It is well known that insect larval midgut cadherin protein serves as a receptor of Bacillus thuringiensis (Bt) crystal Cry1Ac or Cry1Ab toxins, since structural mutations and downregulation of cad gene expression are linked with resistance to Cry1Ac toxin in several lepidopteran insects. However, the role of Spodoptera frugiperda cadherin protein (SfCad) in the mode of action of Bt toxins remains elusive. Here, we investigated whether SfCad is involved in susceptibility to Cry1Ab or Cry1Fa toxins. In vivo, knockout of the SfCad gene by CRISPR/Cas 9 did not increase tolerance to either of these toxins in S. frugiperda larvae. In vitro cytotoxicity assays demonstrated that cultured insect TnHi5 cells expressing GFP-tagged SfCad did not increase susceptibility to activated Cry1Ab or Cry1Fa toxins. In contrast, expression of another well recognized Cry1A receptor in this cell line, the ABCC2 transporter, increased the toxicity of both Cry1Ab and Cry1Fa toxins, suggesting that SfABCC2 functions as a receptor of these toxins. Finally, we showed that the toxin-binding region of SfCad did not bind to activated Cry1Ab, Cry1Ac, nor Cry1Fa. All these results support that SfCad is not involved in the mode of action of Cry1Ab or Cry1Fa toxins in S. frugiperda.
Keywords: Bacillus thuringiensis; Cry1Ab; Cry1Fa; Spodoptera frugiperda; cadherin; mode of action of Cry toxin.