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

Abstract

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 field-evolved 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 loss-of-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.