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[SANET-MG] aflatoxin control



October 20, 2004

Prof. Joe Cummins

“Aflatoxin Control”

Aflatoxin is a natural occurring mycotoxin, because it is a powerful
toxin attacking genes it has gained a great deal of worldwide attention.
Two types pf mold: Aspergillus flavus and Aspergillus parasiticucus can
produce the toxin. Aspergilus flavus is widespread in nature in soil,
moldy grains and nuts commonly are contaminated with the fungus.
Aflatoxin production is favored by moisture and high temperature. At
least 13 different types of aflatoxin are produced and the most potent
of these is aflatoxin B1. grain testing for aphlatoxin is provided by
the Grain Inspection Packers and stockyard administration of USDA at a
cost of $25 per test(1).

Liver cancer is the fifth most prevalent cancer in the world, 80% of the
cases are observed in the developing world. The primary causes of liver
cancer in the developing world is the Hepatitis B virus , aflatoxin and
most ferociously the two combined . Limiting the contamination of
foodstuffs with aflatoxin is a particularly important target for public
health measures (2). However, it is worth pointing out that aflatoxin
pollution of food is also a major problem in the developed world.

The biological strategies being explored to reduce or eliminate
aflatoxin in food and feeds includes us of heavy seed inoculation of
Aspergillus strains selected for being unable to produce aflatoxin to
replace toxin producing strains in the soil. That strategy has been
successfully employed in cotton, as will be discussed below. Crops
resistant to Aspergillus are being selected using traditional genetic
methods, use of molecular marker assisted selection or by direct genetic
modification, as will be discussed below.

A workshop on aflatoxin elimination and fungal genomics provided an
overview and progress report on ecological and genetic approaches to
controlling aflatoxin (3). Cotton seed is an important crop for oil and
feed. Atoxigenic strains of Aspergillus flavus were colonized on sterile
seed. That treatment reduced the proportion of contaminated seed by over
50% the first year and greater reduction in later years of application
providing an economic benefit to the grower (4). Non-toxigenic strains
of Aspergillus flavus and Aspergillus parasiticus alone or in
combination significantly reduce aflatoxin content of peanuts, a mixture
of the two types of fungus was most effective (5).Intra-specific
competition is the basis for the biological control of aflatoxin,
sexually compatible strains produce mycelia that produce aflatoxin,
while vegetative incomaptibilty reactions result in the death of
heterokaryotic ( mycelia with incompatible nuclei).and disrups mycelial
networks resulting in reduced aflatoxin production (6). In common terms
reduced aflatoxin production is achieved by fungal rape and suicide. The
strain of Aspergillus flavus used to reduced aflatoxin in cotton has
been found to be defective in the pathway to aflatoxin synthesis (7).
Growth and mycotoxin production fo Aspergillus flavus and Aspergillus
parasiticus has been inhibeted using extracts of Agave cactus ( the
cactus used in tequila). Scaling up production of such natural products
may be worthwhile in this case (8).

Conventional plant breeding and conventional breeding using molecular
markers are being used to select genes for resistance to Aspergillus
infection of crops. Genetic resistance to Aspergilus and to aflatoxin
production have been identified in maize but fuller work is needed to
produce commercial varieties (9). It is growing clearer that traits for
low aflatoxin production are quantitative trait loci (QTL). Such loci
are the most important kind of genes in plant breeding. QTL govern plant
size, yield of grain, disease resistance , etc. QTL are multiple
additive genes that can be added in many small increments or removed
similarly. QTL for low aflatoxin have been identified in maize (10). QTL
have been pyramided (pyramiding is combining genes from amy strains into
a single strain by crossing) in maize joining QTL for resistance to
invasion by Aspergillus with QTL for resistance to ear feeding insects
which wound the maize and allow fungal infection to take place (11). QTL
provide the most promising long term protection against aflatoxin crop
pollution.

Genetic engineering has focused on Bacillus thuringiensis (Bt) toxin to
reduce wounding of the crop to allow fungal infestation or on more
direct methods to limit fungal infestation. Maize was inoculated with
Aspergillus flavus and infested with corn borers, Bt strains produced
grain with less aflatoxin than did isogenic lines lacking Bt (12). The
experiment was interesting but employed artificial conditions, with
neither natural fungal infection nor borer infestation (12). Peanuts
modified with a Bt cry 1Ac gene were compared and the Bt containing
peanuts had reduced levels of aflatoxin. Peanuts were also modified with
a bacterial choroperoxidase gene that resisted Aspergillusinfection and
showed promise in producing peanuts with reduced aflatoxin (13). The
bacterial chloroperoxidase gene and several other candidate genes have
been used to transform cotton but data on their effectiveness in
reducing aflatoxin has not yet been obtained (14). A gene for a
ribosomal inhibiting protein (RIP) was isolated from maize and used to
transform peanut, RIP blocked fungal ribosomes without inhibiting the
ribosomes of maize, the effectiveness of the modified peanut has not yet
been tested(15). In general the GM crops are not yet fully tested for
their ability to reduce aflatoxin pollution of maize, peanut and cotton
seed.

In conclusion, biological control using fungi unable to produce
aflatoxin to contol those that produce aflatoxin has proved effective in
cotton and conventional breeding using QTL to produce strains resistant
to fungal infestation has proved useful in maize. Genetic modification
has had preliminary success using Bt genes to prevent insect wounding
followed by fungal infestation while none of the other methods has been
had adequate testing even though they may have sound theoretical basis.
For the immediate the biological and conventional methods are most
useful in reducing aflatoxin injury through food and feed.

References

  1. USDA Grain Inspection,Packers and Stockyards Administration
     Aflatoxin 2004
     http://www.usda.gov/gipsa/newsroom/backgrounders/b-aflatox.htm
  2. Wild,C. and Hall,A. Primary prevention of hepatocellular carcinoma
     in developing countries 2000 Mutation Research 462,381-93
  3. Robens,J. and Brown, R. Aflatoxin and fumonisin elimination and
     fungal genomics workshop,San Antonio , Texas,Oct 23-25,2002 2004
     Mycopathologia 157,393-505
  4. Antilla,L. and Cotty,P. Advances in utilization of atoxogenic
     strain technology to manage aflatoxin in commercial cotton page
     448 in Aflatoxin and fumonisin elimination and fungal genomics
     workshop,San Antonio , Texas,Oct 23-25,2002 2004 Mycopathologia
     157,393-505
  5. Dorner,J. and Horn,B. Effect of Nontoxigenic Strains of
     Aspergillus Flavus and A. Parasiticus Applied Separately and in
     Combination on Preharvest Aflatoxin Contamination of Peanuts page
     450 in Aflatoxin and fumonisin elimination and fungal genomics
     workshop,San Antonio , Texas,Oct 23-25,2002 2004 Mycopathologia
     157,393-505
  6. Wicklow,D. and Horn,B. Vegetative compatibility and aflatoxin
     production in Aspergillus page 446 in Aflatoxin and fumonisin
     elimination and fungal genomics workshop,San Antonio , Texas,Oct
     23-25,2002 2004 Mycopathologia 157,393-505
  7. Ehrlich,K. and Cotty,P. An isolate of Aspergillus flavus used to
     reduce aflatoxin contamination in cottonseed has a defective
     polyketide synthase gene 2004 Appl. Microbiol. Biotechnol. 65,473-8
  8. Sanchez,E,Heredia,N. and Garcia,S. Inhibition of growth and
     mycotoxin production of Aspergillus flavus and Aspergillus
     parasiticus by extracts of Agave species 2004 International
     Journal of Food Microbiology in press
  9. Guo,B,Li,R,Widstrom,N,Lynch,R. and Cleveland,T. Genetic variation
     within maize population GT-MAS:gk and the relationship with
     resistance to Aspergillus flavus and aflatoxin production 2001
     Theor.Appl. Genet. 103,533-9
 10. Paul,C,Naidoo,G,Forbes,A,Mikkilineni,V,White,D. and Rocheford,R.
     Quantitative trait loci for low aflatoxin production in two
     related maize populations 2003 Theor Appl Genet 107,263–70
 11. Widstrom,N,Butron,A,Guo,B,Wilson,D,Snook,M,Cleveland,T. and Lynch
     ,R. Control of preharvest aflatoxin contamination in maize by
     pyramiding QTL involved in resistance to ear-feeding insects and
     invasion by Aspergillus spp. 2003 European Journal of Agronomy
     19,563-72
 12. Williams,WmWindham,G,Buckley,P. and Daves,C. Aflatoxin
     accumulation in conventional and transgenic corn hybrids infested
     with southern corn borer page 467 in Aflatoxin and fumonisin
     elimination and fungal genomics workshop,San Antonio , Texas,Oct
     23-25,2002 2004 Mycopathologia 157,393-505
 13. Ozias-Akins,P,Niu,C,Joshi,M,Deng,X,Holbrook,C. and Lynch,R.
     Genetic engineering of peanut for reduction of aflotoxin
     contamination page 437 in Aflatoxin and fumonisin elimination and
     fungal genomics workshop,San Antonio , Texas,Oct 23-25,2002 2004
     Mycopathologia 157,393-505
 14. Chlan,C,Cary,J,Rajasekaran,K. and Cleveland,T. Genetic engineering
     of cotton to confer resistance to A. flavus :An update page 438
     Aflatoxin and fumonisin elimination and fungal genomics
     workshop,San Antonio , Texas,Oct 23-25,2002 2004 Mycopathologia
     157,393-505
 15. Weissinger,A,Wu,M. and Cleveland,T. Transformation of Virginia and
     Runner type peanuts with Mod1,a a gene encoding an anti-fungal RIP
     from maize page 441 in Aflatoxin and fumonisin elimination and
     fungal genomics workshop,San Antonio , Texas,Oct 23-25,2002 2004
     Mycopathologia 157,393-505

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