Spatial organization of silybin biosynthesis in milk thistle [Silybum marianum (L.) Gaertn]

De novo biosynthesis of plant lignans by synthetic yeast consortia

Reconstructing the biosynthesis of complex natural products such as lignans in yeast is challenging and can result in metabolic promiscuity, affecting the biosynthetic efficiency. Here we divide the lignan biosynthetic pathway across a synthetic yeast consortium with obligated mutualism and use ferulic acid as a metabolic bridge. This cooperative system successfully overcomes the metabolic promiscuity and synthesizes the common precursor, coniferyl alcohol. Furthermore, combined with systematic engineering strategies, we achieve the de novo synthesis of key lignan skeletons, pinoresinol and lariciresinol, and verify the scalability of the consortium by synthesizing complex lignans, including antiviral lariciresinol diglucoside. These results provide a starting engineering platform for the heterologous synthesis of lignans. In particular, the study illustrates that the yeast consortium with obligate mutualism is a promising strategy that mimics the metabolic division of labor among multiple plant cells, thereby improving the biosynthesis of long pathways and complex natural products.

De Novo Genome Assembly and Phylogenetic Analysis of Cirsium nipponicum

Background: Cirsium nipponicum, a pharmaceutically valuable plant from the Asteraceae family, has been utilized for over 2000 years. Unlike other thistles, it is native to East Asia and found exclusively on Ulleung Island on the Korea Peninsula. Despite its significance, the genome information of C. nipponicum has remained unclear. Methods: In this study, we assembled the genome of C. nipponicum using both short reads from Illumina sequencing and long reads from Nanopore sequencing. Results: The assembled genome is 929.4 Mb in size with an N50 length of 0.7 Mb, covering 95.1% of BUSCO core groups listed in edicots_odb10. Repeat sequences accounted for 70.94% of the assembled genome. We curated 31,263 protein-coding genes, of which 28,752 were functionally annotated using public databases. Phylogenetic analysis of 11 plant species using single-copy orthologs revealed that C. nipponicum diverged from Cynara cardunculus approximately 15.9 million years ago. Gene family evolutionary analysis revealed significant expansion and contraction in genes involved in abscisic acid biosynthesis, late endosome to vacuole transport, response to nitrate, and abaxial cell fate specification. Conclusions: This study provides a reference genome of C. nipponicum, enhancing our understanding of its genetic background and facilitating an exploration of genetic resources for beneficial phytochemicals.

De Novo Biosynthesis of Dihydroquercetin in Saccharomyces cerevisiae

Dihydroquercetin is a vital flavonoid compound with a wide range of physiological activities. However, factors, such as metabolic regulation, limit the heterologous synthesis of dihydroquercetin in microorganisms. In this study, flavanone 3-hydroxylase (F3H) and flavanone 3'-hydroxylase (F3'H) were screened from different plants, and their co-expression in Saccharomyces cerevisiae was optimized. Promoter engineering and redox partner engineering were used to optimize the corresponding expression of genes involved in the dihydroquercetin synthesis pathway. Dihydroquercetin production was further improved through multicopy integration pathway genes and systems metabolic engineering. By increasing NADPH and α-ketoglutarate supply, the catalytic efficiency of F3'H and F3H was improved, thereby effectively increasing dihydroquercetin production (235.1 mg/L). Finally, 873.1 mg/L dihydroquercetin titer was obtained by fed-batch fermentation in a 5-L bioreactor, which is the highest dihydroquercetin production achieved through de novo microbial synthesis. These results established a pivotal groundwork for flavonoids synthesis.

Deep learning-based elaiosome detection in milk thistle seed for efficient high-throughput phenotyping

Milk thistle, Silybum marianum (L.), is a well-known medicinal plant used for the treatment of liver diseases due to its high content of silymarin. The seeds contain elaiosome, a fleshy structure attached to the seeds, which is believed to be a rich source of many metabolites including silymarin. Segmentation of elaiosomes using only image analysis is difficult, and this makes it impossible to quantify the elaiosome phenotypes. This study proposes a new approach for semi-automated detection and segmentation of elaiosomes in milk thistle seed using the Detectron2 deep learning algorithm. One hundred manually labeled images were used to train the initial elaiosome detection model. This model was used to predict elaiosome from new datasets, and the precise predictions were manually selected and used as new labeled images for retraining the model. Such semi-automatic image labeling, i.e., using the prediction results of the previous stage for retraining the model, allowed the production of sufficient labeled data for retraining. Finally, a total of 6,000 labeled images were used to train Detectron2 for elaiosome detection and attained a promising result. The results demonstrate the effectiveness of Detectron2 in detecting milk thistle seed elaiosomes with an accuracy of 99.9%. The proposed method automatically detects and segments elaiosome from the milk thistle seed. The predicted mask images of elaiosome were used to analyze its area as one of the seed phenotypic traits along with other seed morphological traits by image-based high-throughput phenotyping in ImageJ. Enabling high-throughput phenotyping of elaiosome and other seed morphological traits will be useful for breeding milk thistle cultivars with desirable traits.

A review of the botany, phytochemistry, pharmacology, synthetic biology and comprehensive utilization of Silybum marianum

Silybum marianum (L.) Gaertn, a herbaceous plant with a long history in traditional medicine for the treatment of hepatobiliary diseases, particularly in Europe, which has attracted attention for its remarkable therapeutic effect. This review systematically summarizes the research progress in the botany, phytochemistry, pharmacology, comprehensive utilization and synthetic biology of S. marianum. Up to now, more than 20 types of flavonolignan components have been isolated from S. marianum. In addition, the rearch on fatty acids and triterpenoids is also constantly improving. Among them, silybin is the most active compound in flavonolignans components. Its pharmacological effects in vivo and in vitro include anti-inflammatory, antioxidant, anti-tumour, hypoglycaemic, neuroprotective and immunoregulatory properties. The use of coniferyl alcohol and taxifolin as substrates to produce silybin and isosilybin under the action of enzyme catalysis is the commonly used biosynthetic pathway of silymarin, which provides support for a comprehensive analysis of the synthetic pathway of silymarin. In addition to medicinal use, the extracts of plants also have broad application prospects in the production of food, healthcare products, cosmetics and other aspects. In addition, the chemical composition, pharmacological mechanism and synthetic biology of S. marianum need to be further studied, which is very important for its clinical efficacy and resource development.

Identification of Hydroxylation Enzymes and the Metabolic Analysis of Dihydromyricetin Synthesis in Ampelopsis grossedentata

Ampelopsis grossedentata leaves are highly rich in dihydromyricetin. They have been used to make tea in China for centuries. Dihydromyricetin has many potential applications in foods and medicine. This are because it has five phenolic hydroxyl groups. However, the hydroxylases involving the biosynthesis of dihydromyricetin have not been identified and characterized. In this study, a series of hydroxylases genes, including flavanone 3-hydroxylase (F3H), flavonoid 3'-hydroxylase (F3'H), flavonoid 3',5'-hydroxylase (F3'5'H), and cytochrome P450 reductase (CPR), were identified after RNA sequencing. The full-length CDSs of AgF3H, AgF3'H, AgF3'5'H, and AgCPR genes were amplified from the cDNA library of leaves. The aforementioned enzymes were expressed and verified in Saccharomyces cerevisiae. Through the substrate specificity assay, the functional AgF3'H, AgF3'5'H, and AgCPR in A. grossedentata were identified. The dihydromyricetin hydroxylation process in A. grossedentata was successfully identified. We found that substantial carbon flux occurred through the Naringenin (NAR)-Eriodictyol (ERI)-Dihydroquercetin (DHQ)-Dihydromyricetin (DHM) and NAR-Dihydrokaempferol (DHK)-DHQ-DHM pathways. This study provides some reference for the development and utilization of the germplasm resources and molecular breeding of A. grossedentata.

Microbial cell factories for the production of flavonoids-barriers and opportunities

Flavonoids are natural plant products with important nutritional value, health-promoting benefits, and therapeutic potential. The use of microbial cell factories to generate flavonoids is an appealing option. The microbial biosynthesis of flavonoids is compared to the classic plant extract approach in this review, and the pharmaceutical applications were presented. This paper summarize approaches for effective flavonoid biosynthesis from microorganisms, and discuss the challenges and prospects of microbial flavonoid biosynthesis. Finally, the barriers and strategies for industrial bio-production of flavonoids are highlighted. This review offers guidance on how to create robust microbial cell factories for producing flavonoids and other relevant chemicals.

Mechanistic Insights into the Pharmacological Significance of Silymarin

Medicinal plants are considered the reservoir of diverse therapeutic agents and have been traditionally employed worldwide to heal various ailments for several decades. Silymarin is a plant-derived mixture of polyphenolic flavonoids originating from the fruits and akenes of Silybum marianum and contains three flavonolignans, silibinins (silybins), silychristin and silydianin, along with taxifolin. Silybins are the major constituents in silymarin with almost 70–80% abundance and are accountable for most of the observed therapeutic activity. Silymarin has also been acknowledged from the ancient period and is utilized in European and Asian systems of traditional medicine for treating various liver disorders. The contemporary literature reveals that silymarin is employed significantly as a neuroprotective, hepatoprotective, cardioprotective, antioxidant, anti-cancer, anti-diabetic, anti-viral, anti-hypertensive, immunomodulator, anti-inflammatory, photoprotective and detoxification agent by targeting various cellular and molecular pathways, including MAPK, mTOR, β-catenin and Akt, different receptors and growth factors, as well as inhibiting numerous enzymes and the gene expression of several apoptotic proteins and inflammatory cytokines. Therefore, the current review aims to recapitulate and update the existing knowledge regarding the pharmacological potential of silymarin as evidenced by vast cellular, animal, and clinical studies, with a particular emphasis on its mechanisms of action.

Silybin and its congeners: from traditional medicine to molecular effects

Covering: 2015 up to 2022 (Feb)Silymarin, an extract of milk thistle (Silybum marianum) fruits, has been used in various medicinal applications since ancient times. A major component of silymarin is the flavonolignan silybin and its relatives isosilybin, silychristin, silydianin, 2,3-dehydrosilybin, and some others. Except for silydianin, they occur in nature as two stereomers. This review focuses on recent developments in chemistry, biosynthesis, modern advanced analytical methods, and transformations of flavonolignans specifically reflecting their chirality. Recently described chemotypes of S. marianum, but also the newest findings regarding the pharmacokinetics, hepatoprotective, antiviral, neuroprotective, and cardioprotective activity, modulation of endocrine functions, modulation of multidrug resistance, and safety of flavonolignans are discussed. A growing number of studies show that the respective diastereomers of flavonolignans have significantly different activities in anisotropic biological systems. Moreover, it is now clear that flavonolignans do not act as antioxidants in vivo, but as specific ligands of biological targets and therefore their chirality is crucial. Many controversies often arise, mainly due to the non-standard composition of this phytopreparation, the use of various undefined mixtures, the misattribution of silymarin vs. silybin, and also the failure to consider the chemistry of the respective components of silymarin.

Design of stable and self-regulated microbial consortia for chemical synthesis

Microbial coculture engineering has emerged as a promising strategy for biomanufacturing. Stability and self-regulation pose a significant challenge for the generation of intrinsically robust cocultures for large-scale applications. Here, we introduce the use of multi-metabolite cross-feeding (MMCF) to establish a close correlation between the strains and the design rules for selecting the appropriate metabolic branches. This leads to an intrinicially stable two-strain coculture where the population composition and the product titer are insensitive to the initial inoculation ratios. With an intermediate-responsive biosensor, the population of the microbial coculture is autonomously balanced to minimize intermediate accumulation. This static-dynamic strategy is extendable to three-strain cocultures, as demonstrated with de novo biosynthesis of silybin/isosilybin. This strategy is generally applicable, paving the way to the industrial application of microbial cocultures.

Stability and tunability are two desirable properties of microbial consortia-based bioproduction. Here, the authors integrate a caffeate-responsive biosensor into two and three strains coculture system to achieve autonomous regulation of strain ratios for coniferol and silybin/isosiltbin production, respectively.

Production of phenylpropanoids and flavonolignans from glycerol by metabolically engineered Escherichia coli

Phenylpropanoids are a group of plant natural products with medicinal importance derived from aromatic amino acids. Here, we report the production of two representative phenylpropanoids-coniferyl alcohol (CA) and dihydroquercetin (DHQ)-from glycerol by engineered Escherichia coli. First, an E. coli strain capable of producing 187.7 mg/L of CA from glycerol was constructed by the introduction of hpaBC from E. coli and OMT1, 4CL4, and CCR1 from Arabidopsis thaliana to the p-coumaric acid producer. Next, an E. coli strain capable of producing 239.4 mg/L of DHQ from glycerol was constructed by the introduction of F3H, TT7, and CPR from A. thaliana to the naringenin producer, followed by engineering the signal peptide of a cytochrome P450 TT7. Furthermore, to demonstrate the production of flavonolignans, a group of heterodimeric phenylpropanoids, from glycerol, ascorbate peroxidase 1 from Silybum marianum was employed and engineered to produce 0.04 μg/L of silybin and 1.29 μg/L of isosilybin from glycerol by stepwise culture. Finally, a single strain harboring all the 16 necessary genes was constructed, resulting in 0.12 μg/L of isosilybin production directly from glycerol. The strategies described here will be useful for the production of pharmaceutically important yet complex natural products.

Chirality Matters: Biological Activity of Optically Pure Silybin and Its Congeners

This review focuses on the specific biological effects of optically pure silymarin flavo-nolignans, mainly silybins A and B, isosilybins A and B, silychristins A and B, and their 2,3-dehydro derivatives. The chirality of these flavonolignans is also discussed in terms of their analysis, preparative separation and chemical reactions. We demonstrated the specific activities of the respective diastereomers of flavonolignans and also the enantiomers of their 2,3-dehydro derivatives in the 3D anisotropic systems typically represented by biological systems. In vivo, silymarin flavonolignans do not act as redox antioxidants, but they play a role as specific ligands of biological targets, according to the "lock-and-key" concept. Estrogenic, antidiabetic, anticancer, antiviral, and antiparasitic effects have been demonstrated in optically pure flavonolignans. Potential application of pure flavonolignans has also been shown in cardiovascular and neurological diseases. Inhibition of drug-metabolizing enzymes and modulation of multidrug resistance activity by these compounds are discussed in detail. The future of "silymarin applications" lies in the use of optically pure components that can be applied directly or used as valuable lead structures, and in the exploration of their true molecular effects.

Effect of gamma rays and colchicine on silymarin production in cell suspension cultures of Silybum marianum: A transcriptomic study of key genes involved in the biosynthetic pathway

The aim of this study was to investigate secondary metabolite production in Silybum marianum L. cell suspension cultures obtained from seeds treated with gamma rays (200 and 600 Gy) and 0.05% colchicine. The effects of these treatments on callus induction, growth, viability, and silymarin production were studied, along with the changes in the transcriptome and DNA sequence of chalcone synthase (CHS) genes. The effect of gamma radiation (200 and 600 Gy) on silymarin production in S. marianum dry seeds was also studied using HPLC-UV. All three treatments induced high callus biomass production from leaf segments. The viability of the cell suspension cultures was over 90%. The flavonolignan content measured in the extracellular culture medium of the S. marianum cell suspension was highest after treatment with 600 Gy, followed by 0.05% colchicine, and finally, 200 Gy, after a growth period of 12 days. In general, an increased expression of CHS1, CHS2, and CHS3 genes, accompanied by an increase of silymarin content, was observed in response to all the studied treatments, although the effect was greatest on CHS2 expression. Bioinformatics analysis confirmed that the three CHS2 clones exhibited the highest genetic variation, both in relation to each other and to the CHS1 and CHS3 clones. Based on the results, S. marianum plants obtained from seeds previously exposed to 600 and 200 Gy as well as colchicine constitute a renewable resource with the potential to obtain large amounts of silymarin.

Antifungal Activity against Botryosphaeriaceae Fungi of the Hydro-Methanolic Extract of Silybum marianum Capitula Conjugated with Stevioside

Silybum marianum (L.) Gaertn, viz. milk thistle, has been the focus of research efforts in the past few years, albeit almost exclusively restricted to the medicinal properties of its fruits (achenes). Given that other milk thistle plant organs and tissues have been scarcely investigated for the presence of bioactive compounds, in this study, we present a phytochemical analysis of the extracts of S. marianum capitula during the flowering phenological stage (stage 67). Gas chromatography–mass spectroscopy results evidenced the presence of high contents of coniferyl alcohol (47.4%), and secondarily of ferulic acid ester, opening a new valorization strategy of this plant based on the former high-added-value component. Moreover, the application of the hydro-methanolic extracts as an antifungal agent has been also explored. Specifically, their activity against three fungal species responsible for the so-called Botryosphaeria dieback of grapevine (Neofusicoccum parvum, Dothiorella viticola and Diplodia seriata) has been assayed both in vitro and in vivo. From the mycelial growth inhibition assays, the best results (EC₉₀ values of 303, 366, and 355 μg·mL⁻¹ for N. parvum, D. viticola, and D. seriata, respectively) were not obtained for the hydroalcoholic extract alone, but after its conjugation with stevioside, which resulted in a strong synergistic behavior. Greenhouse experiments confirmed the efficacy of the conjugated complexes, pointing to the potential of the combination of milk thistle extracts with stevioside as a promising plant protection product in organic Viticulture.

Optimum chalcone synthase for flavonoid biosynthesis in microorganisms

Chalcones and the subsequently generated flavonoids, as well as flavonoid derivatives, have been proven to have a variety of physiological activities and are widely used in: the pharmaceutical, food, feed, and cosmetic industries. As the content of chalcones and downstream products in native plants is low, the production of these compounds by microorganisms has gained the attention of many researchers and has a history of more than 20 years. The mining and engineering of chalcone synthase (CHS) could be one of the most important ways to achieve more efficient production of chalcones and downstream products in microorganisms. CHS has a broad spectrum of substrates, and its enzyme activity and expression level can significantly affect the efficiency of the biosynthesis of flavonoids. This review summarizes the recent advances in the: structure, mechanism, evolution, substrate spectrum, transformation, and expression regulation in the flavonoid biosynthesis of this vital enzyme. Future development directions were also suggested. The findings may further promote the research and development of flavonoids and health products, making them vital in the fields of human diet and health.

Selection and validation of reference genes for RT-qPCR normalization in different tissues of milk thistle (Silybum marianum, Gaert.)

Quantitative reverse transcription PCR is a sensitive technique for evaluating transcriptional profiles in different experimental datasets. To obtain a reliable quantification of the transcripts level, data normalization with stable reference genes is required. Stable reference genes are identified after analysis of their transcripts profile in every new experiment and species of interest. In Silybum marianum, a widely cultivated officinal plant, only few gene expression studies exist, and reference genes for RT-qPCR studies in the diverse plant tissues have never been investigated before. In this work, the expression stability of 10 candidate reference genes was evaluated in leaves, roots, stems and fruits of S. marianum grown under physiological environmental condition. The stability values for each candidate reference gene were calculated by four canonical statistical algorithms GeNorm, NormFinder, Bestkeeper and ΔCt method in different subsets of samples, then they were ranked with RefFinder from the most to the least suitable for normalization. Best combinations of reference genes are finally proposed for different experimental data sets, including all tissues, vegetative, and reproductive tissues separately. Three target genes putatively involved in important biosynthetic pathway leading to key metabolites in the fruits of milk thistle, such as silymarin and fatty acids, were analyzed with the chosen panels of reference genes, in comparison to the ones used in previous papers. To the best of our knowledge, this is the first report on a reliable and systematic identification and validation of the reference genes for RT-qPCR normalization to study gene expression in S. marianum.

Chitosan Elicitation Impacts Flavonolignan Biosynthesis in Silybum marianum (L.) Gaertn Cell Suspension and Enhances Antioxidant and Anti-Inflammatory Activities of Cell Extracts

Silybum marianum (L.) Gaertn is a rich source of antioxidants and anti-inflammatory flavonolignans with great potential for use in pharmaceutical and cosmetic products. Its biotechnological production using in vitro culture system has been proposed. Chitosan is a well-known elicitor that strongly affects both secondary metabolites and biomass production by plants. The effect of chitosan on S. marianum cell suspension is not known yet. In the present study, suspension cultures of S. marianum were exploited for their in vitro potential to produce bioactive flavonolignans in the presence of chitosan. Established cell suspension cultures were maintained on the same hormonal media supplemented with 0.5 mg/L BAP (6-benzylaminopurine) and 1.0 mg/L NAA (α-naphthalene acetic acid) under photoperiod 16/8 h (light/dark) and exposed to various treatments of chitosan (ranging from 0.5 to 50.0 mg/L). The highest biomass production was observed for cell suspension treated with 5.0 mg/L chitosan, resulting in 123.3 ± 1.7 g/L fresh weight (FW) and 17.7 ± 0.5 g/L dry weight (DW) productions. All chitosan treatments resulted in an overall increase in the accumulation of total flavonoids (5.0 ± 0.1 mg/g DW for 5.0 mg/L chitosan), total phenolic compounds (11.0 ± 0.2 mg/g DW for 0.5 mg/L chitosan) and silymarin (9.9 ± 0.5 mg/g DW for 0.5 mg/L chitosan). In particular, higher accumulation levels of silybin B (6.3 ± 0.2 mg/g DW), silybin A (1.2 ± 0.1 mg/g DW) and silydianin (1.0 ± 0.0 mg/g DW) were recorded for 0.5 mg/L chitosan. The corresponding extracts displayed enhanced antioxidant and anti-inflammatory capacities: in particular, high ABTS antioxidant activity (741.5 ± 4.4 μM Trolox C equivalent antioxidant capacity) was recorded in extracts obtained in presence of 0.5 mg/L of chitosan, whereas highest inhibitions of cyclooxygenase 2 (COX-2, 30.5 ± 1.3 %), secretory phospholipase A2 (sPLA2, 33.9 ± 1.3 %) and 15-lipoxygenase (15-LOX-2, 31.6 ± 1.2 %) enzymes involved in inflammation process were measured in extracts obtained in the presence of 5.0 mg/L of chitosan. Taken together, these results highlight the high potential of the chitosan elicitation in the S. marianum cell suspension for enhanced production of antioxidant and anti-inflammatory silymarin-rich extracts.

Efficient Biosynthesis of (2S)-Eriodictyol from (2S)-Naringenin in Saccharomyces cerevisiae through a Combination of Promoter Adjustment and Directed Evolution

The compound (2S)-eriodictyol is an important flavonoid that can be derived from (2S)-naringenin through flavonoid 3'-hydroxylase (F3'H) catalyzation. F3'H is a cytochrome P450 enzyme that requires a cytochrome P450 reductase (CPR) to function. However, P450s have limited applications in industrial scale biosynthesis, owing to their low activity. Here, an efficient SmF3'H and a matched SmCPR were identified from Silybum marianum. To improve the efficiency of SmF3'H, we established a high-throughput detection method for (2S)-eriodictyol, in which the promoter combination of SmF3'H and SmCPR were optimized in Saccharomyces cerevisiae. The results revealed that SmF3'H/SmCPR should be expressed by using promoters with similar and strong expression levels. Furthermore, directed evolution was applied to further improve the efficiency of SmF3'H/SmCPR. With the optimized promoter and mutated combinations SmF3'HD285N/SmCPRI453V, the (2S)-eriodictyol titer was improved to 3.3 g/L, the highest titer in currently available reports. These results indicated that S. cerevisiae is an ideal platform for functional expression of flavonoid related P450 enzymes.

Enhanced Biosynthesis of Dihydromyricetin in Saccharomyces cerevisiae by Coexpression of Multiple Hydroxylases

Dihydromyricetin (DHM) is a traditional plant-extracted flavonoid with some health benefits. This study aimed to metabolically engineer the strains for DHM bioproduction. Two strains of BK-11 and BQ-21 were integrated with flavonoid 3-hydroxylase (F3H) or both F3H and flavonoid 3'-hydroxylase (F3'H). The resulting strains have expressed the enzymes of GmCPR and SlF3'5'H, and then, the promoters of INO1p and TDH1p were used to enhance further the DHM production from naringenin in Saccharomyces cerevisiae. Through multiple-copy integration, 709.6 mg/L DHM was obtained by adding 2.5 g/L naringenin in a 5 L bioreactor, implying that the synergistic effect between F3'H and flavonoid 3'5'-hydroxylase is likely to promote the DHM production. An yield of 246.4 mg/L DHM was obtained from glucose by deleting genes for branch pathways and integrating PhCHS, MsCHI, Pc4CL, and FjTAL. To our knowledge, this is the highest production reported for the de novo biosynthesis of DHM.

Gene Expression Analysis and Metabolite Profiling of Silymarin Biosynthesis during Milk Thistle (Silybum marianum (L.) Gaertn.) Fruit Ripening

Mature fruits (i.e., achenes) of milk thistle (Silybum marianum (L.) Gaertn., Asteraceae) accumulate high amounts of silymarin (SILM), a complex mixture of bioactive flavonolignans deriving from taxifolin. Their biological activities in relation with human health promotion and disease prevention are well described. However, the conditions of their biosynthesis in planta are still obscure. To fill this gap, fruit development stages were first precisely defined to study the accumulation kinetics of SILM constituents during fruit ripening. The accumulation profiles of the SILM components during fruit maturation were determined using the LC-MS analysis of these defined developmental phases. The kinetics of phenylalanine ammonia-lyase (PAL), chalcone synthase (CHS) and peroxidase (POX) activities suggest in situ biosynthesis of SILM from l-Phenylalanine during fruit maturation rather than a transport of precursors to the achene. In particular, in contrast to laccase activity, POX activity was associated with the accumulation of silymarin, thus indicating a possible preferential involvement of peroxidase(s) in the oxidative coupling step leading to flavonolignans. Reference genes have been identified, selected and validated to allow accurate gene expression profiling of candidate biosynthetic genes (PAL, CAD, CHS, F3H, F3'H and POX) related to SILM accumulation. Gene expression profiles were correlated with SILM accumulation kinetic and preferential location in pericarp during S. marianum fruit maturation, reaching maximum biosynthesis when desiccation occurs, thus reinforcing the hypothesis of an in situ biosynthesis. This observation led us to consider the involvement of abscisic acid (ABA), a key phytohormone in the control of fruit ripening process. ABA accumulation timing and location during milk thistle fruit ripening appeared in line with a potential regulation of the SLIM accumulation. A possible transcriptional regulation of SILM biosynthesis by ABA was supported by the presence of ABA-responsive cis-acting elements in the promoter regions of the SILM biosynthetic genes studied. These results pave the way for a better understanding of the biosynthetic regulation of SILM during the maturation of S. marianum fruit and offer important insights to better control the production of these medicinally important compounds.

Silibinin and non-melanoma skin cancers

Skin is the largest human organ that shields the inner body from contact with xenobiotic and genotoxic agents, and in this process, the skin's cellular genome faces continuous stress due to direct exposure to these noxious factors. Accumulation of genetic stress results in genomic alterations leading to undesirable gene or protein alteration/expression in skin cells, which eventually causes the formation of non-melanoma skin cancers (NMSCs). Ultraviolet B (UVB) radiation from sun is the most prominent factor contributing to ∼5 million skin cancer cases (which are mostly NMSCs) in the United States (US) and western countries. UVB exposure causes aberrations in a range of biochemical and molecular pathways such as: thymine dimer formation, DNA damage, oxidative stress, inflammatory responses, altered cellular signaling, which ultimately contribute to the development of NMSCs. The focus of this review is to summarize the protective and preventive potential of silymarin and/or silibinin against UVB-induced NMSC in pre-clinical skin cancer studies. Over two decades of research has shown the strong potential of silibinin, a biologically active flavonolignan (crude form Silymarin) derived from milk thistle plant, against a wide range of cancers, including NMSCs. Silibinin protects against UVB-induced thymine dimer formation and in turn promotes DNA repair and/or initiates apoptosis in damaged cells via an increase in p53 levels. Additionally, silibinin has shown strong efficacy against NMSCs via its potential to target aberrant signaling pathways, and induction of anti-inflammatory responses. Overall, completed comprehensive studies suggest the potential use of silibinin to prevent and/or manage NMSCs in humans.

Efficient Biosynthesis of (2S)-Naringenin from p-Coumaric Acid in Saccharomyces cerevisiae

(2S)-Naringenin, a (2S)-flavanone, is widely used in the food, chemical, and pharmaceutical industries because of its diverse physiological activities. The production of (2S)-naringenin in microorganisms provides an ideal source that reduces the cost of the flavonoid. To achieve efficient production of (2S)-naringenin in Saccharomyces cerevisiae (S. cerevisiae), we constructed a biosynthetic pathway from p-coumaric acid, a cost-effective and more efficient precursor. The (2S)-naringenin synthesis pathway genes were integrated into the yeast genome to obtain a (2S)-naringenin production strain. After gene dosage experiments, the genes negatively regulating the shikimate pathway and inefficient chalcone synthase activity were verified as factors limiting (2S)-naringenin biosynthesis. With fed-batch process optimization of the engineered strain, the titer of (2S)-naringenin reached 648.63 mg/L from 2.5 g/L p-coumaric acid. Our results indicate that the constitutive production of (2S)-naringenin from p-coumaric acid in S. cerevisiae is highly promising.

Green production of silybin and isosilybin by merging metabolic engineering approaches and enzymatic catalysis

Silymarin extracted from milk thistle seeds, is used for treating hepatic diseases. Silybin and isosilybin are its main components, and synthesized from coupling of taxifolin and coniferyl alcohol. Here, the biosynthetic pathways of taxifolin and coniferyl alcohol were reconstructed in Saccharomyces cerevisiae for the first time. To alleviate substantial burden caused by a great deal of genetic manipulation, expression of the enzymes (e.g. ZWF1, TYR1 and ARO8) playing multiple roles in the relevant biosynthetic pathways was selectively optimized. The strain YT1035 overexpressing seven heterologous enzymes and five native enzymes and the strain YC1053 overexpressing seven heterologous enzymes and four native enzymes, respectively produce 336.8 mg/L taxifolin and 201.1 mg/L coniferyl alcohol. Silybin and isosilybin are synthesized from taxifolin and coniferyl alcohol under catalysis of APX1t (the truncated milk thistle peroxidase), with a yield of 62.5%. This study demonstrates an approach for producing silybin and isosilybin from glucose for the first time.

Linear regression analysis of silychristin A, silybin A and silybin B contents in Silybum marianum

Quantitative correlations between the contents of the flavonolignans silychristin A and silybins A/B provide biosynthetic clues that support a pathway in which one mesomeric form of a taxifolin radical is undergoing an oxidative coupling with a coniferyl alcohol radical. The flavonolignan content and patterns reported in the literature for 53 samples, representing populations of the Silybum marianum plant growing in different parts of the world, were subject to a meta-analysis. Linear regression analyses were carried out on these data sets, and a mathematical model was derived that predicts the content of silychristin A relative to the metabolomic pattern of its congeners. The validity of the model was verified by applying it to test samples. This approach could potentially become a tool to enhance the understanding of both the relative composition of the silymarin complex and the biosynthetic pathways that underlie its formation.

The Impact of Drought Stress on Antioxidant Responses and Accumulation of Flavonolignans in Milk Thistle (Silybum marianum (L.) Gaertn)

Biosynthesis and accumulation of flavonolignans in plants are influenced by different environmental conditions. Biosynthesis and accumulation of silymarin in milk thistle (Silybum marianum L.) were studied under drought stress with respect to the antioxidant defense system at the physiological and gene expression level. The results revealed a reduction in leaf chlorophyll, ascorbic acid, and glutathione contents. In contrast, H2O2, proline, and antioxidative enzyme activities, such as superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), ascorbate peroxidase (APX), and glutathione reductase (GR), were increased. These results confirmed that milk thistle undergoes oxidative stress under drought stress. Furthermore, transcription levels of APX, SOD, CAT, 1-Cys-Prx, and PrxQ were significantly increased in milk thistle under drought stress. Overall this suggests that protection against reactive oxygen species and peroxidation reactions in milk thistle are provided by enzymatic and non-enzymatic antioxidants. Flavonolignans from milk thistle seeds after different drought treatments were quantified by high-performance liquid chromatography (HPLC) and showed that severe drought stress enhanced the accumulation of silymarin and its components compared with seeds from the control (100% water capacity). Silybin is the major silymarin component and the most bioactive ingredient of the milk thistle extract. Silybin accumulation was the highest among all silymarin components in seeds obtained from drought-stressed plants. The expression of the chalcone synthase (CHS) genes (CHS1, CHS2, and CHS3), which are associated with the silybin biosynthetic pathway, was also increased during drought stress. These results indicated that milk thistle exhibits tolerance to drought stress and that seed derived from severe drought-stressed plants had higher levels of silymarin.

RNA-Seq De Novo Assembly and Differential Transcriptome Analysis of Korean Medicinal Herb Cirsium japonicum var. spinossimum

Cirsium japonicum belongs to the Asteraceae or Compositae family and is a medicinal plant in Asia that has a variety of effects, including tumour inhibition, improved immunity with flavones, and antidiabetic and hepatoprotective effects. Silymarin is synthesized by 4-coumaroyl-CoA via both the flavonoid and phenylpropanoid pathways to produce the immediate precursors taxifolin and coniferyl alcohol. Then, the oxidative radicalization of taxifolin and coniferyl alcohol produces silymarin. We identified the expression of genes related to the synthesis of silymarin in C. japonicum in three different tissues, namely, flowers, leaves, and roots, through RNA sequencing. We obtained 51,133 unigenes from transcriptome sequencing by de novo assembly using Trinity v2.1.1, TransDecoder v2.0.1, and CD-HIT v4.6 software. The differentially expressed gene analysis revealed that the expression of genes related to the flavonoid pathway was higher in the flowers, whereas the phenylpropanoid pathway was more highly expressed in the roots. In this study, we established a global transcriptome dataset for C. japonicum. The data shall not only be useful to focus more deeply on the genes related to product medicinal metabolite including flavolignan but also to study the functional genomics for genetic engineering of C. japonicum.

Milk thistle (Silybum marianum): A concise overview on its chemistry, pharmacological, and nutraceutical uses in liver diseases

Milk thistle (MT; Silybum marianum), a member of the Asteraceae family, is a therapeutic herb with a 2,000-year history of use. MT fruits contain a mixture of flavonolignans collectively known as silymarin, being silybin (also named silibinin) the main component. This article reviews the chemistry of MT, the pharmacokinetics and bioavailability, the pharmacologically relevant actions for liver diseases (e.g., anti-inflammatory, immunomodulating, antifibrotic, antioxidant, and liver-regenerating properties) as well as the clinical potential in patients with alcoholic liver disease, nonalcoholic fatty liver disease, viral hepatitis, drug-induced liver injury, and mushroom poisoning. Overall, literature data suggest that, despite encouraging preclinical data, further well-designed randomized clinical trials are needed to fully substantiate the real value of MT preparations in liver diseases.