The Foxgloves (Digitalis) Revisited

Four enzymes control natural variation in the steroid core of Erysimum cardenolides

Plants commonly produce families of structurally related metabolites with similar defensive functions. This apparent redundancy raises the question of underlying molecular mechanisms and adaptive benefits of such chemical variation. Cardenolides, a class defensive compounds found in the wallflower genus Erysimum (L., Brassicaceae) and scattered across other plant families, show substantial structural variation, with glycosylation and hydroxylation being common modifications of a steroid core, which itself may vary in terms of stereochemistry and saturation. Through a combination of chemical mutagenesis and analysis of gene coexpression networks, we identified four enzymes involved in cardenolide biosynthesis in Erysimum that work together to determine stereochemistry at carbon 5 of the steroid core: Ec3βHSD, a 3β-hydroxysteroid dehydrogenase, Ec3KSI, a ketosteroid isomerase, EcP5βR2, a progesterone 5β-reductase, and EcDET2, a steroid 5α-reductase. We biochemically characterized the activity of these enzymes in vitro and generated CRISPR/Cas9 knockout lines to confirm activity in vivo. Cardenolide biosynthesis was not eliminated in any of the knockouts. Instead, mutant plants accumulated cardenolides with altered saturation and stereochemistry of the steroid core. Furthermore, we found variation in carbon 5 configuration among the cardenolides of 44 species of Erysimum, where the occurrence of some 5β-cardenolides is associated with the expression and sequence of P5βR2. This may have allowed Erysimum species to fine-tune their defensive profiles to target specific herbivore populations over the course of evolution.

SIGNIFICANCE STATEMENT

Plants use an array of toxic compounds to defend themselves from attack against insects and other herbivores. One mechanism through which plants may evolve more toxic compounds is through modifications to the structure of compounds they already produce. In this study, we show how plants in the wallflower genus Erysimum use four enzymes to fine-tune the structure of toxic metabolites called cardenolides. Natural variation in the sequence and expression of a single enzyme called progesterone 5β-reductase 2 partly explains the variation in cardenolides observed across the Erysimum genus. These alterations to cardenolide structure over the course of evolution suggests that there may be context-dependent benefits to Erysimum to invest in one cardenolide variant over another.

Medicinal plants meet modern biodiversity science

Plants have been an essential source of human medicine for millennia. In this review, we argue that a holistic, interdisciplinary approach to the study of medicinal plants that combines methods and insights from three key disciplines - evolutionary ecology, molecular biology/biochemistry, and ethnopharmacology - is poised to facilitate new breakthroughs in science, including pharmacological discoveries and rapid advancements in human health and well-being. Such interdisciplinary research leverages data and methods spanning space, time, and species associated with medicinal plant species evolution, ecology, genomics, and metabolomic trait diversity, all of which build heavily on traditional Indigenous knowledge. Such an interdisciplinary approach contrasts sharply with most well-funded and successful medicinal plant research during the last half-century, which, despite notable advancements, has greatly oversimplified the dynamic relationships between plants and humans, kept hidden the larger human narratives about these relationships, and overlooked potentially important research and discoveries into life-saving medicines. We suggest that medicinal plants and people should be viewed as partners whose relationship involves a complicated and poorly explored set of (socio-)ecological interactions including not only domestication but also commensalisms and mutualisms. In short, medicinal plant species are not just chemical factories for extraction and exploitation. Rather, they may be symbiotic partners that have shaped modern societies, improved human health, and extended human lifespans.

Aphid Resistance Segregates Independently of Cardenolide and Glucosinolate Content in an Erysimum cheiranthoides (Wormseed Wallflower) F2 Population

Plants in the genus Erysimum produce both glucosinolates and cardenolides as a defense mechanism against herbivory. Two natural isolates of Erysimum cheiranthoides (wormseed wallflower) differed in their glucosinolate content, cardenolide content, and their resistance to Myzus persicae (green peach aphid), a broad generalist herbivore. Both classes of defensive metabolites were produced constitutively and were not further induced by aphid feeding. To investigate the relative importance of glucosinolates and cardenolides in E. cheiranthoides defense, we generated an improved genome assembly, genetic map, and segregating F2 population. The genotypic and phenotypic analysis of the F2 plants identified quantitative trait loci, which affected glucosinolates and cardenolides, but not the aphid resistance. The abundance of most glucosinolates and cardenolides was positively correlated in the F2 population, indicating that similar processes regulate their biosynthesis and accumulation. Aphid reproduction was positively correlated with glucosinolate content. Although the overall cardenolide content had little effect on aphid growth and survival, there was a negative correlation between aphid reproduction and helveticoside abundance. However, this variation in defensive metabolites could not explain the differences in aphid growth on the two parental lines, suggesting that processes other than the abundance of glucosinolates and cardenolides have a predominant effect on aphid resistance in E. cheiranthoides.

Aphid resistance segregates independently of cardiac glycoside and glucosinolate content in an Erysimum cheiranthoides (wormseed wallflower) F2 population

Plants in the genus Erysimum produce both glucosinolates and cardiac glycosides as defense against herbivory. Two natural isolates of Erysimum cheiranthoides (wormseed wallflower) differed in their glucosinolate content, cardiac glycoside content, and resistance to Myzus persicae (green peach aphid), a broad generalist herbivore. Both classes of defensive metabolites were produced constitutively and were not induced further by aphid feeding. To investigate the relative importance of glucosinolates and cardiac glycosides in E. cheiranthoides defense, we generated an improved genome assembly, genetic map, and segregating F2 population. Genotypic and phenotypic analysis of the F2 plants identified quantitative trait loci affecting glucosinolates and cardiac glycosides, but not aphid resistance. The abundance of most glucosinolates and cardiac glycosides was positively correlated in the F2 population, indicating that similar processes regulate their biosynthesis and accumulation. Aphid reproduction was positively correlated with glucosinolate content. Although overall cardiac glycoside content had little effect on aphid growth and survival, there was a negative correlation between aphid reproduction and helveticoside abundance. However, this variation in defensive metabolites could not explain the differences in aphid growth on the two parental lines, suggesting that processes other than the abundance of glucosinolates and cardiac glycosides have a predominant effect on aphid resistance in E. cheiranthoides.

Promiscuous CYP87A enzyme activity initiates cardenolide biosynthesis in plants

Cardenolides are specialized, steroidal metabolites produced in a wide array of plant families1,2. Cardenolides play protective roles in plants, but these molecules, including digoxin from foxglove (Digitalis spp.), are better known for treatment of congenital heart failure, atrial arrhythmia, various cancers and other chronic diseases3-9. However, it is still unknown how plants synthesize 'high-value', complex cardenolide structures from, presumably, a sterol precursor. Here we identify two cytochrome P450, family 87, subfamily A (CYP87A) enzymes that act on both cholesterol and phytosterols (campesterol and β-sitosterol) to form pregnenolone, the first committed step in cardenolide biosynthesis in the two phylogenetically distant plants Digitalis purpurea and Calotropis procera. Arabidopsis plants overexpressing these CYP87A enzymes ectopically accumulated pregnenolone, whereas silencing of CYP87A in D. purpurea leaves by RNA interference resulted in substantial reduction of pregnenolone and cardenolides. Our work uncovers the key entry point to the cardenolide pathway, and expands the toolbox for sustainable production of high-value plant steroids via synthetic biology.

A cytochrome P450 CYP87A4 imparts sterol side-chain cleavage in digoxin biosynthesis

Digoxin extracted from the foxglove plant is a widely prescribed natural product for treating heart failure. It is listed as an essential medicine by the World Health Organization. However, how the foxglove plant synthesizes digoxin is mostly unknown, especially the cytochrome P450 sterol side chain cleaving enzyme (P450scc), which catalyzes the first and rate-limiting step. Here we identify the long-speculated foxglove P450scc through differential transcriptomic analysis. This enzyme converts cholesterol and campesterol to pregnenolone, suggesting that digoxin biosynthesis starts from both sterols, unlike previously reported. Phylogenetic analysis indicates that this enzyme arises from a duplicated cytochrome P450 CYP87A gene and is distinct from the well-characterized mammalian P450scc. Protein structural analysis reveals two amino acids in the active site critical for the foxglove P450scc's sterol cleavage ability. Identifying the foxglove P450scc is a crucial step toward completely elucidating digoxin biosynthesis and expanding the therapeutic applications of digoxin analogs in future work.

Lanatoside C decelerates proliferation and induces apoptosis through inhibition of STAT3 and ROS-mediated mitochondrial membrane potential transformation in cholangiocarcinoma

Introduction: The incidence of cholangiocarcinoma (CCA) has increased worldwide in recent years. Given the poor prognosis associated with the current management approach of CCA, new therapeutic agents are warranted to improve the prognosis of this patient population. Methods: In this study, we extracted five cardiac glycosides (CGs) from natural plants: digoxin, lanatoside A, lanatoside C, lanatoside B, and gitoxin. Follow-up experiments were performed to assess the effect of these five extracts on cholangiocarcinoma cells and compounds with the best efficacy were selected. Lanatoside C (Lan C) was selected as the most potent natural extract for subsequent experiments. We explored the potential mechanism underlying the anticancer activity of Lan C on cholangiocarcinoma cells by flow cytometry, western blot, immunofluorescence, transcriptomics sequencing, network pharmacology and in vivo experiments. Results: We found that Lan C time-dependently inhibited the growth and induced apoptosis of HuCCT-1 and TFK-1 cholangiocarcinoma cells. Besides Lan C increased the reactive oxygen species (ROS) content in cholangiocarcinoma cells, decreased the mitochondrial membrane potential (MMP) and resulted in apoptosis. Besides, Lan C downregulated the protein expression of STAT3, leading to decreased expression of Bcl-2 and Bcl-xl, increased expression of Bax, activation of caspase-3, and initiation of apoptosis. N-acetyl-L-cysteine (NAC) pretreatment reversed the effect of Lan C. In vivo, we found that Lan C inhibited the growth of cholangiocarcinoma xenografts without toxic effects on normal cells. Tumor immunohistochemistry showed that nude mice transplanted with human cholangiocarcinoma cells treated with Lan C exhibited decreased STAT3 expression and increased caspase-9 and caspase-3 expression in tumors, consistent with the in vitro results. Conclusion: In summary, our results substantiates that cardiac glycosides have strong anti-CCA effects. Interestingly the biological activity of Lan C provides a new anticancer candidate for the treatment of cholangiocarcinoma.

Overexpression and RNAi-mediated Knockdown of Two 3β-hydroxy-Δ5-steroid dehydrogenase Genes in Digitalis lanata Shoot Cultures Reveal Their Role in Cardenolide Biosynthesis

3β-hydroxy-Δ5-steroid dehydrogenases (3βHSDs) are supposed to be involved in 5β-cardenolide biosynthesis. Here, a novel 3βHSD (Dl3βHSD2) was isolated from Digitalis lanata shoot cultures and expressed in E. coli. Recombinant Dl3βHSD1 and Dl3βHSD2 shared 70% amino acid identity, reduced various 3-oxopregnanes and oxidised 3-hydroxypregnanes, but only rDl3βHSD2 converted small ketones and secondary alcohols efficiently. To explain these differences in substrate specificity, we established homology models using borneol dehydrogenase of Salvia rosmarinus (6zyz) as the template. Hydrophobicity and amino acid residues in the binding pocket may explain the difference in enzyme activities and substrate preferences. Compared to Dl3βHSD1, Dl3βHSD2 is weakly expressed in D. lanata shoots. High constitutive expression of Dl3βHSDs was realised by Agrobacterium-mediated transfer of Dl3βHSD genes fused to the CaMV-35S promotor into the genome of D. lanata wild type shoot cultures. Transformed shoots (35S:Dl3βHSD1 and 35S:Dl3βHSD2) accumulated less cardenolides than controls. The levels of reduced glutathione (GSH), which is known to inhibit cardenolide formation, were higher in the 35S:Dl3βHSD1 lines than in the controls. In the 35S:Dl3βHSD1 lines cardenolide levels were restored after adding of the substrate pregnane-3,20-dione in combination with buthionine-sulfoximine (BSO), an inhibitor of GSH formation. RNAi-mediated knockdown of the Dl3βHSD1 yielded several shoot culture lines with strongly reduced cardenolide levels. In these lines, cardenolide biosynthesis was fully restored after addition of the downstream precursor pregnan-3β-ol-20-one, whereas upstream precursors such as progesterone had no effect, indicating that no shunt pathway could overcome the Dl3βHSD1 knockdown. These results can be taken as the first direct proof that Dl3βHSD1 is indeed involved in 5β-cardenolide biosynthesis.

Variation in the chemical profiles of three foxglove species in the central Balkans

The aim of this study was to determine intra- and interspecies variation in the qualitative and quantitative composition of methanol-soluble metabolites in the leaves of three Digitalis species (D. lanata, D. ferruginea, and D. grandiflora) from the central Balkans. Despite the steady use of foxglove constituents for human health as valuable medicinal products, populations of the genus Digitalis (Plantaginaceae) have been poorly investigated to describe their genetic and phenetic variation. Following untargeted profiling using UHPLC-LTQ Orbitrap MS, by which we identified a total of 115 compounds, 16 compounds were quantified using the UHPLC(-)HESI-QqQ-MS/MS approach. In total, 55 steroid compounds, 15 phenylethanoid glycosides, 27 flavonoids, and 14 phenolic acid derivatives were identified across the samples with D. lanata and D. ferruginea showing a great similarity, while 15 compounds were characteristic only for D. grandiflora. The phytochemical composition of methanol extracts, considered here as complex phenotypes, are further examined along multiple levels of biological organization (intra- and interpopulation) and subsequently subjected to chemometric data analysis. The quantitative composition of the selected set of 16 chemomarkers belonging to the classes of cardenolides (3 compounds) and phenolics (13 compounds) pointed to considerable differences between the taxa studied. D. grandiflora and D. ferruginea were found to be richer in phenolics as compared to cardenolides, which otherwise predominate in D. lanata over other compounds. PCA revealed lanatoside C, deslanoside, hispidulin, and p-coumaric acid to be the main compounds contributing to the differences between D. lanata on one side and D. grandiflora and D. ferruginea on the other, while p-coumaric acid, hispidulin, and digoxin contribute to the diversification between D. grandiflora and D. ferruginea. However, quantitative variation in the metabolite content within species was faint with mild population diversification visible in D. grandiflora and particularly in D. ferruginea. This pointed to the highly conserved content and ratio of targeted compounds within the analyzed species, which was not severely influenced by the geographic origin or environmental conditions. The presented metabolomics approach might have, along with morphometrics and molecular genetics studies, a high information value for further elucidation of the relationships among taxa within the genus Digitalis.

Cardenolide Increase in Foxglove after 2,1,3-Benzothiadiazole Treatment Reveals a Potential Link between Cardenolide and Phytosterol Biosynthesis

Cardenolides are steroidal metabolites in Digitalis lanata with potent cardioactive effects on animals. In plants, cardenolides are likely involved in various stress responses. However, the molecular mechanism of cardenolide increase during stresses is mostly unknown. Additionally, cardenolides are proposed to arise from cholesterol, but indirect results show that phytosterols may also be substrates for cardenolide biosynthesis. Here, we show that cardenolides increased after methyl jasmonate (MJ), sorbitol, potassium chloride (KCl) and salicylic acid analog [2,1,3-benzothiadiazole (BTH)] treatments. However, the expression of three known genes for cardenolide biosynthesis did not correlate well with these increases. Specifically, the expression of progesterone-5β-reductases (P5βR and P5βR2) did not correlate with the cardenolide increase. The expression of 3β-hydroxysteroid dehydrogenase (3βHSD) correlated with changes in cardenolide levels only during the BTH treatment. Mining the D. lanata transcriptome identified genes involved in cholesterol and phytosterol biosynthesis: C24 sterol sidechain reductase 1 (SSR1), C4 sterol methyl oxidase 1, and 3 (SMO1 and SMO3). Surprisingly, the expression of all three genes correlated well with the cardenolide increase after the BTH treatment. Phylogenetic analysis showed that SSR1 is likely involved in both cholesterol and phytosterol biosynthesis. In addition, SMO1 is likely specific to phytosterol biosynthesis, and SMO3 is specific to cholesterol biosynthesis. These results suggest that stress-induced increase of cardenolides in foxglove may correlate with cholesterol and phytosterol biosynthesis. In summary, this work shows that cardenolides are important for stress responses in D. lanata and reveals a potential link between phytosterol and cardenolide biosynthesis.

Quantification of plant cardenolides by HPLC, measurement of Na+/K+-ATPase inhibition activity, and characterization of target enzymes

The biosynthesis of cardiac glycosides, broadly classified as cardenolides and bufadienolides, has evolved repeatedly among flowering plants. Individual species can produce dozens or even hundreds of structurally distinct cardiac glycosides. Although all cardiac glycosides exhibit biological activity by inhibiting the function of the essential Na⁺/K⁺-ATPase in animal cells, they differ in their level of inhibitory activity. For within- and between-species comparisons of cardiac glycosides to address ecological and evolutionary questions, it is necessary to not only quantify their relative abundance, but also their effectiveness in inhibiting the activity of different animal Na⁺/K⁺-ATPases. Here we describe protocols for characterizing the amount and toxicity of cardenolides from plant samples and the degree of insect Na⁺/K⁺-ATPase tolerance to inhibition: (1) an HPLC-based assay to quantify the abundance of individual cardenolides in plant extracts, (2) an assay to quantify inhibition of Na⁺/K⁺-ATPase activity by plant extracts, and (3) extraction of insect Na⁺/K⁺-ATPases for inhibition assays.

Identification of key genes involved in secondary metabolite biosynthesis in Digitalis purpurea

The medicinal plant Digitalis purpurea produces cardiac glycosides that are useful in the pharmaceutical industry. These bioactive compounds are in high demand due to ethnobotany's application to therapeutic procedures. Recent studies have investigated the role of integrative analysis of multi-omics data in understanding cellular metabolic status through systems metabolic engineering approach, as well as its application to genetically engineering metabolic pathways. In spite of numerous omics experiments, most molecular mechanisms involved in metabolic pathways biosynthesis in D. purpurea remain unclear. Using R Package Weighted Gene Co-expression Network Analysis, co-expression analysis was performed on the transcriptome and metabolome data. As a result of our study, we identified transcription factors, transcriptional regulators, protein kinases, transporters, non-coding RNAs, and hub genes that are involved in the production of secondary metabolites. Since jasmonates are involved in the biosynthesis of cardiac glycosides, the candidate genes for Scarecrow-Like Protein 14 (SCL14), Delta24-sterol reductase (DWF1), HYDRA1 (HYD1), and Jasmonate-ZIM domain3 (JAZ3) were validated under methyl jasmonate treatment (MeJA, 100 μM). Despite early induction of JAZ3, which affected downstream genes, it was dramatically suppressed after 48 hours. SCL14, which targets DWF1, and HYD1, which induces cholesterol and cardiac glycoside biosynthesis, were both promoted. The correlation between key genes and main metabolites and validation of expression patterns provide a unique insight into the biosynthesis mechanisms of cardiac glycosides in D. purpurea.

Isolation of the 3β-HSD promoter from Digitalis ferruginea subsp. ferruginea and its functional characterization in Arabidopsis thaliana

BACKGROUND

Although members of the SDR gene family (short chain dehydrogenase) are distributed in kingdom of life, they have diverse roles in stress tolerance mechanism or secondary metabolite biosynthesis. Nevertheless, their precise roles in gene expression or regulation under stress are yet to be understood.

METHODS

As a case study, we isolated, sequenced and functionally characterized the 3β-HSD promoter from Digitalis ferruginea subsp. ferruginea in Arabidopsis thaliana.

RESULTS

The promoter fragment contained light and stress response elements such as Box-4, G-Box, TCT-motif, LAMP element, ABRE, ARE, WUN-motif, MYB, MYC, W box, STRE and Box S. The functional analysis of the 3β-HSD promoter in transgenic Arabidopsis seedlings showed that the promoter was expressed in cotyledon and root elongation zone in 2 days' seedlings. However, this expression was extended to hypocotyl and complete root in 6 days' seedlings. In 20 days-old seedlings, promoter expression was distributed to the whole seedling including hydathodes aperture, vascular bundle, shoot apical meristem, trichomes, midrib, leaf primordia, hypocotyl and xylem tissues. Further, expression of the promoter was enhanced or remained stable under the different abiotic stress conditions like osmotic, heat, cold, cadmium or low pH. In addition, the promoter also showed response to methyl jasmonate (MeJA) application. The expression could not be induced in wounded cotyledon most likely due to lack of interacting elements in the promoter fragment.

CONCLUSIONS

Taken together, the 3β-HSD promoter could be a candidate for the development of transgenic plants especially under changing environmental conditions.

The Decline and Fall of Materia Medica and the Rise of Pharmacology and Therapeutics in Veterinary Medicine

Materia Medica is a Latin term, relating to the history of pharmacy. It describes the sources (vegetable, animal and mineral), nature, preparation, and properties of substances or mixtures of substances, which were used as remedies for the treatment of diseases. Bourgelat authored the first veterinary Materia Medica book. This review describes the evolution and ultimate downfall of Materia Medica concepts and practices. Its survival for more than two millennia reflected the impact of religion and dogmas on therapy. The consignment of Materia Medica to history was signified by publication of the first modern book of veterinary pharmacology and therapeutics by Meyer Jones in 1953. Previously, the dominance of Materia Medica was linked to an hippiatry culture, which was shared with farriers and quacks. The Pasteurian and pharmacological revolutions of the second half of the nineteenth century led to its gradual abandonment. This review explains why the existence of authentically active substances, such as opioid analgesics, cardiotonics and general anesthetics either were not used for those actions or were badly prescribed, in part because of historical precedence and in part from lack of pathophysiological knowledge to justify rational use. The modern concept of dosage, in particular inter-species differences, was not understood. There were also major dogmas, supporting false indications, such as failure to recognize pain as a symptom to be treated, whereas inflammation was only a disease symptom involving excess of activity of the blood system, which had to be vigorously addressed by bleeding and purging. This review covers a well-defined period, ranging from Bourgelat, who wrote the first book of Materia Medica for veterinary studies to the first edition of Meyer Jones textbook in 1953, which marked the end of Materia Medica and the beginning of pharmacology in veterinary medicine.

Acropetal and basipetal cardenolide transport in Erysimum cheiranthoides (wormseed wallflower)

Plant specialized metabolites are often subject to within-plant transport and have tissue-specific distribution patterns. Among plants in the Brassicaceae, the genus Erysimum is unique in producing not only glucosinolates but also cardenolides. Ten cardenolides were detected with varying abundance in different tissues of Erysimum cheiranthoides L (Brassicaceae; wormseed wallflower). As is predicted by the optimal defense theory, cardenolides were most abundant in young leaves and reproductive tissues. The lowest concentrations were observed in senescing leaves and roots. Crosses between wildtype E. cheiranthoides and a mutant line with an altered cardenolide profile showed that the seed cardenolide phenotype is determined entirely by the maternal genotype. Prior to the development of the first true leaves, seedling cotyledons also had the maternal cardenolide profile. Hypocotyl grafting experiments showed that the root cardenolide profile is determined entirely by the aboveground plant genotype. In further grafting experiments, there was no evidence of cardenolide transport into the leaves, but a mixed cardenolide profile was observed in the stems and inflorescences of plants that had been grafted at vegetative and flowering growth stages, respectively. Together, these results indicate that E. cheiranthoides leaves are a site of cardenolide biosynthesis.

Plastidial Expression of 3β-Hydroxysteroid Dehydrogenase and Progesterone 5β-Reductase Genes Confer Enhanced Salt Tolerance in Tobacco

The short-chain dehydrogenase/reductase (SDR) gene family is widely distributed in all kingdoms of life. The SDR genes, 3β-hydroxysteroid dehydrogenase (3β-HSD) and progesterone 5-β-reductases (P5βR1, P5βR2) play a crucial role in cardenolide biosynthesis pathway in the Digitalis species. However, their role in plant stress, especially in salinity stress management, remains unexplored. In the present study, transplastomic tobacco plants were developed by inserting the 3β-HSD, P5βR1 and P5βR2 genes. The integration of transgenes in plastomes, copy number and transgene expression at transcript and protein level in transplastomic plants were confirmed by PCR, end-to-end PCR, qRT-PCR and Western blot analysis, respectively. Subcellular localization analysis showed that 3β-HSD and P5βR1 are cytoplasmic, and P5βR2 is tonoplast-localized. Transplastomic lines showed enhanced growth in terms of biomass and chlorophyll content compared to wild type (WT) under 300 mM salt stress. Under salt stress, transplastomic lines remained greener without negative impact on shoot or root growth compared to the WT. The salt-tolerant transplastomic lines exhibited enhanced levels of a series of metabolites (sucrose, glutamate, glutamine and proline) under control and NaCl stress. Furthermore, a lower Na+/K+ ratio in transplastomic lines was also observed. The salt tolerance, mediated by plastidial expression of the 3β-HSD, P5βR1 and P5βR2 genes, could be due to the involvement in the upregulation of nitrogen assimilation, osmolytes as well as lower Na+/K+ ratio. Taken together, the plastid-based expression of the SDR genes leading to enhanced salt tolerance, which opens a window for developing saline-tolerant plants via plastid genetic engineering.

RNAi-mediated gene knockdown of progesterone 5β-reductases in Digitalis lanata reduces 5β-cardenolide content

Studying RNAi-mediated DlP5βR1 and DlP5βR2 knockdown shoot culture lines of Digitalis lanata, we here provide direct evidence for the participation of PRISEs (progesterone 5β-reductase/iridoid synthase-like enzymes) in 5β-cardenolide formation. Progesterone 5β-reductases (P5βR) are assumed to catalyze the reduction of progesterone to 5β-pregnane-3,20-dione, which is a crucial step in the biosynthesis of the 5β-cardenolides. P5βRs are encoded by VEP1-like genes occurring ubiquitously in embryophytes. P5βRs are substrate-promiscuous enone-1,4-reductases recently termed PRISEs (progesterone 5β-reductase/iridoid synthase-like enzymes). Two PRISE genes, termed DlP5βR1 (AY585867.1) and DlP5βR2 (HM210089.1) were isolated from Digitalis lanata. To give experimental evidence for the participation of PRISEs in 5β-cardenolide formation, we here established several RNAi-mediated DlP5βR1 and DlP5βR2 knockdown shoot culture lines of D. lanata. Cardenolide contents were lower in D. lanata P5βR-RNAi lines than in wild-type shoots. We considered that the gene knockdowns may have had pleiotropic effects such as an increase in glutathione (GSH) which is known to inhibit cardenolide formation. GSH levels and expression of glutathione reductase (GR) were measured. Both were higher in the Dl P5βR-RNAi lines than in the wild-type shoots. Cardenolide biosynthesis was restored by buthionine sulfoximine (BSO) treatment in Dl P5βR2-RNAi lines but not in Dl P5βR1-RNAi lines. Since progesterone is a precursor of cardenolides but can also act as a reactive electrophile species (RES), we here discriminated between these by comparing the effects of progesterone and methyl vinyl ketone, a small RES but not a precursor of cardenolides. To the best of our knowledge, we here demonstrated for the first time that P5βR1 is involved in cardenolide formation. We also provide further evidence that PRISEs are also important for plants dealing with stress by detoxifying reactive electrophile species (RES).

Utilizing Big Data to Identify Tiny Toxic Components: Digitalis

The botanical genus Digitalis is equal parts colorful, toxic, and medicinal, and its bioactive compounds have a long history of therapeutic use. However, with an extremely narrow therapeutic range, even trace amounts of Digitalis can cause adverse effects. Using chemical methods, the United States Food and Drug Administration traced a 1997 case of Digitalis toxicity to a shipment of Plantago (a common ingredient in dietary supplements marketed to improve digestion) contaminated with Digitalis lanata. With increased accessibility to next generation sequencing technology, here we ask whether this case could have been cracked rapidly using shallow genome sequencing strategies (e.g., genome skims). Using a modified implementation of the Site Identification from Short Read Sequences (SISRS) bioinformatics pipeline with whole-genome sequence data, we generated over 2 M genus-level single nucleotide polymorphisms in addition to species-informative single nucleotide polymorphisms. We simulated dietary supplement contamination by spiking low quantities (0-10%) of Digitalis whole-genome sequence data into a background of commonly used ingredients in products marketed for "digestive cleansing" and reliably detected Digitalis at the genus level while also discriminating between Digitalis species. This work serves as a roadmap for the development of novel DNA-based assays to quickly and reliably detect the presence of toxic species such as Digitalis in food products or dietary supplements using genomic methods and highlights the power of harnessing the entire genome to identify botanical species.

Implications of Synthetic Modifications of the Cardiotonic Steroid Lactone Ring on Cytotoxicity

Na,K-ATPase (NKA) and cardiotonic steroids (CTS) have shown potent cytotoxic and anticancer effects. Here, we have synthesized a series of CTS digoxin derivatives (γ-benzylidene) with substitutions in the lactone ring and evaluated the cytotoxicity caused by digoxin derivatives in tumor and non-tumor cells lines, as well as their effects on NKA. The cytotoxicity assay was determined in HeLa, A549, and WI-26 VA4 after they were treated for 48 h with increased concentrations of CTS. The effects of CTS on NKA activity and immunoblotting of α1 and β1 isoforms were evaluated at IC₅₀ concentrations in A549 cell membrane. NKA activity from mouse brain cortex was also measured. The majority of CTS exhibited low cytotoxicity in tumor and non-tumor cells, presenting IC₅₀ values at micromolar concentrations, while digoxin showed cytotoxicity at nanomolar concentrations. BD-15 presented the lowest IC₅₀ value (8 µM) in A549 and reduced its NKA activity in 28%. In contrast, BD-7 was the compound that most inhibited NKA (56% inhibition) and presented high IC₅₀ value for A549. In mouse cortex, only BD-15 modulated the enzyme activity in a concentration-dependent inhibition curve. These results demonstrate that the cytotoxicity of these compounds is not related to NKA inhibition. The substitutions in the lactone ring of digoxin led to an increase in the cytotoxic concentration in tumor cells, which may not be interesting for cancer, but it has the advantage of increasing the therapeutic margin of these molecules when compared to classic CTS, and can be used safely in research for other diseases.

Selective Extraction of Bioactive Phenylethanoids from Digitalis obscura

Cardenolide-free extracts from Digitalis obscura showed significant antifeedant effects against the aphid Myzus persicae and this activity correlated with their phenylethanoid content. The content in phenylethanoids of Digitalis obscura has been studied. Maceration of the aerial parts of D. obscura was used for the selective extraction of the natural compound rengyolone (1) and the aglycone of cornoside (compound 3). Pure rengyolone (1) can be obtained from D. obscura in approximately 90% purity from fresh plant from the CHCl3 soluble fraction of the ethanolic extract (0.8% yield). The ethanol extraction of freshly collected D. obscura showed the presence of compound 3 as the only phenylethanoid. Compound 3 was proven to easily evolve to rengyolone. Due to this instability, and although its presence in plants has been previously reported, the spectroscopical data of 3 are reported herein for the first time. Selective mono-acetylation of compound of 3 led to the active natural compound hallerone (5). The aphid antifeedant (against Myzus persicae) and nematicidal (against root-knot nematode Meloidogyne javanica) activities of these compounds have been evaluated. Here we report for the first time on the aphid antifeedant effects of 1, 3, and 5. Additionally, the nematicidal activity of hallerone (5) is described here for the first time.

Sequencing, de novo assembly and annotation of Digitalis ferruginea subsp. schischkinii transcriptome

There is an increasing demand for elucidating the biosynthetic pathway of medicinal plants, which are capable of producing several metabolites with great potentials for industrial drug production. Digitalis species are important medicinal plants for the production of cardenolide compounds. Advancement on culture techniques is strictly related to our understanding of the genomic background of species. There are a limited number of genomic studies on Digitalis species. The goal of this study is to contribute to the genomic data of Digitalis ferruginea subsp. schischkinii by presenting transcriptome annotation. Digitalis ferruginea subsp. schischkinii has a limited distribution in Turkey and Transcaucasia, and has a high level of lanatoside C, an important cardenolide. In the study, we sequenced the cDNA library prepared from RNA pools of D. ferruginea subsp. schischkinii tissues treated with various stress conditions. Comprehensive bioinformatics approaches were used for de novo assembly and functional annotation of D. ferruginea subsp. schischkinii transcriptome sequence data along with TF families predictions and phylogenetic analysis. In the study, 58,369 unigenes were predicted and unigenes were annotated by analyzing the sequence data in the non-redundant (NR) protein database, the non-redundant nucleotide (NT) database, Gene Orthology (GO), EuKaryotic Orthologous Groups (KOG), Kyoto Encyclopedia of Genes and Genomes (KEGG), SwissProt, and InterPro databases. This study is the first transcriptome data for D. ferruginea subsp. schischkinii.

Semisynthetic Cardenolides Acting as Antiviral Inhibitors of Influenza A Virus Replication by Preventing Polymerase Complex Formation

Influenza virus infections represent a major public health issue by causing annual epidemics and occasional pandemics that affect thousands of people worldwide. Vaccination is the main prophylaxis to prevent these epidemics/pandemics, although the effectiveness of licensed vaccines is rather limited due to the constant mutations of influenza virus antigenic characteristics. The available anti-influenza drugs are still restricted and there is an increasing viral resistance to these compounds, thus highlighting the need for research and development of new antiviral drugs. In this work, two semisynthetic derivatives of digitoxigenin, namely C10 (3β-((N-(2-hydroxyethyl)aminoacetyl)amino-3-deoxydigitoxigenin) and C11 (3β-(hydroxyacetyl)amino-3-deoxydigitoxigenin), showed anti-influenza A virus activity by affecting the expression of viral proteins at the early and late stages of replication cycle, and altering the transcription and synthesis of new viral proteins, thereby inhibiting the formation of new virions. Such antiviral action occurred due to the interference in the assembly of viral polymerase, resulting in an impaired polymerase activity and, therefore, reducing viral replication. Confirming the in vitro results, a clinically relevant ex vivo model of influenza virus infection of human tumor-free lung tissues corroborated the potential of these compounds, especially C10, to completely abrogate influenza A virus replication at the highest concentration tested (2.0 µM). Taken together, these promising results demonstrated that C10 and C11 can be considered as potential new anti-influenza drug candidates.

Exploiting enzyme promiscuity to shape plant specialized metabolism

The amazing variability of plant metabolism and its rapid divergence during evolution pose fundamental questions as to the driving forces, mechanisms, and players in metabolic differentiation. This review examines concepts that help us understand adaptive pathway evolution, with a particular emphasis on plant specialized metabolism, previously often termed secondary metabolism. Following a general introduction to pathway and metabolite evolution, the focus is directed to enzyme promiscuity and its classification. Promiscuous enzymes (or substrates), 'silent' elements of the metabolome, and the 'underground metabolism' may be used and combined to evolve 'new' metabolic pathways. It appears that new pathways rarely appear from scratch, but instead emerge from 'floppy' enzymes and elements of a 'messy' metabolism, and in this way a range of metabolites is generated, some of which may provide benefits to the plant.

New knowledge about old drugs; a cardenolide type glycoside with cytotoxic effect and unusual secondary metabolites from Digitalis grandiflora Miller

Phytochemical investigation of the aerial parts of Digitalis grandiflora Miller (Plantaginaceae) led to the isolation of an undescribed cardenolide type glycoside digigrandifloroside (1) along with five known compounds, rengyoside A (2), rengyoside B (3), cleroindicin A (4), salidroside (5), and cornoside (6), from its aqueous fraction of methanolic extract. Structures of the isolated compounds were determined by means of spectroscopic techniques. 1-6 were isolated for the first time from D. grandiflora. 2 and 3 are being reported for the first time from Digitalis genus and Plantaginaceae family with this study. This is the second report for occurrence of 4 from a Digitalis species. Cytotoxic activity of the aqueous fraction was also tested against HEp-2 (Human larynx epidermoid carcinoma) and HepG2 (Human hepatocellular carcinoma) cancer cell lines and L929 (Mouse fibroblast cell) non-cancerous cell line. Aqueous fraction showed stronger cytotoxicity on HEp-2 cells than HepG2. Therefore, the cytotoxic activity of 1, 2, 4, and 6 were tested against HEp-2 and L929 cell lines. 3 and 5 couldn't be tested due to their insufficient amount. 1 showed the highest cytotoxicity against HEp-2 cells with IC₅₀ value 10.1 μM when compared with the positive control, etoposide and 2-6 (IC₅₀ of etoposide; 39.5 μM).

Cardenolides: Insights from chemical structure and pharmacological utility

Cardiac glycosides (CGs) are a class of naturally occurring steroid-like compounds, and members of this class have been in clinical use for more than 1500 years. They have been used in folk medicine as arrow poisons, abortifacients, heart tonics, emetics, and diuretics as well as in other applications. The major use of CGs today is based on their ability to inhibit the membrane-bound Na⁺/K⁺-ATPase enzyme, and they are regarded as an effective treatment for congestive heart failure (CHF), cardiac arrhythmia and atrial fibrillation. Furthermore, increasing evidence has indicated the potential cytotoxic effects of CGs against various types of cancer. In this review, we highlight some of the structural features of this class of natural products that are crucial for their efficacy, some methods of isolating these compounds from natural resources, and the structural elucidation tools that have been used. We also describe their physicochemical properties and several modern biotechnological approaches for preparing CGs that do not require plant sources.

Influence of auxin and its polar transport inhibitor on the development of somatic embryos in Digitalis trojana

The present study reports the role of auxin and its transport inhibitor during the establishment of an efficient and optimized protocol for the somatic embryogenesis in Digitalis trojana Ivan. Hypocotyl segments (5 mm long) were placed vertically in the Murashige and Skoog medium supplemented with three sets [indole-3-acetic acid (IAA) alone or 2,3,5-triiodobenzoic acid (TIBA) alone or IAA-TIBA combination] of formulations of plant growth regulators, to assess their differential influence on induction and proliferation of somatic embryos (SEs). IAA alone was found to be the most effective, at a concentration of 0.5 mg/l, inducing ~ 10 SEs per explant with 52% induction frequency. On the other hand, the combination of 0.5 mg/l of IAA and 1 mg/l of TIBA produced significantly fewer (~ 3.6 SEs) and abnormal (enlarged, oblong, jar and cup-shaped) SEs per explant with 24% induction frequency in comparison to that in the IAA alone. The explants treated with IAA-TIBA exhibited a delayed response along with the formation of abnormal SEs. Our study revealed that IAA induces high-frequency SE formation when used singly, but the frequency gradually declines when IAA was coupled with increasing levels of TIBA. Eventually, our findings bring new insights into the roles of auxin and its polar transport in somatic embryogenesis of D. trojana.

Anticancer and Immunogenic Properties of Cardiac Glycosides

Cardiac glycosides (CGs) are natural compounds widely used in the treatment of several cardiac conditions and more recently have been recognized as potential antitumor compounds. They are known to be ligands for Na/K-ATPase, which is a promising drug target in cancer. More recently, in addition to their antitumor effects, it has been suggested that CGs activate tumor-specific immune responses. This review summarizes the anticancer aspects of CGs as new strategies for immunotherapy and drug repositioning (new horizons for old players), and the possible new targets for CGs in cancer cells.