Insects

Insect infestation has caused heavy losses in many agricultural crops for years. Depending on the crop, it is estimated that losses range from 5–30% (FAOSTAT 2000). Control of insects has traditionally employed application of pesticides and to some extent biocontrol agents such as Bacillus thuringiensis (Bt). The cloning of the δ-endotoxin of Bt has allowed the generation of transgenic plants containing the gene. For the past ten years, genes coding for δ-endotoxins from different Bacillus thuringiensis (Bt) subspecies individually or in combination, have been used to protect crops against insects (Sanchis and Lereclus 1999). Currently, corn, potato and cotton plants expressing different synthetic Bt are commercially available; they show meaningful protection again different insects such as European corn borer, Colorado potato beetle and bollworm infestations respectively (Dempsey et al. 1998) Recently transgenic rice containing a fusion gene derived from cryIA(b) and cryIA(c) was field-tested in natural and repeated heavy manual infestation of two lepidopteran insects, leaffolder and yellow stem borer (Tu et al. 2000). The transgenic hybrid plants showed high resistance against both insect pests without reduced yield.

In spite of these successes, it is worth mentioning that Bt δ-endotoxins have not been effective against all insects and, most importantly, that insects have developed resistance against different δ-endotoxins (Tabashnik et al. 2000). For that reason alternative insecticidal proteins are being actively pursued. Several such proteins have been identified, including Vip3A and cholesterol oxidases (Dempsey et al. 1998). Various endogenous proteins, which are synthesised in response to insect attack, could potentially be used to engineer pest-resistant plants. One such protein is systemin, the first plant polypeptide hormone discovered. Systemin is phloem-mobile and is an essential component of the wound-inducible systemic signal transduction system leading to the transcriptional activation of the defensive genes (Ryan 2000). Systemin is processed from a larger prohormone protein, called prosystemin, by proteolytic cleavages and it has being suggested that overexpression of prosystemin in transgenic plants may confer protection against insect invasion (Schaller 1999).

A quite different alternative to control insect infestation has been the use of proteinase inhibitors (PIs). These compounds can inhibit various digestive enzymes (proteinases) found in the gut of many insects. The synthesis of some PI is stimulated by wounding, including insect attack, whereas others are induced by pathogen infection (Ryan 1990). The wound-inducible serine PIs from tomato have been studied the most extensively. They were divided into two groups based on sequence and molecular weight (PI-1 = 8kDa; PI-I = 12kDa) (Ryan 1990). Several components in the induction pathway leading to PI synthesis have been identified, including systemin, various intermediates of the octadecanoid pathway and jasmonic acid (Ryan and Pearce 1998). Constitutive expression of different types of PI, including serine (Duan et al. 1996; Hilder and Boulter 1999) and cysteine (Irie et al. 1996) in transgenic plants reduces predation by inhibiting important digestive enzymes in the insect gut. Nevertheless, similarly to the Bt δ-endotoxin, insects have developed resistance against PI (Girard et al. 1998). For a more effective and durable resistance, it may be necessary to combine different strategies.