Contents - Faperta

Transgenic Resistance to Insects: Interactions with Nontarget Organisms 357
lower community diversity and reduce productivity above ground (Van der Heijden et al.,
1998). Organisms in the rhizosphere such as Collembola, nematodes, protozoa, fungi, bacteria,
and earthworms should be included in risk assessment studies, but have received
little attention (Groot and Dicke, 2002).
Toxins from the transgenic plants are introduced into the soil primarily through incorporation
of the crop residues into the soil after crop harvest (Tapp and Stotzky, 1998a) or
through the root exudates (Saxena, Flores, and Stotzky, 1999). Insecticidal proteins produced
by Bt bind rapidly and tightly on clays, both pure mined clay minerals and soil clays, on
humic acids extracted from soil, and on complexes of clay and humic acids. Binding reduces
susceptibility of the proteins to microbial degradation (Stotzky, 2004). Vertical movement
of Cry1Ab protein, either purifi ed or in root exudates, or biomass of Bt corn, decreased as
the concentration of the clay minerals kaolinite or montmorillonite in soil increased. The
toxins produced in Bt plants retain their biological activity when bound to the soil, so
accumulation of these toxins is likely to occur in the ecosystem. Bt toxins are absorbed and
bound around the soil particles in 30 minutes, suggesting that toxins from root exudates
or crop residue remain free in the soil for a short time (Venkateswerulu and Stotzky, 1990,
1992; Crecchio and Stotzky, 1998), which reduces its availability to the microbes. This may
lead to accumulation of the toxins in the environment. The persistence of toxins in the soil
could improve insect control or lead to development of resistance in insects inhabiting the
soil. Bt toxins released from the root exudates bind to the soil particles and remain active
up to 180 days (Saxena and Stotzky, 2001), an association that interferes with biodegradation.
Toxins are present in the soil rhizosphere throughout the crop growth, and for several
months after crop maturity (Saxena and Stotzky, 2000). Purifi ed Cry1Ab protein and
the protein released from Bt corn exhibited binding and persistence in soil. Insecticidal
protein was also released in root exudates of Bt potato (Cry3A protein) and rice (Cry1Ab
protein), but not in root exudates of Bt canola, cotton, and tobacco (Cry1Ac).
Earthworms ingest the bound toxins, but are not affected by them. However, earthworms
may function as intermediaries through which the toxins are passed on to other trophic
levels. When purifi ed toxin from B. thuringiensis ssp. kurstaki is added into the soil, the
pesticidal activity against Manduca sexta L. has been recorded up to 234 days (Tapp and
Stotzky, 1998b). These estimates are quite longer than the persistence reported earlier (8 to
17 days for the purifi ed toxins, and 2 to 41 days from biomass of transgenic corn, cotton,
and potato) (Palm et al., 1994; Sims and Holdan, 1996; Sims and Ream, 1997). The level of
Cry1Ac protein in samples collected three months after the last season’s tillage has been
evaluated using both enzyme-linked immunosorbent assays (ELISA) and bioassays with a
susceptible insect species, H. virescens. Both methods revealed that no detectable Cry1Ac
protein was present in any of the soil samples collected from within or outside the Bollgard
fi elds (Head et al., 2002). Based on the results from reference standards, the limit of detection
for the ELISA was 3.68 ng of extractable protein per gram of soil, and that of the
bioassay (measured by EC 50) was 8 ng of biologically active protein per gram of soil. Cry1Ac
protein accumulated as a result of continuous use of transgenic Bt cotton, and subsequent
incorporation of plant residues into the soil by postharvest tillage is extremely low and
does not result in detectable biological activity. Transgenic Bt corn decomposed at a slower
rate in soil than nontransgenic corn, possibly because Bt corn had more lignins than the
nontransgenic corn. Biomass of Bt canola, cotton, potato, rice, and tobacco also decomposed
at a lower rate than the biomass of respective near-isogenic nontransgenic plants.
However, the lignin content of these Bt plants, which was signifi cantly less than that of
Bt corn, was not signifi cantly different from that of their near-isogenic nontransgenic
counterparts, although it was consistently higher.