Structure and Location Studies on Key Enzymes in Saponins Biosynthesis of Panax notoginseng

Identification of squalene epoxidase in triterpenes biosynthesis in Poria cocos by molecular docking and CRISPR-Cas9 gene editing

BACKGROUND

Squalene epoxidase is one of the rate-limiting enzymes in the biosynthetic pathway of membrane sterols and triterpenoids. The enzyme catalyzes the formation of oxidized squalene, which is a common precursor of sterols and triterpenoids.

RESULT

In this study, the squalene epoxidase gene (PcSE) was evaluated in Poria cocos. Molecular docking between PcSE and squalene was performed and the active amino acids were identified. The sgRNA were designed based on the active site residues. The effect on triterpene synthesis in P. cocos was consistent with the results from ultra-high-performance liquid chromatography-quadruplex time-of-flight-double mass spectrometry (UHPLC-QTOF-MS/MS) analysis. The results showed that deletion of PcSE inhibited triterpene synthesis. In vivo verification of PcSE function was performed using a PEG-mediated protoplast transformation approach.

CONCLUSION

The findings from this study provide a foundation for further studies on heterologous biosynthesis of P. cocos secondary metabolites.

Identification and functional characterization of two trans-isopentenyl diphosphate synthases and one squalene synthase involved in triterpenoid biosynthesis in Platycodon grandiflorus

MAIN CONCLUSION

Two trans-isopentenyl diphosphate synthase and one squalene synthase genes were identified and proved to be involved in the triterpenoid biosynthesis in Platycodon grandiflorus. Platycodon grandiflorus is a commonly used traditional Chinese medicine. The main bioactive compounds of P. grandiflorus are triterpenoid saponins. The biosynthetic pathway of triterpenoid saponins in P. grandiflorus has been preliminarily explored. However, limited functional information on related genes has been reported. A total of three trans-isopentenyl diphosphate synthases (trans-IDSs) genes (PgFPPS, PgGGPPS1 and PgGGPPS2) and one squalene synthase (SQS) gene (PgSQS) in P. grandiflorus were screened and identified from transcriptome dataset. Subcellular localization of the proteins was defined based on the analysis of GFP-tagged. The activity of genes was verified in Escherichia coli, demonstrating that recombinant PgFPPS catalysed the production of farnesyl diphosphate. PgGGPPS1 produced geranylgeranyl diphosphate, whereas PgGGPPS2 did not exhibit catalytic activity. By structural identification of encoding genes, a transmembrane region was found at the C-terminus of the PgSQS gene, which produced an insoluble protein when expressed in E. coli but showed no apparent effect on the enzyme function. Furthermore, some triterpenoid saponin synthesis-related genes were discovered by combining the component content and the gene expression assays at the five growth stages of P. grandiflorus seedlings. The accumulation of active components in P. grandiflorus was closely associated with the expression level of genes related to the synthesis pathway.

Viral infections inhibit saponin biosynthesis and photosynthesis in Panax notoginseng

Virus-infected Panax notoginseng plants with chlorotic, mosaic, and pitted leaves are ubiquitous in the primary P. notoginseng-producing region in Wenshan autonomous prefecture, Yunnan province, China. However, the viruses that infect P. notoginseng and the effects of viral infections on the biosynthesis of secondary metabolites and photosynthesis remain unknown. This study identified a variety of viruses infecting P. notoginseng plants via deep-sequencing of small RNA (sRNA). Of the 10 identified viruses, seven had not previously been detected in P. notoginseng, including Cauliflower mosaic virus and Soybean chlorotic mottle virus. In addition, the simultaneous infection of P. notoginseng by Panax notoginseng virus A (PnVA), Panax cryptic virus 4 (PCV4), and Tomato yellow leaf curl China virus (TYLCCNV) was confirmed by PCR. Moreover, a quantitative PCR analysis showed that the expression levels of key genes related to saponin biosynthesis were generally down-regulated in the virus-infected P. notoginseng. Additionally, high-performance liquid chromatography results indicated the saponin content decreased in the roots of virus-infected P. notoginseng plants. The activities of photosynthesis-related enzymes, including ribulose-1,5-bisphosphate carboxylase/oxygenase, fructose 1,6-bisphosphatase, and fructose 1,6-biphosphate aldolase, decreased significantly in the virus-infected P. notoginseng plants. The viral infections also induced the expression of antioxidant genes and increased antioxidant enzyme activities. Furthermore, the expression levels of many resistance-related genes were up-regulated in P. notoginseng plants inoculated with a viral suspension. The study results provide the foundation for future research on P. notoginseng viral diseases, which may lead to the development of enhanced disease control measures.

Advances in the biosynthesis and metabolic engineering of rare ginsenosides

Rare ginsenosides are the deglycosylated secondary metabolic derivatives of major ginsenosides, and they are more readily absorbed into the bloodstream and function as active substances. The traditional preparation methods hindered the potential application of these effective components. The continuous elucidation of ginsenoside biosynthesis pathways has rendered the production of rare ginsenosides using synthetic biology techniques effective for their large-scale production. Previously, only the progress in the biosynthesis and biotechnological production of major ginsenosides was highlighted. In this review, we summarized the recent advances in the identification of key enzymes involved in the biosynthetic pathways of rare ginsenosides, especially the glycosyltransferases (GTs). Then the construction of microbial chassis for the production of rare ginsenosides, mainly in Saccharomyces cerevisiae, was presented. In the future, discovery of more GTs and improving their catalytic efficiencies are essential for the metabolic engineering of rare ginsenosides. This review will give more clues and be helpful for the characterization of the biosynthesis and metabolic engineering of rare ginsenosides. KEY POINTS: • The key enzymes involved in the biosynthetic pathways of rare ginsenosides are summarized. • The recent progress in metabolic engineering of rare ginsenosides is presented. • The discovery of glycosyltransferases is essential for the microbial production of rare ginsenosides in the future.

Structural and interactions analysis of a transcription factor PnMYB2 in Panax notoginseng

The main active ingredients of the traditional Chinese medicinal plant, Panax notoginseng, are the Panax notoginseng saponins (PNS). They can be synthesized via the mevalonate pathway; PnSS and PnSE1 are the key rate-limiting enzymes in this pathway. In this study, an interaction between PnMYB2 and the key enzymes was identified and characterized from the P. notoginseng cDNA library using the Y1H technique. Subsequently, X-α-gal color reaction confirmed the interaction between PnMYB2 and the upstream sequences of PnSS and PnSE1 promoters. Full-length cDNA sequence of PnMYB2 was isolated and characterized. PnMYB2 has an open reading frame of 864 bp, encoding 287 amino acids. 3D structural analysis of PnMYB2 indicated that its structure was similar to that of the template. Phylogenetic analysis revealed that PnMYB2 and PgMYB2 are highly homologous and belong to the R2R3 MYB transcription factor (TF). Subcellular localization analysis showed that PnMYB2 was localized in the nucleus. The recombinant protein PnMYB2 was successfully obtained through prokaryotic expression and was confirmed to be an inclusion body protein. Furthermore, electrophoretic mobility shift assay (EMSA) experiments demonstrated that PnMYB2 specifically binds to MYB core and AC-rich elements. This study provides a theoretical basis for transcriptional regulation of saponin biosynthesis in P. notoginseng.

Binding proteins PnCOX11 and PnDCD strongly respond to GA and ABA in Panax notoginseng

Panax notoginseng saponins (PNS) are one of the main active ingredients of Panax notoginseng, a representative plant of the genus Panax. However, the detailed regulation mechanism of PNS biosynthesis remains elusive. Therefore, a sequence of upstream promoters of PnSS and PnSE were cloned and analyzed firstly. GUS quantitative results showed that the upstream promoters could specifically and significantly respond to exogenous GA and ABA signals. To further identify the binding proteins that respond to peripheral hormones, PnCOX11 and PnDCD were screened and identified from the P. notoginseng cDNA library. The Y1H experiment verified the interaction between the above two binding proteins and the promoters. Several online software was used to analyze the domains, secondary structures, three-dimensional structures, and phylogenetic trees of the two binding proteins. Subcellular localization analysis exhibited that PnCOX11 was mainly located in the chloroplast, while PnDCD was located in the cytoplasm and nucleus. Prokaryotic expression demonstrated that the recombinant proteins had a high concentration under the induction of IPTG. This study can provide a fundamental date for the subsequent thorough investigation of the transcription regulatory mechanism of PNS biosynthesis.

Activity and function studies of the promoter cis-acting elements of the key enzymes in saponins biosynthesis of DS from Panax notoginseng

Panax notoginseng is a traditional Chinese medicine for the treatment of blood diseases, in which saponins were the main active components. Dammarenediol synthase (DS) is a key enzyme in the saponin synthesis pathway of P. notoginseng. The promoter is an important region to regulate gene expression, and the study of the promoter sequence provides important evidence for revealing the mechanism of gene expression regulation. However, there was still little research on the promoter function of P. notoginseng. In this study, the 1382 bp promoter upstream of DS from P. notoginseng was cloned and sequenced. The promoter sequence was analyzed by online databases. The plant expression vector fused with the β-glucuronidase gene was constructed and transferred into Agrobacterium tumefaciens. Then tobacco was injected, and its response to exogenous hormones (gibberellin and abscisic acid) was studied by transient expression to verify its unique action elements. The results showed that the tobacco leaves transferred with DS promoter had significantly increased GUS protease activity after spraying GA and ABA, indicating that both DS promoter can specifically and significantly respond to exogenous GA and ABA signal. These findings will help us to better understand the regulatory mechanisms of the upstream region of the DS gene and provide a basis for future research on the interaction of cis-acting elements of promoters with related transcription factors.

Molecular cloning and structural analysis of key enzymes in Tetrastigma hemsleyanum for resveratrol biosynthesis

Resveratrol (RES), a plant antitoxin, has antioxidant, anti-inflammatory, anti-cancer and cardiovascular protection effects. It has been reported that RES can be stably detected in a Chinese herbal medicinal plant Tetrastigma hemsleyanum. At present, the research of T. hemsleyanum mainly focused on the discovery of new compounds and pharmacology. However, there were few studies on the molecular mechanism of the synthesis of secondary metabolites in T. hemsleyanum. In this experiment, four key enzymes (ThPAL/ThC4H/Th4CL/ThRS) involved in the RES biosynthesis pathway were cloned and obtained. They contained an open reading frame (ORF) of 2139 bp, 1518 bp, 1716 bp and 1035 bp, encoding 712, 505, 571 and 344 amino acids, separately. Various bioinformatics tools were used to analyze these deduced protein domains, secondary structures, three-dimensional (3D) structures and phylogenetic trees. Subsequently, quantitative primers were designed to conduct the tissue-specific expression. Quantitative results displayed that the four genes were expressed in all tested tissues, and their expression in root tubers was more stable. Moreover, the subcellular localization of the four genes was studied by constructed recombinant green fluorescent expression vectors. Herein, by digging out the key enzyme genes in the biosynthesis of RES in T. hemsleyanum, this experiment tried to reveal the expression patterns of these key enzyme genes. It also provided the basis for the research on the molecular level, which will help people further illuminate and clarify the biosynthesis and regulation mechanism of secondary metabolites in T. hemsleyanum.

Ginsenosides in Panax genus and their biosynthesis

Ginsenosides are a series of glycosylated triterpenoids which belong to protopanaxadiol (PPD)-, protopanaxatriol (PPT)-, ocotillol (OCT)- and oleanane (OA)-type saponins known as active compounds of Panax genus. They are accumulated in plant roots, stems, leaves, and flowers. The content and composition of ginsenosides are varied in different ginseng species, and in different parts of a certain plant. In this review, we summarized the representative saponins structures, their distributions and the contents in nearly 20 Panax species, and updated the biosynthetic pathways of ginsenosides focusing on enzymes responsible for structural diversified ginsenoside biosynthesis. We also emphasized the transcription factors in ginsenoside biosynthesis and non-coding RNAs in the growth of Panax genus plants, and highlighted the current three major biotechnological applications for ginsenosides production. This review covered advances in the past four decades, providing more clues for chemical discrimination and assessment on certain ginseng plants, new perspectives for rational evaluation and utilization of ginseng resource, and potential strategies for production of specific ginsenosides.

Recent Progress in Saikosaponin Biosynthesis in Bupleurum

BACKGROUND

Chaihu is a popular traditional Chinese medicine that has been used for centuries. It is traditionally used to treat cold fever and liver-related diseases. Saikosaponins (SSs) are one of the main active components of chaihu, in addition to essential oils, flavonoids, and polysaccharides. Considerable effort is needed to reveal the biosynthesis and regulation of SSs on the basis of current progress.

OBJECTIVE

The aim of this study is to provide a reference for further studies and arouse attention by summarizing the recent achievements of SS biosynthesis.

METHODS

All the data compiled and presented here were obtained from various online resources, such as PubMed Scopus and Baidu Scholar in Chinese, up to October 2019.

RESULTS

A few genes of the enzymes of SSs participating in the biosynthesis of SSs were isolated. Among these genes, only the P450 gene was verified to catalyze the SS skeleton β-amyrin synthase. Several UDP-glycosyltransferase genes were predicted to be involved in the biosynthesis of SSs. SSs could be largely biosynthesized in the phloem and then transported from the protoplasm, which is the biosynthetic site, to the vacuoles to avoid self-poisoning. As for the other secondary metabolites, the biosynthesis of SSs was strongly affected by environmental factors and the different species belonging to the genus of Bupleurum. Transcriptional regulation was studied at the molecular level.

CONCLUSION

Profound discoveries in SSs may elucidate the mechanism of diverse the monomer formation of SSs and provide a reference for maintaining the stability of SS content in Radix Bupleuri.