Bt Corn

Mycotoxins: Fusarium and fumosins

A mycotoxin is a toxic secondary metabolite produced by organisms of the fungi kingdom. Mycotoxins (particularly fumonisins, produced by Fusarium species mould) are fatal to horses and pigs, suspected human carcinogen Fusarium is usually associated with ECB invasion

Mycotoxins: Aflatoxins and Aspergillus

Aflatoxins are the most toxic secondary metabolites commonly found in maize. Aflatoxins are produced by Aspergillus (mould speceis) predominantly. Associated with insect damage to ears Action limit 20 parts per billion Greater economic significance because they can be passed on into milk

Toxicity to insects

B. thuringiensis produces a protoxin, which accumulates an insoluble crystal protein. This protoxin dissolves at the high pH values (up to 9.5) that are found in the midgut of some insects, and the soluble form of the protoxin is cleaved by proteases to form the active delta endotoxin. Active toxin binds to insect gut brush boarder membranes creating a pore resulting in loss of ions.

Pros of using Bt corn

Bt toxin is not toxic to humans or other animals because the pH of vertebrate guts is too low to allow solubilization of toxin crystals and mammals. Thus, Bt toxin is safe for possible exposure to people in food Very effective against the main pest of corn, European corn borer. Can reduce damage by armyworm and corn earworm Greatest potential to reduce mycotoxins in warm countries Potential for mycotoxin contamination high in sub-Saharan Africa Decreases use of insecticides by <80% in some regions

Stalk rot complex

Development of stalk rot greatly affected by plant stress may or may not be associated with insect damage. May not be able to be helped by Bt corn

Lepidopterans can influence disease

ECB larvae act as a vector and carry spores of Fusarium from plant surface to damaged kernels and into damaged stalks Similar relationship between corn earworm & SW corn borer and Fusarium & Aspergillus Fusarium ear rot Fusarium spp & Aspergillus kernel rot often associated with insect damage to ears or kernels

Bacillus thuringiensis

Gram-positive spore-forming bacterium Bacillus thuringiensis. This bacterium was one of the first prokaryotic biological control agents to be used as a commercial insecticide. Since then, research in the area has led to more and widespread applications, and it has developed into an important agent for the control of insect pests worldwide.

Vegetative Insecticidal protein (Vip3A)

Protein secreted by B. thuringiens Also processed into active form by cleavage in insects gut and also forms ion pores

Cons of Bt corn

Useless against some pests: fall army worm and black cut worm Fungi have other modes of entry into corn Controversy over production & use of GE focused a great deal of attention on Bt corn When conditions are favourable for disease infection may still be high Still need an integrated approach to pest and disease management

Experiments

Using the Bt version of corn reduced the levels of Aspergillus but the concentrations of aflotoxins were still too high

What is Bt corn

With advancements in genetic engineering, it eventually became feasible to move the gene that encodes the insecticidal toxins from B. thuringiensis into crop plants, including maize.

Wounding

spores deposited on wounded tissue are likely to cause disease. Additionally stalk damage by insects causes stress that predisposes to infection Management of insects can play major role in corn root and stalk rot management