Insect pests cause significant damage to crops world-wide. This is despite integrated pest management strategies combining such control measures as chemical control, use of resistant varieties and other measures. Other control measures such as use of genetically modified crops are being adopted. Transgenic crops engineered for enhanced levels of resistance to insect pests have the potential to offer large benefits to agriculture not only through enhanced crop protection, but also from a reduction in the number of insecticide treatments required compared to conventional cropping methods. Insect resistant crops expressing?Bacillus thuringiensis?(Bt) delta-endotoxins are currently being grown in many regions of the world.?B. thuringiensis?is a gram positive, spore-forming bacterium that produces crystalline inclusion bodies during sporulation. These contain insecticidal delta endotoxins, also known as insecticidal crystal proteins. There are two groups of insecticidal crystal proteins (ICP): Cry (crystal delta endotoxins) and Cyt (cytolytic). The specificities of the different insecticidal crystal proteins determine their subsequent toxicity. Cry toxins are classified by their primary amino acid sequence and more than 500 different?Crygene sequences have been classified into 67 groups (Cry1 to Cry67). They are globular molecules composed of three distinct functional domains connected by a short conserved sequence. Two major types of receptors have been identified: transmembrane proteins, such as cadherins, and proteins anchored to the membrane such as the glycosylphosphatidylinisotol (GPI)-anchored proteins that have been proposed to be involved in the action of Cry toxins. The continued use of transgenic crops is threatened by the evolution of resistance in insect populations. It is against this background that research work targeting other candidate genes such as proteinase inhibitors, lectins and secondary metabolites is gaining momentum.
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