8.3.2 Insect Resistance
Insect damage to crops poses a problem for farmers worldwide. In spite of the great
amounts of money and effort spent on attempts to control insect pests, staggering losses
to insects are still incurred before and after harvest. In an effort to control these pests,
synthetic chemical insecticides are widely used where mechanized agriculture is practiced,
but insects nevertheless continue to represent a significant hindrance for food production. In
much of the world, insect damage proceeds unchecked by chemical pesticides, and growers
and consumers suffer significant losses in both yield and quality.
A number of proteins with negative effects on insects have been tested as potential
weapons for use in engineering insect-resistant transgenic crops. Genes for several proteins
have been expressed in transgenic plants and were shown to inhibit insect growth or cause
higher insect death rates. These include genes for protease inhibitors, which interfere with
insect digestion; lectins, which kill insects by binding to specific glycosylated proteins; and
chitinases, enzymes that degrade chitin found in the cuticle of some insects. Although each
of these genes has been shown to have some negative impact when consumed by insects
and may have some utility in insect control, none have been as effective or widely
adopted as genes encoding endotoxins from the bacteriumBacillus thuringiensis(Bt).
The natural insecticidal activity of Bt endotoxin proteins represents an attractive alternative
to synthetic chemical pesticides, which often have nonselective toxic effects on beneficial
insects, birds, fish, and mammals. The transgenic plant produces its own insecticidal protein
that is delivered only to insects that dare eat the plant. Rather than using the entire bacterium
to kill insects, only a single-plant-encoded transgene product is used.
A bacterial species producing Bt toxins was first isolated and described over 100 years
ago. A microbiologist named Ernst Berliner formally named the speciesBacillus thurin-
giensisin 1915. His work followed and confirmed the discovery in 1902 of a bacterial
Figure 8.2.Resistance to glufosinate in LibertyLinkTMplants is engineered by expressing an enzyme
that directly targets and inactivates the herbicide. Glufosinate kills plants by inhibiting glutamine
synthetase. This enzyme is responsible for production of the amino acid glutamine in a reaction
that can sequester excess nitrogen by incorporating ammonia (NH 4 þ). If this enzyme is inactivated
by glufosinate, excess ammonia accumulates and the plant is killed. An enzyme encoded by the bac-
terialbargene in transgenic plants inactivates glufosinate.
200 GENES AND TRAITS OF INTEREST FOR TRANSGENIC PLANTS