Metabolic Regulation during Glyceollin Biosynthesis in Green Soybean Hypocotyls

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Abstract
The accumulation of the isoflavonoid phytoalexin, glyceollin, occurs in hypocotyls of green soybean seedlings in response to the injection of a glucan elicitor isolated from the mycelial walls of the fungas, Phytophthora megasperma f. sp. glycinea. This accumulation, which levels off after 24 h, is preceded by a dramatic, transient rise in extractable activities of 2 early enzymes in the biosynthetic pathway, phenylalanine ammonia-lyase (PAL) and p-coumaryl CoA ligase (pCL). The maximum amount of extractable activity occurs 12-16 h after elicitor treatment and is coincident with the most rapid period of glyceolin accumulation. A regulartory role for these early enzymes in the biosynthesis of this secondary metabolite is suggested. High performance liquid chromatography analysis of the early intermediates in the pathway further corroborates this hypothesis. The relative pool size and rate of turnover of p-coumaric acid, an early intermediate in glyceollin production, increase during the period of rapid increases in enzyme activities. Removal of cotyledons from elicitor-treated seedlings reduces glyceollin accumulation .apprx. 70%. This limitation of phytoalexin accumulation by cotyledon removal is correlated with a similar cotyledon effect on reduction of extractable activities of both PAL and pCL, as well as a decrease in the flux of C through the p-coumaric acid pool. The hypothesis is supported that early enzymic steps in a biosynthetic pathway diverting C from primary to secondary metabolites function as regulatory control points.