Purification and characterization of malonyl-coenzyme A: 21-hydroxypregnane 21-O-malonyltransferase (Dp21MaT) from leaves of Digitalis purpurea L

Identification and analysis of BAHD superfamily related to malonyl ginsenoside biosynthesis in Panax ginseng

Introduction

The BAHD (benzylalcohol O-acetyl transferase, anthocyanin O-hydroxycinnamoyl transferase, N-hydroxycinnamoyl anthranilate benzoyl transferase and deacetylvindoline 4-O-acetyltransferase), has various biological functions in plants, including catalyzing the biosynthesis of terpenes, phenolics and esters, participating in plant stress response, affecting cell stability, and regulating fruit quality.

Methods

Bioinformatics methods, real-time fluorescence quantitative PCR technology, and ultra-high-performance liquid chromatography combined with an Orbitrap mass spectrometer were used to explore the relationship between the BAHD gene family and malonyl ginsenosides in Panax ginseng.

Results

In this study, 103 BAHD genes were identified in P. ginseng, mainly distributed in three major clades. Most PgBAHDs contain cis-acting elements associated with abiotic stress response and plant hormone response. Among the 103 genes, 68 PgBAHDs are WGD (whole-genome duplication) genes. The significance of malonylation in biosynthesis has garnered considerable attention in the study of malonyltransferases. The phylogenetic tree results showed 34 PgBAHDs were clustered with genes that have malonyl characterization. Among them, seven PgBAHDs (PgBAHD4, 45, 65, 74, 90, 97, and 99) showed correlations > 0.9 with crucial enzyme genes involved in ginsenoside biosynthesis and > 0.8 with malonyl ginsenosides. These seven genes were considered potential candidates involved in the biosynthesis of malonyl ginsenosides.

Discussion

These results help elucidate the structure, evolution, and functions of the P. ginseng BAHD gene family, and establish the foundation for further research on the mechanism of BAHD genes in ginsenoside biosynthesis.

21-Hydroxypregnane 21-O-malonylation, a crucial step in cardenolide biosynthesis, can be achieved by substrate-promiscuous BAHD-type phenolic glucoside malonyltransferases from Arabidopsis thaliana and homolog proteins from Digitalis lanata

Three putative 21-hydroxypregnane 21-O-malonyltransferases (21MaT) from Digitalis lanata were partially purified. Two of them were supposed to be BAHD-type enzymes. We were unable to purify them in quantities necessary for reliable sequencing. We identified two genes in A. thaliana coding for substrate-promiscuous BAHD-type phenolic glucoside malonyltransferases (AtPMaT1, AtPMaT2) and docked various 21-hydroxypregnanes into the substrate-binding site of a homology model built on the BAHD template 2XR7 (NtMaT1 from N. tabacum). Recombinant forms of Atpmat1 and Atpmat2 were expressed in E. coli and the recombinant enzymes characterized with regard to their substrate preferences. They were shown to malonylate various 21-hydroxypregnanes. The Atpmat1 sequence was used to identify candidate genes in Digitalis lanata (Dlmat1 to Dlmat4). Dlmat1 and Dlmat2 were also expressed in E. coli and shown to possess 21-hydroxypregnane 21-O-malonyltransferase activity.

Ionotropin is the mammalian digoxin-like material (DLM). It is a phosphocholine ester of a steroid with 23 carbon atoms

We describe a novel steroid, which we have named "Ionotropin". Its unique features are: [1] it has 23 carbon atoms and [2] it is a phosphocholine ester. There are no other known mammalian steroids with either structural feature. Ionotropin cross reacts with digoxin-specific antibodies and may be the long-sought, endogenous, mammalian digoxin-like material (DLM). Using LC-MS, we identified three other phosphocholine steroids in serum. Two of these steroids also cross-react with digoxin specific antibodies. In adrenal extracts, we found both phosphocholine esters and corresponding phospho-ethanolamine steroid esters. There are no other known phosphoethanolamine steroid esters. Together, these 8 compounds define a biosynthetic pathway from 7-dehydropregnenolone to Ionotropin. Ionotropin may be the only steroid hormone not synthesized with cholesterol as a precursor. Finally, we propose that Ionotropin serves as the endogenous potassium sparing hormone. Ionotropin provides a new understanding of renal, cardiac, gonadal and placental function.

Comprehensive analysis of alternative splicing in Digitalis purpurea by strand-specific RNA-Seq

Digitalis purpurea (D. purpurea) is one of the most important medicinal plants and is well known in the treatment of heart failure because of the cardiac glycosides that are its main active compounds. However, in the absence of strand specific sequencing information, the post-transcriptional mechanism of gene regulation in D. purpurea thus far remains unknown. In this study, a strand-specific RNA-Seq library was constructed and sequenced using Illumina HiSeq platforms to characterize the transcriptome of D. purpurea with a focus on alternative splicing (AS) events and the effect of AS on protein domains. De novo RNA-Seq assembly resulted in 48,475 genes. Based on the assembled transcripts, we reported a list of 3,265 AS genes, including 5,408 AS events in D. purpurea. Interestingly, both glycosyltransferases and monooxygenase, which were involved in the biosynthesis of cardiac glycosides, are regulated by AS. A total of 2,422 AS events occurred in coding regions, and 959 AS events were located in the regions of 882 unique protein domains, which could affect protein function. This D. purpurea transcriptome study substantially increased the expressed sequence resource and presented a better understanding of post-transcriptional regulation to further facilitate the medicinal applications of D. purpurea for human health.

A stable and efficient Agrobacterium tumefaciens-mediated genetic transformation of the medicinal plant Digitalis purpurea L

In this study, we developed a rapid and efficient method for in vitro propagation and Agrobacterium tumefaciens-mediated transformation of Digitalis purpurea L. (syn. foxglove), an important medicinal plant. Mature leaf explants of D. purpurea were used for 100 % adventitious shoot regeneration on Murashige and Skoog (MS) medium supplemented with 1 mg L(-1) thidiazuron (TDZ) (a cytokine) and 0.1 mg L(-1) 1-naphthaleneacetic acid (NAA) (an auxin). Transformation was achieved by inoculating leaf explants with the A. tumefaciens strains GV2260/pBI121 or GV3101/pBI121. The binary vector pBI121 contained the reporter β-glucuronidase gene (GUS) and kanamycin selection marker nptII. Kanamycin-resistant shoots were regenerated directly on the selection medium 4-6 weeks after co-cultivation. Approximately, 52.2 and 60 % of kanamycin-resistant shoots transformed with Agrobacterium strains GV2260 and GV3101, respectively, showed strong GUS staining by histochemical assay. Furthermore, PCR and Southern blot analysis confirmed the presence of nptII and GUS on the chromosome of the transformed D. purpurea plants, and stable GUS expression was detected in the transformants by RT-PCR analysis. This efficient method of shoot regeneration and genetic transformation of D. purpurea will provide a powerful tool to increase and produce valuable components such as digitoxin, digoxin, and digoxigenin in D. purpurea through improved secondary metabolic pathways via a biotechnological approach.

In vitro propagation and production of cardiotonic glycosides in shoot cultures of Digitalis purpurea L. by elicitation and precursor feeding

Digitalis purpurea L. (Scrophulariaceae; Foxglove) is a source of cardiotonic glycosides such as digitoxin and digoxin which are commercially applied in the treatment to strengthen cardiac diffusion and to regulate heart rhythm. This investigation deals with in vitro propagation and elicited production of cardiotonic glycosides digitoxin and digoxin in shoot cultures of D. purpurea L. In vitro germinated seedlings were used as a primary source of explants. Multiple shoot formation was achieved for three explant types (nodal, internodal, and leaf) cultured on Murashige and Skoog (MS) medium with several treatments of cytokinins (6-benzyladenine-BA; kinetin-Kin; and thidiazuron-TDZ) and auxins (indole-3-acetic acid-IAA; α-naphthaleneacetic acid-NAA; and 2,4-dichlorophenoxy acetic acid-2,4-D). Maximum multiple shoots (12.7 ± 0.6) were produced from nodal explants on MS + 7.5 μM BA. Shoots were rooted in vitro on MS containing 15 μM IAA. Rooted plantlets were successfully acclimatized. To further maintain the multiple shoot induction, mother tissue was cut into four equal parts and repeatedly sub-cultured on fresh shoot induction liquid medium after each harvest. On adaptation of this strategy, an average of 18 shoots per explant could be produced. This strategy was applied for the production of biomass and glycosides digitoxin and digoxin in shoot cultures on MS medium supplemented with 7.5 μM BA and several treatments with plant growth regulators, incubation period, abiotic (salicylic acid, mannitol, sorbitol, PEG-6000, NaCl, and KCl), biotic (Aspergillus niger, Helminthosporium sp., Alternaria sp., chitin, and yeast extract) elicitors, and precursors (progesterone, cholesterol, and squalene). The treatment of KCl, mycelial mass of Helminthosporium sp., and progesterone were highly effective for the production of cardenolides. In the presence of progesterone (200 to 300 mg/l), digitoxin and digoxin accumulation was enhanced by 9.1- and 11.9-folds respectively.

Comprehensive transcriptome analysis reveals novel genes involved in cardiac glycoside biosynthesis and mlncRNAs associated with secondary metabolism and stress response in Digitalis purpurea

Abstract

Conclusions

Through comprehensive transcriptome analysis, we not only identified 29 novel gene families potentially involved in the biosynthesis of cardiac glycosides but also characterized a large number of mlncRNAs. Our results suggest the importance of mlncRNAs in secondary metabolism and stress response in D. purpurea.

A simple chemical method for synthesizing malonyl hemiesters of 21-hydroxypregnanes, potential intermediates in cardenolide biosynthesis

A simple and versatile method for the chemical synthesis of 21-hydroxypregnane 21-O-malonyl hemiesters which may be important intermediates of cardenolide biosynthesis is described. Starting from commercial beta-methyldigitoxin, acid hydrolysis followed by 3beta-O-acetylation and ozonolysis with reductive cleavage of the ozonides afforded 3beta-acetoxy-5beta-pregnane-14beta,21-diol-20-one which was finally converted into the target compound by treatment with malonyl chloride. The malonylation protocol was optimized using deoxycorticosterone (DOC) as the pregnane educt.

Synthesis of a putative substrate for malonyl-coenzyme A: 21-hydroxypregnane 21-O-malonyltransferase and development of an HPLC method for the quantification of the enzyme reaction

The butenolide ring is the main common characteristic of all cardenolides. Its formation is supposed to be initiated by the transfer of a malonyl moiety from malonyl-coenzyme A to an appropriate 21-hydroxypregnane. A new, reliable, fast and sensitive method to determine malonyl-coenzyme A: 21-hydroxypregnane 21-O-malonyltransferase activity had to be developed since previous attempts employing HPLC, TLC or GC did not prove successful. A surrogate substrate was synthesized containing a side chain resembling the sugar side chain attached to C-3 of putative cardenolide precursors and containing a chromophor allowing UV detection. 3beta-benzoyloxy-5beta-pregnane-14beta,21-dihydroxy-20-one and its 21-O-malonylated derivative were synthesized, the latter being the expected product of the enzyme reaction. The new substrate was well accepted by the enzyme. An HPLC method has been established to detect and quantify 3beta-benzoyloxy-5beta-pregnane-14beta,21-dihydroxy-20-one and its 21-O-malonylated derivative, 3beta-benzoyloxy-5beta-pregnane-14beta-hydroxy-20-one 21-O-malonylhemiester. The method was validated.