ABSTRACT
CONTENTS 14.1 Introduction ............................................................................................. 395 14.2 Genetics of Aspergillus flavus ................................................................. 397
14.2.1 Aflatoxin Biosynthesis ............................................................. 398 14.2.2 Regulatory Elements in Aflatoxin Biosynthesis .................. 399 14.2.3 Genomics of A. flavus ............................................................... 402
14.2.3.1 Expressed Sequence Tag Technology................... 403 14.2.3.2 DNA Microarrays .................................................... 403 14.2.3.3 Whole Genome Sequencing ................................... 406
14.3 Strategies to Control Preharvest Aflatoxin Contamination ............. 406 14.3.1 Biological Control ..................................................................... 406 14.3.2 Enhancement of Host-Plant Resistance................................. 407
14.3.2.1 Plant Breeding Strategies........................................ 408 14.3.2.2 Genetic Engineering for Resistance
to Aspergillus flavus .................................................. 413 14.4 Conclusion ............................................................................................... 417 References ........................................................................................................... 418
Mycotoxins are typically low molecular weight organic compounds produced by fungi and are not considered to be essential to maintaining the life of the fungal cell (reviewed in Bhatnagar et al., 2002). They are called ‘‘secondary metabolites’’ because they are not involved in obtaining energy or synthesizing structural components of the cell. The well-studied mycotoxins are the aflatoxins because they are highly toxic and carcinogenic compounds that frequently contaminate foods and feeds. These compounds
(Figure 14.1) are produced by several species of fungi in the genus Aspergillus. Aspergillus flavus, the most common species causing crop contamination, is an opportunistic pathogen, which routinely produces aflatoxins B1 and B2. Aspergillus parasiticus produces two additional toxins, namely G1, and G2. Aflatoxin B1 is both the most common and the most toxic and, as a result, is the aflatoxin most stringently regulated because it is the major toxicosis threat. Presence of aflatoxins in human foods may cause chronic health effects including immune-system suppression, impaired childhood development, and cancer. Acute poisoning can result in death (Wild and Turner,
2002; Gong et al., 2004). In developing countries, where detection and decontamination policies and practices are impractical, aflatoxin contamination is predominantly a food safety issue. Recently Kenya reported a minimum of 125 people killed by consuming aflatoxin-contaminated maize (http:==www.cdc.gov=mmwr=preview=mmwr.html=mm533a4.htm). In developed countries, however, aflatoxin remains an economic problem because the contamination is most frequently recognized due to loss in crop value (estimated to be as high as $300 million in direct cost in the United States alone) resulting from strict government regulations on maximum permissible levels in crops and crop-related products (Cotty, 2001; Richard and Payne, 2003; Wu, 2004). In crops intended for human consumption, maximum permitted aflatoxin levels range from 2 ppb aflatoxin B1 (4 ppb total aflatoxins) in the European Union to 20 ppb total aflatoxins in the United States. Small proportions of crop aflatoxin content are transferred from feed to milk (aflatoxin M1) causing even food crops to have stringent regulations for use at dairies (van Egmond, 2004; Wu, 2004). Aflatoxin contamination occurrence frustrates not only the agricultural community but also all those that handle, market, or utilize crops from highly vulnerable regions. The crop may have no visible symptoms, and yet contain enough aflatoxins to either severely reduce value or completely eliminate available markets. The same fungal isolates may cause contamination of several crops grown in a given area, such as cottonseed, corn, peanuts, and tree nuts.
