Non-indolyl cruciferous phytoalexins: Nasturlexins and tridentatols, a striking convergent evolution of defenses in terrestrial plants and marine animals?

Anticancer Potential of Indole Phytoalexins and Their Analogues

Indole phytoalexins, found in economically significant Cruciferae family plants, are synthesized in response to pathogen attacks or stress, serving as crucial components of plant defense mechanisms against bacterial and fungal infections. Furthermore, recent research indicates that these compounds hold promise for improving human health, particularly in terms of potential anticancer effects that have been observed in various studies. Since our last comprehensive overview in 2016 focusing on the antiproliferative effects of these substances, brassinin and camalexin have been the most extensively studied. This review analyses the multifaceted pharmacological effects of brassinin and camalexin, highlighting their anticancer potential. In this article, we also provide an overview of the antiproliferative activity of new synthetic analogs of indole phytoalexins, which were synthesized and tested at our university with the aim of enhancing efficacy compared to the parent compound.

Phytoalexins of the crucifer Barbarea vulgaris: Structural profile and correlation with glucosinolate turnover

Phytoalexins are antimicrobial plant metabolites elicited by microbial attack or abiotic stress. We investigated phytoalexin profiles after foliar abiotic elicitation in the crucifer Barbarea vulgaris and interactions with the glucosinolate-myrosinase system. The treatment for abiotic elicitation was a foliar spray with CuCl2 solution, a usual eliciting agent, and three independent experiments were carried out. Two genotypes of B. vulgaris (G-type and P-type) accumulated the same three major phytoalexins in rosette leaves after treatment: phenyl-containing nasturlexin D and indole-containing cyclonasturlexin and cyclobrassinin. Phytoalexin levels were investigated daily by UHPLC-QToF MS and tended to differ among plant types and individual phytoalexins. In roots, phytoalexins were low or not detected. In treated leaves, typical total phytoalexin levels were in the range 1-10 nmol/g fresh wt. during three days after treatment while typical total glucosinolate (GSL) levels were three orders of magnitude higher. Levels of some minor GSLs responded to the treatment: phenethylGSL (PE) and 4-substituted indole GSLs. Levels of PE, a suggested nasturlexin D precursor, were lower in treated plants than controls. Another suggested precursor GSL, 3-hydroxyPE, was not detected, suggesting PE hydrolysis to be a key biosynthetic step. Levels of 4-substituted indole GSLs differed markedly between treated and control plants in most experiments, but not in a consistent way. The dominant GSLs, glucobarbarins, are not believed to be phytoalexin precursors. We observed statistically significant linear correlations between total major phytoalexins and the glucobarbarin products barbarin and resedine, suggesting that GSL turnover for phytoalexin biosynthesis was unspecific. In contrast, we did not find correlations between total major phytoalexins and raphanusamic acid or total glucobarbarins and barbarin. In conclusion, two groups of phytoalexins were detected in B. vulgaris, apparently derived from the GSLs PE and indol-3-ylmethylGSL. Phytoalexin biosynthesis was accompanied by depletion of the precursor PE and by turnover of major non-precursor GSLs to resedine. This work paves the way for identifying and characterizing genes and enzymes in the biosyntheses of phytoalexins and resedine.

Facile access to S-methyl dithiocarbamates with sulfonium or sulfoxonium iodide as a methylation reagent

Efficient access to S-methyl dithiocarbamates was achieved with sulfonium or sulfoxonium iodide as a methylation reagent. This method is reliable for the synthesis of dithiocarbamates from primary or secondary amines, with sulfoxonium iodide demonstrating more robust methylation capability than sulfonium iodide. Moreover, it also enables facile access to S-trideuteromethyl dithiocarbamates via sulfoxonium metathesis between sulfoxonium iodide and DMSO-d6 with high yields.

Impact of Marine Chemical Ecology Research on the Discovery and Development of New Pharmaceuticals

Diverse ecologically important metabolites, such as allelochemicals, infochemicals and volatile organic chemicals, are involved in marine organismal interactions. Chemically mediated interactions between intra- and interspecific organisms can have a significant impact on community organization, population structure and ecosystem functioning. Advances in analytical techniques, microscopy and genomics are providing insights on the chemistry and functional roles of the metabolites involved in such interactions. This review highlights the targeted translational value of several marine chemical ecology-driven research studies and their impact on the sustainable discovery of novel therapeutic agents. These chemical ecology-based approaches include activated defense, allelochemicals arising from organismal interactions, spatio-temporal variations of allelochemicals and phylogeny-based approaches. In addition, innovative analytical techniques used in the mapping of surface metabolites as well as in metabolite translocation within marine holobionts are summarized. Chemical information related to the maintenance of the marine symbioses and biosyntheses of specialized compounds can be harnessed for biomedical applications, particularly in microbial fermentation and compound production. Furthermore, the impact of climate change on the chemical ecology of marine organisms—especially on the production, functionality and perception of allelochemicals—and its implications on drug discovery efforts will be presented.

Specialized metabolites as versatile tools in shaping plant-microbe associations

Plants are rich repository of a large number of chemical compounds collectively referred to as specialized metabolites. These compounds are of importance for adaptive processes including responses against changing abiotic conditions and interactions with various co-existing organisms. One of the strikingly affirmed functions of these specialized metabolites is their involvement in plants' life-long interactions with complex multi-kingdom microbiomes including both beneficial and harmful microorganisms. Recent developments in genomic and molecular biology tools not only help to generate well-curated information about regulatory and structural components of biosynthetic pathways of plant specialized metabolites but also to create and screen mutant lines defective in their synthesis. In this review, we have comprehensively surveyed the function of these specialized metabolites and discussed recent research findings demonstrating the responses of various microbes on tested mutant lines having defective biosynthesis of particular metabolites. In addition, we attempt to provide key clues about the impact of these metabolites on the assembly of the plant microbiome by summarizing the major findings of recent comparative metagenomic analyses of available mutant lines under customized and natural microbial niches. Subsequently, we delineate benchmark initiatives that aim to engineer or manipulate the biosynthetic pathways to produce specialized metabolites in heterologous systems but also to diversify their immune function. While denoting the function of these metabolites, we also discuss the critical bottlenecks associated with understanding and exploiting their function in improving plant adaptation to the environment.

Responses of the Necrotrophic Fungus Alternaria brassisicola to the Indolic Phytoalexin Brassinin

Alternaria brassicicola causes black spot disease in Brassicaceae. During host infection, this necrotrophic fungus is exposed to various antimicrobial compounds, such as the phytoalexin brassinin which is produced by many cultivated Brassica species. To investigate the cellular mechanisms by which this compound causes toxicity and the corresponding fungal adaptive strategies, we first analyzed fungal transcriptional responses to short-term exposure to brassinin and then used additional functional approaches. This study supports the hypothesis that indolic phytoalexin primarily targets mitochondrial functions in fungal cells. Indeed, we notably observed that phytoalexin treatment of A. brassicicola disrupted the mitochondrial membrane potential and resulted in a significant and rapid decrease in the oxygen consumption rates. Secondary effects, such as Reactive oxygen species production, changes in lipid and endoplasmic reticulum homeostasis were then found to be induced. Consequently, the fungus has to adapt its metabolism to protect itself against the toxic effects of these molecules, especially via the activation of high osmolarity glycerol and cell wall integrity signaling pathways and by induction of the unfolded protein response.

Engineering Plant Synthetic Pathways for the Biosynthesis of Novel Antifungals

Plants produce a wealth of biologically active compounds, many of which are used to defend themselves from various pests and pathogens. We explore the possibility of expanding upon the natural chemical diversity of plants and create molecules that have enhanced properties, by engineering metabolic pathways new to nature. We rationally broaden the set of primary metabolites that can be utilized by the core biosynthetic pathway of the natural biopesticide, brassinin, producing in planta a novel class of compounds that we call crucifalexins. Two of our new-to-nature crucifalexins are more potent antifungals than brassinin and, in some instances, comparable to commercially used fungicides. Our findings highlight the potential to push the boundaries of plant metabolism for the biosynthesis of new biopesticides.

Glucosinolate structural diversity, identification, chemical synthesis and metabolism in plants

The glucosinolates (GSLs) is a well-defined group of plant metabolites characterized by having an S-β-d-glucopyrano unit anomerically connected to an O-sulfated (Z)-thiohydroximate function. After enzymatic hydrolysis, the sulfated aglucone can undergo rearrangement to an isothiocyanate, or form a nitrile or other products. The number of GSLs known from plants, satisfactorily characterized by modern spectroscopic methods (NMR and MS) by mid-2018, is 88. In addition, a group of partially characterized structures with highly variable evidence counts for approximately a further 49. This means that the total number of characterized GSLs from plants is somewhere between 88 and 137. The diversity of GSLs in plants is critically reviewed here, resulting in significant discrepancies with previous reviews. In general, the well-characterized GSLs show resemblance to C-skeletons of the amino acids Ala, Val, Leu, Trp, Ile, Phe/Tyr and Met, or to homologs of Ile, Phe/Tyr or Met. Insufficiently characterized, still hypothetic GSLs include straight-chain alkyl GSLs and chain-elongated GSLs derived from Leu. Additional reports (since 2011) of insufficiently characterized GSLs are reviewed. Usually the crucial missing information is correctly interpreted NMR, which is the most effective tool for GSL identification. Hence, modern use of NMR for GSL identification is also reviewed and exemplified. Apart from isolation, GSLs may be obtained by organic synthesis, allowing isotopically labeled GSLs and any kind of side chain. Enzymatic turnover of GSLs in plants depends on a considerable number of enzymes and other protein factors and furthermore depends on GSL structure. Identification of GSLs must be presented transparently and live up to standard requirements in natural product chemistry. Unfortunately, many recent reports fail in these respects, including reports based on chromatography hyphenated to MS. In particular, the possibility of isomers and isobaric structures is frequently ignored. Recent reports are re-evaluated and interpreted as evidence of the existence of "isoGSLs", i.e. non-GSL isomers of GSLs in plants. For GSL analysis, also with MS-detection, we stress the importance of using authentic standards.

Glucosinolate turnover in Brassicales species to an oxazolidin-2-one, formed via the 2-thione and without formation of thioamide

Glucosinolates are found in plants of the order Brassicales and hydrolyzed to different breakdown products, particularly after tissue damage. In Barbarea vulgaris R.Br. (Brassicaceae), the dominant glucosinolate in the investigated "G-type" is glucobarbarin, (S)-2-hydroxy-2-phenylethylglucosinolate. Formation of the nitrile from glucobarbarin was observed in vitro, while a previously suggested thioamide (synonym thionamide) was not confirmed. Resedine (5-phenyl-1,3-oxazolidin-2-one) was detected after glucobarbarin hydrolysis in crushed B. vulgaris leaves and siliques, but not in intact parts. The abundance increased for several hours after completion of hydrolysis. The corresponding 1,3-oxazolidine-2-thione (OAT), with the common name barbarin, was also formed, and appeared to be the precursor of resedine. Addition of each of two non-endogenous OATs, (S)-5-ethyl-5-methylOAT and (R)-5-vinylOAT (R-goitrin), to a leaf homogenate resulted in formation of the corresponding 1,3-oxazolidin-2-ones (OAOs), confirming the metabolic connection of OAT to OAO. Formation of OAOs was inhibited by prior brief heating of the homogenate, suggesting enzyme involvement. We suggest the conversion of OATs to OAOs to be catalyzed by an enzyme ("oxazolidinethionase") responsible for turnover of OAT formed in intact plants. Resedine had been reported as an alkaloid from another species - Reseda luteola L. (Resedaceae) - naturally containing the glucosinolate glucobarbarin. However, resedine was not detected in intact R. luteola plants, but formed after tissue damage. The formation of resedine in two families suggests a broad distribution of putative OATases in the Brassicales; potentially involved in glucosinolate turnover that needs myrosinase activity as the committed step. In agreement with the proposed function of OATase, several candidate genes for myrosinases in glucosinolate turnover in intact plants were discovered in the B. vulgaris genome. We also suggest that biotechnological conversion of OATs to OAOs might improve the nutritional value of Brassicales protein. HPLC-MS/MS methods for detection of these glucobarbarin products are described.

Synthesis of stable isotope-labeled nasturlexins and potential precursors to probe biosynthetic pathways of cruciferous phytoalexins

The syntheses of perdeuterated phytoalexins nasturlexins A and C, and putative biosynthetic precursors, including phenylethyl isothiocyanates and phenylethyl dithiocarbamates, using commercially available [2,3,4,5,6-D₅ ]phenylalanine, [2,3,4,5,6-D₅ ]nitrobenzene, and [2,3,4,5,6-D₅ ]benzaldehyde are described. In addition, application of an efficient deuterium-hydrogen exchange transformation to nonlabeled starting materials allowed access to new deuterated compounds, including 3-hydroxyphenylethyl glucosinolate.

Interrogation of biosynthetic pathways of the cruciferous phytoalexins nasturlexins with isotopically labelled compounds

A biosynthetic map of the precursors of the phytoalexins nasturlexins and cyclonasturlexin is established.

Defense and signalling metabolites of the crucifer Erucastrum canariense: Synchronized abiotic induction of phytoalexins and galacto-oxylipins

Erucastrum canariense Webb & Berthel. (Brassicaceae) is a wild crucifer that grows in rocky soils, in salt and water stressed habitats, namely in the Canary Islands and similar environments. Abiotic stress induced by copper chloride triggered formation of a phytoalexin and galacto-oxylipins in E. canariense, whereas wounding induced galacto-oxylipins but not phytoalexins. Analysis of the metabolite profiles of leaves of E. canariense followed by isolation and structure determination afforded the phytoalexin erucalexin, the phytoanticipin indolyl-3-acetonitrile, the galacto-oxylipins arabidopsides A, C, and D, and the oxylipin 12-oxophytodienoic acid. In addition, arabidopsides A and D were also identified in extracts of leaves of Nasturtium officinale R. Br.

Muscarine, imidazole, oxazole and thiazole alkaloids

Covering: July 2012 to June 2015. Previous review: Nat. Prod. Rep., 2013, 30, 869-915The structurally diverse imidazole-, oxazole-, and thiazole-containing secondary metabolites are widely distributed in terrestrial and marine environments, and exhibit extensive pharmacological activities. In this review the latest progress involving the isolation, biological activities, and chemical and biogenetic synthesis studies on these natural products has been summarized.

Pathogen inactivation of cruciferous phytoalexins: detoxification reactions, enzymes and inhibitors

This review covers the detoxification pathways of cruciferous phytoalexins, the corresponding detoxifying enzymes and their natural and synthetic inhibitors. Paldoxins are examined as a potentially sustainable strategy to control plant pathogenic fungi.

Glucosinolate diversity within a phylogenetic framework of the tribe Cardamineae (Brassicaceae) unraveled with HPLC-MS/MS and NMR-based analytical distinction of 70 desulfoglucosinolates

As a basis for future investigations of evolutionary trajectories and biosynthetic mechanisms underlying variations in glucosinolate structures, we screened members of the crucifer tribe Cardamineae by HPLC-MS/MS, isolated and identified glucosinolates by NMR, searched the literature for previous data for the tribe, and collected HPLC-MS/MS data for nearly all glucosinolates known from the tribe as well as some related structures (70 in total). This is a considerable proportion of the approximately 142 currently documented natural glucosinolates. Calibration with authentic references allowed distinction (or elucidation) of isomers in many cases, such as distinction of β-hydroxyls, methylthios, methylsulfinyls and methylsulfonyls. A mechanism for fragmentation of secondary β-hydroxyls in MS was elucidated, and two novel glucosinolates were discovered: 2-hydroxy-3-methylpentylglucosinolate in roots of Cardamine pratensis and 2-hydroxy-8-(methylsulfinyl)octylglucosinolate in seeds of Rorippa amphibia. A large number of glucosinolates (ca. 54 with high structural certainty and a further 28 or more suggested from tandem MS), representing a wide structural variation, is documented from the tribe. This included glucosinolates apparently derived from Met, Phe, Trp, Val/Leu, Ile and higher homologues. Normal side chain elongation and side chain decoration by oxidation or methylation was observed, as well as rare abnormal side chain decoration (hydroxylation of aliphatics at the δ rather than β-position). Some species had diverse profiles, e.g. R. amphibia and C. pratensis (19 and 16 individual glucosinolates, respectively), comparable to total diversity in literature reports of Armoracia rusticana (17?), Barbarea vulgaris (20-24), and Rorippa indica (>20?). The ancestor or the tribe would appear to have used Trp, Met, and homoPhe as glucosinolate precursor amino acids, and to exhibit oxidation of thio to sulfinyl, formation of alkenyls, β-hydroxylation of aliphatic chains and hydroxylation and methylation of indole glucosinolates. Two hotspots of apparent biochemical innovation and loss were identified: C. pratensis and the genus Barbarea. Diversity in other species mainly included structures also known from other crucifers. In addition to a role of gene duplication, two contrasting genetic/biochemical mechanisms for evolution of such combined diversity and redundancy are discussed: (i) involvement of widespread genes with expression varying during evolution, and (ii) mutational changes in substrate specificities of CYP79F and GS-OH enzymes.

Unveiling the first indole-fused thiazepine: structure, synthesis and biosynthesis of cyclonasturlexin, a remarkable cruciferous phytoalexin

The structure, synthesis, biosynthesis and antifungal activity of cyclonasturlexin, the most intriguing indolyl phytoalexin isolated from watercress plants, are reported.

Expanding the nasturlexin family: Nasturlexins C and D and their sulfoxides are phytoalexins of the crucifers Barbarea vulgaris and B. verna

The metabolites produced in leaves of the crucifers winter cress (Barbarea vulgaris) and upland cress (Barbarea verna) abiotically elicited were investigated and their chemical structures were elucidated by analyses of spectroscopic data and confirmed by syntheses. Nasturlexins C and D and their sulfoxides are cruciferous phytoalexins displaying antifungal activity against the crucifer pathogens Alternaria brassicicola, Leptosphaeria maculans and Sclerotinia sclerotiorum. The biosynthesis of these metabolites is proposed based on pathways of cruciferous indolyl phytoalexins. This work indicates that B. vulgaris and B. verna have great potential as sources of defense pathways transferable to agriculturally important crops within the Brassica species.

Two cytochromes P450 catalyze S-heterocyclizations in cabbage phytoalexin biosynthesis

Phytoalexins are abundant in edible crucifers and have important biological activities, yet no dedicated gene for their biosynthesis is known. Here, we report two new cytochromes P450 from Brassica rapa (Chinese cabbage) that catalyze unprecedented S-heterocyclizations in cyclobrassinin and spirobrassinin biosynthesis. Our results reveal the first genetic and biochemical insights into the biosynthesis of a prominent pair of dietary metabolites, and have implications for pathway discovery across >20 recently sequenced crucifers.

Glucosinolate hydrolysis products in the crucifer Barbarea vulgaris include a thiazolidine-2-one from a specific phenolic isomer as well as oxazolidine-2-thiones

Two isomeric phenolic glucosinolates, m- and p-hydroxyl derivatives of epiglucobarbarin [(R)-2-hydroxy-2-phenylethylglucosinolate], co-occur in an eastern chemotype (P-type) of the crucifer Barbarea vulgaris along with epiglucobarbarin itself. Levels of the phenolic derivatives in B. vulgaris were low in summer but higher during fall and winter, allowing isolation of all three glucosinolates. Hydrolysis in vitro, catalyzed by Sinapis alba myrosinase at near neutral pH, resulted in expectable oxazolidine-2-thione type hydrolysis products of epiglucobarbarin and its m-hydroxyl derivative. In contrast, a thiazolidine-2-one type product was formed in vitro from p-hydroxy epiglucobarbarin and characterized by UV, IR, MS/MS and 2D NMR. Maceration of leaf material resulted in disappearance of the glucosinolates and formation of the same oxazolidine-2-thione and thiazolidine-2-one products as found in vitro. The detected amounts were comparable to initial amounts of precursor glucosinolates. The corresponding oxazolidine-2-thione type product was also detected quantitatively from glucobarbarin in foliage of a western genotype (G-type). We suggest that p-hydroxy epiglucobarbarin is initially converted into the conventional oxazolidine-2-thione, which would further rearrange to a thiazolidine-2-one due to the activating effect of the p-hydroxyl group. We conclude that a subtle difference between isomeric phenolic glucosinolates results in significantly different natural hydrolysis products.

Specific glucosinolate analysis reveals variable levels of epimeric glucobarbarins, dietary precursors of 5-phenyloxazolidine-2-thiones, in watercress types with contrasting chromosome numbers

Watercress obtained in food stores in the United States contained significant levels of epiglucobarbarin [(R)-2-hydroxy-2-phenylethylglucosinolate] and low levels of the 2S-epimer glucobarbarin identified by an HPLC+NMR+MS/MS approach. Typical combined levels were 4-7 μmol/g dry wt. The hydrolysis product, 5-phenyloxazolidine-2-thione (barbarin), was detected at similar levels as the precursor glucosinolates after autolysis of fresh watercress in water. Fragmentation patterns in MS(2) of reference desulfoglucosinolates were side chain specific and suitable for routine identification. Watercress was of two main glucosinolate chemotypes: Material from U.S. food stores had a complex profile including glucobarbarins, gluconasturtiin, indole glucosinolates and high levels (6-28 μmol/g dry wt.) of long-chain methylsulfinylalkyl and methylthioalkyl glucosinolates. Material from European food stores had a simple profile dominated by gluconasturtiin, with low levels of epiglucobarbarin and moderate levels of indole glucosinolates. Some wild U.S. material was similar to the U.S. food store type. Both types were found to be Nasturtium officinale by floral parts morphology. Cytological analysis of one U.S. food store accession indicated that it represented a chromosome-doubled variant within N. officinale. The nutritional consequences and invasive potential of the U.S. food store chemotype are discussed.