|TABASHNIK, BRUCE - University Of Arizona|
|ZHANG, MIN - University Of Arizona|
|WU, YIDONG - Nanjing Agricultural University|
|GAO, MEIJING - Nanjing Agricultural University|
|HUANG, FANGNENG - Louisana State University|
|WEI, JIZHEN - University Of Arizona|
|ZHANG, JIE - Chinese Academy Of Agricultural Sciences|
|YELICH, ALEXANDER - University Of Arizona|
|UNNITHAN, CHANDRAN - University Of Arizona|
|BRAVO, ALEJANDRA - Universidad Nacional Autonoma De Mexico|
|SOBERON, MARIO - Universidad Nacional Autonoma De Mexico|
|CARRIERE, YVES - University Of Arizona|
|LI, XIANCHUN - University Of Arizona|
Submitted to: Scientific Reports
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/16/2015
Publication Date: 10/12/2015
Publication URL: http://handle.nal.usda.gov/10113/61617
Citation: Tabashnik, B.E., Zhang, M., Fabrick, J.A., Wu, Y., Gao, M., Huang, F., Wei, J., Zhang, J., Yelich, A., Unnithan, C., Bravo, A., Soberon, M., Carriere, Y., Li, X. 2015. Dual mode of action of Bt proteins: Protoxin efficacy against resistant insects. Scientific Reports. 5:15107. doi: 10.1038/srep15107.
Interpretive Summary: Genetically modified or transgenic crops producing insecticidal toxins of the bacterium, Bacillus thuringiensis (Bt), are key management tools for many insect pests. These Bt toxins kill a number of major insect pests while having little impact on beneficial insects or other plants or animals. Established models of Bt mode of action assert that Bt toxins are partially digested or "activated" by digestive enzymes within the insect digestive system. These activated Bt proteins then bind and damage the insect gut, eventually killing the insect. Contrary to this paradigm, an ARS scientist at Maricopa, AZ and collaborators report that precursor Bt toxin is more potent than the corresponding active toxin in seven resistant strains of three major crop pests. These data suggest a new model of Bt intoxication, in which the precursor Bt toxin can kill insects more effectively than the active toxin. Recognizing that precursor Bt toxins can be more potent than activated toxins against Bt resistant insects may help delay resistance, enhance efficacy of Bt toxins, and improve the rational design of more effective transgenic Bt crops.
Technical Abstract: 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 in an inactive protoxin form that requires 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 some previously reported results support a new model in which both protoxins and activated toxins can kill insects directly. 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.