Effects of Oilseed Storage Proteins on Aflatoxin Production by Aspergillus flavus

Potential involvement of seed storage proteins in susceptibility to aflatoxin contamination was assessed with in vitro tests. Initially, two oilseed storage proteins [cottonseed storage protein (CSP) and zein] were compared with bovine serum albumin (BSA) and collagen. Supplementation of a complete defined medium with either oilseed storage protein resulted in significantly more aflatoxin production by Aspergillus flavus than supplementation with either BSA or collagen. Little or no afla-toxin was produced when either BSA, CSP, or zein was employed (at 0.5%) as both the sole carbon and the sole nitrogen source. Media with collagen (0.5%) as the sole nitrogen and carbon source supported aflatoxin production similar to the complete defined medium. Although lower than levels observed with defined medium, aflatoxin production increased with both increasing CSP concentration (0 to 2.0%) and increasing zein concentration (0 to 6.0%) when these proteins served as both the sole carbon and sole nitrogen source. Denaturing polyacrylamide gel elec-trophoresis and protease activity assays indicated that fungal acquisition of protein carbon was probably via hydrolysis mediated by the 35 kD metalloprotease of A. flavus. Media lacking nitrogen but containing sucrose (5.0%) and supplemented with either zein (1.7%) or CSP (2.0%) supported three-to eightfold more afla-toxin production than the complete defined medium. The results suggest seed storage proteins, when present with an accessible carbon source, may predispose oilseed crops to support production of high levels of aflatoxins by A. flavus during seed infection. The toxigenic fungus Aspergillus flavus is a widely distributed saprophyte that, under specific environmental conditions, is capable of opportunistic pathogenesis in plants. As a result, the value of the oilseed crops corn and cottonseed is diminished due to fungal production of the potent carcinogen aflatoxin (1). Although storage proteins are prominent components of both crops, the contribution of these proteins to crop vulnerability to aflatoxin contamination has not been addressed. Zein, the prolamin (ethanol-soluble) storage protein of corn, is localized in the endosperm, whereas cottonseed storage protein (CSP), a globulin (salt-soluble), is localized in the developing cotyledons. The dry weight composition of protein is 11 to 12% in corn and 39% in cottonseed (2). Zein comprises half of all seed protein in corn and CSP is the major protein in cottonseed (2). In addition, CSP contains 18% nitrogen (3). These seed protein reserves are potential carbon and nitrogen resources for the fungus. Fungal invasion of oilseed crops may produce localized, elevated levels of seed storage …