1988). For example, the most widely deployedcrygenes in transgenic plants are members
of thecry1A gene family, which are toxic to a broad range of Lepidoptera pests. However,
this form of Bt has relatively little effect on Coleoptera species because the insects lack the
specific receptors that recognize Cry1A proteins. Likewise, some beetle species, such as the
Colorado potato beetle (Leptinotarsa decemlineata), are targeted by the Cry3A Bt toxin,
whereas most lepidopterans are unaffected. Therefore, specific crygenes have been
expressed in transgenic crops to tailor varieties to control specific pests and not affect non-
target species. For example, several variations ofcry1A genes have been transferred to corn
to control European corn borer (Ostrinia nubilali), a lepidopteran pest that feeds on the
insides of corn stems; whereascry3Bb1 expression has been used in corn varieties to
control western corn rootworm (Diabrotica virgifera)larvae, a coleopteran species that
feeds primarily on roots. By using this strategy, varieties resistant to a particular insect
pest can be effective in growing regions where particular pests are problematic.
Because of the steps necessary to activate them and their target sites in the digestive tract,
the Cry toxins are not effective as contact insecticides. Rather, insects are killed only when
the toxins are ingested. This means that most nontarget and beneficial insects are not affected
in fields of Bt crops. Furthermore, most insect and noninsect species lack the specific
membrane receptors for Bt and often have digestive conditions that degrade the Bt toxin
if it is consumed; therefore Bt is essentially nontoxic for most arthropods, animals, and
birds (and humans). In fact, Bt sprays (the intact microbes) are considered to be so safe
that certified organic food production in the United States allows for the direct application
of Bt crystalline spores on plants immediately prior to harvest as a control for insects.
Organic growers use Bt in this form as a valuable tool for insect control. One disadvantage
of this approach in comparison to transgenic Bt production in plants is that Bt applied exter-
nally to plant surfaces does not penetrate the plant tissue and is not very stable, since it
breaks down with time and exposure to ultraviolet light. Even so, because organic producers
sometimes depend on application of Bt as a management tool, they are especially concerned
about the possibility of Bt-resistant insect populations developing because of the growing
and widespread application of engineered Bt crops.
As with herbicide-resistant crops, adoption of Bt transgenic crops has also been extensive.
Damage by insects can be a severe problem in cotton, and this crop is heavily treated with
synthetic chemical pesticides in many production schemes. In 2005 transgenic cotton rep-
resented almost 80% of the total of that crop grown in the United States, and it is widely
grown in other parts of the world, including China. Transgenic corn is now grown on well
over 50% of all the acreage in the United States. In the case of both cotton and corn, traits
of herbicide and insect resistance are often combined in the same plant lines as “stacked” traits.
8.3.3 Pathogen Resistance
Plant pathogens such as viruses, fungi, and bacteria are a severe and constant threat to agri-
cultural crop production. Multiple transgenic approaches have been used to attempt plant
disease control, although relatively few of these have yet made their way into the field of
production.
The most effective way to control pathogens in a field setting is to use plants that are
resistant to the problem pathogen. Resistance to a particular pathogen can often be con-
ferred by a single plant gene (anRgene), the product of which is active in recognition
of the presence or activity of a single virulence factor from the pathogen (encoded by an
Avr gene). In plant pathogen systems, this relationship is known as a gene-for-gene
202 GENES AND TRAITS OF INTEREST FOR TRANSGENIC PLANTS