Current status of Bacillus thuringiensis: Insecticidal crystal proteins and transgenic crops

Abstract

Bacillus thuringiensis (Bt) is used to control agriculturally-important pests. It is a Gram positive spore-forming bacterium which produces parasporal proteinaceous inclusions during the sporulation phase. These crystalline parasporal inclusions are toxic to a wide spectrum of insects including the orders Lepidoptera, Coleopteran, Diptera, etc. The Bt insecticide proteins are toxic only after ingestion by the susceptible insects. The main steps involved when the Cry protein is ingested by the insect is comprised of solubilization of the protoxin, its enzymatic activation by terminal cleavage, receptor binding in brush border membrane of the midgut, pore formation, consequent disruption of ionic potential and destruction of the epithelial membrane leading to cell death. The first discovery of Bt was in 1901 when Ishiwata discovered a bacterium in Japan and in 1915, Berliner in Germany renamed it as Bacillus thuringiensis. Following a brief introduction, this chapter addresses the classification, the general structure of Cry toxin, its mode of action, strategies to improve the insecticidal activity of Cry proteins, transgenic plants developed using Bt genes, resistance to Bt toxins and resistance management, and an overall brief account of Bt and its insecticidal proteins, from 1901 to the present.