GENETICS AND BIOCHEMISTRY OF AFLATOXIN FORMATION AND GENOMICS APPROACH FOR PREVENTING AFLATOXIN CONTAMINATION

摘要

Mycotoxins are toxic, small molecular weight secondary metabolites produced by fungi. Research on mycotoxins gained worldwide attention alter the notorious "Turkey X disease" in 1962 near London, Hngland, when approximately 100,000 turkey poults died.1'2 This mysterious disease was later found to be caused by feeding peanut (groundnut) meal contaminated with the toxin of Aspergillus Jlavus named "aflaloxin." Chemically, aflatoxins arc dil'uranocoumarin derivatives. Aflatoxins B,, B2, G,, and Ci2 (AFB,, AFG,, AFB2, and AFG2) (Fig. 11.1) are the four major allatoxins named based on their colors of fluorescence emission under ultraviolet light (blue or green) after thin layer chromatographic separation. Allatoxins are produced primarily by the filamentous fungi Aspergillus Jlavus and A. parasiticus}'4 as well as some strains of A, nomius, two isolates of A. pseudotamurii? nine of A. hombveis}' as well as one isolate of A. oehracearoseus x and Emericella venezuelensis (Klich, unpublished data). A. Jlavus produces allatoxins B, and B2. Other toxic compounds produced by A. Jlavus are cyclopiazonic acid, kojic acid, [5-nitropropionic acid, aspertoxin, aflatrem, and aspergillic acid. A. parasiticus produces aflaloxin G; and G2, in addition to B| and B2. but not cyclopiazonic acid. A. jlavus is more persistent in crop debris, produces higher levels of conidia early in the growing season, and is most commonly associated with preharvcsl aflatoxin contamination of food and feed crops. Although A. Jlavus is not an aggressive pathogen, under weather conditions favorable to its growth, the fungus can cause ear rot on maize, thus demonstrating characteristics of an "opportunistic" pathogen. Because of its ability to grow at low water activity, A. Jlavus is also well adapted to colonize seeds of grain and oil crops in storage, where exposure of seed to moisture is purposely limited. Control methods have been developed for post-harvest control of aflatoxin contamination, but there are no effective control strategies to prevent pre-harvesl aflaloxin contamination.10