|Zhu, Kun yan|
Submitted to: Journal of Economic Entomology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/5/2007
Publication Date: 12/1/2007
Citation: Li, H., Buschman, L.L., Huang, F., Zhu, K., Bonning, B., Oppert, B.S. 2007. Dipel-selected Ostrinia nubilalis larvae are not resistant to transgenic corn expressing Bacillus thuringiensis Cry1Ab. Journal of Economic Entomology 100: 1862-1870. Interpretive Summary: Bacillus thuringiensis is a bacterium used to control insect pests, and it acts through toxins (one of which is cry1Ab) which is activated in the insect gut. Resistant European corn borer larvae can survive Bacillus thuringiensis Cry1Ab protoxin in bioassays, but not Cry1Ab-transgenic plants or plant extracts in bioassays. To examine the paradox, Cry1Ab protoxin was incubated with enzymes in corn extracts, and the protoxin was hydrolyzed to a 58 kDa protein, similar to the product obtained from incubating Cry1Ab protoxin with either corn borer gut enzymes or mammalian trypsin. Therefore, although the purported form of Cry1Ab in transgenic hybrids is not fully activated, the insect ingests a functionally active form of the toxin, either as a result of corn and/or insect gut enzymes. These data explain why Bt-resistant corn borer larvae cannot survive on Bt-transgenic corn and provide assurance that proteinase-mediated resistance in field corn borer populations poses no threat to the success of Bt transgenic technology.
Technical Abstract: The survival of KS-SC DipelTM-resistant and -susceptible European corn borer, Ostrinia nubilalis Hübner, was evaluated on different tissues from corn hybrids, including a non-transgenic and two transgenic corn plants (events MON810 and Bt11) expressing high doses of Bacillus thuringiensis (Bt) delta-endotoxin. After 5 d exposure to transgenic plant tissues, survival of third instar Bt-resistant and -susceptible larvae was similar. The survivors eventually died when returned to Bt corn tissues, but many were able to continue development when transferred to non-Bt corn tissues. Survival of resistant and susceptible larvae also was evaluated in bioassays with dilutions of leaf extracts from the three corn hybrids incorporated in an artificial diet. In this case, survival was significantly higher for resistant O. nubilalis neonates at three of the five dilutions when compared to susceptible strain, but the resistance ratio was only 2.2- and 2.4-fold for MON810 and Bt11, respectively. The data demonstrate that resistant O. nubilalis were as susceptible to MON810 and Bt11 event corn hybrids as an unselected control strain. To evaluate the Cry1Ab protein that larvae were exposed to in the transgenic tissue, Cry1Ab protoxin was incubated with extracts from non-Bt corn leaf. The protein was hydrolyzed rapidly into peptide fragments with molecular masses ranging from 132 to 74 kDa, and eventually 58 kDa. These data suggest that during the preparation of Bt-leaf extracts, enzymes in the corn leaf tissues hydrolyze the transgenic toxin to one that is functionally activated. Therefore, resistant insect populations with reduced proteinase activity pose no threat to the efficacy of commercial MON810 and Bt11 corn hybrids.