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Buildup of Bt toxins in soil
THE GENE EXCHANGE
A Public Voice on Biotechnology and Agriculture
Union of Concerned Scientists
Fall/Winter 1998

Buildup of active Bt toxins in soil

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.


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