Aspergillus flavus hydrolases: their roles in pathogenesis and substrate utilization

Authors: Mellon, Jay; Cotty, Peter; Dowd, Michael

Submitted to: Applied Microbiology and Biotechnology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/12/2007
Publication Date: 10/16/2007
Citation: Mellon, J.E., Cotty, P.J., Dowd, M.K. 2007. Aspergillus flavus hydrolases: their roles in pathogenesis and substrate utilization. Applied Microbiology and Biotechnology. 77(3):497-504.

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 primary mode by which A. flavus obtains nutrients is by using non-living materials (saprophytic). As such, the fungus is capable of producing a wide variety of hydrolytic enzymes to help access nutrients from complex substrates (proteins, polysaccharides, lipids). But, the fungus also occupies the niche of an opportunistic pathogen. That is, it can invade living tissues if easy access is available or the tissue displays poor viability. Examples of such tissues are cotton seeds with low viability or human lung tissue from immuno-compromised patients. Thus, A. flavus requires an additional set of hydrolases to assist in accessing nutrients from living tissues. This manuscript reviews the published work on A. flavus hydrolases. Most investigative work has concentrated on proteinases, pectinases, and amylases. The roles these enzymes play in assisting the fungus in nutrient capture are discussed. This research 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: Aspergillus flavus is a fungus that principally obtains resources for growth in a saprophytic mode. Yet, it also possesses the characteristics of an opportunistic pathogen with a wide, non-specific host range (plants, animals, insects). It has attained a high level of agricultural significance due to its ability to produce the very potent mycotoxic carcinogen aflatoxin which significantly reduces the value of contaminated oilseed crops and corn. In order to access a large variety of nutrient substrates, as well as penetrate host tissues, A. flavus appears to possess the capacity for the production of numerous extracellular hydrolases. The majority of investigative work has concentrated on the serine and metalloproteinases, pectinase P2c, and amylase. The proteinases are employed by the fungus to help access protein substrates, such as elastin that is found in mammals and insects. In addition to hydrolases secreted for nutrient capture, A. flavus also can secrete endopolygalacturonase (P2c) that is strongly correlated with isolate virulence (against plants) and assists in the maceration of cotton boll tissues. In certain metabolic environments, secretion of a-amylase is critical for starch digestion and may play a critical role in induction of aflatoxin biosynthesis. In spite of the work that has been completed, much remains to be learned regarding the capacity for A. flavus production of hydrolases, and how they are employed by the fungus. This information will be critical for the formulation of successful strategies to control aflatoxin contamination in oilseed commodities.