Endophytes, biotransforming microorganisms, and engineering microbial factories for triterpenoid saponins production

Chemical characterization, pathway enrichments and bioactive potentials of catechin-producing endophytic fungi isolated from tea leaves

Endophytes acquire flavonoid biosynthetic genes from the host medicinal plants. Despite tea (Camellia sinensis (L.) Kuntze) being the major source of bioactive catechins, catechin-producing endophytic fungi have never been reported from the tea plant. Here, we report the isolation and characterization of catechin-producing endophytic fungi isolated from tea leaves, their chemical characterization, and associated bioactivities. Among the nine isolated endophytes, two (CSPL6 and CSPL5b) produced catechin (381.48 and 166.40 μg per mg extract) and epigallocatechin-o-gallate (EGCG; 484.41 and 281.99 μg per mg extract) as quantified by high-performance liquid chromatography (HPLC). The isolates were identified as Pseudopestalotiopsis camelliae-sinensis and Didymella sinensis based on molecular and morphological characterization. Untargeted metabolomics using gas-chromatography mass spectroscopy (GCMS) revealed the presence of several bioactive phytochemicals mostly belonging to tyrosols, pyridoxines, fatty acids, aminopyrimidine, and benzenetriol classes. Metabolic pathways pertaining to the biosynthesis of unsaturated fatty acids (UFAs), butanoate metabolism, and linoleic acid metabolism were highly enriched in both catechin-producing isolates. The isolates were able to differentially scavenge intracellular O2 and N2 free-radicals, but CSPL5b demonstrated relatively superior bioactivities compared to CSPL6. Both isolates stimulated the growth of various probiotic strains, indicating prebiotic effects that are otherwise known to be associated with catechins. Collectively, the current study demonstrated that fungal endophytes CSPL6 and CSPL5b, isolated from tea leaves, could be used as alternative sources of catechins, and hold promising potential in evidence-based therapeutics.

Hosts engineering and in vitro enzymatic synthesis for the discovery of novel natural products and their derivatives

Novel natural products (NPs) and their derivatives are important sources for drug discovery, which have been broadly applied in the fields of agriculture, livestock, and medicine, making the synthesis of NPs and their derivatives necessarily important. In recent years, biosynthesis technology has received increasing attention due to its high efficiency in the synthesis of high value-added novel products and its advantages of green, environmental protection, and controllability. In this review, the technological advances of biosynthesis strategies in the discovery of novel NPs and their derivatives are outlined, with an emphasis on two areas of host engineering and in vitro enzymatic synthesis. In terms of hosts engineering, multiple microorganisms, including Streptomyces, Aspergillus, and Penicillium, have been used as the biosynthetic gene clusters (BGCs) provider and host strain for the expression of BGCs to discover new compounds over the past years. In addition, the use of in vitro enzymatic synthesis strategy to generate novel compounds such as triterpenoid saponins and flavonoids is also hereby described.

Endophytic fungi of Panax sokpayensis produce bioactive ginsenoside Compound K in flask fermentation

Endophytes of Panax have the potential to produce their host plant secondary metabolites, ginsenosides. Panax sokpayensis, an endemic traditional medicinal plant of the Sikkim Himalayas was explored for the isolation of endophytic fungi. In the present study, we have isolated 35 endophytic fungal cultures from the rhizome of P. sokpayensis and screened for ginsenosides production by HPLC by comparing the peak retention time with that of standard ginsenosides. The HPLC analysis revealed that out of 35 isolates, the mycelial extracts of four fungal endophytes (PSRF52, PSRF53, PSRF49 and PSRF58) exhibited peaks with a similar retention time of the standard ginsenoside, Compound K (CK). LC-ESI-MS/MS analysis led to the confirmation of ginsenoside CK production by the four fungal endophytes which showed a compound with m/z 639.6278, similar to that of standard ginsenoside CK with yield in potato dextrose broth flask fermentation ranging from 0.0019 to 0.0386 mg/g of mycelial mass in dry weight basis. The four prospective fungal endophyte isolates were identified as Thermothielavioides terrestris PSRF52, Aspergillus sp. PSRF49, Rutstroemiaceae sp. strain PSRF53, and Phaeosphaeriaceae sp. strain PSRF58 based on ITS sequencing. The present finding highlights the need for further study on growth optimization and other culture parameters to exploit the endophytes as an alternative source for ginsenoside CK production.

Recent advances in triterpenoid pathway elucidation and engineering

Triterpenoids are among the most assorted class of specialized metabolites found in all the taxa of living organisms. Triterpenoids are the leading active ingredients sourced from plant species and are utilized in pharmaceutical and cosmetic industries. The triterpenoid precursor 2,3-oxidosqualene, which is biosynthesized via the mevalonate (MVA) pathway is structurally diversified by the oxidosqualene cyclases (OSCs) and other scaffold-decorating enzymes such as cytochrome P450 monooxygenases (P450s), UDP-glycosyltransferases (UGTs) and acyltransferases (ATs). A majority of the bioactive triterpenoids are harvested from the native hosts using the traditional methods of extraction and occasionally semi-synthesized. These methods of supply are time-consuming and do not often align with sustainability goals. Recent advancements in metabolic engineering and synthetic biology have shown prospects for the green routes of triterpenoid pathway reconstruction in heterologous hosts such as Escherichia coli, Saccharomyces cerevisiae and Nicotiana benthamiana, which appear to be quite promising and might lead to the development of alternative source of triterpenoids. The present review describes the biotechnological strategies used to elucidate complex biosynthetic pathways and to understand their regulation and also discusses how the advances in triterpenoid pathway engineering might aid in the scale-up of triterpenoid production in engineered hosts.

An overview of tea saponin as a surfactant in food applications

The residue of Camellia seeds after oil extraction contains many bioactive ingredients, including tea saponin. Tea saponin has many pharmacological effects and is an excellent nonionic surfactant. The development of natural surfactants has become a hot topic in food research. This review gathers the applications of tea saponin as a surfactant in food. It focuses on the application of tea saponin in emulsions, delivery systems, extraction and fermentation, as well as the challenges and development prospects in food applications. Tea saponin shows great potential as a surfactant in food applications, which can replace some synthetic surfactants. The full utilization of tea saponin improves the comprehensive utilization value of Camellia seed residue, contributes to the sustainable development of Camellia industry and avoids resource waste.

Compound K Production: Achievements and Perspectives

Compound K (CK) is one of the major metabolites found in mammalian blood and organs following oral administration of Panax plants. CK, also known as minor ginsenoside, can be absorbed in the systemic circulation. It has garnered significant attention in healthcare and medical products due to its pharmacological activities, such as antioxidation, anticancer, antiproliferation, antidiabetics, neuroprotection, and anti-atherogenic activities. However, CK is not found in natural ginseng plants but in traditional chemical synthesis, which uses toxic solvents and leads to environmental pollution during the harvest process. Moreover, enzymatic reactions are impractical for industrial CK production due to low yield and high costs. Although CK could be generated from major ginsenosides, most ginsenosides, including protopanaxatriol-oleanane and ocotillol-type, are not converted into CK by catalyzing β-glucosidase. Therefore, microbial cell systems have been used as a promising solution, providing a safe and efficient approach to CK production. This review provides a summary of various approaches for the production of CK, including chemical and enzymatic reactions, biotransformation by the human intestinal bacteria and endophytes as well as engineered microbes. Moreover, the approaches for CK production have been discussed to improve the productivity of target compounds.

High-yield production of protopanaxadiol from sugarcane molasses by metabolically engineered Saccharomyces cerevisiae

Background

Ginsenosides are Panax plant-derived triterpenoid with wide applications in cardiovascular protection and immunity-boosting. However, the saponins content of Panax plants is fairly low, making it time-consuming and unsustainable by direct extraction. Protopanaxadiol (PPD) is a common precursor of dammarane-type saponins, and its sufficient supply is necessary for the efficient synthesis of ginsenoside.

Results

In this study, a combinational strategy was used for the construction of an efficient yeast cell factory for PPD production. Firstly, a PPD-producing strain was successfully constructed by modular engineering in Saccharomyces cerevisiae BY4742 at the multi-copy sites. Then, the INO2 gene, encoding a transcriptional activator of the phospholipid biosynthesis, was fine-tuned to promote the endoplasmic reticulum (ER) proliferation and improve the catalytic efficiency of ER-localized enzymes. To increase the metabolic flux of PPD, dynamic control, based on a carbon-source regulated promoter P_HXT1, was introduced to repress the competition of sterols. Furthermore, the global transcription factor UPC2-1 was introduced to sterol homeostasis and up-regulate the MVA pathway, and the resulting strain BY-V achieved a PPD production of 78.13 ± 0.38 mg/g DCW (563.60 ± 1.65 mg/L). Finally, sugarcane molasses was used as an inexpensive substrate for the first time in PPD synthesis. The PPD titers reached 1.55 ± 0.02 and 15.88 ± 0.65 g/L in shake flasks and a 5-L bioreactor, respectively. To the best of our knowledge, these results were new records on PPD production.

Conclusion

The high-level of PPD production in this study and the successful comprehensive utilization of low-cost carbon source -sugarcane molassesindicate that the constructed yeast cell factory is an excellent candidate strain for the production of high-value-added PPD and its derivativeswith great industrial potential.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-022-01949-4.

Diversity and Ginsenoside Biotransformation Potential of Cultivable Endophytic Fungi Associated With Panax bipinnatifidus var. bipinnatifidus in Qinling Mountains, China

To obtain novel fungi with potent β-glucosidase for minor ginsenoside production, Panax bipinnatifidus var. bipinnatifidus, which is a traditional medicinal plant containing various ginsenosides, was first employed to isolate endophytic fungi in this study. A total of 93 representative morphotype strains were isolated and identified according to ITS rDNA sequence analyses, and they were grouped into three phyla (Ascomycota, Basidiomycota, and Mucoromycota), five classes (Dothideomycetes, Sordariomycetes, Eurotiomycetes, Agaricomycetes, and Mucoromycetes), and 24 genera. Plectosphaerella (RA, 19.35%) was the most abundant genus, followed by Paraphoma (RA, 11.83%) and Fusarium (RA, 9.70%). The species richness index (S, 34) and the Shannon–Wiener index (H’, 3.004) indicated that P. bipinnatifidus harbored abundant fungal resources. A total of 26 endophytic fungal ethyl acetate extracts exhibited inhibitory activities against at least one pathogenic bacterium or fungus. In total, 11 strains showed strong β-glucosidase activities and also presented with the ability of ginsenoside biotransformation with varied glycoside-hydrolyzing pathways. Excitingly, three genera, namely, Ilyonectria, Sarocladium, and Lecanicillium, and all 11 taxa were first found to have the ability to transform ginsenosides in our study. The results indicated that P. bipinnatifidus could be a new fungi resource with potential novel natural compounds with antimicrobial activity and potent β-glucosidase for varied minor ginsenoside production.

High-yield bioactive triterpenoid production by heterologous expression in Nicotiana benthamiana using the Tsukuba system

Oleanolic acid is a pentacyclic triterpenoid found in numerous plant species and is a precursor to several bioactive triterpenoids with commercial potential. However, oleanolic acid accumulates at low levels in plants, and its chemical synthesis is challenging. Here, we established a method for producing oleanolic acid in substantial quantities via heterologous expression of pathway enzymes in Nicotiana benthamiana. The "Tsukuba system" is one of the most efficient agroinfiltration-based transient protein expression systems using the vector pBYR2HS, which contains geminiviral replication machinery and a double terminator for boosting expression. Additionally, the pBYR2HS vector contains an expression cassette for the gene-silencing suppressor p19 protein from tomato bushy stunt virus, which can also contribute to enhancing the expression of target proteins. In this study, we evaluated the applicability of this system to heterologous triterpenoid production in N. benthamiana. Medicago truncatula cytochrome P450 monooxygenase (CYP) 716A12 is the first enzyme to be functionally characterized as β-amyrin C-28 oxidase producing oleanolic acid. A mutant CYP716A12 (D122Q) with improved catalytic activity engineered in our previous study was co-expressed with other enzymes in N. benthamiana leaves. Using pBYR2HS, oleanolic acid yield was increased 13.1-fold compared with that using the conventional binary vector, indicating the advantage of the Tsukuba system. We also demonstrated the efficacy of co-expressing a mutant Arabidopsis thaliana HMGR1 catalytic domain, additional NADPH-cytochrome P450 reductase (CPR) transferring electrons to heterologous CYPs, and application of ascorbic acid for preventing leaf necrosis after agroinfiltration, to improve product yield. As a result, the product yields of both simple (β-amyrin) and oxidized (oleanolic acid and maslinic acid) triterpenoids were significantly improved compared with the previously reported yield in heterologous triterpenoid production in N. benthamiana leaves.