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Dual mode of action of Bt proteins: protoxin efficacy against resistant insects

Bruce E. Tabashnik, Min Zhang, Jeffrey A. Fabrick, Yidong Wu, Meijing Gao, Fangneng Huang, Jizhen Wei, Jie Zhang, Alexander Yelich, Gopalan C. Unnithan, Alejandra Bravo, Mario Soberón, Yves Carrière, Xianchun Li
Scientific reports 2015 v.5 article no.15107 pp. -
Bacillus thuringiensis, bacterial proteins, crops, crystal proteins, insecticidal properties, insects, mechanism of action, models, pest control, pesticide resistance, plant pests, toxicity, toxins, transgenic plants
Transgenic crops that produce Bacillus thuringiensis (Bt) proteins for pest control are grown extensively, but insect adaptation can reduce their effectiveness. Established mode of action models assert that Bt proteins Cry1Ab and Cry1Ac are produced as inactive protoxins that require conversion to a smaller activated form to exert toxicity. However, contrary to this widely accepted paradigm, we report evidence from seven resistant strains of three major crop pests showing that Cry1Ab and Cry1Ac protoxins were generally more potent than the corresponding activated toxins. Moreover, resistance was higher to activated toxins than protoxins in eight of nine cases evaluated in this study. These data and previously reported results support a new model in which protoxins and activated toxins kill insects via different pathways. Recognizing that protoxins can be more potent than activated toxins against resistant insects may help to enhance and sustain the efficacy of transgenic Bt crops.