Calystegines in Calystegia sepium derive from the tropane alkaloid pathway

The chemical composition of the essential oils of two Mediterranean species of Convolvulaceae: Convolvulus althaeoides subsp. tenuissimus collected in Sicily (Italy) and Calystegia silvatica collected in Algeria

Convolvulus L. and Calystegia R.Br. are two closely related genera of the Convolvulaceae family distributed in Asia, Mediterranean, Macaronesia, East Africa, and Arabia, including about 210 and 30 accepted species, respectively, of flowering plants, present as trees, shrubs, and herbs. The ethnomedical use of Convolvulus species dates to 1730s as they displayed profuse medicinal properties. In the present study, the not previously investigated chemical compositions of the essential oils from aerial parts of Convolvulus althaeoides subsp. tenuissimus (Sm.) Bat., collected in Sicily, and Calystegia sylvatica (Kit.) Griseb., collected in Algeria, were evaluated by GC-MS. The main components of the essential oil of the first one were β-caryophyllene (28.68%), γ-muurolene (23.75%), and γ-elemene (17.55%), whereas the C. silvatica essential oil was shown to be rich of valeranone (10.77%), viridiflorol (9.45%), and germacrene D (8.61%). Furthermore, a complete literature review on the ethno-pharmacological uses of Convolvulus and Calystegia species was performed.

Plant Secondary Metabolites: The Weapons for Biotic Stress Management

The rise in global temperature also favors the multiplication of pests and pathogens, which calls into question global food security. Plants have developed special coping mechanisms since they are sessile and lack an immune system. These mechanisms use a variety of secondary metabolites as weapons to avoid obstacles, adapt to their changing environment, and survive in less-than-ideal circumstances. Plant secondary metabolites include phenolic compounds, alkaloids, glycosides, and terpenoids, which are stored in specialized structures such as latex, trichomes, resin ducts, etc. Secondary metabolites help the plants to be safe from biotic stressors, either by repelling them or attracting their enemies, or exerting toxic effects on them. Modern omics technologies enable the elucidation of the structural and functional properties of these metabolites along with their biosynthesis. A better understanding of the enzymatic regulations and molecular mechanisms aids in the exploitation of secondary metabolites in modern pest management approaches such as biopesticides and integrated pest management. The current review provides an overview of the major plant secondary metabolites that play significant roles in enhancing biotic stress tolerance. It examines their involvement in both indirect and direct defense mechanisms, as well as their storage within plant tissues. Additionally, this review explores the importance of metabolomics approaches in elucidating the significance of secondary metabolites in biotic stress tolerance. The application of metabolic engineering in breeding for biotic stress resistance is discussed, along with the exploitation of secondary metabolites for sustainable pest management.

De Novo Production of the Plant-Derived Tropine and Pseudotropine in Yeast

Tropine and pseudotropine with opposite stereospecific configurations as platform compounds are central building blocks in both biosynthesis and chemical synthesis of pharmacologically important tropane and nortropane alkaloids. The supply of plant-derived tropine and pseudotropine still heavily depends on either plant extraction or chemical synthesis. Advances in synthetic biology prompt the microbial synthesis of various valuable chemicals. With the biosynthetic pathway elucidation of tropine and pseudotropine in several Solanaceae plants, the key genes were sequentially identified. Here, the enzymes responsible for converting N-methylpyrrolinium into tropine and pseudotropine from Anisodus acutangulus were characterized. Reconstruction of the six-step biosynthetic pathways into Saccharomyces cerevisiae provides cell chassis producing tropine and pseudotropine with 0.13 and 0.08 mg/L titers from simple feedstocks in a shake flask, respectively. The strains described not only offer alternative sources of these central intermediates and their derived alkaloids but also provide platforms for pathway enzyme discovery.

Tropane Alkaloids: Chemistry, Pharmacology, Biosynthesis and Production

Tropane alkaloids (TA) are valuable secondary plant metabolites which are mostly found in high concentrations in the Solanaceae and Erythroxylaceae families. The TAs, which are characterized by their unique bicyclic tropane ring system, can be divided into three major groups: hyoscyamine and scopolamine, cocaine and calystegines. Although all TAs have the same basic structure, they differ immensely in their biological, chemical and pharmacological properties. Scopolamine, also known as hyoscine, has the largest legitimate market as a pharmacological agent due to its treatment of nausea, vomiting, motion sickness, as well as smooth muscle spasms while cocaine is the 2nd most frequently consumed illicit drug globally. This review provides a comprehensive overview of TAs, highlighting their structural diversity, use in pharmaceutical therapy from both historical and modern perspectives, natural biosynthesis in planta and emerging production possibilities using tissue culture and microbial biosynthesis of these compounds.

Determination of Calystegines in Several Tomato Varieties Based on GC-Q-Orbitrap Analysis and Their Classification by ANOVA

In this study, several calystegines (A₃, A₅, B₁, B₂, B₃, B₄, and C₁) were determined in tomato. A simple extraction followed by a derivatization step with silylating agents was performed prior to their analysis by gas chromatography coupled to high resolution mass spectrometry (GC-HRMS-Q-Orbitrap), which allowed the monitoring of several ions at accurate mass. The validation of the method has provided suitable values of linearity, trueness (73.7-120.0%), and precision (≤20.0%, except for calystegines B₃ and B₄ at 0.5 mg/kg). The limit of quantitation was set at 0.5 mg/kg for all analytes. The validated method was successfully applied to the analysis of nine different tomato varieties, and calystegines A₃, A₅, B₂, and C₁ were found at concentrations ranging between 0.65 mg/kg (C₁) and 12.47 mg/kg (B₂). Tomato varieties were classified according to their calystegines content by applying an analysis of variance (ANOVA).

Potato plants with genetically engineered tropane alkaloid precursors

Solanum tuberosum tropinone reductase I reduced tropinone in vivo. Suppression of tropinone reductase II strongly reduced calystegines in sprouts. Overexpression of putrescine N -methyltransferase did not alter calystegine accumulation. Calystegines are hydroxylated alkaloids formed by the tropane alkaloid pathway. They accumulate in potato (Solanum tuberosum L., Solanaceae) roots and sprouting tubers. Calystegines inhibit various glycosidases in vitro due to their sugar-mimic structure, but functions of calystegines in plants are not understood. Enzymes participating in or competing with calystegine biosynthesis, including putrescine N-methyltransferase (PMT) and tropinone reductases (TRI and TRII), were altered in their activity in potato plants by RNA interference (RNAi) and by overexpression. The genetically altered potato plants were investigated for the accumulation of calystegines and for intermediates of their biosynthesis. An increase in N-methylputrescine provided by DsPMT expression was not sufficient to increase calystegine accumulation. Overexpression and gene knockdown of StTRI proved that S. tuberosum TRI is a functional tropinone reductase in vivo, but no influence on calystegine accumulation was observed. When StTRII expression was suppressed by RNAi, calystegine formation was severely compromised in the transformed plants. Under phytochamber and green house conditions, the StTRII RNAi plants did not show phenotypic alterations. Further investigation of calystegines function in potato plants under natural conditions is enabled by the calystegine deprived StTRII RNAi plants.

Fifty years of alkaloid biosynthesis in Phytochemistry

An overview is presented of the studies related to the biosynthesis of alkaloids published in Phytochemistry in the past 50 years.

Putrescine N-methyltransferases--a structure-function analysis

Putrescine N-methyltransferase (PMT) is a key enzyme of plant secondary metabolism at the start of the specific biosynthesis of nicotine, of tropane alkaloids, and of calystegines that are glycosidase inhibitors with nortropane structure. PMT is assumed to have developed from spermidine synthases (SPDS) participating in ubiquitous polyamine metabolism. In this study decisive differences between both enzyme families are elucidated. PMT sequences were known from four Solanaceae genera only, therefore additional eight PMT cDNA sequences were cloned from five Solanaceae and a Convolvulaceae. The encoded polypeptides displayed between 76% and 97% identity and typical amino acids different from plant spermidine synthase protein sequences. Heterologous expression of all enzymes proved catalytic activity exclusively as PMT and K (cat) values between 0.16 s(-1) and 0.39 s(-1). The active site of PMT was initially inferred from a protein structure of spermidine synthase obtained by protein crystallisation. Those amino acids of the active site that were continuously different between PMTs and SPDS were mutated in one of the PMT sequences with the idea of changing PMT activity into spermidine synthase. Mutagenesis of active site residues unexpectedly resulted in a complete loss of catalytic activity. A protein model of PMT was based on the crystal structure of SPDS and suggests that overall protein folds are comparable. The respective cosubstrates S-adenosylmethionine and decarboxylated S-adenosylmethionine, however, appear to bind differentially to the active sites of both enzymes, and the substrate putrescine adopts a different position.

Immunolocalisation of two tropinone reductases in potato (Solanum tuberosum L.) root, stolon, and tuber sprouts

Tropinone reductases (TRs) are essential enzymes in the tropane alkaloid biosynthesis, providing either tropine for hyoscyamine and scopolamine formation or providing pseudotropine for calystegines. Two cDNAs coding for TRs were isolated from potato (Solanum tuberosum L.) tuber sprouts and expressed in E. coli. One reductase formed pseudotropine, the other formed tropine and showed kinetic properties typical for tropine-forming tropinone reductases (TRI) involved in hyoscyamine formation. Hyoscyamine and tropine are not found in S. tuberosum plants. Potatoes contain calystegines as the only products of the tropane alkaloid pathway. Polyclonal antibodies raised against both enzymes were purified to exclude cross reactions and were used for Western-blot analysis and immunolocalisation. The TRI (EC 1.1.1.206) was detected in protein extracts of tuber tissues, but mostly in levels too low to be localised in individual cells. The function of this enzyme in potato that does not form hyoscyamine is not clear. The pseudotropine-forming tropinone reductase (EC 1.1.1.236) was detected in potato roots, stolons, and tuber sprouts. Cortex cells of root and stolon contained the protein; additional strong immuno-labelling was located in phloem parenchyma. In tuber spouts, however, the protein was detected in companion cells.

Tropinone reductases, enzymes at the branch point of tropane alkaloid metabolism

Two stereospecific oxidoreductases constitute a branch point in tropane alkaloid metabolism. Products of tropane metabolism are the alkaloids hyoscyamine, scopolamine, cocaine, and polyhydroxylated nortropane alkaloids, the calystegines. Both tropinone reductases reduce the precursor tropinone to yield either tropine or pseudotropine. In Solanaceae, tropine is incorporated into hyoscyamine and scopolamine; pseudotropine is the first specific metabolite on the way to the calystegines. Isolation, cloning and heterologous expression of both tropinone reductases enabled kinetic characterisation, protein crystallisation, and structure elucidation. Stereospecificity of reduction is achieved by binding tropinone in the respective enzyme active centre in opposite orientation. Immunolocalisation of both enzyme proteins in cultured roots revealed a tissue-specific protein accumulation. Metabolite flux through both arms of the tropane alkaloid pathway appears to be regulated by the activity of both enzymes and by their access to the precursor tropinone. Both tropinone reductases are NADPH-dependent short-chain dehydrogenases with amino acid sequence similarity of more than 50% suggesting their descent from a common ancestor. Putative tropinone reductase sequences annotated in plant genomes other that Solanaceae await functional characterisation.

Putrescine N-methyltransferase in Solanum tuberosum L., a calystegine-forming plant

Putrescine N-methyltransferase (PMT, EC 2.1.1.53) catalyses the first specific step in the biosynthesis of tropane and nicotine alkaloids. Potato (Solanum tuberosum L.) contains neither nicotine nor the medicinal tropane alkaloids hyoscyamine or scopolamine, but calystegines. They are nortropane alkaloids with glycosidase inhibitory activity. Based on the assumption of calystegine formation by the tropane alkaloid pathway, PMT genes and enzymes were investigated in potato. Sprouting tubers contained both N-methylputrescine and PMT activity. Two cDNA clones coding for PMTs were obtained together with a cDNA clone for spermidine synthase (SPDS, EC 2.5.1.16). The pmt sequences resemble those from Nicotiana tabacum (85% identity) and those from tropane alkaloid plants, Atropa belladonna (80% identity) and Hyoscyamus niger (79% identity). They are less similar to SPDS of S. tuberosum (66% identity). Expression of pmt1 and spds cDNA in Escherichia coli yielded active enzymes, while pmt2 expression resulted in insoluble protein. Chimera proteins obtained by fusion of fragments of S. tuberosum pmt2 and H. niger pmt were active as PMT, if the initial part of pmt2 was used, indicating that a mutation in the terminal part of the gene caused insolubility of the enzyme. PMT1 was purified after expression in E. coli and proved to be an active N-methyltransferase without SPDS activity. The enzyme was specific for putrescine (K (M) 250 microM) and inhibited by n-butylamine and cadaverine. While spds was transcribed in all plant organs, pmt transcripts were found in small tuber sprouts only. The results confirm that in potato genes and enzymes specific for the tropane alkaloid metabolism are expressed and active.

Calystegines in wild and cultivated Erythroxylum species

Calystegines were identified in the genus Erythroxylum for the first time. Erythroxylum novogranatense var. novogranatense, a species cultivated for cocaine production, contained 0.2% total calystegines in dry leaves. Forty six Erythroxylum herbarium species consisting mostly of leaf tissue were analysed for calystegines, and 38 were found positive. Calystegines were compared qualitatively and quantitatively between individual Erythroxylum species. Calystegines A(3) and B(2) were the major calystegines in most species. Total calystegine content reached up to 0.32% dry mass. The simultaneous occurrence of calystegines, cocaine, other alkaloids of a 3alpha-hydroxy- or 3beta-hydroxytropane structure together with nicotine supports the concept of common biosynthetic steps of these alkaloids in Erythroxylum. The present results are the basis for further investigations of the phylogenetic origin of tropane alkaloid biosynthesis in the taxonomically remote families Solanaceae and Erythroxylaceae.

Calystegines as chemotaxonomic markers in the Convolvulaceae

An extended GC-MS study of 129 convolvulaceous species belonging to 29 genera (all 12 tribes) including the results of a previous survey (65 spp.) revealed the occurrence of one to six polyhydroxy alkaloids of the nortropane type (calystegines) in 62 species belonging to 22 genera of all tribes except the unique parasitic Cuscuteae. The large genus Ipomoea turned out to comprise calystegine-positive species in at least eight out of ten sections checked. The number of the calystegines used as reference compounds has been increased from seven (previous survey) to 11 (present study). Furthermore, the results concerning these additional four alkaloids could also be completed for all species of the previous survey. The plant material (epigeal vegetative parts and/or roots, flowers, fruits/seeds) was obtained from collections in the wild from a wide range of tropical, subtropical, and temperate locations of all continents as well as from cultivation in the greenhouse. All plant organs turned out to be potential locations for the occurrence of these metabolites though they are detectable often only in certain organs of a given species. Three genera (Cuscuta, Operculina, Polymeria) might have lost the ability to synthesize these plesiomorphic characters in the course of the evolution since the examination of several different organs and/or provenances of five species each failed to show calystegines as constituents. Nevertheless, the present data clearly demonstrate that the occurrence of calystegines is an almost consistent trait in the Convolvulaceae in principle, from basal to most advanced tribes.

Calystegines in Calystegia sepium do not inhibit fungal growth and invertase activity but interact with plant invertase

Calystegines are alkaloidal glycosidase inhibitors. They accumulate predominantly in young and meristemic parts of Calystegia sepium (Convolvulaceae). C. sepium, bindweed, infests meadows and cereal fields and is difficult to control chemically. Fungal pathogens against C. sepium are established as mycoherbicides. Stagonospora convolvuli LA39 attacks C. sepium and does not affect crop plants, but young plants of C. sepium are less susceptible to the fungus. The interaction of Stagonospora convolvuli with calystegines was investigated. Further, endophytic fungi of several classes were isolated from wild-grown Calystegia sepium leaves, and selected strains were tested for interaction with calystegines. Fungal growth on agar containing calystegines was not affected considerably. Plants in climate chambers were infected with an endophyte, Phomopsis, and with the fungal pathogen, Stagonospora convolvuli. Calystegine levels were measured in infected and non-infected plant tissues. Accumulation depended on developmental stage of the plant tissue and was not influenced by infection. Acid invertase was measured from fungal mycelia and from infected and non-infected plant tissues. Fungal acid invertase activity was not inhibited by 10 mM calystegine B (2), while invertase from C. sepium leaves was inhibited. It is concluded that calystegines do not inhibit fungal development and sucrose consumption under the conditions of the present investigation, but may act by redirection of plant carbohydrate metabolism.

Biosynthesis of calystegines: 15N NMR and kinetics of formation in root cultures of Calystegia sepium

Calystegines are nortropane alkaloids bearing between three and five hydroxyl groups in various positions. [15N]Tropinone was administered to root cultures of Calystegia sepium and the incorporation into calystegines was followed. Increase of label in calystegines was measured by one-dimensional 15N NMR and inverse-detected 2D NMR techniques. The results show that tropinone and pseudotropine are metabolites in the biosynthetic pathway of calystegines. The velocity of calystegine accumulation was followed kinetically by transfer of root cultures from 15N-enriched medium to 14N-medium and analysis by GC-MS. A constant calystegine formation with no interference by excretion or degradation was observed. A biosynthetic rate for individual calystegines at each time point was calculated, the maximum was 0.4 mg/day/g of biomass. This allowed the velocity of individual biosynthetic steps to be estimated.

Analysis of tropane and related alkaloids

The current methods for tropane alkaloid chromatographic separation and determination are summarised. The alkaloids included are: the medicinally applied tropic acid esters hyoscyamine and scopolamine and their derivatives, cocaine and derivatives, the metabolites and degradation products of these compounds occurring in plant material, calystegines as nortropane alkaloids, anatoxins as homonortropane alkaloids, pelletierines and pseudopelletierines as alkaloids with isomeric structures. Developments in GC, HPLC, CE and TLC are presented and the advantages of each method for plant analysis are discussed. A summary for each chromatographic method lists the instrumentation and parameters applied for tropane alkaloids.