Phenylalanine ammonia-lyase activity in soybean hypocotyls and leaves following infection with Phytophthora megasperma f.sp. glycinea

Abstract

Phenylalanine ammonia-lyase activity increased rapidly beginning 2 h after inoculation with Phytophthora megasperma (Drechs.) f.sp. glycinea (Hildeb.) Kuan & Erwin race 1 in unwounded hypocotyls of soybean cv. Harosoy 63 (resistant) but did not change significantly in cv. Harosoy (susceptible). Small increases in phenylalanine ammonia-lyase activity also were caused by wounding. Activity increased more slowly in hypocotyls (cv. Harosoy 63) wounded just before inoculation than in intact inoculated hypocotyls, but most activity developed in hypocotyls wounded 12 h before inoculation. There were comparable effects of wounding on symptom development. Trifoliolate leaves of 14-day-old cv. Harosoy 63 plants are resistant, but trifoliolate leaves of 12-day-old cv. Harosoy 63 plants and of 14-day-old cv. Harosoy plants are susceptible to race 1. Increases in phenylalanine ammonia-lyase activity following inoculation were demonstrated only in 14-day-old Harosoy 63 plants but not until 24–36 h after the inoculation. Significant accumulation of glyceollin occurred by 24 h. Susceptible trifoliolate leaves of 12-day-old cv. Harosoy 63 plants produced only low levels of glyceollin following either infection or treatment with the abiotic elicitor AgNO3, whereas trifoliolate leaves of 14-day-old cv. Harosoy plants produced high levels of glyceollin in response to AgNO3. It is concluded that trifoliolate leaves of 12-day-old, as opposed to 14-day-old, cv. Harosoy 63 plants have not developed mechanisms that trigger responses to either infection or the abiotic elicitor or they are deficient in metabolic processes that support glyceollin biosynthesis or other defense-related responses.