Synergistic selection of a Helicoverpa armigera cadherin fragment with Cry1Ac in different cells and insects

The midgut cadherin fragments were extensively studied as Bt synergists in insects, while their synergistic selection modes with Bt toxins in different mechanisms of resistance or insects have never been determined. Here, a soluble Helicoverpa armigera cadherin fragment which corresponds to the Cry1Ac binding region (HaCad-TBR) was expressed in Escherichia coli and its synergism with Cry1Ac toxin in H. armigera and Plutella xylostella larvae as well as Sf9 cells expressing different cadherins was tested. HaCad-TBR exhibited higher synergism factor in P. xylostella larvae (4.84-fold) than in H. armigera larvae (2.45-fold). Among the cells expressing HaCad alleles, HaCad-TBR enhanced the Cry1Ac toxicity only in the cells expressing the mutant lacking the extracellular domain. Moreover, HaCad-TBR had a weak enhancement of Cry1Ac toxicity in Sf9 cells expressing the P. xylostella cadherin. Further researches revealed that the enhancement of toxicity in Sf9 cells was correlated with increased toxin binding. These results suggested that cadherin fragments which have high binding level with Cry1Ac are more likely to enhance toxin toxicity well against the cells or larvae where the cadherin has lower binding level with Cry1Ac, especially in the cases lacking the toxin binding domain.