Agriculture Reference
In-Depth Information
Insect-resistant
crops are genetically engineered to express insecticidal toxins derived
from the spore-forming soil bacterium
Bacillus thuringiensis
(
Bt
). Naturally occurring
Bt
soil organisms produce insecticidal crystalline proteins (called Cry proteins) during spor-
ulation that are toxic to the larvae of certain insects (Hofte and Whiteley, 1989; reviewed
in Schnepf et al., 1998; USDA, 2010). To date, more than 60 different Cry proteins have
been identified that exhibit a high degree of specificity toward Lepidoptera (e.g., moths
and butterflies); Coleoptera (e.g., beetles); Diptera (e.g., flies and mosquitoes); Homoptera
(e.g., cicadas, leafhoppers, aphids, scales); Hymenoptera (e.g., wasps, bees, ants, sawflies);
Orthoptera (e.g., grasshoppers, crickets, and locusts); Mallophaga (e.g., lice); and nema-
todes (reviewed in Schnepf et al., 1998; Federici, 2002; Stotzky, 2002; Lee et al., 2003; Icoz
and Stotzky, 2008b; Sanchis, 2011). Some
Bt
proteins have even been used for targeted treat-
ment of some types of cancer cells in humans (e.g., Ito et al., 2004; Yamashita et al., 2005;
Ohba et al., 2009; Tharakan et al., 2009; Nagamatsu et al., 2010; Poornima et al., 2010; Wong
et al., 2010).
Formulations of
Bt
carrying the parasporal crystals have been used as a natural insec-
ticide in agricultural systems since the 1930s (Hofte and Whiteley, 1989; reviewed in Beegle
and Yamamoto, 1992; Sanchis, 2011), but success is often compromised by the poor survival
of the natural form of
Bt
in the environment (Griego and Spence, 1978; West, 1984; West et
al., 1985; Clark et al., 2005). Moreover, the
Bt
toxin present in the soil bacteria is not acti-
vated until cleaved by alkaline hydrolysis in the gut of a susceptible insect larva (Hofte
and Whiteley, 1989), and activation may also require the presence of indigenous bacteria
in the midgut of susceptible insects (Broderick et al., 2006, 2009; reviewed by Then, 2010).
The
Bt
gene that is genetically engineered into plants, however, is truncated and constitu-
tively produces only the preactivated Cry protein in the cells of the GM plant (e.g., Shu et
al., 2002; Xu et al., 2006).
At present, the two major crops that contain genes coding for insecticidal
Bt
toxin
are
Bt
maize and
Bt
cotton. Other
Bt
crops that have been developed include
Bt
potato,
Bt
tobacco,
Bt
spruce,
Bt
tomato,
Bt
rice,
Bt
eggplant,
Bt
sunflower, and
Bt
canola, although
not all of these are presently commercially available. In 2010, 86% of the maize and 93%
of the cotton cultivated in the United States was genetically modified to express herbicide
making up 26% and 49% of the global GM crop acreage, respectively (James, 2010). The
dramatic rise in the adoption rate of GM crops resulted primarily from the development of
GM varieties containing “stacked traits” or “pyramided traits” (as opposed to single traits
in one variety or hybrid). The term
stacked trait
refers to a plant that has been engineered
to express multiple toxins against different pests (e.g., protection against European corn
borer and corn root worm) or contains multiple plant protection properties (e.g., herbicide
tolerance plus insect resistance), whereas a
pyramided trait
is one in which multiple toxins
are expressed to target the same pest (U.S. Environmental Protection Agency [EPA], 2011).
In 2009, 75% of the GM maize hybrids in the United States were engineered with double
or triple stacked traits (James, 2010). One of the newest GM maize hybrids, SmartStax
®
,
was engineered to express eight different genes coding for pest resistance and herbicide
tolerance and produces six different types of Cry proteins—Cry1A.105, Cry2Ab2, Cry1F,
Cry3Bb1, Cry34Ab1, and Cry35Ab1—to protect plants against 13 different insect pests
(European corn borer, southwestern corn borer, southern cornstalk borer, corn earworm,
fall armyworm, stalk borer, lesser corn stalk borer, sugarcane borer, western bean cut-
worm, black cutworm, western corn rootworm, northern corn rootworm, Mexican corn
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