Glyceollin: its rôle in restricting fungal growth in resistant soybean hypocotyls infected with Phytophthora megasperma var. sojae

Abstract

Race 1 of Phytophthora megasperma var. sojae grew at similar rates in Harosoy (susceptible) and Harosoy 63 (resistant) soybean hypocotyls until 9 h after inoculation, but then was rapidly inhibited in Harosoy 63 hypocotyls. Low levels of the soybean phytoalexin glyceollin (<its ED 50 value for the inhibition of mycelial growth of the fungus) were detected at 9 or 10 h in extracts of whole hypocotyls of both cultivars. However, when glyceollin was analyzed in thin sections of the infected hypocotyls, levels at or above the ED 90 value were found at 8 h after inoculation in tissue layers of Harosoy 63 which contained advancing hyphae. Such high and localized accumulation of glyceollin was not detected in the susceptible inoculated hypocotyls of their Harosoy or Harosoy 63 treated with blasticidin S. These results strongly suggested that glyceollin accumulation accounts for cessation of fungal growth in the resistant soybean hypocotyls.