Research from New York University indicates that active Bt toxins genetically engineered into crops may accumulate in soil. In laboratory experiments, Guenther Stotzky and his colleagues have shown that purified Bt toxins, similar to ones found in some lines of transgenic Bt crops, do not disappear when added to soil but instead become rapidly bound to clay and humic acid soil particles. The bound Bt toxins, unlike free toxins, are not degraded by soil microbes, nor do they lose their capacity to kill insects.
The accumulation of active Bt toxins in soils could represent a risk to soil ecosystems. Typically, toxins in naturally occurring Bt bacteria, and sprays made from them, are not active-they exist in the form of inactive, so-called protoxins. Before they are able to kill an insect, the protoxins must be dissolved in its gut and cut by protein-digesting enzymes liberating the active toxins. By contrast, the toxin in many Bt crops needs no activation. It is already in an active form.
Stotzky suggests that active Bt toxins might be released to the soil as farmers incorporate plant material into the ground after harvest. Active toxins, which might build up with time, could kill known Bt-sensitive soil insects. In addition, a broader range of insects and other organisms may be susceptible to engineered toxins than to toxins from naturally occurring bacteria. Organisms unable to dissolve or cut the protoxin but sensitive to the active toxin, would not be harmed by the bacterial toxin but would be vulnerable to the engineered active form. Soil-inhabiting insect pests, already exposed to the toxin in their plant-eating phase, may be under continuing pressure to evolve resistance to Bt.
Stotzky's results, if they hold true under field conditions, should sound an alarm to regulators and others concerned about the risks of genetically engineered crops. To the extent that Bt crops containing active toxins are planted in the United States, soil organisms may be newly exposed to active Bt toxin. Sprays contribute far less active toxin to soil ecosystems because, for the most part, they exist in an inactive form. In addition, unlike the engineered toxins, which are protected inside the plant, spray toxins on the surfaces of leaves and soil are subject to inactivation by UV light before they have a chance to be incorporated into soils.
Sources: C. Crecchio and G. Stotzky, "Insecticidal activity and biodegradation of the toxin from Bacillus thuringiensis subsp. kurstaki bound to humic acids from soil," Soil Biology and Biochemistry 30: 463-70, 1998, and references therein.