World wide damage due to insect pest attack accounts for 15% crop losses, despite the use of insecticides which represents over US $ 100 billion (Krattiger, 1997). The annual cost of insect control itself amounts to US $ 8 billion, thus warranting immediate and economical control measures. Unfortunately the crop varieties developed in the past 30 years were high yielders, but had poor storage characteristics and were highly susceptible to pest damage. Insect pests are capable of evolving to biotypes that can adapt to new situations, for instance they overcome the effect of toxic materials or bypass natural or artificial plant resistance, which further confounds the problem (Roush and McKenzie, 1987). Under these circumstances, provision of food to the rapidly expanding population has always been a challenge facing mankind. This problem is more acute in the tropics and sub-tropics, where the climate provides a highly condusive environment for a wide range of insects and necessitates massive efforts to suppress the population densities of different pests in order to achieve an adequate supply of food. In developing countries, the problem of competition from insect pests is further complicated with a rapid annual increase in human population (2.5-3.0 percentage) against a mearge 1.0 percent increase in food production. In order to feed the ever expanding population, crop protection plays a vital and integral role in the present day agricultural production to minimise yield losses. Currently, the crop protection practice in agricultural systems relies exclusively on the use of agrochemicals, although a few specific cases do exist, where inherent varietal resistance and biological control have been successfully employed. The exclusive use of chemical pesticides not only results in rapid build-up of resistance to such compounds, but their non-selectivity affects the balance between pests and natural predators, and is generally in favour of pests (Metcalf, 1986). Therefore, an integrated pest management (IPM) programme, comprising a combination of practices including the judicious use of pesticides, crop rotation, field sanitation and above all exploitation of inherently resistant plant varieties would provide the best option (Meiners and Elden, 1978). The last option includes the use of transgenic crops, expressing foreign insecticidal genes which could make a significant contribution to sustainable agriculture and thus could be an integral component of IPM (Boulter, 1993). The production of transgenic crops has seen rapid advances during the last decade with the commercial introduction of Bt transgenics, but the major concern with these crops has been the development of resistance by pest and public acceptability. Hence, there has been a need to discover new effective plant genes which would offer resistance/protection against these pests. Plant protease inhibitors (PIs) are one of the prime candidates with highly proven inhibitory activity against insect pests and also known to improve the nutritional quality of food.

Plant proteinase inhibitors (PIs) have been well established to play a potent defensive  role against predators and pathogens. Although diverse endogenous functions for these proteins has been proposed, ranging from regulators of endogenous proteinases to act as storage proteins, evidence for many of these roles are partial, or confined to isolated examples. On the other hand, many PIs have been shown to act as defensive compounds against pests by direct assay or by expression in transgenic crop plants, and a body of evidence for their role in plant defense has been accumulated consistently. The role and mechanism of action for most of these inhibitors are being studied in detail and their respective genes isolated. These genes have been used for the construction of transgenic crop plants to be incorporated in integrated pest management programmes. This article describes the classes of protease inhibitors, their regulation and genes used to construct transgenic plants against phytophagous insects.

The protease inhibitor genes have practical advantages over genes encoding for complex pathways i.e. by transferring single defensive gene from one plant species to another and expressing them from their own wound inducible or constitutive promoters thereby imparting resistance against insect pests (Boulter, 1993). This was first demonstrated by Hilder et al. 1987 by transferring trypsin inhibitor gene from Vigna unguiculata to tobacco, which conferred resistance to wide range of lepidopteran insect pests. Further, there is no evidence that it had toxic or deleterious effects on mammals.  Many of these protease inhibitors are rich in cysteine and lysine, contributing to better and enhanced nutritional quality (Ryan, 1989). Protease inhibitors also exhibit a very broad spectrum of activity including suppression of pathogenic nematodes (Williamson and Hussey, 1996), inhibition of spore germination and mycelium growth of Alternaria alternata (Dunaevskii et al. 1997). These advantages make protease inhibitors an ideal choice to be used in developing transgenic crops resistant to insect pests. Further, transformation of plant genomes with PI-encoding cDNA clones appears attractive not only for the control of plant pests and pathogens, but also as a means to produce PIs, useful in alternative systems and the use of plants as factories for the production of heterologous proteins (Sardana et al. 1998).

A large number of protease inhibitor genes with distinct modes of action have been isolated from a wide range of crop species. Development of transgenic crops have come a long way from the first transgenic developed by Hilder et al. 1987. Many transgenic crops have been generated using various plant PIs as listed in Table 1 of the article and are found to be resistant against wide range of pests, pathogens and nematodes. Further, the availability of diverse genes from different plant species makes it a possibility to use one or more genes in combination, whose products are targeted at different biochemical and physiological processes within the insect. These packages will not only contain protease inhibitor genes but also lectins, alpha-amylase inhibitors, or other plant genes encoding insecticidal proteins. This technology may not replace the use of chemical pesticides in near future but effectively complement it. However, in future non-scientific issues such as regulatory approval, propriety rights and public perception will be decisive in releasing crop plants produced by genetic engineering using recombinant DNA technology.

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METCALF, R.L. The ecology of insecticides and the chemical control of insects. In: KOGAN, M. ed. Ecological theory and integrated pest management. New York, John Wiley and Sons, 1986, p. 251-297.

ROUSH, R.T. and MCKENZIE, J.A. Ecological and genetics of insecticide and araricide resistance. Annual Review of Entomology, 1987, vol. 32, p. 361-381.

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SARDANA, R.K.; GANZ, P.R.; DUDANI, A.K.; TACKABERRY, E.S.; CHENG, X. and ALTOSAAR, I. Synthesis of recombinant human cytokine GMCSF in the seeds of transgenic tobacco plants. In: CUNNINGHAM, C. and PORTER A.J.R., eds. Recombinant proteins from plants. Production and isolation of clinically useful compounds. Totowa NJ, Humana Press, 1998, p. 77-87.

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