Soybean phytoalexin, glyceollin, prevents accumulation of aflatoxin B₁ in cultures ofAspergillus flavus

Abstract

The soybean phytoalexin, glyceollin, suppresses the accumulation of aflatoxin B₁ in cultures ofAspergillus flavus. At concentrations of 6.25μg/ ml and 62.5μg/ml, glyceollin causes 70% and 95% decreases in the maximum observed levels of aflatoxin B₁, respectively. In contrast to the dramatic effect on aflatoxin B₁ levels, these concentrations have little effect on fungal growth. For example, at 62.5μg/ml in liquid culture, glyceollin causes a barely discernible lag in the beginning of growth and a 11.5% decrease in maximum fungal mass. When the same concentration of glyceollin is added to the colony margin on semisolid medium, an inhibition zone is formed and then overgrown in one day. Glyceollin appears to act by inhibiting aflatoxin B₁ synthesis, since the rate of aflatoxin B₁ breakdown is not increased in fungal cultures that have been grown in the presence of glyceollin. Glyceollin does accumulate in viable soybean seeds that have been infected withAspergillus flavus. Such seeds accumulate aflatoxin B₁ at one-third the rate of non-glyceollin-producing, nonviable seeds. These results suggest that the synthesis of glyceollin in infected seeds may explain, at least in part, why aflatoxin contamination of soybeans is not a common problem.