APPROACHES TO INSECT RESISTANCE USING TRANSGENIC PLANTS

Crops resistant to insect attack offer a different strategy of pest co ntrol to indiscriminate pesticide usage, which has undesirable effects on both the environment and humans. Transgenic plant technology can b e a useful tool in producing resistant crops, by introducing entirely novel resistance genes into a plant species. Although most work in thi s area has focused on the use of genes encoding insecticidal Bacillus thuringiensis delta-endotoxins in transgenic plants, an alternative ap proach is to use plant genes which encode proteins with insecticidal p roperties. Protease inhibitors are involved in endogenous plant defenc e against insects. Over-expression of several inhibitors from constitu tive promoters has been shown to afford protection in transgenic tobac co plants against attack by lepidopteran larvae. However, the degree o f protection is not sufficiently high, and shows species- and inhibito r-specific effects. By assaying the interactions of protease inhibitor s with insect gut proteases in vitro, the most effective inhibitor can be selected for a particular insect species. Data from bioassays of i nsects using artificial diets, and with transgenic plants, suggest tha t the in vitro assay of relative inhibitor effectiveness is consistent with the effects of different inhibitors on insect development and su rvival in vivo. Development of this techniology is considered. A diffe rent approach must be taken with sucking insect pests, as they do not rely on proteolysis for nutrition, and as Bt toxins effective against homopterans have not been reported to date. Bioassay in artificial die t was used to identify plant proteins with insecticidal effects on the rice brown planthopper (a model homopteran). The lectin from snowdrop (GNA) was found to be the most effective of the proteins tested. GNA was shown to be present in the phloem sap of a transgenic tobacco plan t transformed with a chimeric gene construct, containing the rice sucr ose synthase-I gene promoter and the GNA coding sequence, by immunoass ay of honeydew produced by aphids feeding on it. GNA is also insectici dal to the aphid Myzus persicae, which will feed on tobacco, and thus a bioassay of transgenic tobacco, to 'prove' the technology, can be ca rried out. The effects of combining different resistance genes in the same transgenic plant to improve the effectiveness of protection are d iscussed, and exemplified.