Knockdown of two Cadherin genes confers resistance to Cry2A and Cry1C in Chilo suppressalis

Bacillus thuringiensis (Bt) Cry toxins play an important role in the management of insect pests. Resistance to Bt toxins has been reported in many pest insects but the mechanism responsible for this resistance in rice crop pests remains largely unknown. Cadherin is one of several Bt toxin receptors. At present, only one cadherin gene, CsCAD1, has been documented in the striped rice stem borer, Chilo suppressalis. We amplified a nearly full-length transcript of another C. suppressalis cadherin gene, CsCAD2, and found that it has a different expression pattern to CsCAD1. CsCAD1 was highly expressed in fifth and sixth instar larvae, especially in the midgut, while the expression levels of CsCA2 were equably in each developmental stage. Newly hatched larvae were fed on rice smeared with synthesized siRNA to knockdown either CsCAD1 or CsCAD2, and then were fed transgenic rice expressing either the Cry2A or Cry1C toxins. The siRNA-treatment groups had lower mortality and higher survival rates than the control group, suggesting that reduced expression of CsCAD1 or CsCAD2 increased resistance to Cry2A and Cry1C. We conclude that CsCAD1 and CsCAD2 interact with Bt toxins in C. suppressalis and that this interaction could be the mechanism underlying Bt resistance in this insect.