Age-Related Changes in Specificity and Glyceollin Production in the Hypocotyl Reaction of Soybeans toPhytophthora megaspermavar.sojae

Abstract

Intact, 6 day old soybean hypocotyls (G. max [L.] Merr.) increased in resistance to inoculation with zoospores of P. megasperma Drechs. var. sojae Hildeb. (Pms) from the top (youngest part) to the bottom (oldest part). The interaction with the compatible race at the top of the hypocotyl was characterized by watersoaking and unrestricted tissue colonization with low glyceollin production whereas at the middle and bottom of the hypocotyl, the interaction was necrotic and glyceollin production was increased. This change in host response corresponded to a difference in tisue age of 1-2 days. The incompatible interaction, which typically was necrotic with high glyceollin production at the top of the hypocotyl, became even more incompatible lower down, where there was a reduction in both necrosis and glyceollin production. With cultivar Altona and Pms races 4 (incompatible) and 6 (compatible) the number of appressoria formed and penetrations were similar regardless of race or inoculation site. Similarities in symptoms and in the time-course of glyceollin production suggested that similar mechanisms govern the incompatible response whether it occurred on the incompatible host in the normal way, or on the compatible host at resistant-responding sites on the lower parts of the hypocotyl.