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.