Mechanisms of Resistance to Aflatoxin Accumulation by Aspergillus Flavus in Maize Genotype GT-MAS:gk.

Maize genotype differences in aflatoxin production were observed when kernels of resistant GT-MAS:gk and thirteen susceptible commercial hybrids were inoculated with Aspergillus flavus using a laboratory kernel screening assay. GT-MAS:gk supported the lowest levels of aflatoxin in both intact and endosperm-wounded kernels. Treating intact kernels with KOH effected substantial increases in aflatoxin accumulation in GT-MAS:gk, but only marginal increases in Pioneer 3154. Removing wax from the surface of GT-MAS:gk kernels greatly increased aflatoxin accumulation. These results indicated that GT-MAS:gk resistance was associated with an intact pericarp (wax and cutin layers), acting as a physical barrier, along with internal biochemical factors. Kernels of GT-MAS:gk and Pioneer 3154 were tested for resistance to aflatoxin accumulation by A. flavus under different relative humidities (RH). Resistance in GT-MAS:gk was consistent across all RH levels. Preincubation at 100% RH for three days increased germination. In germinated kernels, aflatoxin levels decreased markedly in Pioneer 3154 but not GT-MAS:gk. When eight susceptible hybrids were evaluated under preincubation conditions, seven supported significantly lower aflatoxin levels than kernels not subjected to preincubation. Data suggested that an inhibitor of aflatoxin biosynthesis may be induced during kernel germination process. A. flavus can utilize cutin as sole carbon source, which suggests production of extracellular cutinase in vitro. This production was optimal at pH 8.0. Two esterases (cutinases) (36 kD and 22-23 kD) were isolated from A. flavus liquid cultures. Treatments with exogenous cutinase or DFP, an fungal cutinase inhibitor, markedly increased or decreased aflatoxin production, respectively. Results suggested that A. flavus produces cutinase and that this enzyme may play an important role in pathogenicity of A. flavus. Studies demonstrated that GT-MAS:gk kernels have more surface wax than do susceptible hybrids, and that this wax has antifungal activity against A. flavus. Wax from GT-MAS:gk kernels had a unique component, visualized using thin layer chromatography. This component was purified and examined using gas chromatography and mass spectrometry. A peak was identified that was absent in other genotypes. This compound may be a triterpenoid