Boosting understanding of Bt-BoosterTM: modes of action of cadherin-based synergism of Bacillus thuringiensis Cry toxins

The Cry toxins produced by Bacillus thuringiensis (Bt) are the most widely used biopesticides effective against a wide range of crop pests and disease vectors. Like chemical insecticides, development of resistance is a primary threat to the long-term efficacy of Bt toxins. Also, there is the problem that some pests, such as Spodoptera frugiperda (fall armyworm) are relatively tolerant to Cry proteins. The serendipitous discovery of Bt-booster, an insect receptor (cadherin)-based synergist of Bt toxin, provides a way to surmount Bt resistance and kill relatively Bt-tolerant insects. However, a lack of understanding of Bt-booster’s mode of action in the insect midgut presents a major challenge to fully realizing its potential. Here we have elucidated the mechanism of cadherin-based Bt synergism by utilizing the differential Cry1Fa toxin enhancing properties of Spodoptera frugiperda (SfCad) and Manduca sexta (MsCad) cadherin fragments. Both fragments exhibited high affinity for Cry1Fa toxin and increased rate of Cry1Fa-induced pore-formation, and in vitro and in vivo stabilization of Cry1Fa toxin in S. frugiperda. However, only SfCad bound S. frugiperda BBMV and more effectively prolonged stability of Cry1Fa in the gut explaining higher Cry1Fa enhancement by this peptide. This study shows for the first time that cadherin fragments enhance Bt toxicity by at least 2 different mechanisms or a combination thereof: (1) increasing probability of toxin-receptor interactions by prolonging toxin stability in the protease-laden insect gut environment, and (2) increasing rate of pore formation in the midgut epithelium.