Mode of Action of Cry Toxins from Bacillus thuringiensis and Resistance Mechanisms

Abstract

Bacillus thuringiensis (Bt) insecticidal Cry toxins have been shown to be effective in controlling insect pests either in spray products or expressed in transgenic crops. All Cry toxins are expressed as protoxins that undergo proteolytic processing in the insect gut releasing the activated toxin. It has been shown that activated toxin binds to different insect protein molecules in gut cells leading to oligomerization, membrane insertion, and pore formation. However, it was recently shown that not only the activated toxin is able to specifically interact with receptors, since Cry1A protoxins bind gut receptor molecules leading also to oligomerization, membrane insertion, and pore formation. The final pores induced by protoxin or by activated toxin have different characteristics, suggesting dual mode of action of Cry proteins. In addition it was shown that different Cry1A resistant populations from different insect species are significantly more susceptible to Cry1A protoxins than to Cry1A activated toxins, supporting that Cry1A proteins may undergo two toxic pathways involving protoxin binding to receptors and binding of activated Cry toxins to gut receptor molecules. Here the authors will revise this dual mode of action of Cry proteins and discuss implications of the dual mode of action of Cry proteins for insect pest management in transgenic plants.