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Title: Cis-mediated down-regulation of a trypsin gene associated with Bt resistance in cotton bollworm

Author
item LIU, CHENXI - Chinese Academy Of Agricultural Sciences
item XIAO, YUTAO - Chinese Academy Of Agricultural Sciences
item LI, XIANCHUN - University Of Arizona
item Oppert, Brenda
item TABASHNIK, BRUCE - University Of Arizona
item WU, KONGMING - Chinese Academy Of Agricultural Sciences

Submitted to: Scientific Reports
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/26/2014
Publication Date: 11/27/2014
Publication URL: http://handle.nal.usda.gov/10113/60070
Citation: Liu, C., Xiao, Y., Li, X., Oppert, B.S., Tabashnik, B.E., Wu, K. 2014. Cis-mediated down-regulation of a trypsin gene associated with Bt resistance in cotton bollworm. Scientific Reports. 4:7219. DOI: http://dx.doi.org/10.1038/srep07219.

Interpretive Summary: Microbial toxins have been very effective in controlling some insect pests, but resistant populations of insects threatens long-term efficacy of Bt crops. The cotton bollworm is effectively controlled by these toxins, but there are laboratory strains that can be selected for resistance. By studying one of these resistant bollworm strains, we found that a gut enzyme was greatly decreased in the resistant strain, and this was found to be a factor in why this insect pest was not killed as effectively by the toxin. The mechanism of the decreased production of the enzyme was linked to changes in the genetics of the resistant strain. These studies will help to refine resistance management and prolong the use of Bt crops to control major pests.

Technical Abstract: Transgenic plants producing insecticidal proteins from the bacterium Bacillus thuringiensis (Bt) are useful for pest control, but their efficacy is reduced when pests evolve resistance. Previously identified mechanisms of resistance to Bt toxins include reduced binding of activated Bt toxins to midgut receptors and reduced conversion of Bt protoxins to activated toxins by insect midgut proteases such as trypsin. Here we examined the mechanism of resistance to Bt toxin Cry1Ac in the laboratory-selected LF5 strain of the cotton bollworm, Helicoverpa armigera. This strain had 110-fold resistance to Cry1Ac protoxin and 39-fold resistance to Cry1Ac activated toxin. Evaluation of five trypsin genes revealed 99% reduced transcription of one trypsin gene (HaTryR) was associated with resistance. Silencing of this gene with RNA interference in susceptible larvae increased their survival on diets containing Cry1Ac. Bioassays of progeny from crosses revealed that resistance to Cry1Ac was genetically linked with HaTryR. We identified mutations in the promoter region of HaTryR in the resistant strain. In transfected insect cell lines, transcription was lower when driven by the resistant promoter compared with the susceptible promoter, implicating cis-mediated down-regulation of HaTryR transcription as a mechanism of resistance. The results suggest that H. armigera can adapt to Bt toxin Cry1Ac by decreased expression of trypsin. Because trypsin activation of protoxin is a critical step in toxicity, transgenic plants with activated toxins rather than protoxins might increase the durability of Bt crops.