Synergism of Cry1Aa protein against Grapholita molesta (Busck) by a cadherin fragment from Spodoptera exigua (Hübner)

Bacillus thuringiensis (Bt) is currently the most commercialized biopesticide worldwide. Despite the success of Bt application in biological control programs, diverse strategies have been developed to enhance the toxicity of Bt toxins and delay the emergence of insect resistance, including the addition of cadherin fragments as a synergistic agent. In this study, the synergistic effect of two different cadherin fragments, one from a lepidopteran species, Spodoptera exigua (rSeCad1bp), and the other from a coleopteran species, Tenebrio molitor (rTmCad1p), was evaluated on the toxicity of Cry1Aa, Cry1Ab, and Cry1Ia against S. exigua and Grapholita molesta. Our results show that while the toxicity of Cry1 proteins to S. exigua was not affected by rSeCad1bp, the toxicity of Cry1Aa against G. molesta increased about 2.6-fold. No other synergistic or antagonist effects were observed. The potential mechanisms for the detected toxicity enhancement in G. molesta were studied, including protection against proteolysis, promotion of oligomerization, and increased binding to receptors on midgut brush border membrane vesicles (BBMV). Results revealed a slight increase in Cry1Aa binding to BBMV. These data highlight the great potential of using rSeCad1bp in combination with Cry1Aa for control of G. molesta. Key ContributionOur results show that the cadherin fragment from Spodoptera exigua synergized Cry1Aa toxicity against Grapholita molesta, but not to S. exigua. The synergistic effect observed might be partly associated with an increase in Cry1Aa binding to the target midgut membrane.