Acifluorfen-Induced Isoflavonoids and Enzymes of Their Biosynthesis in Mature Soybean Leaves

Abstract

Mature soybean (G. max L. cv Harosoy 63) leaves normally contain kaempferol-3-glycosides but they accumulate no other flavonoids. Whole leaves sprayed with the diphenyl ether herbicide Acifluorfen and maintained in the light developed small necrotic lesions and accumulated isoflavone aglycones, isoflavone glucosides, and pterocarpans. Isoflavonoid accumulation was preceded by induced activity for chalcone synthase (CHS) and by increased activity for phenylalanine ammonia-lyase (PAL) and UDP-glucoseisoflavone 7-O-glucosyl transferase (IGT). PAL and CHS activity was highest between 24 and 30 h after treatment, isoflavone aglycones and pterocarpans at 48 h, IGT at 72 h and isoflavone glucosides at 96 h. Mesophyll cells isolated from control leaves contained no activity for PAL, CHS, or IGT and no flavonoids of any class. Cells isolated from treated leaves at the stage of maximal enzyme activity or isoflavonoid content contained PAL (12% of the whole leaf activity), CHS (24%), IGT (20%), and 25% of the whole leaf isoflavone glucosides, but only traces, presumably as contaminants, of the other flavonoids. The isoflavone glucosides were probably synthesized and accumulated in intact mesophyll cells as soluble detoxification products, while the isoflavone aglycones and pterocarpans accumulated in the epidermis or extracellularly within the mesophyll. This is the 1st report of tissue-specific induction of isoflavonoid glucosides and key enzymes of their biosynthesis in any plant.