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Published on December 17, 2008; 10.1104/pp.108.123679

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Received May 29, 2008
Accepted December 3, 2008

Molecular Cloning and Characterization of a cDNA for Pterocarpan 4-Dimethylallyltransferase Catalyzing the Key Prenylation Step in the Biosynthesis of Glyceollin, a Soybean Phytoalexin

Tomoyoshi Akashi , Kanako Sasaki , Toshio Aoki , Shin-ichi Ayabe , and Kazufumi Yazaki *

Department of Applied Biological Sciences, Nihon University, Fujisawa, Kanagawa 252-8501, Japan; and Laboratory of Plant Gene Expression, Research Institute for Sustainable Humanosphere, Kyoto University, Gokasho, Uji 611-0011, Japan

* Corresponding author; email: yazaki{at}rish.kyoto-u.ac.jp.

Glyceollins are soybean 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 EST 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 5-24 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 [1-13C]glucose and NMR 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.






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