Abstract
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.
Highlights
Insecticidal Cry proteins from Bacillus thuringiensis (Bt) have been widely used to develop transgenic crops that have become an important part of agricultural pest management[1,2,3]
In contrast to CsCAD1, which was almost exclusively expressed in the midgut, CsCAD2 were highly expressed in a number of different organs
After the Cry1A toxin has been activated by enzymes in the midgut, it binds to the first receptor with high affinity, which facilitates oligomerization of the toxins via a proteolytic process resulting in cell lysis
Summary
Insecticidal Cry proteins from Bacillus thuringiensis (Bt) have been widely used to develop transgenic crops that have become an important part of agricultural pest management[1,2,3]. A calcium-dependent cell adhesion protein[14], is thought to be one of several such receptor proteins that bind to Cry toxins[15, 16] The first such cadherin protein to be identified was the Cry1A toxin-binding protein in Manduca sexta[17], which was found to be involved in binding Cry toxins in other Lepidopteran, coleopteran and dipteran insects[4, 18,19,20,21,22,23]. Transgenic rice strains expressing the Cry toxins Cry2A and Cry1C have been developed to protect rice crops from this notorious pest. It is likely that C. suppressalis will develop resistance to these toxins once transgenic rice becomes more widely grown. It is, important to understand the mechanisms that confer resistance to Cry toxins in this species. We found that knockdown of these genes reduced sensitivity to both Cry2A and Cry1C in C. suppressalis
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