Crop Protection Group, Biological Sciences, University of Durham, Durham DH1 3LE, UK

Background and objectives
Crop protection plays an integral role in modern-day agricultural production where the ever-increasing demands on yield and the intensification of farming practice have increased the problem of pest damage, and hence control. Transgenic plant technology can provide a major contribution to the production of inherently resistant/tolerant varieties since it opens up a virtually limitless source of germplasm variability from which to select insect control genes for introduction into elite crop varieties. The entomicidal bacterium Bacillus thuringiensis (Bt) has proved to be a valuable source of resistance genes, particularly in the control of lepidopteran and, to a lesser extent, coleopteran insects. However, there is still a need to develop alternative strategies, particularly for control of homopteran insects, many of which transmit agronomically important diseases and are not targeted by existing Bt toxins.

We have investigated the use of defensive genes taken from various plant species for control of insect pests in transgenic crop plants. Further, we have also investigated the modes of activity of the products of these genes at the molecular and biochemical levels, in order to better understand, and hence exploit, this technology.

Results and conclusions
Several different classes of plant proteins have been shown to be insecticidal towards a range of economically important insect pests from different orders in artificial diet bioassays. Genes encoding these proteins have been isolated from various sources, and expressed in transgenic crops through the use of plant genetic engineering technology. Amongst these genes are those that encode inhibitors of proteases and alpha-amylase, lectins and enzymes such as chitinases and lipoxygenases. Protease inhibitors from a variety of plant sources have been used to produce transgenic plants with enhanced resistance to lepidopteran and coleopteran larvae. Transgenic legumes such as peas and mung beans expressing the l-amylase inhibitor from Phaseolus vulgaris are highly resistant to larvae of bruchid seed weevils. The snowdrop lectin gene has been successfully expressed in potato to give protection against aphids and against lepidopteran pests [1], and in rice where it gives significant levels of protection against the rice brown planthopper. The mechanisms of toxicity of plant proteins towards insects are not well understoood. Although inhibition of digestion of ingested proteins by protease inhibitors can readily be demonstrated in gut extracts from lepidopteran and coleopteran larvae, some insects are able to respond and adapt to protease inhibitors. In lepidopteran larvae the soyabean Kunitz inhibitor causes changes in expression of a family of serine protease-encoding genes, resulting in the induction of novel 'insensitive' proteases [2]. This phenomenon may explain the limited protection against insect attack afforded in some transgenic plants expressing foreign protease inhibitors. Lectin toxicity, on the other hand, is not directly mediated by effects on enzyme activities in the insect gut. Ingestion of snowdrop lectin by lepidopteran larvae and homopteran insects results in both binding of the lectin to proteins in the insect gut tissue, and transport to the haemolymph, and may result in systemic effects on insect development. Better understanding of the mechanisms of action of insecticidal plant proteins is likely to enable their effectiveness as protective agents to be increased.

1. Gatehouse AMR, Davison GM, Newell CA et al., 1997. Molecular Breeding 3, 49-63.
2. Brown DP, Wilkinson HS, Gatehouse JA, 1997. Insect Biochemistry and Molecular Biology 27, 625-638.