Expressed sequence tags and molecular cloning and characterization of gene encoding pinoresinol/lariciresinol reductase from Podophyllum hexandrum

Physiological and omics-based insights for underpinning the molecular regulation of secondary metabolite production in medicinal plants: UV stress resilience

Despite complex phytoconstituents, the commercial potential of medicinal plants under ultraviolet (UV) stress environment hasn't been fully comprehended. Due to sessile nature, these plants are constantly exposed to damaging radiation, which disturbs their natural physiological and biochemical processes. To combat with UV stress, plants synthesized several small organic molecules (natural products of low molecular mass like alkaloids, terpenoids, flavonoids and phenolics, etc.) known as plant secondary metabolites (PSMs) that come into play to counteract the adverse effect of stress. Plants adapted a stress response by organizing the expression of several genes, enzymes, transcription factors, and proteins involved in the synthesis of chemical substances and by making the signaling cascade (a series of chemical reactions induced by a stimulus within a biological cell) flexible to boost the defensive response. To neutralize UV exposure, secondary metabolites and their signaling network regulate cellular processes at the molecular level. Conventional breeding methods are time-consuming and difficult to reveal the molecular pattern of the stress tolerance medicinal plants. Acquiring in-depth knowledge of the molecular drivers behind the defensive mechanism of medicinal plants against UV radiation would yield advantages (economical and biological) that will bring prosperity to the burgeoning world's population. Thus, this review article emphasized the comprehensive information and clues to identify several potential genes, transcription factors (TFs), proteins, biosynthetic pathways, and biological networks which are involved in resilience mechanism under UV stress in medicinal plants of high-altitudes.

Lignans dramatically enhance the resistance of Fraxinus velutina Torr. by adjusting the dominant bacterium group of Agrilus planipennis Fairmaire

BACKGROUND

Velvet ash (Fraxinus velutina Torr.) is an important wood and ornamental tree species. Emerald ash borer (EAB), Agrilus planipennis Fairmaire, is a major wood borer of velvet ash. The aim of this study was to identify the secondary metabolites of velvet ash involved in regulating the dominant bacterium group of EAB.

RESULTS

The amount of lignans in the phloem of infested trees had increased by 290.96% because of A. planipennis infection. The addition of lignans to the artificial diet significantly reduced the weight of the larvae and decreased the dominant bacterial group in the larval midgut, such as Pseudomonadaceae, Xanthomonadaceae, and Enterobacteriaceae. The FvPLR1, a key gene for lignan synthesis, was obtained based on the phloem transcriptome of velvet ash. The expression of FvPLR1 in the phloem of the infested tree was significantly higher than that in the noninfested tree. Meanwhile, FvPLR1 silenced by virus-induced gene silencing showed that its expression level and the lignan content were decreased by 69.91% and 31.65%, respectively. Interestingly, silencing FvPLR1 induced alterations in the dominant bacteria group in the larvae, with the reverse trend in the lignan-fed treatment.

CONCLUSION

The evidence showed that FvPLR1 was a positive regulator. The increasing synthesis of lignans leads to resistance improvement in velvet ash, which will provide comprehensive insights into the tree defense system to wood borer infestation. © 2021 Society of Chemical Industry.

Candidate genes involved in the biosynthesis of lignan in Schisandra chinensis fruit based on transcriptome and metabolomes analysis

Schisandra chinensis Turcz. (Baill.) is a plant species with fruits that have been well known in Far Eastern medicine for a long time. It has traditionally been used as a stimulating and fortifying agent in cases of physical exhaustion and to inhibit fatigue. The major bioactive compounds found in S. chinensis are lignans with a dibenzocyclooctadiene skeleton, but little is known about their biosynthesis in plants. S. chinensis is the ideal medicinal plant for studying the biosynthesis of lignans, especially the dibenzocyclooctadiene skeleton. Genomic information for this important herbal plant is unavailable. To better understand the lignan biosynthesis pathway, we generated transcriptome sequences from the fruit during ripening and performed de novo sequence assembly, yielding 136 843 unique transcripts with N50 of 1778 bp. Putative functions could be assigned to 41 824 transcripts (51.57%) based on BLAST searches against annotation databases including GO (Gene ontology) and KEGG (Kyoto Encyclopedia of Genes and Genomes). Furthermore, 22 candidate cytochrome P450 genes and 15 candidate dirigent proteins genes that were most likely involved in the lignan biosynthesis pathway were discovered based on transcriptome sequencing of S. chinensis. The genomic data obtained from S. chinensis, especially the identification of putative genes involved in the lignan biosynthesis pathway, will facilitate our understanding of lignan biosynthesis at the molecular level. The lignan metabolite profiles were analyzed by metabolomes, the accumulation patterns of 30 metabolites involved in the lignan pathway were studied. Co-expression network of lignan contents and transcriptional changes showed 355 strong correlations (correlation coefficient, R² > 0.9) between 21 compounds and 153 transcripts. Furthermore, the comprehensive analysis and characterization of the genes involved in lignan pathways and the metabolite profiles of lignans are expected to provide better insight regarding the diversity of the chemical composition, synthetic characteristics, and regulatory mechanisms of this medical herb.

Effect of UV-B radiation on growth, flavonoid and podophyllotoxin accumulation, and related gene expression in Sinopodophyllum hexandrum

Sinopodophyllum hexandrum is an alpine medicinal plant that produces the anticancer compound podophyllotoxin (PPT). Although a positive relationship between PPT content and altitude has been proved and low temperature enhances plant growth and PPT accumulation has also been revealed, the role of UV radiation in regulating growth and PPT accumulation is still unclear In this study, morphophysiological traits, metabolites content and related genes expression were investigated by exposing S. hexandrum seedlings to treatment with UV-B radiation. The results showed that the contents of soluble sugars and flavonoids, and the expression levels of genes involved in glycometabolism (XET and β-1,3-glucanase) and flavonoid biosynthesis (PAL,C4H,4CL,CHS1 and DTX41) were enhanced in response to UV-B compared to CK. Moreover, genes involved in stress tolerance (MYB, WRKY,APX3 and EX2) were also upregulated in response to UV-B radiation. Although the whole plant biomass exhibited slightly increased values that depended largely on root development, the contents of chlorophyll and PPT and the expression levels of genes involved in photosynthesis (matK, ndhF,rbcL and ycf5) and PPT biosynthesis (C3H,CCoAMT,CCR,CAD, DPO, PLR,SDH, CPY719A23,OMT3,CYP71CU1,OMT1and 2-ODD) were significantly decreased in response to UV-B compared to CK. It can be concluded that UV-B radiation promotes soluble sugars and flavonoids accumulation, but inhibits PPT biosynthesis in S. hexandrum.

Insights into Lignan Composition and Biosynthesis in Stinging Nettle (Urtica dioica L.)

Stinging nettle (Urtica dioica L.) has been used as herbal medicine to treat various ailments since ancient times. The biological activity of nettle is chiefly attributed to a large group of phenylpropanoid dimers, namely lignans. Despite the pharmacological importance of nettle lignans, there are no studies addressing lignan biosynthesis in this plant. We herein identified 14 genes encoding dirigent proteins (UdDIRs) and 3 pinoresinol-lariciresinol reductase genes (UdPLRs) in nettle, which are two gene families known to be associated with lignan biosynthesis. Expression profiling of these genes on different organs/tissues revealed a specific expression pattern. Particularly, UdDIR7, 12 and 13 displayed a remarkable high expression in the top internode, fibre tissues of bottom internodes and roots, respectively. The relatively high expression of UdPLR1 and UdPLR2 in the young internodes, core tissue of bottom internode and roots is consistent with the high accumulation of lariciresinol and secoisolariciresinol in these tissues. Lignan quantification showed a high abundance of pinoresinol in roots and pinoresinol diglucosides in young internodes and leaves. This study sheds light on lignan composition and biosynthesis in nettle, providing a good basis for further functional analysis of DIRs and PLRs and, ultimately, engineering lignan metabolism in planta and in cell cultures.

Pinoresinol-lariciresinol reductases, key to the lignan synthesis in plants

This paper provides an overview on activity, stereospecificity, expression and regulation of pinoresinol-lariciresinol reductases in plants. These enzymes are shared by the pathways to all 8-8' lignans derived from pinoresinol. Pinoresinol-lariciresinol reductases (PLR) are enzymes involved in the lignan biosynthesis after the initial dimerization of two monolignols. They catalyze two successive reduction steps leading to the production of lariciresinol or secoisolariciresinol from pinoresinol. Two secoisolariciresinol enantiomers can be synthetized with different fates. Depending on the plant species, these enantiomers are either final products (e.g., in the flaxseed where it is stored after glycosylation) or are the starting point for the synthesis of a wide range of lignans, among which the aryltetralin type lignans are used to semisynthesize anticancer drugs such as Etoposide^®. Thus, the regulation of the gene expression of PLRs as well as the possible specificities of these reductases for one reduction step or one enantiomer are key factors to fine-tune the lignan synthesis. Results published in the last decade have shed light on the presence of more than one PLR in each plant and revealed various modes of action. Nevertheless, there are not many results published on the PLRs and most of them were obtained in a limited range of species. Indeed, a number of them deal with wild and cultivated flax belonging to the genus Linum. Despite the occurrence of lignans in bryophytes, pteridophytes and monocots, data on PLRs in these taxa are still missing and indeed the whole diversity of PLRs is still unknown. This review summarizes the data, published mainly in the last decade, on the PLR gene expression, enzymatic activity and biological function.

Recent Advances in the Chemistry and Biology of Podophyllotoxins

Podophyllotoxin and its related aryltetralin cyclolignans belong to a family of important products that exhibit various biological properties (e.g., cytotoxic, insecticidal, antifungal, antiviral, anti-inflammatory, neurotoxic, immunosuppressive, antirheumatic, antioxidative, antispasmogenic, and hypolipidemic activities). This Review provides a survey of podophyllotoxin and its analogues isolated from plants. In particular, recent developments in the elegant total chemical synthesis, structural modifications, biosynthesis, and biotransformation of podophyllotoxin and its analogues are summarized. Moreover, a deoxypodophyllotoxin-based chemosensor for selective detection of mercury ion is described. In addition to the most active podophyllotoxin derivatives in each series against human cancer cell lines and insect pests listed in the tables, the structure-activity relationships of podophyllotoxin derivatives as cytotoxic and insecticidal agents are also outlined. Future prospects and further developments in this area are covered at the end of the Review. We believe that this Review will provide necessary information for synthetic, medicinal, and pesticidal chemistry researchers who are interested in the chemistry and biology of podophyllotoxins.

Comparative whole-transcriptome analysis in Podophyllum species identifies key transcription factors contributing to biosynthesis of podophyllotoxin in P. hexandrum

Podophyllum species (Podophyllum hexandrum Royle and Podophyllum peltatum) are a major source of deriving anticancer drugs from their major chemical constituent, podophyllotoxin. However, information lacks on regulatory components of podophyllotoxin biosynthesis; therefore, different classes of transcription factors were identified through mining transcriptomes of Podophyllum species and validated through qRT-PCR analysis vis-à-vis podophyllotoxin contents in different tissues/organs of Podophyllum hexandrum. A total of 82, 278, 70, and 90 transcripts were identified in shoots and 89, 273, 72, and 91 transcripts in rhizomes of P. hexandrum transcriptome; 70, 268, 48, and 92 transcripts were in shoots and 58, 245, 41, and 85 transcripts in rhizomes of P. peltatum transcriptome corresponding to bZIP, MYB, WRKY, and bHLH families of transcription factors, which have been shown in regulating biosynthesis of secondary metabolites. Two unique transcripts encoding bHLH and MYB/SANT TFs in shoots of P. peltatum (medp_podpe_41091 and medp_podpe_2547) and bZIP and MYB TFs in rhizomes of P. hexandrum (medp_podhe_163581 and medp_podhe_147614) correlated with podophyllotoxin content. Quantification of podophyllotoxin and comparative expression analysis between high (2.51 %) versus low (0.59) podophyllotoxin content accessions revealed 0.04 to ~16-folds increase in transcripts of transcription factors, thereby further supporting the association of identified transcription factors with podophyllotoxin content. bZIP TF showed the highest transcript abundance (19.60-folds) in P. hexandrum rhizomes (2.51 % podophyllotoxin) compared to shoots (0.01 %). In silico analysis of putative promoter regions of pathway genes in other plant species revealed the presence of sequence elements for MYB and WRKY transcription factors, thereby suggesting their role in controlling the production of podophyllotoxin. A repertoire of additional transcription factors has been provided, which can be functionally validated and used in designing a suitable genetic intervention strategy towards enhanced production of podophyllotoxin.

Molecular Approaches to Screen Bioactive Compounds from Endophytic Fungi

Endophytic fungi are capable of producing plant associated metabolites and their analogs with therapeutic value. In order to identify the potential endophytic isolates producing bioactive compounds, one need to screen all isolated endophytes, which may run into hundreds. Isolation of endophytic fungi is relatively a simple process; but screening of the isolated fungi for required metabolite production is a cumbersome process. Endophytic fungi producing plant associated metabolites may contain genes involved in the entire biosynthetic pathway(s). Therefore, ascertaining the presence of key enzymes of a particular biosynthetic pathway could serve as a molecular marker for screening of these endophytes to produce that metabolite. In absence of entire biosynthetic pathways in endophytic fungi, plant genes associated with that metabolic pathway could serve as markers. This review focuses on the impact of molecular approaches to screen the endophytic fungi for the production of bioactive compounds. An attempt has been made on screening of anticancer compounds like taxol (paclitaxel), podophyllotoxin, and camptothecin using molecular markers. The advantages of molecular approaches over conventional methods to screen endophytic fungi and also identification of endophytic fungi are discussed.

Chilling temperature stimulates growth, gene over-expression and podophyllotoxin biosynthesis in Podophyllum hexandrum Royle

Podophyllotoxin (PPT) and its derivatives, isolated from the rhizome of Podophyllum hexandrum Royle (P. hexandrum), are typically used in clinical settings for anti-cancer and anti-virus treatments. Empirical studies have verified that P. hexandrum had stronger tolerance to chilling, due to involving PPT accumulation in rhizome induced by cold stress. However, the cold-adaptive mechanism and its association with PPT accumulation at a molecular level in P. hexandrum are still limited. In this study, the morpho-physiological traits related to plant growth, PPT accumulation and key gene expressions controlling PPT biosynthesis were assessed by exposing P. hexandrum seedlings to different temperatures (4 °C and 10 °C as chilling stress and 22 °C as the control). The results showed that chilling significantly increased chlorophyll content, net photosynthetic rate, stomatal conductance, and plant biomass, whereas it greatly decreased transpiration rates and intercellular CO2 concentration. Compared to the control, the chilling treatments under 4 °C and 10 °C conditions induced a 5.00- and 3.33-fold increase in PPT contents, respectively. The mRNA expressions of six key genes were also up-regulated by chilling stresses. The findings are useful in understanding the molecular basis of P. hexandrum response to chilling.

Three-steps in one-pot: whole-cell biocatalytic synthesis of enantiopure (+)- and (-)-pinoresinol via kinetic resolution

Background

Pinoresinol is a high-value plant-derived lignan with multiple health supporting effects. Enantiomerically pure pinoresinol can be isolated from natural sources, but with low efficiency. Most chemical and biocatalytic approaches that have been described for the synthesis of pinoresinol furnish the racemic mixture. In this study we devised a three-step biocatalytic cascade for the production of enantiomerically pure pinoresinol from the cheap compound eugenol. Two consecutive oxidations of eugenol through vanillyl-alcohol oxidase and laccase are followed by kinetic resolution of racemic pinoresinol by enantiospecific pinoresinol reductases.

Results

The addition of the enantiospecific pinoresinol reductase from Arabidopsis thaliana for kinetic resolution of (±)-pinoresinol to an in vitro cascade involving the vanillyl-alcohol oxidase from Penicillium simplicissimum and the bacterial laccase CgL1 from Corynebacterium glutamicum resulted in increasing ee values for (+)-pinoresinol; however, an ee value of 34 % was achieved in the best case. The ee value could be increased up to ≥99 % by applying Escherichia coli-based whole-cell biocatalysts. The optimized process operated in a one-pot “two-cell” sequential mode and yielded 876 µM (+)-pinoresinol with an ee value of 98 %. Switching the reductase to the enantiospecific pinoresinol lariciresinol reductase from Forsythia intermedia enabled the production of 610 µM (−)-pinoresinol with an ee value of 97 %.

Conclusion

A new approach for the synthesis of enantiomerically pure (+)- and (−)-pinoresinol is described that combines three biotransformation steps in one pot. By switching the reductase in the last step, the whole-cell biocatalysts can be directed to produce either (+)- or (−)-pinoresinol. The products of the reductases’ activity, (−)-lariciresinol and (−)-secoisolariciresinol, are valuable precursors that can also be applied for the synthesis of further lignans.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-016-0472-0) contains supplementary material, which is available to authorized users.

Six enzymes from mayapple that complete the biosynthetic pathway to the etoposide aglycone

Podophyllotoxin is the natural product precursor of the chemotherapeutic etoposide, yet only part of its biosynthetic pathway is known. We used transcriptome mining in Podophyllum hexandrum (mayapple) to identify biosynthetic genes in the podophyllotoxin pathway. We selected 29 candidate genes to combinatorially express in Nicotiana benthamiana (tobacco) and identified six pathway enzymes, including an oxoglutarate-dependent dioxygenase that closes the core cyclohexane ring of the aryltetralin scaffold. By coexpressing 10 genes in tobacco-these 6 plus 4 previously discovered-we reconstitute the pathway to (-)-4'-desmethylepipodophyllotoxin (the etoposide aglycone), a naturally occurring lignan that is the immediate precursor of etoposide and, unlike podophyllotoxin, a potent topoisomerase inhibitor. Our results enable production of the etoposide aglycone in tobacco and circumvent the need for cultivation of mayapple and semisynthetic epimerization and demethylation of podophyllotoxin.

Expression analysis of biosynthetic pathway genes vis-à-vis podophyllotoxin content in Podophyllum hexandrum Royle

Podophyllum hexandrum Royle is known for its vast medicinal properties, particularly anticancer. It contains higher amount of podophyllotoxin (4.3 %), compared to Podophyllum peltatum (0.025 %) and other plant species; as a result, it has been used worldwide in the preparation of various drugs including anticancer, antimalarial, antiviral, antioxidant, antifungal, and so on. Currently, Etoposide (VP-16-213), Vumon® (Teniposide; VM-26), Etopophos®, Pod-Ben- 25, Condofil, Verrusol, and Warticon are available in the market. Due to highly complex synthesis and low cell culture yields of podophyllotoxin (0.3 %), the supply of raw material cannot be met due to increasing industrial demands. The knowledge on podophyllotoxin biosynthetic pathway vis-à-vis expression status of genes is fragmentary. Quantitative expression analysis of 21 pathway genes has revealed 9 genes, namely SD, PD, PCH, CM, CMT, CAD, CCR, C4H, and ADH, that showed increase in transcript abundance up to 1.4 to 23.05 folds, respectively, vis-à-vis podophyllotoxin content in roots (1.37 %) and rhizomes (3.05 %) of P. hexandrum. In silico analysis of putative cis-regulatory elements in promoter regions of overexpressed genes showed the presence of common Skn-1 motif and MBS elements in CMT, CAD, CCR, C4H, and ADH genes, thereby, suggesting their common regulation. The outcome of the study has resulted in the identification of suitable candidate genes which might be contributing to podophyllotoxin biosynthesis that can act as potential targets for any genetic intervention strategies aimed at its enhanced production.

Essences in metabolic engineering of lignan biosynthesis

Lignans are structurally and functionally diverse phytochemicals biosynthesized in diverse plant species and have received wide attentions as leading compounds of novel drugs for tumor treatment and healthy diets to reduce of the risks of lifestyle-related non-communicable diseases. However, the lineage-specific distribution and the low-amount of production in natural plants, some of which are endangered species, hinder the efficient and stable production of beneficial lignans. Accordingly, the development of new procedures for lignan production is of keen interest. Recent marked advances in the molecular and functional characterization of lignan biosynthetic enzymes and endogenous and exogenous factors for lignan biosynthesis have suggested new methods for the metabolic engineering of lignan biosynthesis cascades leading to the efficient, sustainable, and stable lignan production in plants, including plant cell/organ cultures. Optimization of light conditions, utilization of a wide range of elicitor treatments, and construction of transiently gene-transfected or transgenic lignan-biosynthesizing plants are mainly being attempted. This review will present the basic and latest knowledge regarding metabolic engineering of lignans based on their biosynthetic pathways and biological activities, and the perspectives in lignan production via metabolic engineering.