Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/14/2000
Publication Date: N/A
Interpretive Summary: The fungus named Aspergillus flavus produces a poison called aflatoxin when it infects corn. Aflatoxin prevents the corn from being used commercially. In previous studies, we identified a protein that is expressed in high amounts in corn kernels of lines resistant to Aspergillus flavus infection and subsequent aflatoxin contamination, but either is not expressed or expressed in low amounts in kernels of susceptible corn lines. We also determined that this protein strongly inhibits the growth of Aspergillus flavus. In the present study, we found that this antifungal protein inhibits the production of a-amylase enzyme by Aspergillus flavus and thus its ability to breakdown starch and use it as a carbon source for growth. This information suggests that resistance in lines of corn with high levels of this protein may be the result of the inhibition of fungal ability to convert starch to sugars in corn kernel endosperm tissue, thereby limiting the substrate available for fungal growth and aflatoxin production. This discovery provides us with a mechanism through which resistance via plant breeding and/or genetic engineering, could be enhanced in commercially important lines of corn. Providing resistant corn varieties to growers will prevent millions of dollars in losses due to aflatoxin contamination of this commodity.
Technical Abstract: In this study, we found that the inhibition of fungal growth in potato dextrose broth (PDB) medium by the 14kDa corn trypsin inhibitor protein, previously found to be associated with host resistance to aflatoxin production and active against various fungi, was relieved/reversed when exogenous a-amylase was added along with trypsin inhibitor. No inhibitory effect of trypsin inhibitor on fungal growth was observed when A. flavus was grown on A&M medium containing either 5% glucose or 1% gelatin as carbon source. Further investigation found that trypsin inhibitor not only reduced the enzymatic activity of A. flavus a-amylase by 27%, but also inhibited fungal production of extracellular a-amylase when A. flavus was grown in PDB medium containing trypsin inhibitor. However, the effect of trypsin inhibitor on the production of amyloglucosidase, another enzyme involved in starch metabolism by the fungus, was quite different. It stimulated the production of this enzyme. The higher the trypsin inhibitor concentration in the growth medium, the greater the induction of amyloglucosidase during the first 10 h. These studies suggest that the resistance of certain corn genotypes to A. flavus infection may be partially due to the ability of trypsin inhibitor to reduce the production of extracellular fungal a-amylase and its activity, therefore, limiting the availability of simple sugars for fungal growth.