Phytoalexin Production as a Chemosystematic Parameter Within the Genus Glycine

Asymmetric one-pot transformation of isoflavones to pterocarpans and its application in phytoalexin synthesis

Phytoalexins have attracted much attention due to their health-promoting effects and their vital role in plant health during the last years. Especially the 6a-hydroxypterocarpans glyceollin I and glyceollin II, which may be isolated from stressed soy plants, possess a broad spectrum of bioactivities such as anticancer activity and beneficial contributions against western diseases by anti-oxidative and anti-cholesterolemic effects. Aiming for a catalytic asymmetric access to these natural products, we establish the asymmetric syntheses of the natural isoflavonoids (−)-variabilin, (−)-homopterocarpin, (−)-medicarpin, (−)-3,9-dihydroxypterocarpan, and (−)-vestitol by means of an asymmetric transfer hydrogenation (ATH) reaction. We successfully adapt this pathway to the first catalytic asymmetric total synthesis of (−)-glyceollin I and (−)-glyceollin II. This eight-step synthesis features an efficient one-pot transformation of a 2′-hydroxyl-substituted isoflavone to a virtually enantiopure pterocarpan by means of an ATH and a regioselective benzylic oxidation under aerobic conditions to afford the susceptible 6a-hydroxypterocarpan skeleton.

Concise total syntheses of 6a-hydroxypterocarpans are sought after due to their broad spectrum of bioactivities. Here, the authors report the asymmetric syntheses of several natural isoflavonoids, including (−)-glyceollin I and (−)-glyceollin II, by means of an asymmetric transfer hydrogenation (ATH) reaction.

Flavonoids from three Wild Glycine Species in Japan and Taiwan

Fourteen flavonols, four flavones and six isoflavones were isolated from the aerial parts of two Japanese Glycine species, G. tabacina and G. koidzumii, and the leaves of Taiwanese G. max subsp. formosana. Of their flavonoids, twelve flavonols were identified as kaempferol 3- O-sophoroside (1), 3- O-rutinoside (2), 3- O-robinobioside (3) and 3- O-rhamnosyl-(1→4)-[rhamnosyl-(1→6)-galactoside] (4), quercetin 3- O-gentiobioside (5), 3- O-glucoside (6), 3- O-galactoside (7), 3- O-rutinoside (8), 3- O-robinobioside (9) and 3- O-rhamnosyl-(1→4)-[rhamnosyl-(1→6)-galactoside] (10), and isorhamnetin 3- O-rutinoside (11) and 3- O-robinobioside (12). Other two flavonols were characterized as isorhamnetin 3- O-rhamnosylrhamnosylglucoside (13) and 3- O-rhamnosylrhamnosylgalactoside (14). Four flavones and six isoflavones were estimated as schaftoside (15), apigenin 6,8-di- C-arabinoside (16), luteolin 7- O-glucoside (17) and chrysoeriol 7- O-glucoside (18), and daidzein 7- O-glucoside (19), 4'- O-glucoside (20) and 7- O-xylosylglucoside (21), genistein 7- O-glucoside (22) and 4'- O-glucoside (23), and 3'- O-methylorobol 7- O-glucoside (24). Although flavonoid composition of G. tabacina and G. koidzumii was similar to each other, that of G. max subsp. formosana was different with those of two Japanese Glycine species described above. Flavonoids of their Glycine species were reported for the first time except for those of G. tabacina.

Molecular Characterization of Soybean Pterocarpan 2-Dimethylallyltransferase in Glyceollin Biosynthesis: Local Gene and Whole-Genome Duplications of Prenyltransferase Genes Led to the Structural Diversity of Soybean Prenylated Isoflavonoids

Soybean (Glycine max) accumulates several prenylated isoflavonoid phytoalexins, collectively referred to as glyceollins. Glyceollins (I, II, III, IV and V) possess modified pterocarpan skeletons with C5 moieties from dimethylallyl diphosphate, and they are commonly produced from (6aS, 11aS)-3,9,6a-trihydroxypterocarpan [(-)-glycinol]. The metabolic fate of (-)-glycinol is determined by the enzymatic introduction of a dimethylallyl group into C-4 or C-2, which is reportedly catalyzed by regiospecific prenyltransferases (PTs). 4-Dimethylallyl (-)-glycinol and 2-dimethylallyl (-)-glycinol are precursors of glyceollin I and other glyceollins, respectively. Although multiple genes encoding (-)-glycinol biosynthetic enzymes have been identified, those involved in the later steps of glyceollin formation mostly remain unidentified, except for (-)-glycinol 4-dimethylallyltransferase (G4DT), which is involved in glyceollin I biosynthesis. In this study, we identified four genes that encode isoflavonoid PTs, including (-)-glycinol 2-dimethylallyltransferase (G2DT), using homology-based in silico screening and biochemical characterization in yeast expression systems. Transcript analyses illustrated that changes in G2DT gene expression were correlated with the induction of glyceollins II, III, IV and V in elicitor-treated soybean cells and leaves, suggesting its involvement in glyceollin biosynthesis. Moreover, the genomic signatures of these PT genes revealed that G4DT and G2DT are paralogs derived from whole-genome duplications of the soybean genome, whereas other PT genes [isoflavone dimethylallyltransferase 1 (IDT1) and IDT2] were derived via local gene duplication on soybean chromosome 11.

Increasing soy isoflavonoid content and diversity by simultaneous malting and challenging by a fungus to modulate estrogenicity

Soybeans were germinated on a kilogram-scale, by the application of malting technology used in the brewing industry, and concomitantly challenged with Rhizopus microsporus var. oryzae. In a time-course experiment, samples were taken every 24 h for 10 days, and the isoflavonoid profile was analyzed by RP-UHPLC-MS. Upon induction with R. microsporus, the isoflavonoid composition changed drastically with the formation of phytoalexins belonging to the subclasses of the pterocarpans and coumestans and by prenylation of the various isoflavonoids. The pterocarpan content stabilized at 2.24 mg of daidzein equivalents (DE) per g after ∼9 days. The levels of the less common glyceofuran, glyceollin IV, and V/VI ranged from 0.18 to 0.35 mg DE/g and were comparable to those of the more commonly reported glyceollins I, II, and III (0.22-0.32 mg DE/g) and glycinol (0.42 mg DE/g). The content of prenylated isoflavones after the induction process was 0.30 mg DE/g. The total isoflavonoid content increased by a factor of 10-12 on DW basis after 9 days, which was suggested to be ascribable to de novo synthesis. These changes were accompanied by a gradual increase in agonistic activity of the extracts toward both the estrogen receptor α (ERα) and ERβ during the 10-day induction, with a more pronounced activity toward ERβ. Thus, the induction process yielded a completely different spectrum of isoflavonoids, with a much higher bioactivity toward the estrogen receptors. This, together with the over 10-fold increase in potential bioactives, offers promising perspectives for producing more, novel, and higher potency nutraceuticals by malting under stressed conditions.

A single nuclear locus phylogeny of soybean based on DNA sequence

Soybean [Glycine max (L.) Merr.] evolution was examined by sequencing portions of the restriction fragment length polymorphism (RFLP) locus A-199a of 21 taxa from the Glycininae and 1 from the Phaseoleae. Four hundred nucleotides were determined in each, aligned, and then compared for these taxa. Within the annual soybean subgenus (Soja), the four accessions differed at as many as 2.2% of the nucleotides. Among 13 perennial soybean species (subgenus Glycine), nucleotide variation ranged from 1.7% to 8.4%. The nucleotide difference between the two soybean subgenera was 3.0-7.0%. Nucleotide variation between the genus Glycine and the related genera of Neonotonia, Amphicarpa, Teramnus, and Phaseolus ranged from 8.2% to 16.4%. In addition to nucleotide substitutions, insertions/deletions (indels) differences were also observed and were consistent with nucleotide-based analysis. Cladistic analysis of the A-199a sequences was performed using Wagner parsimony to construct a soybean phylogeny. Sixteen equally parsimonious trees were produced from these data. The trees were 246 steps in length with a consistency index of 0.78. Indels distribution upon the consensus topology revealed a pattern congruent with the nucleotide-based phylogeny. The current taxonomic status of the soybean subgenera and the related genera of Neonotonia, Amphicarpa, and Teramnus were well-supported and appear monophyletic in this analysis. Homoplasy within the subgenus Glycine led to a lack of resolved topology for many of these 13 taxa. However, the Glycine clade topology was consistent with phylogenies proposed using crossing experiments and cpDNA RFLPs. These genera were arranged from ancestral to derived as: Teramnus, Amphicarpa, Neonotonia, and Glycine when Phaseolus vulgaris was used as an outgroup.

Genomic relationships among diploid wild perennial species of the genus Glycine Willd. subgenus Glycine revealed by crossability, meiotic chromosome pairing and seed protein electrophoresis

The nomenclature of species beased on classical taxonomy can be verified from cytogenetic, biochemical and molecular studies. The objective of the study presented here was to provide further information on genomic affinities among species of the genus Glycine Willd. based on crossability, meiotic chromosome pairing of F1 hybrids and seed-protein profiles. Meiotic chromosome pairing data revealed no genomic similarity between G. microphylla (BB) and G. falcata (FF), nor between G. tomentella (2n = 38; EE) and G. microphylla (BB). Despite morphological similarity between G. cyrtoloba (CC) and G. curvata no F1 hybrid was obtained, although 748 flowers were pollinated. The seed-protein banding patterns showed G. latrobeana to be closer to the A-genome species than to others. Based on these results we assign genome symbol A3A3 to G. latrobeana. Likewise, G. curvata was allotted the designation C1C1 because the seed-protein banding patterns of G. curvata and G. cyrtoloba are similar. The genome designations of Glycine species based on cytogenetic investigations may be further extended by results obtained from biochemical and molecular approaches.

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.