Biosynthesis and biodegradation of glyceollin by soybean hypocotyls infected with Phytophthora megasperma var. sojae

Abstract

The soybean phytoalexin glyceollin accumulated more rapidly in Harosoy 63 soybean hypocotyls infected with an incompatible race of Phytophthora megasperma var. sojae than with a compatible race of the fungus. However, when the rate of glyceollin biosynthesis was estimated by pulse-labelling experiments with [ 14C]phenylalanine, no significant differences were observed with hypocotyls infected with the two races of the fungus at any stage of infection. In contrast, “pulse-chase” experiments with [ 14C]phenylalanine revealed that glyceollin biodegrading activity, which was normally present in the uninoculated hypocotyls, appeared to be inhibited more strongly in the hypocotyls infected with the incompatible race than in the hypocotyls infected with the compatible race. The fungus did not appear to contribute directly to the differential glyceollin degrading activity in the infected hypocotyls since neither race degraded glyceollin in vitro. These results suggested that differential glyceollin degrading activity of the hypocotyls following infection was of importance in determining ultimate levels of glyceollin accumulation in the infected hypocotyls and consequently in determining disease reaction in this host-pathogen interaction. Uninoculated hypocotyls also synthesized glyceollin after wounding, but glyceollin did not accumulate, apparently due to the presence of active glyceollin degrading activity.