Functional Divergence of Diterpene Syntheses in the Medicinal Plant Salvia miltiorrhiza

Genome-Wide Identification of the AP2/ERF Gene Family Involved in Active Constituent Biosynthesis in

Tanshinones and phenolic acids are the major bioactive constituents in the traditional medicinal crop ; however, transcription factors (TFs) are seldom investigated with regard to their regulation of the biosynthesis of these compounds. Here a complete overview of the APETALA2/ethylene-responsive factor (AP2/ERF) transcription factor family in is provided, including phylogeny, gene structure, conserved motifs, and gene expression profiles of different organs (root, stem, leaf, flower) and root tissues (periderm, phloem, xylem). In total, 170 AP2/ERF genes were identified and divided into five relatively conserved subfamilies, including AP2 (25 genes), DREB (61 genes), ethylene responsive factor (ERF; 79 genes), RAV (4 genes), and Soloist (1 gene). According to the distribution of bioactive constituents and the expression patterns of AP2/ERF genes in different organs and root tissues, the genes related to the biosynthesis of bioactive constituents were selected. On the basis of quantitative real-time polymerase chain reaction (qRT-PCR) analysis, coexpression analysis, and the prediction of -regulatory elements in the promoters, we propose that two genes ( and ) regulate tanshinone biosynthesis and two genes ( and ) participate in controlling phenolic acid biosynthesis. The genes related to tanshinone biosynthesis belong to the ERF-B3 subgroup. In contrast, the genes predicted to regulate phenolic acid biosynthesis belong to the ERF-B1 and ERF-B4 subgroups. These results provide a foundation for future functional characterization of AP2/ERF genes to enhance the biosynthesis of the bioactive compounds of .

Arabidopsis DREB1B in transgenic Salvia miltiorrhiza increased tolerance to drought stress without stunting growth

Multiple stress response genes are controlled by transcription factors in a coordinated manner; therefore, these factors can be used for molecular plant breeding. CBF1/DREB1B, a known stress-inducible gene, was isolated from Arabidopsis thaliana and introduced into Salvia miltiorrhiza under the control of the CaMV35S or RD29A promoter. Under drought stress, relative water content, chlorophyll content, and the net photosynthetic rate were observed to be higher in the transgenic lines than in the wild type (WT). Moreover, O2(-) and H2O2 accumulation was observed to be lower in the transgenic lines. Additional analyses revealed that the AtDREB1B transgenic plants generally displayed lesser malondialdehyde (MDA) but higher superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) activities than the WT under drought stress. Quantitative real-time polymerase chain reaction of a subset of genes involved in photosynthesis, stress response, carbohydrate metabolism, and cell protection further verified that AtDREB1B could enhance tolerance to drought by activating different downstream DREB/CBF genes in the transgenic plants. Furthermore, no growth inhibition was detected in transgenic S. miltiorrhiza plants that expressed AtDREB1B driven by either the constitutive CaMV35S promoter or the stress-inducible RD29A promoter. Together, these results suggest that AtDREB1B is a good candidate gene for increasing drought tolerance in transgenic S. miltiorrhiza.

Genome-wide analysis of auxin response factor gene family members in medicinal model plant Salvia miltiorrhiza

Auxin response factors (ARFs) can function as transcriptional activators or repressors to regulate the expression of auxin response genes by specifically binding to auxin response elements (AuxREs) during plant development. Based on a genome-wide strategy using the medicinal model plant Salvia miltiorrhiza, 25 S. miltiorrhiza ARF (SmARF) gene family members in four classes (class Ia, IIa, IIb and III) were comprehensively analyzed to identify characteristics including gene structures, conserved domains, phylogenetic relationships and expression patterns. In a hybrid analysis of the phylogenetic tree, microRNA targets, and expression patterns of SmARFs in different organs, root tissues, and methyl jasmonate or indole-3-acetic acid treatment conditions, we screened for candidate SmARFs involved in various developmental processes of S. miltiorrhiza. Based on this analysis, we predicted that SmARF25, SmARF7, SmARF16 and SmARF20 are involved in flower, leaf, stem and root development, respectively. With the further insight into the targets of miR160 and miR167, specific SmARF genes in S. miltiorrhiza might encode products that participate in biological processes as described for ARF genes in Arabidopsis. Our results provide a foundation for understanding the molecular basis and regulatory mechanisms of SmARFs in S. miltiorrhiza.

Summary: Genome-wide analysis identified 25 ARF gene members (seven transcriptional activators and 18 repressors) in S. miltiorrhiza. The gene structures, functional domains, miRNA targets and expression patterns were analyzed in detail.

Extreme promiscuity of a bacterial and a plant diterpene synthase enables combinatorial biosynthesis

Diterpenes are widely distributed across many biological kingdoms, where they serve a diverse range of physiological functions, and some have significant industrial utility. Their biosynthesis involves class I diterpene synthases (DTSs), whose activity can be preceded by that of class II diterpene cyclases (DTCs). Here, a modular metabolic engineering system was used to examine the promiscuity of DTSs. Strikingly, both a bacterial and plant DTS were found to exhibit extreme promiscuity, reacting with all available precursors with orthogonal activity, producing an olefin or hydroxyl group, respectively. Such DTS promiscuity enables combinatorial biosynthesis, with remarkably high yields for these unoptimized non-native enzymatic combinations (up to 15mg/L). Indeed, it was possible to readily characterize the 13 unknown products. Notably, 16 of the observed diterpenes were previously inaccessible, and these results provide biosynthetic routes that are further expected to enable assembly of more extended pathways to produce additionally elaborated 'non-natural' diterpenoids.

Cytochrome P450 promiscuity leads to a bifurcating biosynthetic pathway for tanshinones

Cytochromes P450 (CYPs) play a key role in generating the structural diversity of terpenoids, the largest group of plant natural products. However, functional characterization of CYPs has been challenging because of the expansive families found in plant genomes, diverse reactivity and inaccessibility of their substrates and products. Here we present the characterization of two CYPs, CYP76AH3 and CYP76AK1, which act sequentially to form a bifurcating pathway for the biosynthesis of tanshinones, the oxygenated diterpenoids from the Chinese medicinal plant Danshen (Salvia miltiorrhiza). These CYPs had similar transcription profiles to that of the known gene responsible for tanshinone production in elicited Danshen hairy roots. Biochemical and RNA interference studies demonstrated that both CYPs are promiscuous. CYP76AH3 oxidizes ferruginol at two different carbon centers, and CYP76AK1 hydroxylates C-20 of two of the resulting intermediates. Together, these convert ferruginol into 11,20-dihydroxy ferruginol and 11,20-dihydroxy sugiol en route to tanshinones. Moreover, we demonstrated the utility of these CYPs by engineering yeast for heterologous production of six oxygenated diterpenoids, which in turn enabled structural characterization of three novel compounds produced by CYP-mediated oxidation. Our results highlight the incorporation of multiple CYPs into diterpenoid metabolic engineering, and a continuing trend of CYP promiscuity generating complex networks in terpenoid biosynthesis.

Enhanced Diterpene Tanshinone Accumulation and Bioactivity of Transgenic Salvia miltiorrhiza Hairy Roots by Pathway Engineering

Tanshinones are health-promoting diterpenoids found in Salvia miltiorrhiza and have wide applications. Here, SmGGPPS (geranylgeranyl diphosphate synthase) and SmDXSII (1-deoxy-D-xylulose-5-phosphate synthase) were introduced into hairy roots of S. miltiorrhiza. Overexpression of SmGGPPS and SmDXSII in hairy roots produces higher levels of tanshinone than control and single-gene transformed lines; tanshinone production in the double-gene transformed line GDII10 reached 12.93 mg/g dry weight, which is the highest tanshinone content that has been achieved through genetic engineering. Furthermore, transgenic hairy root lines showed higher antioxidant and antitumor activities than control lines. In addition, contents of chlorophylls, carotenoids, indoleacetic acid, and gibberellins were significantly elevated in transgenic Arabidopsis thaliana plants. These results demonstrate a promising method to improve the production of diterpenoids including tanshinone as well as other natural plastid-derived isoprenoids in plants by genetic manipulation of the 2-C-methyl-D-erythritol-4-phosphate (MEP) pathway.

Functional characterization of ent-copalyl diphosphate synthase, kaurene synthase and kaurene oxidase in the Salvia miltiorrhiza gibberellin biosynthetic pathway

Salvia miltiorrhiza Bunge is highly valued in traditional Chinese medicine for its roots and rhizomes. Its bioactive diterpenoid tanshinones have been reported to have many pharmaceutical activities, including antibacterial, anti-inflammatory, and anticancer properties. Previous studies found four different diterpenoid biosynthetic pathways from the universal diterpenoid precursor (E,E,E)-geranylgeranyl diphosphate (GGPP) in S. miltiorrhiza. Here, we describe the functional characterization of ent-copalyl diphosphate synthase (SmCPS_ent), kaurene synthase (SmKS) and kaurene oxidase (SmKO) in the gibberellin (GA) biosynthetic pathway. SmCPS_ent catalyzes the cyclization of GGPP to ent-copalyl diphosphate (ent-CPP), which is converted to ent-kaurene by SmKS. Then, SmKO catalyzes the three-step oxidation of ent-kaurene to ent-kaurenoic acid. Our results show that the fused enzyme SmKS-SmCPS_ent increases ent-kaurene production by several fold compared with separate expression of SmCPS_ent and SmKS in yeast strains. In this study, we clarify the GA biosynthetic pathway from GGPP to ent-kaurenoic acid and provide a foundation for further characterization of the subsequent enzymes involved in this pathway. These insights may allow for better growth and the improved accumulation of bioactive tanshinones in S. miltiorrhiza through the regulation of the expression of these genes during developmental processes.

Comparative Transcriptomics Unravel Biochemical Specialization of Leaf Tissues of Stevia for Diterpenoid Production

Stevia (Stevia rebaudiana) produces not only a group of diterpenoid glycosides known as steviol glycosides (SGs), but also other labdane-type diterpenoids that may be spatially separated from SGs. However, their biosynthetic routes and spatial distribution in leaf tissues have not yet been elucidated. Here, we integrate metabolome and transcriptome analyses of Stevia to explore the biosynthetic capacity of leaf tissues for diterpenoid metabolism. Tissue-specific chemical analyses confirmed that SGs were accumulated in leaf cells but not in trichomes. On the other hand, Stevia leaf trichomes stored other labdane-type diterpenoids such as oxomanoyl oxide and agatholic acid. RNA sequencing analyses from two different tissues of Stevia provided a comprehensive overview of dynamic metabolic activities in trichomes and leaf without trichomes. These metabolite-guided transcriptomics and phylogenetic and gene expression analyses clearly identified specific gene members encoding enzymes involved in the 2-C-methyl-d-erythritol 4-phosphate pathway and the biosynthesis of steviol or other labdane-type diterpenoids. Additionally, our RNA sequencing analysis uncovered copalyl diphosphate synthase (SrCPS) and kaurene synthase1 (SrKS1) homologs, SrCPS2 and KS-like (SrKSL), which were specifically expressed in trichomes. In vitro and in planta assays showed that unlike SrCPS and SrKS1, SrCPS2 synthesized labda-13-en-8-ol diphosphate and successively catalyzed the formation of manoyl oxide and epi-manoyl oxide in combination with SrKSL. Our findings suggest that Stevia may have evolved to use distinct metabolic pathways to avoid metabolic interferences in leaf tissues for efficient production of diverse secondary metabolites.

Recent advances in biosynthesis of bioactive compounds in traditional Chinese medicinal plants

Plants synthesize and accumulate large amount of specialized (or secondary) metabolites also known as natural products, which provide a rich source for modern pharmacy. In China, plants have been used in traditional medicine for thousands of years. Recent development of molecular biology, genomics and functional genomics as well as high-throughput analytical chemical technologies has greatly promoted the research on medicinal plants. In this article, we review recent advances in the elucidation of biosynthesis of specialized metabolites in medicinal plants, including phenylpropanoids, terpenoids and alkaloids. These natural products may share a common upstream pathway to form a limited numbers of common precursors, but are characteristic in distinct modifications leading to highly variable structures. Although this review is focused on traditional Chinese medicine, other plants with a great medicinal interest or potential are also discussed. Understanding of their biosynthesis processes is critical for producing these highly value molecules at large scale and low cost in microbes and will benefit to not only human health but also plant resource conservation.

The Biosynthetic Pathways of Tanshinones and Phenolic Acids in Salvia miltiorrhiza

Secondary metabolites from plants play key roles in human medicine and chemical industries. Due to limited accumulation of secondary metabolites in plants and their important roles, characterization of key enzymes involved in biosynthetic pathway will enable metabolic engineering or synthetic biology to improve or produce the compounds in plants or microorganisms, which provides an alternative for production of these valuable compounds. Salvia miltiorrhiza, containing tanshinones and phenolic acids as its active compounds, has been widely used for the treatment of cardiovascular and cerebrovascular diseases. The biosynthetic analysis of secondary metabolites in S. miltiorrhiza has made great progress due to the successful genetic transformation system, simplified hairy roots system, and high-throughput sequencing. The cloned genes in S. miltiorrhiza had provided references for functional characterization of the post-modification steps involved in biosynthesis of tanshinones and phenolic acids, and further utilization of these steps in metabolic engineering. The strategies used in these studies could provide solid foundation for elucidation of biosynthetic pathways of diterpenoids and phenolic acids in other species. The present review systematically summarizes recent advances in biosynthetic pathway analysis of tanshinones and phenolic acids as well as synthetic biology and metabolic engineering applications of the rate-limiting genes involved in the secondary metabolism in S. miltiorrhiza.