Proteins RAPs and Natural Compounds in Corn That Inhibit Aspergillus flavus Growth and Aflatoxin Contamination

Developing resistance to fungal infection in wounded as well as intact kernels would go a long way toward solving the aflatoxin problem (Payne 1992). Studies demonstrating subpericarp (wounded-kernel) resistance in corn kernels have led to research for identification of subpericarp resistance mechanisms. Examinations of kernel proteins of several genotypes revealed differences between genotypes resistant and susceptible to aflatoxin contamination (Guo et al. 1998). Imbibed susceptible kernels, for example, showed decreased aflatoxin levels and contained germination-induced ribosome inactivating protein (RIP) and zeamatin (Guo et al. 1997). Both zeamatin and RIP have been shown to inhibit A. flavus growth in vitro (Guo et al. 1997). In another study, two kernel proteins were identified from a resistant corn inbred (Tex6) which may contribute to resistance to aflatoxin contamination (Huang et al. 1997). One protein, 28kDa in size, inhibited A. flavus growth, while a second, over 100 kDa in size, primarily inhibited toxin formation. When a commercial corn hybrid was inoculated with aflatoxin and nonaflatoxin-producing strains of A. flavus at milk stage, one induced chitinase and one b-1,3-glucanase isoform was detected in maturing infected kernels, while another isoform was detected in maturing uninfected kernels (Ji et al. 2000).

In another investigation, an examination of kernel protein profiles of 13 corn genotypes revealed that a 14 kDa trypsin inhibitor protein (TI) is present at relatively high concentrations in seven resistant corn lines, but at low concentrations or is absent in six susceptible lines (Chen et al. 1998). The mode of action of TI against fungal growth may be partially due to its inhibition of fungal-amylase, limiting A. flavus access to simple sugars (Chen et al. 1999b) required not only for fungal growth, but also for toxin production (Woloshuk et al. 1997). The TI also demonstrated antifungal activity against other mycotoxigenic species (Chen et al. 1999a). The identification of these proteins may provide markers for plant breeders, and may facilitate the cloning and introduction of



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