Candidate Genes Involved in the Biosynthesis of Triterpenoid Saponins in Platycodon grandiflorum Identified by Transcriptome Analysis

Genome-wide identification of bZIP transcription factors and their expression analysis in Platycodon grandiflorus under abiotic stress

The Basic Leucine Zipper (bZIP) transcription factors (TFs) family is among of the largest and most diverse gene families found in plant species, and members of the bZIP TFs family perform important functions in plant developmental processes and stress response. To date, bZIP genes in Platycodon grandiflorus have not been characterized. In this work, a number of 47 PgbZIP genes were identified from the genome of P. grandiflorus, divided into 11 subfamilies. The distribution of these PgbZIP genes on the chromosome and gene replication events were analyzed. The motif, gene structure, cis-elements, and collinearity relationships of the PgbZIP genes were simultaneously analyzed. In addition, gene expression pattern analysis identified ten candidate genes involved in the developmental process of different tissue parts of P. grandiflorus. Among them, Four genes (PgbZIP5, PgbZIP21, PgbZIP25 and PgbZIP28) responded to drought and salt stress, which may have potential biological roles in P. grandiflorus development under salt and drought stress. Four hub genes (PgbZIP13, PgbZIP30, PgbZIP32 and PgbZIP45) mined in correlation network analysis, suggesting that these PgbZIP genes may form a regulatory network with other transcription factors to participate in regulating the growth and development of P. grandiflorus. This study provides new insights regarding the understanding of the comprehensive characterization of the PgbZIP TFs for further exploration of the functions of growth and developmental regulation in P. grandiflorus and the mechanisms for coping with abiotic stress response.

Cloning, Expression Characteristics of Farnesyl Pyrophosphate Synthase Gene from Platycodon grandiflorus and Functional Identification in Triterpenoid Synthesis

Platycodon grandiflorus is a medicinal plant whose main component is platycodins, which have a variety of pharmacological effects and nutritional values. The farnesyl pyrophosphate synthase (FPS) is a key enzyme in the isoprenoid biosynthesis pathway, which catalyzes the synthesis of farnesyl diphosphate (FPP). In this study, we cloned the FPS gene from P. grandiflorus (PgFPS) with an ORF of 1260 bp, encoding 419 amino acids with a deduced molecular weight and theoretical pI of 46,200.98 Da and 6.52, respectively. The squalene content of overexpressed PgFPS in tobacco leaves and yeast cells extract was 1.88-fold and 1.21-fold higher than that of the control group, respectively, and the total saponin content was also increased by 1.15 times in yeast cells extract, which verified the biological function of PgFPS in terpenoid synthesis. After 48 h of MeJA treatment and 6 h of ethephon treatment, the expression of the PgFPS gene in roots and stems reached its peak, showing a 3.125-fold and 3.236-fold increase compared to the untreated group, respectively. Interestingly, the expression of the PgFPS gene in leaves showed a decreasing trend after exogenous elicitors treatment. The discovery of this enzyme will provide a novel perspective for enhancing the efficient synthesis of platycodins.

Mining genomic regions associated with agronomic and biochemical traits in quinoa through GWAS

Quinoa (Chenopodium quinoa Willd.), an Andean crop, is a facultative halophyte food crop recognized globally for its high nutritional value and plasticity to adapt to harsh conditions. We conducted a genome-wide association study on a diverse set of quinoa germplasm accessions. These accessions were evaluated for the following agronomic and biochemical traits: days to 50% flowering (DTF), plant height (PH), panicle length (PL), stem diameter (SD), seed yield (SY), grain diameter (GD), and thousand-grain weight (TGW). These accessions underwent genotyping-by-sequencing using the DNBSeq-G400R platform. Among all evaluated traits, TGW represented maximum broad-sense heritability. Our study revealed average SNP density of ≈ 3.11 SNPs/10 kb for the whole genome, with the lowest and highest on chromosomes Cq1B and Cq9A, respectively. Principal component analysis clustered the quinoa population in three main clusters, one clearly representing lowland Chilean accessions, whereas the other two groups corresponded to germplasm from the highlands of Peru and Bolivia. In our germplasm set, we estimated linkage disequilibrium decay to be ≈ 118.5 kb. Marker-trait analyses revealed major and consistent effect associations for DTF on chromosomes 3A, 4B, 5B, 6A, 7A, 7B and 8B, with phenotypic variance explained (PVE) as high as 19.15%. Nine associations across eight chromosomes were also found for saponin content with 20% PVE by qSPN5A.1. More QTLs were identified for PL and TGW on multiple chromosomal locations. We identified putative candidate genes in the genomic regions associated with DTF and saponin content. The consistent and major-effect genomic associations can be used in fast-tracking quinoa breeding for wider adaptation across marginal environments.

Transcriptomic Analysis of Cyclamen persicum to Identify Invovled Genes in Triterpene Secondary Metabolites Pathway

Saponins are considered as a diverse group of natural active compounds, which are widely found in crops. Mevalonate pathway (MVA) is regarded as the main pathway for synthesis of saponins in crops. This study aims to compare transcriptome of the leaf with tuber of crop including tubers and roots. First, more than 166 million reads were generated. The existence of 36,678 unigenes in the two samples out of 48,936 assembled ones showed a significant difference in expression. Finally, 310 and 290 highly up-regulated genes in leaf and tuber were selected for the next analysis. In addition, the expression profiles of 13 key genes in the MVA pathway were compared in RNA sequencing and reverse transcription-quantitative polymerase chain reaction analysis. The results indicated that cyclamen tuber has a higher level of expression of MVA pathway genes. The topological analysis for gene co-expression network involved in triterpenoid synthesis represented that the genes at the beginning of such pathway play a critical role so that the reduction of their expression challenges triterpenoid synthesis severely. The tuber of the cyclamen appears to be the major site of triterpene synthesis, and transfer of excess Isopentenyl pyrophosphate (IPP) from tuber to leaf activates downstream genes in leaf of crop.

Identification and expression analysis of YABBY family genes in Platycodon grandiflorus

Platycodon grandiflorus set ornamental, edible, and medicinal plant with broad prospects for further application development. However, there are no reports on the YABBY transcription factor in P. grandiflorus. Identification and analysis of the YABBY gene family of P. grandiflorus using bioinformatics means. Six YABBY genes were identified and divided into five subgroups. Transcriptome data and qRT-PCR were used to analyze the expression patterns of YABBY. YABBY genes exhibited organ-specific patterns in expression in P grandiflorus. Upon salt stress and drought induction, P. grandiflorus presented different morphological and physiological changes with some dynamic changes. Under salt treatment, the YABBY gene family was down-regulated; PgYABBY5 was up-regulated in leaves at 24 h. In drought treatment, PgYABBY1, PgYABBY2, and PgYABBY3 were down-regulated to varying degrees, but PgYABBY3 was significantly up-regulated in the roots. PgYABBY5 was up-regulated gradually after being down-regulated. PgYABBY5 was significantly up-regulated in stem and leaf at 48 h. PgYABBY6 was down-regulated at first and then significantly up-regulated. The dynamic changes of salt stress and drought stress can be regarded as the responses of plants to resist damage. During the whole process of salt and drought stress treatment, the protein content of each tissue part of P grandiflorus changed continuously. At the same time, we found that the promoter region of the PgYABBY gene contains stress-resistant elements, and the regulatory role of YABBY transcription factor in the anti-stress mechanism of P grandiflorus remains to be studied. PgYABBY1, PgYABBY2, and PgYABBY5 may be involved in the regulation of saponins in P. grandiflorus. PgYABBY5 may be involved in the drought resistance mechanism in P. grandiflorus stems and leaves. This study may provide a theoretical basis for studying the regulation of terpenoids by the YABBY transcription factor and its resistance to abiotic stress.

Chromosome-Scale Genome Assembly and Triterpenoid Saponin Biosynthesis in Korean Bellflower (Platycodon grandiflorum)

Platycodon grandiflorum belongs to the Campanulaceae family and is an important medicinal and food plant in East Asia. However, on the whole, the genome evolution of P. grandiflorum and the molecular basis of its major biochemical pathways are poorly understood. We reported a chromosome-scale genome assembly of P. grandiflorum based on a hybrid method using Oxford Nanopore Technologies, Illumina sequences, and high-throughput chromosome conformation capture (Hi-C) analysis. The assembled genome was finalized as 574 Mb, containing 41,355 protein-coding genes, and the genome completeness was assessed as 97.6% using a Benchmarking Universal Single-Copy Orthologs analysis. The P. grandiflorum genome comprises nine pseudo-chromosomes with 56.9% repeat sequences, and the transcriptome analysis revealed an expansion of the 14 beta-amylin genes related to triterpenoid saponin biosynthesis. Our findings provide an understanding of P. grandiflorum genome evolution and enable genomic-assisted breeding for the mass production of important components such as triterpenoid saponins.

De Novo Transcriptome Sequencing of Codonopsis lanceolata for Identification of Triterpene Synthase and Triterpene Acetyltransferase

Codonopsis lanceolata (Campanulaceae) is a perennial plant commonly known as the bonnet bellflower. This species is widely used in traditional medicine and is considered to have multiple medicinal properties. In this study, we found that shoots and roots of C. lanceolata contained various types of free triterpenes (taraxerol, β-amyrin, α-amyrin, and friedelin) and triterpene acetates (taraxerol acetate, β-amyrin acetate, and α-amyrin acetate). The content of triterpenes and triterpene acetates by GC analysis was higher in the shoot than in the roots. To investigate the transcriptional activity of genes involved in triterpenes and triterpene acetate biosynthesis, we performed de novo transcriptome analysis of shoots and roots of C. lanceolata by sequencing using the Illumina platform. A total of 39,523 representative transcripts were obtained. After functional annotation of the transcripts, the differential expression of genes involved in triterpene biosynthetic pathways was investigated. Generally, the transcriptional activity of unigenes in the upstream region (MVA and MEP pathway) of triterpene biosynthetic pathways was higher in shoots than in roots. Various triterpene synthases (2,3-oxidosqualene cyclase, OSC) participate to produce triterpene skeletons by the cyclization of 2,3-oxidosqualene. A total of fifteen contigs were obtained in annotated OSCs in the representative transcripts. Functional characterization of four OSC sequences by heterologous expression in yeast revealed that ClOSC1 was determined as taraxerol synthase, and ClOSC2 was a mixed-amyrin synthase producing α-amyrin and β-amyrin. Five putative contigs of triterpene acetyltransferases showed high homology to the lettuce triterpene acetyltransferases. Conclusively, this study provides the basis of molecular information, particularly for the biosynthesis of triterpenes and triterpene acetates in C. lanceolata.

Biotransformation of Platycosides, Saponins from Balloon Flower Root, into Bioactive Deglycosylated Platycosides

Platycosides, saponins from balloon flower root (Platycodi radix), have diverse health benefits, such as antioxidant, anti-inflammatory, anti-tussive, anti-cancer, anti-obesity, anti-diabetes, and whitening activities. Deglycosylated platycosides, which show greater biological effects than glycosylated platycosides, are produced by the hydrolysis of glycoside moieties in glycosylated platycosides. In this review, platycosides are classified according to the chemical structures of the aglycone sapogenins and also divided into natural platycosides, including major, minor, and rare platycosides, depending on the content in Platycodi radix extract and biotransformed platycosides. The biological activities of platycosides are summarized and methods for deglycosylation of saponins, including physical, chemical, and biological methods, are introduced. The biotransformation of glycosylated platycosides into deglycosylated platycosides was described based on the hydrolytic pathways of glycosides, substrate specificity of glycosidases, and specific productivities of deglycosylated platycosides. Methods for producing diverse and/or new deglycosylated platycosides are also proposed.

Platycodon grandiflorum (Jacq.) A. DC.: A review of phytochemistry, pharmacology, toxicology and traditional use

BACKGROUND

The traditional Chinese medicine Platycodon grandiflorum (Jacq.) A. DC. (PG, balloon flower) has medicinal and culinary value. It consists of a variety of chemical components including triterpenoid saponins, polysaccharides, flavonoids, polyphenols, polyethylene glycols, volatile oils and mineral components, which have medicinal and edible value.

PURPOSE

The ultimate goal of this review is to summarize the phytochemistry, pharmacological activities, safety and uses of PG in local and traditional medicine.

METHODS

A comprehensive search of published literature up to March 2022 was conducted using the PubMed, China Knowledge Network and Web of Science databases to identify original research related to PG, its active ingredients and pharmacological activities.

RESULTS

Triterpene saponins are the primary bioactive compounds of PG. To date, 76 triterpene saponin compounds have been isolated and identified from PG. In addition, there are other biological components, such as flavonoids, polyacetylene and phenolic acids. These extracts possess antitussive, immunostimulatory, anti-inflammatory, antioxidant, antitumor, antiobesity, antidepressant, and cardiovascular system activities. The mechanisms of expression of these pharmacological effects include inhibition of the expression of proteins such as MDM and p53, inhibition of the activation of enzymes, such as AKT, the secretion of inflammatory factors, such as IFN-γ, TNF-α, IL-2 and IL-1β, and activation of the AMPK pathway.

CONCLUSION

This review summarizes the chemical composition, pharmacological activities, molecular mechanism, toxicity and uses of PG in local and traditional medicine over the last 12 years. PG contains a wide range of chemical components, among which triterpene saponins, especially platycoside D (PD), play a strong role in pharmacological activity, representing a natural phytomedicine with low toxicity that has applications in food, animal feed and cosmetics. Therefore, PG has value for exploitation and is an excellent choice for treating various diseases.

Comparative de novo transcriptome analysis of flower and root of Oliveria decumbens Vent. to identify putative genes in terpenes biosynthesis pathway

The Oliveria decumbens Vent. is a wild, rare, annual medicinal plant and endemic plant of Iran that has metabolites (mostly terpenes) which make it a precious plant in Persian Traditional Medicine and also a potential chemotherapeutic agent. The lack of genetic resources has slowed the discovery of genes involved in the terpenes biosynthesis pathway. It is a wild relative of Daucus carota. In this research, we performed the transcriptomic differences between two samples, flower and root of Oliveria decumbens, and also analyze the expression value of the genes involved in terpenoid biosynthesis by RNA-seq and its essential oil’s phytochemicals analyzed by GC/MS. In total, 136,031,188 reads from two samples of flower and root have been produced. The result shows that the MEP pathway is mostly active in the flower and the MVA in the root. Three genes of GPP, FPPS, and GGPP that are the precursors in the synthesis of mono, di, and triterpenes are upregulated in root and 23 key genes were identified that are involved in the biosynthesis of terpenes. Three genes had the highest upregulation in the root including, and on the other hand, another three genes had the expression only in the flower. Meanwhile, 191 and 185 upregulated genes in the flower and root of the plant, respectively, were selected for the gene ontology analysis and reconstruction of co-expression networks. The current research is the first of its kind on Oliveria decumbens transcriptome and discussed 67 genes that have been deposited into the NCBI database. Collectively, the information obtained in this study unveils the new insights into characterizing the genetic blueprint of Oliveria decumbens Vent. which paved the way for medical/plant biotechnology and the pharmaceutical industry in the future.

Identification of Putative Candidate Genes from Galphimia spp. Encoding Enzymes of the Galphimines Triterpenoids Synthesis Pathway with Anxiolytic and Sedative Effects

Galphimia spp. is popularly used in Mexican traditional medicine. Some populations of Galphimia exert anxiolytic and sedative effects due to the presence of the modified triterpenoids galphimines. However, the galphimine synthesis pathway has not yet been elucidated. Hence, in this study, a comparative transcriptome analysis between two contrasting populations of Galphimia spp., a galphimine-producer, and a non-galphimine-producer, is performed using RNA-Seq in the Illumina Next Seq 550 platform to identify putative candidates genes that encode enzymes of this metabolic pathway. Transcriptome functional annotation was performed using the Blast2GO in levels of gene ontology. For differential expression analysis, edgeR, pheatmap, and Genie3 library were used. To validate transcriptome data, qPCR was conducted. In producer and non-producer plants of both populations of Galphimia spp., most of the transcripts were grouped in the Molecular Function level of gene ontology. A total of 680 differentially expressed transcripts between producer and non-producer plants were detected. In galphimine-producer plants, a larger number of highly expressed transcripts related to acyclic and polycyclic terpene synthesis were identified. As putative candidate genes involved in the galphimine synthesis pathway, P450 family members and enzymes with kinase activity were identified.

Comparative transcriptomic analysis of genes in the triterpene saponin biosynthesis pathway in leaves and roots of Ardisia kteniophylla A. DC., a plant used in traditional Chinese medicine

Ardisia kteniophylla (Primulaceae) is highly valued in traditional medicine due to its production of the pharmacologically active secondary metabolites, especially triterpenoid saponins in its roots. Although A. kteniophylla is very important in traditional medicine, the genetic basis for its production of triterpenoid saponins remains largely unknown. Therefore, we sequenced transcriptomes of A. kteniophylla to identify putative genes involved in production of triterpenoid saponins in both leaves and roots, and we used the transcriptomes to compare expression levels of these genes between the two organ systems. The production of triterpenoid saponins in plants is usually induced through hormonal signaling on account of the presence of pests. Thus, we treated plants with the hormones salicylic acid (SA) and methyl jasmonate (MeJA) and used quantitative real‐time PCR (qRT‐PCR) to investigate expression levels of genes involved in triterpenoid saponin biosynthesis. In total, we obtained transcriptomes for leaf and root tissues representing 52,454 unigenes. Compared with the leaf transcriptome, we found that 6092 unigenes were upregulated in the root, especially enzymes involved in the direct synthesis of triterpenoid saponins, while 6001 genes appeared downregulated, including those involved in precursory steps in the triterpenoid saponin biosynthesis pathway. Our results from qRT‐PCR indicate that genes within the upstream parts of the triterpenoid saponin biosynthesis pathway may be upregulated under exposure to the applied hormones, but downstream genes are downregulated. This suggests possible conflicting effects of SA and MeJA in promoting the production of secondary metabolites on the one hand, and, on the other, limiting plant growth processes to devote energy to combating pests. We also performed an analysis of transcription factors (TFs) and found 997 unique transcripts belonging to 16 TF families. Our data may help to facilitate future work on triterpene saponins biosynthesis in A. kteniophylla with potential pharmacological and molecular breeding applications.

De novo assembly and analysis of Polygonatum cyrtonema Hua and identification of genes involved in polysaccharide and saponin biosynthesis

Background

The investigation of molecular mechanisms involved in polysaccharides and saponin metabolism is critical for genetic engineering of Polygonatum cyrtonema Hua to raise major active ingredient content. Up to now, the transcript sequences are available for different tissues of P. cyrtonema, a wide range scanning about temporal transcript at different ages’ rhizomes was still absent in P. cyrtonema.

Results

Transcriptome sequencing for rhizomes at different ages was performed. Sixty-two thousand six hundred thirty-five unigenes were generated by assembling transcripts from all samples. A total of 89 unigenes encoding key enzymes involved in polysaccharide biosynthesis and 56 unigenes encoding key enzymes involved in saponin biosynthesis. The content of total polysaccharide and total saponin was positively correlated with the expression patterns of mannose-6-phosphate isomerase (MPI), GDP-L-fucose synthase (TSTA3), UDP-apiose/xylose synthase (AXS), UDP-glucose 6-dehydrogenase (UGDH), Hydroxymethylglutaryl CoA synthase (HMGS), Mevalonate kinase (MVK), 2-C-methyl-D-erythritol 2,4-cyclodiphosphate synthase (ispF), (E)-4-hydroxy-3-methylbut-2-enyl-diphosphate synthase (ispG), 4-hydroxy-3-methylbut-2-enyl diphosphate reductase (ispH), Farnesyl diphosphate synthase (FPPS). Finally, a number of key genes were selected and quantitative real-time PCR were performed to validate the transcriptome analysis results.

Conclusions

These results create the link between polysaccharides and saponin biosynthesis and gene expression, provide insight for underlying key active substances, and reveal novel candidate genes including TFs that are worth further exploration for their functions and values.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08421-y.

Characterization of Saponins from Various Parts of Platycodon grandiflorum Using UPLC-QToF/MS

Platycodon grandiflorum (PG) is known as a high-potential material in terms of its biological activity. The objective of this report is to provide chromatographic and mass fragment ion data of 38 simultaneously identified saponins, including novel compounds, by analyzing them through ultra-performance liquid chromatography coupled with quadrupole time-of-flight mass spectrometry (UPLC-QToF/MS). In so doing, we investigated their diverse conditions, including morphological parts (stems, roots, buds, and leaves), peeling (or not), and blanching of PG. The total contents of individual saponins indicated an order of roots (containing peel, 1674.60 mg/100 g, dry weight) > buds (1364.05) > roots (without peel, 1058.83) ≈ blanched roots (without peel, 945.17) ≈ stems (993.71) ≈ leaves (881.16). When considering three types of aglycone, the platycodigenin group (55.04 ~ 68.34%) accounted for the largest proportion of the total content, whereas the platycogenic acid A group accounted for 17.83 ~ 22.61%, and the polygalacic acid group represented 12.06 ~ 22.35%. As they are classified as major compounds, novel saponins might be utilized for their role in healthy food for human consumption. Additionally, during blanching, the core temperature of PG was satisfied with the optimal condition, thus activating the enzymes related to biotransformation. Furthermore, through the use of this comprehensive data, additional studies related to buds, as well as roots or the characterization of individual saponins, can be conducted in a rapid and achievable manner.

Transcriptome analysis identifies putative genes involved in triterpenoid biosynthesis in Platycodon grandiflorus

Comprehensive transcriptome analysis of different Platycodon grandiflorus tissues discovered genes related to triterpenoid saponin biosynthesis. Platycodon grandiflorus (Jacq.) A. DC. (P. grandiflorus), a traditional Chinese medicine, contains considerable triterpenoid saponins with broad pharmacological activities. Triterpenoid saponins are the major components of P. grandiflorus. Here, single-molecule real-time and next-generation sequencing technologies were combined to comprehensively analyse the transcriptome and identify genes involved in triterpenoid saponin biosynthesis in P. grandiflorus. We quantified four saponins in P. grandiflorus and found that their total content was highest in the roots and lowest in the stems and leaves. A total of 173,354 non-redundant transcripts were generated from the PacBio platform, and three full-length transcripts of β-amyrin synthase, the key synthase of β-amyrin, were identified. A total of 132,610 clean reads obtained from the DNBSEQ platform were utilised to explore key genes related to the triterpenoid saponin biosynthetic pathway in P. grandiflorus, and 96 differentially expressed genes were selected as candidates. The expression levels of these genes were verified by quantitative real-time PCR. Our reliable transcriptome data provide valuable information on the related biosynthesis pathway and may provide insights into the molecular mechanisms of triterpenoid saponin biosynthesis in P. grandiflorus.

Transcriptomic Analysis, Cloning, Characterization, and Expression Analysis of Triterpene Biosynthetic Genes and Triterpene Accumulation in the Hairy Roots of Platycodon grandiflorum Exposed to Methyl Jasmonate

Platycodon grandiflorum is a perennial plant that has been used for medicinal purposes. Specifically, it is widely used in Northern China and Korea for the treatment of various diseases. Terpenoids belong to a group called secondary metabolites and have attracted a wide range of interest. Here, we determined the expressed sequence tag (EST) library of the methyl jasmonate (MeJA)-treated hairy root of P. grandiflorum. In total, 5760 ESTs were obtained, but after deleting the vector sequences and removing poor-quality sequences, a total of 2536 ESTs were attained. Of these, 811 contigs and 1725 singletons were annotated. The data were further analyzed with a focus on the gene families of the terpenoid biosynthetic pathway (TBP). We identified and characterized four TBP genes; among these were three full-length cDNAs encoding PgHMGS, PgMK, and PgMVD, whereas PgHMGR had a partial sequence based on the EST library database. Phylogenetic analysis and a pairwise identity matrix showed that these identified genes were closely related to those of other higher plants. Moreover, the tertiary structure and multiple alignment analysis showed that several distinct conserved motifs were present. Quantitative reverse transcription-polymerase chain reaction results revealed that TBP genes were constitutively expressed in all organs of P. grandiflorum, while the expression of transcript levels of these genes varied distinctly among the organs. Additionally, the total amount of platycosides was highly detected in the root, accumulating in concentrations 7.8 and 2.6 times higher than in the hairy root and stem, respectively, and 1.4 times higher than in the leaf and flower. The highest amount of total phytosterols was found to accumulate in the leaves at 9.3, 9.1, 1.8, and 1.6 times higher than that of the stem, root, hairy root, and flower, respectively. After the hairy root was exposed to the MeJA treatment, the transcript levels of PgHMGS, PgHMGR, PgMK, and PgMVD had significantly increased. The highest level of transcript accumulation occurred at 3 h after initial exposure for most of the genes. Meanwhile, triterpene saponin accumulation increased with the increase in the time of exposure, and at 48 h after initial exposure, the total saponin content was the highest recorded.

A candidate gene identified in converting platycoside E to platycodin D from Platycodon grandiflorus by transcriptome and main metabolites analysis

Platycodin D and platycoside E are two triterpenoid saponins in Platycodon grandiflorus, differing only by two glycosyl groups structurally. Studies have shown β-Glucosidase from bacteria can convert platycoside E to platycodin D, indicating the potential existence of similar enzymes in P. grandiflorus. An L₉(3⁴) orthogonal experiment was performed to establish a protocol for calli induction as follows: the optimal explant is stems with nodes and the optimum medium formula is MS + NAA 1.0 mg/L + 6-BA 0.5 mg/L to obtain callus for experimental use. The platycodin D, platycoside E and total polysaccharides content between callus and plant organs varied wildly. Platycodin D and total polysaccharide content of calli was found higher than that of leaves. While, platycoside E and total polysaccharide content of calli was found lower than that of leaves. Associating platycodin D and platycoside E content with the expression level of genes involved in triterpenoid saponin biosynthesis between calli and leaves, three contigs were screened as putative sequences of β-Glucosidase gene converting platycoside E to platycodin D. Besides, we inferred that some transcription factors can regulate the expression of key enzymes involved in triterpernoid saponins and polysaccharides biosynthesis pathway of P. grandiflorus. Totally, a candidate gene encoding enzyme involved in converting platycoside E to platycodin D, and putative genes involved in polysaccharide synthesis in P. grandiflorus had been identified. This study will help uncover the molecular mechanism of triterpenoid saponins biosynthesis in P. grandiflorus.

Production of Bioactive Deapiosylated Platycosides from Glycosylated Platycosides in Balloon Flower Root Using the Crude Enzyme from the Food-Available Fungus Rhizopus oryzae

Extract from balloon flower root (Platycodi radix) containing platycosides as saponins is a beneficial food additive and is used for their savory taste and the alleviation of respiratory diseases. Deglycosylated platycosides show greater pharmacological effects than glycosylated platycosides. However, there are no reports on the conversion of glycosylated platycosides into deapiosylated platycosides. In this study, we showed that the crude enzyme from Rhizopus oryzae, a generally recognized as safe (GRAS) fungus isolated from meju (fermented soybean brick), completely converted glycosylated platycosides in Platycodi radix extract into deapiosylated platycosides: deapiosylated platycodin D (deapi-PD), deapiosylated platycodin A (deapi-PA), deapiosylated polygalacin D (deapi-PGD), and deapiosylated platyconic acid A (deapi-PCA). Among these, deapi-PA and deapi-PCA were first identified using liquid chromatography/mass spectrometry. The anti-inflammatory and antioxidant effects of deapiosylated platycosides were greater than those of the precursor glycosylated platycosides. These deapiosylated platycosides could improve the properties of functional food additives.

Full-Length Transcriptome Analyses of Genes Involved in Triterpenoid Saponin Biosynthesis of Psammosilene tunicoides Hairy Root Cultures With Exogenous Salicylic Acid

Triterpenoid saponins constitute a diverse class of bioactive compounds in medicinal plants. Salicylic acid (SA) is an efficient elicitor for secondary metabolite production, but a transcriptome-wide regulatory network of SA-promoted triterpenoid saponin biosynthesis remains little understood. In the current study, we described the establishment of the hairy root culture system for Psammosilene tunicoides, a triterpenoid saponin-producing medicinal herb in China, using genetic transformation by Agrobacterium rhizogenes. Compared to controls, we found that total saponin content was dramatically increased (up to 2.49-fold) by the addition of 5 mg/L SA in hairy roots for 1 day. A combination of single-molecule real-time (SMRT) and next-generation sequencing (Illumina RNA-seq) was generated to analyze the full-length transcriptome data for P. tunicoides, as well as the transcript profiles in treated (8 and 24 h) and non-treated (0 h) groups with 5 mg/L SA in hairy roots. A total of 430,117 circular consensus sequence (CCS) reads, 16,375 unigenes and 4,678 long non-coding RNAs (lncRNAs) were obtained. The average length of unigenes (2,776 bp) was much higher in full-length transcriptome than that derived from single RNA-seq (1,457 bp). The differentially expressed genes (DEGs) were mainly enriched in the metabolic process. SA up-regulated the unigenes encoding SA-binding proteins and antioxidant enzymes in comparison with controls. Additionally, we identified 89 full-length transcripts encoding enzymes putatively involved in saponin biosynthesis. The candidate transcription factors (WRKY, NAC) and structural genes (AACT, DXS, SE, CYP72A) might be the key regulators in SA-elicited saponin accumulation. Their expression was further validated by quantitative real-time PCR (qRT-PCR). These findings preliminarily elucidate the regulatory mechanisms of SA on triterpenoid saponin biosynthesis in the transcriptomic level, laying a foundation for SA-elicited saponin augmentation in P. tunicoides.

Whole-genome, transcriptome, and methylome analyses provide insights into the evolution of platycoside biosynthesis in Platycodon grandiflorus, a medicinal plant

Triterpenoid saponins (TSs) are common plant defense phytochemicals with potential pharmaceutical properties. Platycodon grandiflorus (Campanulaceae) has been traditionally used to treat bronchitis and asthma in East Asia. The oleanane-type TSs, platycosides, are a major component of the P. grandiflorus root extract. Recent studies show that platycosides exhibit anti-inflammatory, antiobesity, anticancer, antiviral, and antiallergy properties. However, the evolutionary history of platycoside biosynthesis genes remains unknown. In this study, we sequenced the genome of P. grandiflorus and investigated the genes involved in platycoside biosynthesis. The draft genome of P. grandiflorus is 680.1 Mb long and contains 40,017 protein-coding genes. Genomic analysis revealed that the CYP716 family genes play a major role in platycoside oxidation. The CYP716 gene family of P. grandiflorus was much larger than that of other Asterid species. Orthologous gene annotation also revealed the expansion of β-amyrin synthases (bASs) in P. grandiflorus, which was confirmed by tissue-specific gene expression. In these expanded gene families, we identified key genes showing preferential expression in roots and association with platycoside biosynthesis. In addition, whole-genome bisulfite sequencing showed that CYP716 and bAS genes are hypomethylated in P. grandiflorus, suggesting that epigenetic modification of these two gene families affects platycoside biosynthesis. Thus whole-genome, transcriptome, and methylome data of P. grandiflorus provide novel insights into the regulation of platycoside biosynthesis by CYP716 and bAS gene families.

Comparative transcriptomic analysis of rhizomes, stems, and leaves of Polygonatum odoratum (Mill.) Druce reveals candidate genes associated with polysaccharide synthesis

Polygonatum odoratum (Mill.) Druce is a well-known traditional Chinese herb. Polysaccharides are major bioactive components of Polygonatum odoratum, which can improve immunity, and are used to treat rheumatic heart disease, cardiovascular disease, and diabetes. This study identified potential genes and transcription factors (TFs) that regulate polysaccharide synthesis in Polygonatum odoratum (Mill.) Druce using RNA sequencing data from leaf, stem, and rhizome tissues. 76,714 unigenes were annotated in public databases. Analysis of KEGG annotations identified 18 key enzymes responsible for polysaccharide biosynthesis and the most of the upregulated expressed unigenes were enriched in rhizome tissue compared with leaf or stem tissue. 73 TFs involved in polysaccharide synthesis were predicted. In addition, key enzyme genes were verified by quantitative real-time PCR. This study substantially enlarged the public transcriptome datasets of this species, and provided insight into detection of novel genes involved in synthesis of polysaccharides and other secondary metabolites.

NAA and 6-BA promote accumulation of oleanolic acid by JA regulation in Achyranthes bidentata Bl

Application of plant growth regulators has become one of the most important means of improving yield and quality of medicinal plants. To understand the molecular basis of phytohormone-regulated oleanolic acid metabolism, RNA-seq was used to analyze global gene expression in Achyranthes bidentata treated with 2.0 mg/L 1-naphthaleneacetic acid (NAA) and 1.0 mg/L 6-benzyladenine (6-BA). Compared with untreated controls, the expression levels of 20,896 genes were significantly altered with phytohormone treatment. We found that 13071 (62.5%) unigenes were up-regulated, and a lot of differentially expressed genes involved in hormone or terpenoid biosynthesis, or transcription factors were significantly up-regulated. These results suggest that oleanolic acid biosynthesis induced by NAA and 6-BA occurs due to the expression of key genes involved in jasmonic acid signal transduction. This study is the first to analyze the production and hormonal regulation of medicinal A. bidentata metabolites at the molecular level. The results herein contribute to a better understanding of the regulation of oleanane-type triterpenoid saponins accumulation and define strategies to improve the yield of these useful metabolites.

Role of Saponins in Plant Defense Against Specialist Herbivores

The diamondback moth (DBM), Plutella xylostella (Lepidoptera: Plutellidae) is a very destructive crucifer-specialized pest that has resulted in significant crop losses worldwide. DBM is well attracted to glucosinolates (which act as fingerprints and essential for herbivores in host plant recognition) containing crucifers such as wintercress, Barbarea vulgaris (Brassicaceae) despite poor larval survival on it due to high-to-low concentration of saponins and generally to other plants in the genus Barbarea. B. vulgaris build up resistance against DBM and other herbivorous insects using glucosinulates which are used in plant defense. Aside glucosinolates, Barbarea genus also contains triterpenoid saponins, which are toxic to insects and act as feeding deterrents for plant specialist herbivores (such as DBM). Previous studies have found interesting relationship between the host plant and secondary metabolite contents, which indicate that attraction or resistance to specialist herbivore DBM, is due to higher concentrations of glucosinolates and saponins in younger leaves in contrast to the older leaves of Barbarea genus. As a response to this phenomenon, herbivores as DBM has developed a strategy of defense against these plant biochemicals. Because there is a lack of full knowledge in understanding bioactive molecules (such as saponins) role in plant defense against plant herbivores. Thus, in this review, we discuss the role of secondary plant metabolites in plant defense mechanisms against the specialist herbivores. In the future, trials by plant breeders could aim at transferring these bioactive molecules against herbivore to cash crops.

De novo sequencing and transcriptome assembly of Arisaema heterophyllum Blume and identification of genes involved in isoflavonoid biosynthesis

Arisaema heterophyllum Blume (AhBl) is one of the valued medicinal plants. However, its genetic information is limited, which impedes further studies of this valuable resource. To investigate the genes involved in the isoflavonoid biosynthesis, we deeply performed transcriptome sequencing for AhBl. An average of 10.98 Gb clean reads were obtained based on root, tuber and leaf tissues, and 109,937 unigenes were yielded after de novo assembly. In total, 72,287 of those unigenes were annotated in at least one public database. The numbers of expressed unigenes in each tissue were 35,686, 43,363 and 47,783, respectively. The overall expression levels of transcripts in leaf were higher than those in root and tuber. Differentially expressed genes analysis indicated that a total of 12,448 shared unigenes were detected in all three tissues, 10,215 of which were higher expressed in tuber than that in root and leaf. Besides, 87 candidate unigenes that encode for enzymes involved in biosynthesis of isoflavonoid were identified and analyzed, and some key enzyme genes were experimentally validated by quantitative Real-Time PCR (qRT-PCR). This study provides a unique dataset for the systematic analysis of AhBl functional genes and expression characteristics, and facilitates the future study of the pharmacological mechanism of AhBl.