Genetically modified corn exudes toxin
December 21, 1999: An article in Nature this month presents and reviews evidence that the insecticidal toxin produced by genetically modified corn (Bt corn) persists in the soil for considerable lengths of time, and can also affect insects that have not fed on the crop (1).
Bacillus thuringiensis is a ubiquitous soil bacterium that produces an insecticidal endotoxin during sporulation. There are different forms of this toxin, produced by different subspecies, each of which is highly specific for one or two insect orders. For example, in the subsp. kurtstaki, the cry1Ab gene encodes a precursor form of an endotoxin that is specific for lepidopteran pests such as moths and butterflies. When this protoxin is ingested by larvae of the target species, it is converted into its active form by proteases within the insect gut. The toxin then binds to receptors on the surface of endothelial cells, causing cell lysis and paralysis of the insect mouth and gut. This in turn prevents the larvae from feeding and causes death.
Bt toxin is considered to be an ideal tool for biological pest control for several reasons. It is highly specific for particular insect species; therefore, non-target organisms will not be affected. It is non-toxic to vertebrates, and target-insect resistance is slow to develop. Finally, due to its light-sensitivity, it does not persist in an exposed environment. Bt toxin has been used to control gypsy moth and spruce budworm populations; in this application, the protoxin is applied as a component of proteinaceous inclusion bodies produced by the bacterium, along with chemicals that attract feeding insects. However, the genes encoding the various forms of Bt toxin can also be inserted into other bacteria or plant species. For example, Bt corn is corn that contains a truncated form of the cry1Ab gene, and therefore produces an active form of Bt toxin, rather than the proform normally produced by the bacterium.
In 1998, almost 20% of the total acreage of corn planted in the United States was Bt corn (2). Deepak Saxena and colleagues (1) have found that this corn exudes toxin into the rhizosphere soil, where it binds surface-active soil particles, retains its larvicidal activity, and is protected against microbial degradation. These findings are in agreement with those of Tapp and Stotzky (1998), who observed that the toxin persisted in soil and retained its larvicidal activity for as long as 234 days (the longest period studied) (3). These root exudates add to the Bt toxin already introduced into the soil by pollen, and by incorporation of plant material during crop harvesting. It is not known if the presence of Bt toxin in the soil over long periods of time will affect the soil community or promote selection of toxin-resistant insects, but it is a possibility.
In addition to the persistance of Bt toxin in the environment, the toxin has been observed to affect insects which have not fed on Bt corn plants. In one laboratory study, caterpillars of the monarch butterfly were killed by feeding on milkweed that had been artificially contaminated by pollen from Bt corn (4). In a second case, green lacewings died as a result of ingesting European corn borers reared on Bt corn (5). The death of insect predators such as the green lacewing is certainly counterproductive in the struggle for control of insect pests.
It is likely that Bt corn will be the subject of further investigation. It remains to be determined if the persistence and spread of Bt toxin in the environment will have negative effects on soil conditions, patterns of resistance in insect pests, and organisms at higher levels within the food chain.