase. Zinc extractability increased drastically with germination. Germination at 28 °C for 72 h resulted in higher reduction of antinutrients with optimal retention of antioxidant activity and better functional characteristics, as validated by principal component analysis.

Novelty and scientific contribution

Dhaincha is an unknown crop in Europe, and even in Asia it is predominantly used as green manure and animal feed. This research demonstrated that the intervention in germination can transform dhaincha into a promising crop for food industry. Germinated dhaincha exhibited enhanced technobiofunctionality for utilisation in various food formulations.

Secretion dynamics of soyasaponins in soybean roots and effects to modify the bacterial composition

Soyasaponins are triterpenoid saponins widely found in legume plants. These compounds have drawn considerable attention because they have various activities beneficial for human health, and their biosynthesis has been actively studied. In our previous study, we found that legume plants including soybean secrete soyasaponins from the roots in hydroponic culture throughout the growth period, but the physiological roles of soyasaponins in the rhizosphere and their fate in soil after exudation have remained unknown. This study demonstrates that soyasaponins are secreted from the roots of field-grown soybean, and soyasaponin Bb is the major soyasaponin detected in the rhizosphere. In vitro analysis of the distribution coefficient suggested that soyasaponin Bb can diffuse over longer distances in the soil in comparison with daidzein, which is a typical isoflavone secreted from soybean roots. The degradation rate of soyasaponin Bb in soil was slightly faster than that of daidzein, whereas no soyasaponin Bb degradation was observed in autoclaved soil, suggesting that microbes utilize soyasaponins in the rhizosphere. Bacterial community composition was clearly influenced by soyasaponin Bb, and potential plant growth-promoting rhizobacteria such as Novosphingobium were significantly enriched in both soyasaponin Bb-treated soil and the soybean rhizosphere. These results strongly suggest that soyasaponin Bb plays an important role in the enrichment of certain microbes in the soybean rhizosphere.

Plant terpenoid metabolism co-opts a component of the cell wall biosynthesis machinery

Glycosylation is one of the most prevalent molecular modifications in nature. Single or multiple sugars can decorate a wide range of acceptors from proteins to lipids, cell wall glycans and small molecules, dramatically affecting their activity. Here, we discovered that by 'hijacking' an enzyme of the cellulose synthesis machinery involved in cell wall assembly, plants evolved cellulose synthase-like enzymes (Csls) and acquired the capacity to glucuronidate specialized metabolites, that is, triterpenoid saponins. Apparently, endoplasmic reticulum-membrane localization of Csls and of other pathway proteins was part of evolving a new glycosyltransferase function, as plant metabolite glycosyltransferases typically act in the cytosol. Discovery of glucuronic acid transferases across several plant orders uncovered the long-pursued enzymatic reaction in the production of a low-calorie sweetener from licorice roots. Our work opens the way for engineering potent saponins through microbial fermentation and plant-based systems.

Triterpenoid-biosynthetic UDP-glycosyltransferases from plants

Triterpenoid saponins are naturally occurring structurally diverse glycosides of triterpenes that are widely distributed among plant species. Great interest has been expressed by pharmaceutical and agriculture industries for the glycosylation of triterpenes. Such modifications alter their taste and bio-absorbability, affect their intra-/extracellular transport and storage in plants, and induce novel biological activities in the human body. Uridine diphosphate (UDP)-glycosyltransferases (UGTs) catalyze glycosylation using UDP sugar donors. These enzymes belong to a multigene family and recognize diverse natural products, including triterpenes, as the acceptor molecules. For this review, we collected and analyzed all of the UGT sequences found in Arabidopsis thaliana as well as 31 other species of triterpene-producing plants. To identify potential UGTs with novel functions in triterpene glycosylation, we screened and classified those candidates based on similarity with UGTs from Panax ginseng, Glycine max, Medicago truncatula, Saponaria vaccaria, and Barbarea vulgaris that are known to function in glycosylate triterpenes. We highlight recent findings on UGT inducibility by methyl jasmonate, tissue-specific expression, and subcellular localization, while also describing their catalytic activity in terms of regioselectivity for potential key UGTs dedicated to triterpene glycosylation in plants. Discovering these new UGTs expands our capacity to manipulate the biological and physicochemical properties of such valuable molecules.

Comparative analysis of proteomic and metabolomic profiles of different species of Paris

An extract prepared from species of Paris is the most widely consumed herbal product in China. The genus Paris includes a variety of genotypes with different medicinal component contents but only two are defined as official sources. Closely related species have different medicinal properties because of differential expression of proteins and metabolites. To better understand the molecular basis of these differences, we examined proteomic and metabolomic changes in rhizomes of P. polyphylla var. chinensis, P. polyphylla var. yunnanensis, and P. fargesii var. fargesii using a technique known as sequential window acquisition of all theoretical mass spectra as well as gas chromatography-time-of-flight mass spectrometry. In total, 419 proteins showed significant abundance changes, and 33 metabolites could be used to discriminate Paris species. A complex analysis of proteomic and metabolomic data revealed a higher efficiency of sucrose utilization and an elevated protein abundance in the sugar metabolic pathway of P. polyphylla var. chinensis. The pyruvate content and efficiency of acetyl-CoA-utilization in saponin biosynthesis were also higher in P. polyphylla var. chinensis than in the other two species. The results expand our understanding of the proteome and metabolome of Paris and offer new insights into the species-specific traits of these herbaceous plants. SIGNIFICANCE: The traditional Chinese medicine Paris is the most widely consumed herbal product for the treatment of joint pain, rheumatoid arthritis and antineoplastic. All Paris species have roughly the same morphological characteristics; however, different members have different medicinal compound contents. Efficient exploitation of genetic diversity is a key factor in the development of rare medicinal plants with improved agronomic traits and malleability to challenging environmental conditions. Nevertheless, only a partial understanding of physiological and molecular mechanisms of different plants of Paris can be achieved without proteomics. To better understand the molecular basis of these differences and facilitate the use of other Paris species, we examine proteomic metabolomic changes in rhizomes of Paris using the technique known as SWATH-MS and GC/TOF-MS. Our research has provided information that can be used in other studies to compare metabolic traits in different Paris species. Our findings can also serve as a theoretical basis for the selection and cultivation of other Paris species with a higher medicinal value.

Transcriptome analysis of Panax zingiberensis identifies genes encoding oleanolic acid glucuronosyltransferase involved in the biosynthesis of oleanane-type ginsenosides

Oleanolic acid glucuronosyltransferase (OAGT) genes synthesizing the direct precursor of oleanane-type ginsenosides were discovered. The four recombinant proteins of OAGT were able to transfer glucuronic acid at C-3 of oleanolic acid that yields oleanolic acid 3-O-β-glucuronide. Ginsenosides are the primary active components in the genus Panax, and great efforts have been made to elucidate the mechanisms underlying dammarane-type ginsenoside biosynthesis. However, there is limited information on oleanane-type ginsenosides. Here, high-performance liquid chromatography analysis demonstrated that oleanane-type ginsenosides (particularly ginsenoside Ro and chikusetsusaponin IV and IVa) are the abundant ginsenosides in Panax zingiberensis, an extremely endangered Panax species in southwest China. These ginsenosides are derived from oleanolic acid 3-O-β-glucuronide, which may be formed from oleanolic acid catalyzed by an unknown oleanolic acid glucuronosyltransferase (OAGT). Transcriptomic analysis of leaves, stems, main roots, and fibrous roots of P. zingiberensis was performed, and a total of 46,098 unigenes were obtained, including all the identified homologous genes involved in ginsenoside biosynthesis. The most upstream genes were highly expressed in the leaves, and the UDP-glucosyltransferase genes were highly expressed in the roots. This finding indicated that the precursors of ginsenosides are mainly synthesized in the leaves and transported to different parts for the formation of particular ginsenosides. For the first time, enzyme activity assay characterized four genes (three from P. zingiberensis and one from P. japonicus var. major, another Panax species with oleanane-type ginsenosides) encoding OAGT, which particularly transfer glucuronic acid at C-3 of oleanolic acid to form oleanolic acid 3-O-β-glucuronide. Taken together, our study provides valuable genetic information for P. zingiberensis and the genes responsible for synthesizing the direct precursor of oleanane-type ginsenosides.

Soyasaponins: A New Class of Root Exudates in Soybean (Glycine max)

Root exudates are plant metabolites secreted from the roots into the soil. These exudates are involved in many important biological processes, including acquisition of nutrients, defense and signaling to rhizosphere bacteria, such as isoflavones of soybean crucial for the symbiosis with rhizobium. Less is known, however, about other types of root exudates. This study shows that soybean roots secrete large amounts of soyasaponins (triterpenoid glycosides) as root exudates. The soyasaponins are classified into four groups, with group A being the most secreted of these compounds, whereas DDMP (2,3-dihydro-2,5-dihydroxy-6-methyl-4H-pyran-4-one) soyasaponins is the group showing greatest accumulation in root tissues, suggesting a selection system for secreted compounds. Time-course experiments showed that the soyasaponin secretion peaked during early vegetative stages. In particular, soyasaponin Ah was the major compound secreted by soybean roots, whereas the deacetylated derivative Af was the major compound secreted specifically during the VE stage. The secretion of soyasaponins containing glycosyl moieties is an apparent loss of photosynthates. This phenomenon has been also observed in other legume species, although the composition of secreted soyasaponins is plant species dependent. The identification of triterpenoid saponins as major metabolites in legume root exudates will provide novel insights into chemical signaling in the rhizosphere between plants and other organisms.

Functional characterization of naturally occurring wild soybean mutant (sg-5) lacking astringent saponins using whole genome sequencing approach

Triterpenoid saponins are one of the most highly accumulated groups of functional components in soybean (Glycine max) and the oxidative reactions during their biosynthesis are required for their aglycone diversity. Natural mutants of soyasaponins in wild soybean (Glycine soja) are valuable resources for establishing the soyasaponin biosynthesis pathway and breeding new soybean varieties. In this study, we investigated the genetic mechanism behind the absence of group A saponins in a Korean wild soybean mutant, CWS5095. Whole genome sequencing (WGS) of CWS5095 identified four point mutations [Val6 → Asp, Ile231 → Thr, His294 → Gln, and Arg376 → Lys] in CYP72A69 (Glyma15g39090), which oxygenate the C-21 position of soyasapogenol B or other intermediates to produce soyasapogenol A, leading to group A saponin production. An in vitro enzyme activity assay of single-sited mutated clones indicated that the Arg376 > Lys mutation (a highly conserved mutation based on a nucleotide change from G → A at the 1,127th position) may lead to loss of gene function in the sg-5 mutant. A very high normalized expression value of 377 reads per kilo base per million (RPKM) of Glyma15g39090 in the hypocotyl axis at the early maturation seed-development stage confirmed their abundant presence in seed hypocotyls. A molecular dynamics analysis of the Arg376 > Lys mutation based on the CYP3A4 (a human CYP450) protein structure found that it was responsible for the increase in axis length toward the heme (active site), which is critically important for biological activity and ligand binding. Our results provide important information on how to eradicate bitter and astringent saponins in soybean by utilizing the reported mutation in Glyma15g39090, and its importance for seed hypocotyl development based on transcript abundance.

Determination of soyasaponins in Fagioli di Sarconi beans (Phaseolus vulgaris L.) by LC-ESI-FTICR-MS and evaluation of their hypoglycemic activity

Soyasaponins are oleanene-type triterpenoid saponins, naturally occurring in many edible plants that have attracted a great deal of attention for their role in preventing chronic diseases. The aim of this study was to establish the distribution and the content of soyasaponins in 21 ecotypes of Fagioli di Sarconi beans (Phaseolus vulgaris, Leguminosae). High-performance reversed-phase liquid chromatography (RPLC) with positive electrospray ionization (ESI(+)) and Fourier transform ion cyclotron resonance (FTICR) mass spectrometry (MS) in conjunction with infrared multiphoton dissociation (IRMPD) was applied for the unambiguous identification of soyasaponins Ba (m/z 959.5213, [C₄₈H₇₉O₁₉]⁺), Bb (m/z 943.5273, [C₄₈H₇₉O₁₈]⁺), Bd (m/z 957.5122, [C₄₈H₇₇O₁₉]⁺), and Be (m/z 941.5166, [C₄₈H₇₇O₁₈]⁺), which are the only commercially available reference standards. In addition, the several diagnostic product ions generated by IRMPD in the ICR-MS cell allowed us the putative identification of soyasaponins Bb' (m/z 797.4680, [C₄₂H₆₉O₁₄]⁺), αg (m/z 1085.5544, [C₅₄H₈₅O₂₂]⁺), βg (m/z 1069.5600, [C₅₄H₈₅O₂₁]⁺), and γg (m/z 923.5009, [C₄₈H₇₅O₁₇]⁺), establishing thus their membership in the soyasaponin group. Quantitative and semiquantitative analysis of identified soyasaponins were also performed by RPLC-ESI(+) FTICR-MS; the total concentration levels were found ranging from 83.6 ± 9.3 to 767 ± 37 mg/kg. In vitro hypoglycemic outcomes of four soyasaponin standards were evaluated; significant inhibitory activities were obtained with IC₅₀ values ranging from 1.5 ± 0.1 to 2.3 ± 0.2 μg/mL and 12.0 ± 1.1 to 29.4 ± 1.4 μg/mL for α-glucosidase and α-amylase, respectively. This study represents the first detailed investigation on the antidiabetic activity of bioactive constituents found in Fagioli di Sarconi beans. Graphical abstract The first detailed RPLC-ESI(+) FTICR-MS investigation of the qualitative and semiquantitative profile of soyasaponins, occurring in 21 ecotypes of Fagioli di Sarconi beans (P. vulgaris L.).

Transcriptome Analysis to Identify the Putative Biosynthesis and Transport Genes Associated with the Medicinal Components of Achyranthes bidentata Bl

Achyranthes bidentata is a popular perennial medicine herb used for 1000s of years in China to treat various diseases. Although this herb has multiple pharmaceutical purposes in China, no transcriptomic information has been reported for this species. In addition, the understanding of several key pathways and enzymes involved in the biosynthesis of oleanolic acid and ecdysterone, two pharmacologically active classes of metabolites and major chemical constituents of A. bidentata root extracts, is limited. The aim of the present study was to characterize the transcriptome profile of the roots and leaves of A. bidentata to uncover the biosynthetic and transport mechanisms of the active components. In this study, we identified 100,987 transcripts, with an average length of 1146.8 base pairs. A total of 31,634 (31.33%) unigenes were annotated, and 12,762 unigenes were mapped to 303 pathways according to the Kyoto Encyclopedia of Genes and Genomes pathway database. Moreover, we identified a total of 260 oleanolic acid and ecdysterone genes encoding biosynthetic enzymes. Furthermore, the key enzymes involved in the oleanolic acid and ecdysterone synthesis pathways were analyzed using quantitative real-time polymerase chain reaction, revealing that the roots expressed these enzymes to a greater extent than the leaves. In addition, we identified 85 ATP-binding cassette transporters, some of which might be involved in the translocation of secondary metabolites.

Glycyrrhiza uralensis transcriptome landscape and study of phytochemicals

Medicinal and industrial properties of phytochemicals (e.g. glycyrrhizin) from the root of Glycyrrhiza uralensis (licorice plant) made it an attractive, multimillion-dollar trade item. Bioengineering is one of the solutions to overcome such high market demand and to protect plants from extinction. Unfortunately, limited genomic information on medicinal plants restricts their research and thus biosynthetic mechanisms of many important phytochemicals are still poorly understood. In this work we utilized the de novo (no reference genome sequence available) assembly of Illumina RNA-Seq data to study the transcriptome of the licorice plant. Our analysis is based on sequencing results of libraries constructed from samples belonging to different tissues (root and leaf) and collected in different seasons and from two distinct strains (low and high glycyrrhizin producers). We provide functional annotations and the expression profile of 43,882 assembled unigenes, which are suitable for various further studies. Here, we searched for G. uralensis-specific enzymes involved in isoflavonoid biosynthesis as well as elucidated putative cytochrome P450 enzymes and putative vacuolar saponin transporters involved in glycyrrhizin production in the licorice root. To disseminate the data and the analysis results, we constructed a publicly available G. uralensis database. This work will contribute to a better understanding of the biosynthetic pathways of secondary metabolites in licorice plants, and possibly in other medicinal plants, and will provide an important resource to further advance transcriptomic studies in legumes.

Characterization and quantification of saponins and flavonoids in sprouts, seed coats and cotyledons of germinated black beans

Saponins, flavonols and isoflavones were quantified in sprouts, cotyledons and seed coats of black beans (Phaseolus vulgaris L.) subjected to germination over five days. Sprouts had a higher concentration of saponins compared to cotyledons or seed coats (p<0.05). The saponins concentration in hilum increased 2.3-fold after soaking. After the first day of germination, the saponin concentration in sprouts and cotyledons increased 1.9 and 2.1-fold, respectively. Additional germination days decreased the amount of the most abundant soyasaponins in black bean sprouts. Flavonols and isoflavones were associated with seed coats and less than one third of the initial amount remained after the soaking process. The concentrations of flavonols were also reduced during germination process. Aglycones were detected only after soaking and their concentration remained unchanged during germination. Genistein was detected only after three days of germination. In general, one-day germinated black beans could be recommended for increasing the concentration of saponins and non-glycosylated flavonols in sprouts and seed coats, respectively.

Genetic analysis of variations in the sugar chain composition at the C-3 position of soybean seed saponins

Saponins are sterols or triterpene glycosides that are widely distributed in plants. The biosynthesis of soybean saponins is thought to involve many kinds of glycosyltransferases, which is reflected in their structural diversity. Here, we performed linkage analyses of the Sg-3 and Sg-4 loci, which may control the sugar chain composition at the C-3 sugar moieties of the soybean saponin aglycones soyasapogenols A and B. The Sg-3 locus, which controls the production of group A saponin Af, was mapped to chromosome (Chr-) 10. The Sg-4 locus, which controls the production of DDMP saponin βa, was mapped to Chr-1. To elucidate the preference of sugar chain formation at the C-3 and C-22 positions, we analyzed the F(2) population derived from a cross between a mutant variety, Kinusayaka (sg-1(0)), for the sugar chain structure at C-22 position, and Mikuriya-ao (sg-3), with respect to the segregation of the composition of the group A saponins, and found that the formation of these sugar chains was independently regulated. Furthermore, a novel saponin, predicted to be A0-γg, 3-O-[β-d-galactopyranosyl (1→2)-β-d-glucuronopyranosyl]-22-O-α-l-arabinopyranosyl-soyasapogenol A, appeared in the hypocotyl of F(2) individuals with genotype sg-1(0)/sg-1(0)sg-3/sg-3.

An efficient approach to finding Siraitia grosvenorii triterpene biosynthetic genes by RNA-seq and digital gene expression analysis

Background

Siraitia grosvenorii (Luohanguo) is an herbaceous perennial plant native to southern China and most prevalent in Guilin city. Its fruit contains a sweet, fleshy, edible pulp that is widely used in traditional Chinese medicine. The major bioactive constituents in the fruit extract are the cucurbitane-type triterpene saponins known as mogrosides. Among them, mogroside V is nearly 300 times sweeter than sucrose. However, little is known about mogrosides biosynthesis in S. grosvenorii, especially the late steps of the pathway.

Results

In this study, a cDNA library generated from of equal amount of RNA taken from S. grosvenorii fruit at 50 days after flowering (DAF) and 70 DAF were sequenced using Illumina/Solexa platform. More than 48,755,516 high-quality reads from a cDNA library were generated that was assembled into 43,891 unigenes. De novo assembly and gap-filling generated 43,891 unigenes with an average sequence length of 668 base pairs. A total of 26,308 (59.9%) unique sequences were annotated and 11,476 of the unique sequences were assigned to specific metabolic pathways by the Kyoto Encyclopedia of Genes and Genomes. cDNA sequences for all of the known enzymes involved in mogrosides backbone synthesis were identified from our library. Additionally, a total of eighty-five cytochrome P450 (CYP450) and ninety UDP-glucosyltransferase (UDPG) unigenes were identified, some of which appear to encode enzymes responsible for the conversion of the mogroside backbone into the various mogrosides. Digital gene expression profile (DGE) analysis using Solexa sequencing was performed on three important stages of fruit development, and based on their expression pattern, seven CYP450s and five UDPGs were selected as the candidates most likely to be involved in mogrosides biosynthesis.

Conclusion

A combination of RNA-seq and DGE analysis based on the next generation sequencing technology was shown to be a powerful method for identifying candidate genes encoding enzymes responsible for the biosynthesis of novel secondary metabolites in a non-model plant. Seven CYP450s and five UDPGs were selected as potential candidates involved in mogrosides biosynthesis. The transcriptome data from this study provides an important resource for understanding the formation of major bioactive constituents in the fruit extract from S. grosvenorii.

Sterol glycosyltransferases--the enzymes that modify sterols

Sterols are important components of cell membranes, hormones, signalling molecules and defense-related biotic and abiotic chemicals. Sterol glycosyltransferases (SGTs) are enzymes involved in sterol modifications and play an important role in metabolic plasticity during adaptive responses. The enzymes are classified as a subset of family 1 glycosyltransferases due to the presence of a signature motif in their primary sequence. These enzymes follow a compulsory order sequential mechanism forming a ternary complex. The diverse applications of sterol glycosides, like cytotoxic and apoptotic activity, anticancer activity, medicinal values, anti-stress roles and anti-insect and antibacterial properties, draws attention towards their synthesis mechanisms. Many secondary metabolites are derived from sterol pathways, which are important in defense mechanisms against pathogens. SGTs in plants are involved in changed sensitivity to stress hormones and their agrochemical analogs and changed tolerance to biotic and abiotic stresses. SGTs that glycosylate steroidal hormones, such as brassinosteroids, function as growth and development regulators in plants. In terms of metabolic roles, it can be said that SGTs occupy important position in plant metabolism and may offer future tools for crop improvement.

Molecular activities, biosynthesis and evolution of triterpenoid saponins

Saponins are bioactive compounds generally considered to be produced by plants to counteract pathogens and herbivores. Besides their role in plant defense, saponins are of growing interest for drug research as they are active constituents of several folk medicines and provide valuable pharmacological properties. Accordingly, much effort has been put into unraveling the modes of action of saponins, as well as in exploration of their potential for industrial processes and pharmacology. However, the exploitation of saponins for bioengineering crop plants with improved resistances against pests as well as circumvention of laborious and uneconomical extraction procedures for industrial production from plants is hampered by the lack of knowledge and availability of genes in saponin biosynthesis. Although the ability to produce saponins is rather widespread among plants, a complete synthetic pathway has not been elucidated in any single species. Current conceptions consider saponins to be derived from intermediates of the phytosterol pathway, and predominantly enzymes belonging to the multigene families of oxidosqualene cyclases (OSCs), cytochromes P450 (P450s) and family 1 UDP-glycosyltransferases (UGTs) are thought to be involved in their biosynthesis. Formation of unique structural features involves additional biosynthetical enzymes of diverse phylogenetic background. As an example of this, a serine carboxypeptidase-like acyltransferase (SCPL) was recently found to be involved in synthesis of triterpenoid saponins in oats. However, the total number of identified genes in saponin biosynthesis remains low as the complexity and diversity of these multigene families impede gene discovery based on sequence analysis and phylogeny. This review summarizes current knowledge of triterpenoid saponin biosynthesis in plants, molecular activities, evolutionary aspects and perspectives for further gene discovery.

Triterpenoid biosynthesis and engineering in plants

Triterpenoid saponins are a diverse group of natural products in plants and are considered defensive compounds against pathogenic microbes and herbivores. Because of their various beneficial properties for humans, saponins are used in wide-ranging applications in addition to medicinally. Saponin biosynthesis involves three key enzymes: oxidosqualene cyclases, which construct the basic triterpenoid skeletons; cytochrome P450 monooxygenases, which mediate oxidations; and uridine diphosphate-dependent glycosyltransferases, which catalyze glycosylations. The discovery of genes committed to saponin biosynthesis is important for the stable supply and biotechnological application of these compounds. Here, we review the identified genes involved in triterpenoid biosynthesis, summarize the recent advances in the biotechnological production of useful plant terpenoids, and discuss the bioengineering of plant triterpenoids.

Key Enzymes of Triterpenoid Saponin Biosynthesis and the Induction of Their Activities and Gene Expressions in Plants

Triterpenoid saponins are one of the key active components of many medicinal plants. The biosynthetic pathway of triterpenoid saponins in higher plants and a lot of experimental results both indicated that the key enzymes involved in triterpenoid saponin synthesis are squalene synthase (SS), squalene epoxidase (SE), lupeol synthase (LS), dammarenediol synthase (DS), β-amyrin synthase (β-AS), cytochrome P450-dependent monooxygenase (PDMO), and glycosyltransferase (GT). The activities and coding genes of the key enzymes could be induced by a range of factors in various plant species. However, the effects of the factors on the content and composition of the triterpenoid saponins in specific plants are not certainly coincident, and different factors appear to induce the gene expressions of the key enzymes by different signal pathways and at different levels. This paper could provide a reference for strengthening the triterpenoid saponin-synthesizing capability of specific medicinal plants at enzyme and/or gene expression levels in order to improve the plants’ commercial values.

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.

Local differentiation of sugar donor specificity of flavonoid glycosyltransferase in Lamiales

Flavonoids are most commonly conjugated with various sugar moieties by UDP-sugar:glycosyltransferases (UGTs) in a lineage-specific manner. Generally, the phylogenetics and regiospecificity of flavonoid UGTs are correlated, indicating that the regiospecificity of UGT differentiated prior to speciation. By contrast, it is unclear how the sugar donor specificity of UGTs evolved. Here, we report the biochemical, homology-modeled, and phylogenetic characterization of flavonoid 7-O-glucuronosyltransferases (F7GAT), which is responsible for producing specialized metabolites in Lamiales plants. All of the Lamiales F7GATs were found to be members of the UGT88-related cluster and specifically used UDP-glucuronic acid (UDPGA). We identified an Arg residue that is specifically conserved in the PSPG box in the Lamiales F7GATs. Substitution of this Arg with Trp was sufficient to convert the sugar donor specificity of the Lamiales F7GATs from UDPGA to UDP-glucose. Homology modeling of the Lamiales F7GAT suggested that the Arg residue plays a critical role in the specific recognition of anionic carboxylate of the glucuronic acid moiety of UDPGA with its cationic guanidinium moiety. These results support the hypothesis that differentiation of sugar donor specificity of UGTs occurred locally, in specific plant lineages, after establishment of general regiospecificity for the sugar acceptor. Thus, the plasticity of sugar donor specificity explains, in part, the extraordinary structural diversification of phytochemicals.

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.

Impact of external calcium and calcium sensors on ginsenoside Rb1 biosynthesis byPanax notoginseng cells

The effects of external calcium concentrations on biosynthesis of ginsenoside Rb1 and several calcium signal sensors were quantitatively investigated in suspension cultures of Panax notoginseng cells. It was observed that the synthesis of intracellular ginsenoside Rb1 in 3-day incubation was dependent on the medium Ca2+ concentration (0-13 mM). At an optimal Ca2+ concentration of 8 mM, a maximal ginsenoside Rb1 content of 1.88 +/- 0.03 mg g(-1) dry weight was reached, which was about 60% and 25% higher than that at Ca2+ concentrations of 0 and 3 mM, respectively. Ca2+ feeding experiments confirmed the Ca2+ concentration-dependent Rb1 biosynthesis. In order to understand the mechanism of the signal transduction from external Ca2+ to ginsenoside biosynthesis, the intracellular content of calcium and calmodulin (CaM), activities of calcium/calmodulin-dependent NAD kinase (CCDNK) and calcium-dependent protein kinase (CDPK), and activity of a new biosynthetic enzyme of ginsenoside Rb1, i.e., UDPG:ginsenoside Rd glucosyltransferase (UGRdGT), in the cultured cells were all analyzed. The intracellular calcium content and CCDNK activity were increased with an increase of external Ca2+ concentration within 0-13 mM. In contrast, the CaM content and activities of CDPK and UGRdGT reached their highest levels at 8 mM of initial Ca2+ concentration, which was also optimal to the ginsenoside Rb1 synthesis. A similar Ca2+ concentration-dependency of the intracellular contents of calcium and CaM and activities of CCDNK, CDPK, and UGRdGT was confirmed in Ca2+ feeding experiments. Finally, a possible model on the effect of external calcium on ginsenoside Rb1 biosynthesis via the signal transduction pathway of CaM, CDPK, and UGRdGT is proposed. Regulation of external Ca2+ concentration is considered a useful strategy for manipulating ginsenoside Rb1 biosynthesis by P. notoginseng cells.

UDP-glucuronic acid:anthocyanin glucuronosyltransferase from red daisy (Bellis perennis) flowers. Enzymology and phylogenetics of a novel glucuronosyltransferase involved in flower pigment biosynthesis

In contrast to the wealth of biochemical and genetic information on vertebrate glucuronosyltransferases (UGATs), only limited information is available on the role and phylogenetics of plant UGATs. Here we report on the purification, characterization, and cDNA cloning of a novel UGAT involved in the biosynthesis of flower pigments in the red daisy (Bellis perennis). The purified enzyme, BpUGAT, was a soluble monomeric enzyme with a molecular mass of 54 kDa and catalyzed the regiospecific transfer of a glucuronosyl unit from UDP-glucuronate to the 2''-hydroxyl group of the 3-glucosyl moiety of cyanidin 3-O-6''-O-malonylglucoside with a kcat value of 34 s(-1) at pH 7.0 and 30 degrees C. BpUGAT was highlyspecific for cyanidin 3-O-glucosides (e.g. Km for cyanidin 3-O-6''-O-malonylglucoside, 19 microM) and UDP-glucuronate (Km, 476 microM). The BpUGAT cDNA was isolated on the basis of the amino acid sequence of the purified enzyme. Quantitative PCR analysis showed that transcripts of BpUGAT could be specifically detected in red petals, consistent with the temporal and spatial distributions of enzyme activity in the plant and also consistent with the role of the enzyme in pigment biosynthesis. A sequence analysis revealed that BpUGAT is related to the glycosyltransferase 1 (GT1) family of the glycosyltransferase superfamily (according to the Carbohydrate-Active Enzymes (CAZy) data base). Among GT1 family members that encompass vertebrate UGATs and plant secondary product glycosyltransferases, the highest sequence similarity was found with flavonoid rhamnosyltransferases of plants (28-40% identity). Although the biological role (pigment biosynthesis) and enzymatic properties of BpUGAT are significantly different from those of vertebrate UGATs, both of these UGATs share a similarity in that the products produced by these enzymes are more water-soluble, thus facilitating their accumulation in vacuoles (in BpUGAT) or their excretion from cells (in vertebrate UGATs), corroborating the proposed general significance of GT1 family members in the metabolism of small lipophilic molecules.

Biochemical and molecular characterization of a novel UDP-glucose:anthocyanin 3'-O-glucosyltransferase, a key enzyme for blue anthocyanin biosynthesis, from gentian

Gentian (Gentiana triflora) blue petals predominantly contain an unusually blue and stable anthocyanin, delphinidin 3-O-glucosyl-5-O-(6-O-caffeoyl-glucosyl)-3'-O-(6-O-caffeoyl-glucoside) (gentiodelphin). Glucosylation and the subsequent acylation of the 3'-hydroxy group of the B-ring of anthocyanins are important to the stabilization of and the imparting of bluer color to these anthocyanins. The enzymes and their genes involved in these modifications of the B-ring, however, have not been characterized, purified, or isolated to date. In this study, we purified a UDP-glucose (Glc):anthocyanin 3'-O-glucosyltransferase (3'GT) enzyme to homogeneity from gentian blue petals and isolated a cDNA encoding a 3'GT based on the internal amino acid sequences of the purified 3'GT. The deduced amino acid sequence indicates that 3'GT belongs to the same subfamily as a flavonoid 7-O-glucosyltransferase from Schutellaria baicalensis in the plant glucosyltransferase superfamily. Characterization of the enzymatic properties using the recombinant 3'GT protein revealed that, in contrast to most of flavonoid glucosyltransferases, it has strict substrate specificity: 3'GT specifically glucosylates the 3'-hydroxy group of delphinidin-type anthocyanins containing Glc groups at 3 and 5 positions. The enzyme specifically uses UDP-Glc as the sugar donor. The specificity was confirmed by expression of the 3'GT cDNA in transgenic petunia (Petunia hybrida). This is the first report of the gene isolation of a B-ring-specific glucosyltransferase of anthocyanins, which paves the way to modification of flower color by production of blue anthocyanins.

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.