Functional assessment of cadherin as a shared mechanism for cross/dual resistance to Cry1Ac and Cry2Ab in Helicoverpa zea

Helicoverpa zea is a major target pest of pyramided transgenic crops expressing Cry1, Cry2 and/or Vip3Aa proteins from Bacillus thuringiensis (Bt) in the United States. Laboratory -selected Cry1Ac/Cry2Ab cross resistance and fieldevolved practical dual resistance of H . zea to these two toxins have been widely reported. Whether the widespread Cry1Ac/Cy2Ab dual resistance of H . zea has resulted from the selection of one shared or two independent resistance mechanisms by pyramided Bt crops remains unclear. Cadherin is a well -confirmed receptor of Cry1Ac and a suggested receptor of Cry2Ab in at least three Lepidopteran species. To test whether cadherin may serve as one shared mechanism for the cross and dual resistance of H . zea to Cry1Ac and Cry2Ab, we cloned H . zea cadherin ( HzCadherin ) cDNA and studied its functional roles in the mode of action of Cry1Ac and Cry2Ab by gain- and lossof -function analyses. Heterologous expression of HzCadherin in H . zea midgut, H . zea fat body and Sf9 cells made all three of these cell lines more susceptible to activated Cry1Ac but not activated Cry2Ab, whereas silencing HzCadherin of H . zea midgut and fat body cells significantly reduced the susceptibility to Cry1Ac but not Cry2Ab. Likewise, suppressing HzCadherin with siRNA made H . zea larvae resistant to Cry1Ac. These results clearly demonstrate that HzCadherin is not a receptor for Cry2Ab, and thus it is unlikely to serve as one shared mechanism for the cross and dual resistance of H . zea to Cry1Ac and Cry2Ab.