Pterocarpan phytoalexins

In some species, such as C. arietinum and M. saliva, the products of VR and DMID (maackiain and medicarpin) are the main pterocarpan phytoalexins. They are typically glucosylated and malonylated and stored in the vacuole. In species such as G. max, P. sativum, and P. vulgaris, the pterocarpans are further converted by a series of reactions to species-specific compounds. For G. max and P. sativum, the initial reaction is a hydro-xylation catalyzed by P6aH. 

Tiemann, K. et al., Pterocarpan phytoalexin biosynthesis in elicitor-challenged chickpea (Cicer arietinum L.) cell cultures. Purification, characterization and cDNA cloning of NADPHnsofiavone oxidoreductase. Eur. J. Biochem., 200, 751, 1991. 

Hinderer W, Flentje U, Barz W. 1987. Microsomal isoflavone 2- and 3-hydroxylases from chickpea (Cicer arietinum L.) cell suspensions induced for pterocarpan phytoalexin formation. FEBS Lett 214 101-106. 

A feature of some pterocarpan phytoalexins (e.g., pisatin and glyceollin of pea and soybean, respectively) is their hydroxylation at position 6a, a reaction catalyzed by a microsomal cytochrome P450 monooxygenase.49 50 A cDNA encoding this enzyme was recently characterized from elicited soybean cell cultures.51 The microsomal protein, expressed in yeast cells, catalyzed the stereoselective hydroxylation of (6a/ , lla/ )-3,9-dihydroxypterocarpan to its 6a-hydroxy derivative. It was also demonstrated that the enzyme expression is regulated at the transcriptional level.51... 

GUNIA, W., HINDERER, W., WITTKAMPF, U., BARZ, W Elicitor induction of cytochrome P450 monooxygenases in cell suspension cultures of chickpea and their involvement in pterocarpan phytoalexin biosynthesis. Z Natwforsch, 1991, 36c, 58-66. 

Flavonoids stimulate spore germination in the soilborne phytopathogenic fungus Fusarium solani. Spore germination of F. solani f. sp. pisi, which causes disease in pea, is stimulated by flavanone hesperetin (141), flavone apigenin (101), the pterocarpan phytoalexin pisatin (142), and so on. In contrast, germination of the bean pathogen F. solani f. sp. phaseoli is stimulated by the pterocarpans maackiain (143) and medicarpin (129), and isoflavones biochanin A (144), and so on but not by pisatin.85... 

Alternatively, chalcone reductase (CHR also known as deoxychalcone synthase) together with chalcone synthase and NADPH as a cofactor act in the formation of isoliquiritigenin, which is then isomerized, again by the enzyme chalcone isomerase, to form liquiritigenin, the precursor to daidzein, and the pterocarpan phytoalexins. A type II chalcone isomerase that seems to be found exclusively in the legumes catalyzes this isomerization reaction. Glycitein synthesis is not yet clearly defined, but is likely derived from liquiritigenin via flavonoid 6-hydroxylase, and an unidentified methyltransferase. 

BARZ, W., WELLE, R., Biosynthesis and metabolism of isoflavones and pterocarpan phytoalexins in chiekpea, soybean and phytopathogenic fungi, in ... 

Barz, W. and R. Welle, Biosynthesis and metabolism of isofla-vones and pterocarpan phytoalexins in chickpea, soybean, and phytopathogenic fungi, in Phenolic Metabolism in Plants (H. A. Stafford and R. K. Ibrahim, eds.), Recent Advances in Phytochemistry Vol. 26, 139-164, Plenum Press, New York, 1992. 

The formation of phytoalexins such as glyceollins and phaseollins requires C-prenylation by a range of pterocarpan prenyltransferase (PTP) activities, with dimethylallyl pyrophosphate (DMAPP) as the prenyl donor. For glyceollins and phaseollins, prenylation occurs at position C-2 or C-4 of glycinol or C-10 of 3,9-dihydroxypterocarpan. ° ° However, there are differing activities in other species. For example, in Lupinus albus (white lupin) a prenyltransferase acting at the C-6, -8, and -3 positions of isoflavones has been identified.PTPs have also been characterized in detail for the formation of prenylated flavanones in Sophora flavescens (see, e.g., Ref. 207). However, no cDNA clones for flavonoid-related prenyltransferases have been published to date. 

Welle, R. and Grisebach, H., Induction of phytoalexin synthesis in soybean enzymatic cyclization of prenylated pterocarpans to glyceollin isomers. Arch. Biochem. Biophys., 263, 191, 1988. 

FIGURE 8.6 Structure of the phytoalexin isoflavonoid pterocarpans, maackianin, and pisatin from garden pea, and the isoflavones daidzein and genistein from soybean. 

The biosynthesis of pterocarpans, which include well-known phytoalexins such as medicarpin in alfalfa (Medicago sativa) [Baker et al., 1989 Higgins, 1972 Oommen et al., 1994 He and Dixon, 2000] (Fig. 21.2), pisatin in pea... 

Akashi T, Koshimizu S, Aoki T, Ayabe S-I. 2006. Identification of cDNAs encoding pterocarpan reductase involved in isoflavan phytoalexins biosynthesis in Lotus japonicus by EST mining. FEBS Lett 580 5666-5670. 

The pterocarpan pisatin was first isolated and characterized as a phytoalexin in Pi sum sativum in the early 1960s.234 235 Since then, structurally diverse phytoalexins have been reported from more than 30 families of higher plants. Much progress is also being made in studies on phytoalexin biosynthesis based on genomic information. 

As shown in the following scheme, a SEM-ether derived phenyl acetylene was treated with PtClj in the presence of CO to give a benzo[4>]furan, which was first subjected to desilylation, followed by Pd-catalyzed intramolecular etherification, affording the tetracyclic skeleton of pterocarpane family of phytoalexins <05JA15024>. A similar type of synthetic transformation was so applied to make a key intermediate in the total synthesis of vibsanol <05JA15022>. 2,3-Disubstituted benzo[h]furans were prepared under very mild reaction conditions by the Pd/Cu-catalyzed cross-coupling of various o-iodoanisoles and terminal... 

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