Agriculture Reference
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real possibility of gene flow between GM and wild-type varieties in other regions of the
world. For example, gene flow has been detected between
Bt
maize and native maize land-
races in Mexico (Quist and Chapela, 2001; Pineyro-Nelson et al., 2009), and despite limits
on where
Bt
cotton can be cultivated in the United States,
Bt
cotton is also grown in at least
12 other countries, several of which are in tropical regions associated with areas that are
within the center of origin for New World cotton species (
TableĀ 8.1
).
As more and different types of GM crops are developed, the likelihood of gene trans-
fer between SCWR and GM crops is expected to increase, especially if grown in regions
where the level of government oversight is not as extensive as it is in the United States.
Thus, while the safety testing and approval process can minimize much of the environ-
mental and health/safety risk of GM crops in the United States, it is still possible for some
GM crops to have nontarget effects in the environment or on organisms that have either
not been tested or may have unexpected effects under certain environmental conditions. It
is therefore important that GM crops continue to be evaluated for nontarget effects under a
variety of environmental and experimental scenarios, even after they have been approved
for commercial use in the United States.
Numerous studies have shown that insecticidal
Bt
proteins are released from transgenic
plants into soil through root exudates (e.g., Saxena et al., 1999; Saxena and Stotzky, 2000,
2001a; Saxena et al., 2002a, 2002b, 2004; Stotzky, 2004; Icoz and Stotzky, 2008a; Li et al., 2009),
pollen (Losey et al., 1999; Zangerl et al., 2001), and plant residue decomposition (Zwahlen,
Hilbeck, Gugerli, et al., 2003). Once in soil, the
Bt
toxins bind to clay particles (Tapp et
al., 1994; Tapp and Stotzky, 1995) and humic acids (Crecchio and Stotzky, 1998) and can
retain their insecticidal properties for at least up to 234 days (Tapp and Stotzky, 1998). In
laboratory studies, Cry1Ab protein from
Bt
maize root exudates persisted in soil for at
least 180 days and for 350 days in soil amended with
Bt
maize plant material (Saxena and
Stotzky, 2002). In a greenhouse pot study in which one
Bt
cotton line, two stacked
Bt
and
cowpea trypsin inhibitor (
Bt
+
CpTI
) cotton lines, and their non-GM isolines were con-
secutively cultivated for 4 years, Cry1Ac and
CpTI
proteins persisted in soil throughout the
experiment (Chen et al., 2011), supporting a previous study in which 41% and 60% of the
introduced amounts of
Bt
protein from stems and leaves of two
Bt
cottons (Events
Bt
-Zk
and
Bt
-GK, respectively) incorporated into soil under laboratory conditions remained after
56 days (Sun et al., 2007). In field studies, Cry1Ab protein from transgenic maize litter
has been shown to persist for at least 8 months (Zwahlen, Hilbeck, Gugerli, et al., 2003),
although
Bt
protein in soil does not appear to accumulate over time (e.g., Hopkins and
Gregorich, 2003; Baumgarte and Tebbe, 2005; Icoz and Stotzky, 2008a).
A higher lignin content has been reported in some
Bt
crops, including several different
lines of
Bt
maize (Saxena and Stotzky, 2001c; Flores et al., 2005; Poerschmann et al., 2005; Fang
et al., 2007). Higher lignin content has also been reported for
Bt
tobacco,
Bt
cotton,
Bt
canola,
Bt
potato, and
Bt
rice, although these differences were not statistically significant when com-
pared with the non-
Bt
isolines (Flores et al., 2005). The slower decomposition of
Bt
organic
material in soil, in some cases, has been attributed to higher lignin in transgenic plant resi-
dues (Saxena and Stotzky, 2001c; Stotzky, 2004; Flores et al., 2005). As a result, soil organisms
may have a longer exposure to the
Bt
toxins as they are slowly released from organic matter
and soil particles over time (Zwahlen, Hilbeck, Gugerli, et al., 2003; Stotzky, 2004).
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