Increased accumulation of isoflavonoids in common bean (Phaseolus vulgaris L.) tissues treated with 1-oxo-indane-4-carboxylic acid derivatives

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Eleven indanoyl derivatives as potential elicitors were synthesized and characterized.

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Accumulation of nine isoflavonoid phytoalexins in two common bean (P. vulgaris L.) cultivars grown in Colombia was analyzed.

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Dose-response and time-course experiments were performed on cotyledons and hypocotyls-roots of common bean treated with the potential elicitors.

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Application of indanoyl-type elicitors increased the concentration of isoflavonoids in tissues of common bean.

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Accumulation of isoflavonoid phytoalexins was dependent on the cultivar, the tissue type, the elicitor structure and the post-induction time.

Isoflavonoid phytoalexins (isoflavones: genistein, 2′-hydroxygenistein, and daidzein; isoflavanones: dalbergioidin and kievitone; coumestrol; pterocarpans: phaseollidin and phaseollin; and the isoflavan: phaseollinisoflavan) production in response to the application of eleven 1-oxo-indane-4-carboxylic acid derivatives (indanoyl esters and indanoyl amino acids conjugates), in cotyledons and hypocotyl/root of two common bean (Phaseolus vulgaris L.) cultivars was evaluated. The content of isoflavonoids depended on the cultivar, the treated tissue, the time after induction, the structure and concentration of the elicitor. The highest isoflavonoid contents were found when 1-oxo-indanoyl-amino acids conjugates were used as elicitors. Cotyledons and hypocotyl/root of the anthracnose-resistant cultivar produced significantly higher isoflavonoid contents as compared to the susceptible one. Maximum levels of phaseollin were obtained using 0.66 mM 1-oxo-indanoyl-l-isoleucyl methyl ester and between 72 and 96 h post-induction. So, 1-oxo-indane-4-carboxylic acid derivatives, may be used to enhance the amount of isoflavonoid phytoalexins in common bean and protect crops from phytopathogenic microorganisms.

Coumestrol Confers Partial Resistance in Soybean Plants Against Cercospora Leaf Blight

Although previous research showed that the purple symptom of Cercospora leaf blight (CLB) is associated with lower biomass of Cercospora cf. flagellaris and lower concentrations of cercosporin, a reactive oxygen species producer, as compared with blighted leaves, the hypothesis that the purple symptom is a plant reaction to the pathogen has never been tested. In this study, we demonstrated that high levels of coumestrol (COU) were associated with purple symptoms of CLB and that COU has strong antioxidant activity. Additionally, we found that COU is restricted to the pigmented areas of purple leaves, and the pigmentation is restricted to the adaxial surfaces, suggesting that COU may be acting as a sunscreen. Even though COU is associated with the purple leaf symptom, this coumestan is not the direct cause of discoloration in that COU is colorless. Quantification of chlorophyll a and b and total carotenoids suggested that blighted but not purple or asymptomatic leaves were undergoing photooxidative stress. Because the purple symptom is associated with high COU concentrations, lower biomass of C. cf. flagellaris, and lower cercosporin concentrations, we conclude that the purple symptom is a disease resistance reaction, mediated in part by COU, which provides a high level of antioxidant activity and, hence, partial resistance.

Bioactivity of natural O-prenylated phenylpropenes from Illicium anisatum leaves and their derivatives against spider mites and fungal pathogens

A variety of volatile phenylpropenes, C6-C3 compounds are widely distributed in the plant kingdom, whereas prenylated phenylpropenes are limited to a few plant species. In this study, we analysed the volatile profiles from Illicium anisatum leaves and identified two O-prenylated phenylpropenes, 4-allyl-2-methoxy-1-[(3-methylbut-2-en-1-yl)oxy]benzene [O-dimethylallyleugenol (9)] and 5-allyl-1,3-dimethoxy-2-(3-methylbut-2-en-1-yl)oxy]benzene [O-dimethylallyl-6-methoxyeugenol (11)] as major constituents. The structure-activity relationship of a series of eugenol derivatives showed that specific phenylpropenes, including eugenol (1), isoeugenol (2) and 6-methoxyeugenol (6), with a phenolic hydroxy group had antifungal activity for a fungal pathogen, whereas guaiacol, a simple phenolic compound, and allylbenzene had no such activity. The eugenol derivatives that exhibited antifungal activity, in turn, had no significant toxicant property for mite oviposition. Interestingly, O-dimethylallyleugenol (9) in which the phenolic oxygen was masked with a dimethylallyl group exhibited a specific, potent oviposition deterrent activity for mites. The sharp contrast in structural requirements of phenylpropenes suggested distinct mechanisms underlying the two biological activities and the importance of a phenolic hydroxy group and its dimethylallylation for the structure-based design of new functional properties of phenylpropenes.

Prenylated isoflavonoids from plants as selective estrogen receptor modulators (phytoSERMs)

Isoflavonoids are a class of secondary metabolites, which comprise amongst others the subclasses of isoflavones, isoflavans, pterocarpans and coumestans. Isoflavonoids are abundant in Leguminosae, and many of them can bind to the human estrogen receptor (hER) with affinities similar to or lower than that of estradiol. Dietary intake of these so-called phytoestrogens has been associated with positive effects on menopausal complaints, hormone-related cancers, and osteoporosis. Therefore, phytoestrogens are used as nutraceuticals in functional foods or food supplements. Most of the isoflavonoids show agonistic activity towards both hERα and hERβ, the extent of which is modulated by the substitution pattern of their skeleton (i.e.-OH, -OCH(3)). Interestingly, substitutions consisting of a five-carbon prenyl group often seem to result in an antiestrogenic activity. There is growing evidence that the action of some of these prenylated isoflavonoids is tissue-specific, suggesting that they act like selective estrogen receptor modulators (SERMs), such as the well-known chemically synthesized raloxifene and tamoxifen. These so-called phytoSERMS might have high potential for realizing new food and pharma applications. In this review, the structural features of isoflavonoids (i.e. the kind of skeleton and prenylation (e.g. chain or pyran), position of the prenyl group on the skeleton, and the extent of prenylation (single, double)) are discussed in relation to their estrogenic activity. Anti-estrogenic and SERM activity of isoflavonoids was always associated with prenylation, but these activities did not seem to be confined to one particular kind/position of prenylation or isoflavonoid subclass. Few estrogens with agonistic activity were prenylated, but these were not tested for antagonistic activity; possibly, these molecules will turn out to be phytoSERMs as well. Furthermore, the data on the dietary occurrence, bioavailability and metabolism of prenylated isoflavonoids are discussed.

Identification of prenylated pterocarpans and other isoflavonoids in Rhizopus spp. elicited soya bean seedlings by electrospray ionisation mass spectrometry

Phytoalexins from soya are mainly characterised as prenylated pterocarpans, the glyceollins. Extracts of non-soaked and soaked soya beans, as well as that of soya seedlings, grown in the presence of Rhizopus microsporus var. oryzae, were screened for the presence of prenylated flavonoids with a liquid chromatography/mass spectrometry (LC/MS)-based screening method. The glyceollins I-III and glyceollidins I-II, belonging to the isoflavonoid subclass of the pterocarpans, were tentatively assigned. The formation of these prenylated pterocarpans was accompanied by that of other prenylated isoflavonoids of the subclasses of the isoflavones and the coumestans. It was estimated that approx. 40% of the total isoflavonoid content in Rhizopus-challenged soya bean seedlings were prenylated pterocarpans, whereas 7% comprised prenylated isoflavones and prenylated coumestans. The site of prenylation (A-ring or B-ring) of the prenylated isoflavones was tentatively annotated using positive-ion mode MS by comparing the (1,3) A(+) retro-Diels-Alder (RDA) fragments of prenylated and non-prenylated isoflavones. Furthermore, the fragmentation pathways of the five pterocarpans in negative-ion (NI) mode were proposed, which involved the cleavage of the C-ring and/or D-ring. The absence of the ring-closed prenyl (pyran or furan) gave exclusively -H(2) O(x,y) RDA fragments, whereas its presence gave predominantly the common RDA fragments.

Glyceollin I, a novel antiestrogenic phytoalexin isolated from activated soy

Glyceollins, a group of novel phytoalexins isolated from activated soy, have recently been demonstrated to be novel antiestrogens that bind to the estrogen receptor (ER) and inhibit estrogen-induced tumor progression. Our previous publications have focused specifically on inhibition of tumor formation and growth by the glyceollin mixture, which contains three glyceollin isomers (I, II, and III). Here, we show the glyceollin mixture is also effective as a potential antiestrogenic, therapeutic agent that prevents estrogen-stimulated tumorigenesis and displays a differential pattern of gene expression from tamoxifen. By isolating the individual glyceollin isomers (I, II, and III), we have identified the active antiestrogenic component by using competition binding assays with human ERalpha and in an estrogen-responsive element-based luciferase reporter assay. We identified glyceollin I as the active component of the combined glyceollin mixture. Ligand-receptor modeling (docking) of glyceollin I, II, and III within the ERalpha ligand binding cavity demonstrates a unique type II antiestrogenic confirmation adopted by glyceollin I but not isomers II and III. We further compared the effects of glyceollin I to the antiestrogens, 4-hydroxytamoxifen and ICI 182,780 (fulvestrant), in MCF-7 breast cancer cells and BG-1 ovarian cancer cells on 17beta-estradiol-stimulated expression of progesterone receptor and stromal derived factor-1alpha. Our results establish a novel inhibition of ER-mediated gene expression and cell proliferation/survival. Glyceollin I may represent an important component of a phytoalexin-enriched food (activated) diet in terms of chemoprevention as well as a novel therapeutic agent for hormone-dependent tumors.

Total syntheses of racemic and natural glycinol

Total syntheses of racemic and (-)-glycinol (1) are described. A Wittig reaction produced the isoflav-3-ene from which a Sharpless dihydroxylation introduced either the racemic or enantiomeric 6a-hydroxy group. A 5.5% overall yield of racemic material was obtained after 12 steps. A method was devised for a one-pot switch of protecting groups masking a sensitive resorcinolic para-functionality, and conditions were optimized to prompt spontaneous closure of the pterocarpanolic dihydrofuran upon subsequent exposure of its ortho-functionality. These improvements eliminated two steps and increased the overall yield to 9.8% during production of the natural enantiomer.

Identification of the potent phytoestrogen glycinol in elicited soybean (Glycine max)

The primary induced isoflavones in soybean, the glyceollins, have been shown to be potent estrogen antagonists in vitro and in vivo. The discovery of the glyceollins' ability to inhibit cancer cell proliferation has led to the analysis of estrogenic activities of other induced isoflavones. In this study, we investigated a novel isoflavone, glycinol, a precursor to glyceollin that is produced in elicited soy. Sensitive and specific in vitro bioassays were used to determine that glycinol exhibits potent estrogenic activity. Estrogen-based reporter assays were performed, and glycinol displayed a marked estrogenic effect on estrogen receptor (ER) signaling between 1 and 10 microM, which correlated with comparable colony formation of MCF-7 cells at 10 microM. Glycinol also induced the expression of estrogen-responsive genes (progesterone receptor and stromal-cell-derived factor-1). Competitive binding assays revealed a high affinity of glycinol for both ER alpha (IC(50) = 13.8 nM) and ER beta (IC(50) = 9.1 nM). In addition, ligand receptor modeling (docking) studies were performed and glycinol was shown to bind similarly to both ER alpha and ER beta. Taken together, these results suggest for the first time that glycinol is estrogenic and may represent an important component of the health effects of soy-based foods.

Molecular cloning and characterization of a cDNA for pterocarpan 4-dimethylallyltransferase catalyzing the key prenylation step in the biosynthesis of glyceollin, a soybean phytoalexin

Glyceollins are soybean (Glycine max) phytoalexins possessing pterocarpanoid skeletons with cyclic ether decoration originating from a C5 prenyl moiety. Enzymes involved in glyceollin biosynthesis have been thoroughly characterized during the early era of modern plant biochemistry, and many genes encoding enzymes of isoflavonoid biosynthesis have been cloned, but some genes for later biosynthetic steps are still unidentified. In particular, the prenyltransferase responsible for the addition of the dimethylallyl chain to pterocarpan has drawn a large amount of attention from many researchers due to the crucial coupling process of the polyphenol core and isoprenoid moiety. This study narrowed down the candidate genes to three soybean expressed sequence tag sequences homologous to genes encoding homogentisate phytyltransferase of the tocopherol biosynthetic pathway and identified among them a cDNA encoding dimethylallyl diphosphate: (6aS, 11aS)-3,9,6a-trihydroxypterocarpan [(-)-glycinol] 4-dimethylallyltransferase (G4DT) yielding the direct precursor of glyceollin I. The full-length cDNA encoding a protein led by a plastid targeting signal sequence was isolated from young soybean seedlings, and the catalytic function of the gene product was verified using recombinant yeast microsomes. Expression of the G4DT gene was strongly up-regulated in 5 to 24 h after elicitation of phytoalexin biosynthesis in cultured soybean cells similarly to genes associated with isoflavonoid pathway. The prenyl part of glyceollin I was demonstrated to originate from the methylerythritol pathway by a tracer experiment using [1-(13)C]Glc and nuclear magnetic resonance measurement, which coincided with the presumed plastid localization of G4DT. The first identification of a pterocarpan-specific prenyltransferase provides new insights into plant secondary metabolism and in particular those reactions involved in the disease resistance mechanism of soybean as the penultimate gene of glyceollin biosynthesis.

Nitric oxide synthase-mediated phytoalexin accumulation in soybean cotyledons in response to the Diaporthe phaseolorum f. sp. meridionalis elicitor

Phytoalexin biosynthesis is part of the defense mechanism of soybean (Glycine max) plants against attack by the fungus Diaporthe phaseolorum f. sp. meridionalis (Dpm), the causal agent of stem canker disease. The treatment of soybean cotyledons with Dpm elicitor or with sodium nitroprusside (SNP), a nitric oxide (NO) donor, resulted in a high accumulation of phytoalexins. This response did not occur when SNP was replaced by ferricyanide, a structural analog of SNP devoid of the NO moiety. Phytoalexin accumulation induced by the fungal elicitor, but not by SNP, was prevented when cotyledons were pretreated with NO synthase (NOS) inhibitors. The Dpm elicitor also induced NOS activity in soybean tissues proximal to the site of inoculation. The induced NOS activity was Ca(2+)- and NADPH-dependent and was sensitive to the NOS inhibitors N(G)-nitro-L-arginine methyl ester, aminoguanidine, and L-N(6)-(iminoethyl) lysine. NOS activity was not observed in SNP-elicited tissues. An antibody to brain NOS labeled a 166-kD protein in elicited and nonelicited cotyledons. Isoflavones (daidzein and genistein), pterocarpans (glyceollins), and flavones (apigenin and luteolin) were identified after exposure to the elicitor or SNP, although the accumulation of glyceollins and apigenin was limited in SNP-elicited compared with fungal-elicited cotyledons. NOS activity preceded the accumulation of these flavonoids in tissues treated with the Dpm elicitor. The accumulation of these metabolites was faster in SNP-elicited than in fungal-elicited cotyledons. We conclude that the response of soybean cotyledons to Dpm elicitor involves NO formation via a constitutive NOS-like enzyme that triggers the biosynthesis of antimicrobial flavonoids.

Intracellular localization of prenyltransferases of isoflavonoid phytoalexin biosynthesis in bean and soybean

The intracellular localization of prenyltransferases involved in the biosynthesis of the phytoalexins glyceollin in soybean (Glycine max L.) and phaseollin in French bean (Phaseolus vulgaris L.) has been investigated. By sucrose- and Percoll-gradient centrifugation of microsomes of an elicitor-challenged soybean cell culture, the membranes containing prenyltransferase were separated from the endoplasmic reticulum and shown to be lighter in density. In a continuous Percoll gradient the peak of prenyltransferase activity coincided with the peak of galactolipid synthesis, as determined by incorporation of uridine 5'-diphospho-[(14)C]galactose (UDP-[(14)C]galactose). Intact chloroplasts isolated from cupricchloride-treated bean leaves contained both prenyltransferase and UDP-galactose transferase activity. Both activities increased during chloroplast isolation. Fractionation of swollen chloroplasts on a discontinuous sucrose gradient showed prenyltransferase and UDP-galactose transferase activity in the envelope membrane subfraction. It is concluded that in both plants prenyltransferase is located in the envelope membrane of plastids.

Induction of phytoalexin synthesis in soybean: enzymatic cyclization of prenylated pterocarpans to glyceollin isomers

A microsome preparation from elicitor-challenged soybean cell suspension cultures catalyzed an NADPH-dependent and oxygen-dependent cyclization of a mixture of 2- and 4-dimethylallylglycinols to the glyceollin isomers I-III. This is the last committed step in glyceollin biosynthesis. The cyclase was inhibited in a light-reversible manner by carbon monoxide in the presence of oxygen. Cyclase was also inhibited by cytochrome c, NADP+, and a number of inhibitors of cytochrome P-450 enzymes. NADH in the presence of low concentrations of NADPH had a synergistic effect. On a Percoll gradient, the position of cyclase coincided with marker enzymes for the endoplasmic reticulum. These properties identify the cyclase as a cytochrome P-450-dependent monooxygenase. Unstimulated soybean cell culture did not contain detectable cyclase activity. Challenge with either a glucan elicitor from Phytophthora megasperma f.sp. glycinea or with yeast extract caused strong stimulation of cyclase activity with a maximum at about 24 h after elicitor addition.

Differential induction of enzyme in soybean cell cultures by elicitor or osmotic stress

Soybean cell cultures were challenged either by glucan elicitor from Phytophthora megasperma f.sp. glycinea or by osmotic stress (0.4 M glucose). Osmotic stress induced production of a microsomal NADPH-dependent flavone synthase (flavone synthase II) which catalyses conversion of (2S)-naringenin to apigenin. In one of our cell-lines this enzyme activity was not detected either in unchallenged cells or in cells treated with glucan elicitor. Inducibility of flavone synthase II by 0.4 M glucose was highest at the end of the linear growth phase. Changes in the activities of a number of other enzymes were determined after treatment of the cells with elicitor or 0.4 M glucose. The activities of phenylalanine ammonialyase, cinnamate 4-hydroxylase, chalcone synthase and dihydroxypterocarpan 6a-hydroxylase all increased with elicitor and with osmoticum, albeit to a different degree. The rise in enzyme activity occurred later with osmoticum than with elicitor. The prenyltransferase involved in glyceollin synthesis was induced strongly by elicitor but only very weakly by osmoticum, whereas isoflavone synthase and NADPH: cytochrome-c reductase were only induced by elicitor. The activity of glucose-6-phosphate dehydrogenase did not change with elicitor or with osmoticum. Different product patterns were also obtained: whereas with elicitor, glyceollin I was the major product, intermediates of the glyceollin pathway (7,4'-dihydroxyflavanone, trihydroxypterocarpan) accumulated with osmoticum.

Induction of phytoalexin synthesis in soybean. Stereospecific 3,9-dihydroxypterocarpan 6a-hydroxylase from elicitor-induced soybean cell cultures

A microsomal preparation from elicitor-challenged soybean cell suspension cultures catalyzes an NADPH-dependent and dioxygen-dependent 6a-hydroxylation of 3,9-dihydroxypterocarpan to 3,6a,9-trihydroxypterocarpan. The latter is a precursor for the soybean phytoalexin glyceollin. No reaction is observed with NADH. The 6a-hydroxylase is inhibited by cytochrome c. Optical rotatory dispersion spectra of the enzymatic product formed from racemic dihydroxypterocarpan and of the remaining unreacted substrate proved that the product has the natural (6aS, 11aS)-configuration and that hydroxylation proceeds with retention of configuration. The 6a-hydroxylase was also found in elicitor-challenged soybean seedlings. The results indicate that the 6a-hydroxylase is specifically involved in the biosynthesis of glyceollin.

Induction of enzymes of phytoalexin synthesis in cultured soybean cells by an elicitor from Phytophthora megasperma f. sp. glycinea

The glucan elicitor from cell walls of the fungal pathogen, Phytophthora megasperma f. sp. glycinea, induced rapid but transient increases in enzyme activities of general phenylpropanoid metabolism (phenylalanine ammonia-lyase and 4-coumarate: CoA ligase) and of the flavonoid pathway (chalcone synthase) in cell suspension cultures of soybean (Glycine max). After transferring cells into fresh medium, two peaks of inducibility for the enzymes by elicitor were observed, one shortly after transfer (stage I), and one at the end of the linear growth phase (stage II). Only one of the two isoenzymes of 4-coumarate: CoA ligase ("isoenzyme 2"), for which a specific involvement in flavonoid biosynthesis has been postulated, was affected by the elicitor. For two of the induced enzymes, phenylalanine ammonia-lyase and chalcone synthase, the changes in activity at stage I were shown to be preceded by large changes in their rates of synthesis, as determined by in vivo labelling with [(35)S] methionine and immunoprecipitation.