Identification and accumulation of isoflavonoids and isoflavone glucosides in soybean leaves and hypocotyls in resistance responses to Phytophthora megasperma f.sp. glycinea

Abstract

Levels of isoflavones and isoflavone glucosides in young soybean leaves, susceptible to Phytophthora megasperma f.sp. glycinea, were low (total amounts < 100 μg g −1 fresh weight). Similar low levels were found in leaves with race-specific resistance and in older leaves with non-specific age-related resistance. Therefore neither form of resistance is due to a constitutive accumulation of these compounds. However, several of these compounds accumulated in much higher concentrations in leaves and in hypocotyls following inoculation. In leaves daidzin, malonyl-daidzin, glycitein-7- O-β-glucoside, genistin, malonyl-genistin and the glyceollins accumulated in the border zones surrounding necrode lesions, but only the glyceollins were found in abundance in the lesions themselves. Although the glyceollins were the most abundant group of isoflavonoids, the combined mass of the simple isoflavones and glucosides exceeded that of the glyceollins in the border zones 36 h after inoculation. Thus, the general defence response in soybeans involves the induction of several isoflavone biosynthetic pathways, of which only one leads to the glyceollins. In in vitro bioassays, daidzein, formononetin and genistein did not significantly inhibit growth of P. megasperma f.sp. glycinea at 200 μg ml −1. Although the possibility that the daidzein glucosides eventually serve as secondary precursors for glyceollin biosynthesis was not ruled out, no evidence for this was obtained. Addition of daidzin to hypocotyl lesions caused a dramatic increase in the level of daidzein but not of the glyceollins.