Submitted to: Mycopathologia
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 21, 2003
Publication Date: April 1, 2004
Citation: Mellon, J.E., Cotty, P.J. 2004. Expression of pectinase activity among Aspergillus flavus isolates from southwestern and southeastern United States. Mycopathologia. 157(3):333-338. Interpretive Summary: Aflatoxin is a very potent carcinogen and toxin that is produced by the fungus Aspergillus flavus. When this fungus infects oilseed crops (corn, cotton, peanuts, tree nuts), the developing seed can become contaminated with this toxin, rendering the product unusable for food or feed. The fungus can produce an enzyme called pectinase that helps it spread through plant tissues. A survey of several hundred isolates of the fungus reveale much variability in pectinase production, depending upon which region of the U.S. the isolate is derived. In addition, although this enzyme aids fungal spread through cotton bolls, its presence is not required for aflatoxin contamination of seed. Thus, the ability of pectinase to contribute to fungal aggressiveness is distinct from fungal seed pathogenicity. This information will benefit oilseed breeders, producers and pathologists, and will aid in the formulation of methods to prevent aflatoxin contamination of food and feedstuffs.
Technical Abstract: A survey of pectinase expression in soil isolates of Aspergillus flavus derived from different regions of the United States revealed geographic polymorphisms. Aspergillus flavus isolates of L strain (large sclerotia) derived from southwestern U.S. (Arizona) produced moderate to high levels of a specific pectinase, P2c, while S strain (small sclerotia) isolates produced variable amounts of P2c. In contrast, A. flavus isolates of L strain from southeastern U.S. yielded variable P2c production, while S strain isolates consistently gave high P2c levels. These results were corroborated by pectinase surveys of a second collection of soil A. flavus isolates (resampled) and a collection of A. flavus isolates derived from cottonseed. Expression patterns for P2c and pectinesterase were evaluated for a select number of isolates using an isoelectric focusing technique. Clear zone reactions from the pectinase plate assay (surveys) corresponded to the presence of P2c, while red ring reactions corresponded to the lack of P2c. Single cotton seed assays for aflatoxin B1 levels and pectinase production of resident A. flavus isolates (S strain) revealed a lack of correlation between P2c production and aflatoxin contamination, suggesting a distinction between fungal invasiveness and seed pathogenicity.