Genetic transformation of crops for insect resistance: Potential and limitations

Transgenic resistance to insects has been demonstrated in plants expressing insecticidal genes such as delta-endotoxins from Bacillus thuringiensis (Bt), protease inhibitors, enzymes, secondary plant metabolites, and plant lectins. While transgenic plants with introduced Bt genes have been deployed in several crops on a global scale, the alternative genes have received considerably less attention. The protease inhibitor and lectin genes largely affect insect growth and development and, in most instances, do not result in insect mortality. The effective concentrations of these proteins are much greater than the Bt toxin proteins. Therefore, the potential of some of the alternative genes can only be realized by deploying them in combination with conventional host plant resistance and Bt genes. Genes conferring resistance to insects can also be deployed as multilines or synthetic varieties. Initial indications from deployment of transgenics with insect resistance in diverse cropping systems in USA, Canada, Argentina, China, India, Australia, and South Africa suggest that single transgene products in standard cultivar backgrounds are not a recipe for sustainable pest management. Instead, a much more complex approach may be needed, one which may involve deployment of a combination of different transgenes in different backgrounds. Under diverse climatic conditions and cropping systems of tropics, the success in the utilization of transgenics for pest management may involve decentralized national breeding programs and several small-scale seed companies. While several transgenic crops with insecticidal genes have been introduced in the temperate regions, very little has been done to use this technology for improving crop productivity in the harsh environments of the tropics, where the need for increasing food production is most urgent. There is a need to develop appropriate strategies for deployment of transgenics for pest management, keeping in view the pest spectrum involved, and the effects on nontarget organisms in the ecosystem.