Performance of microencapsulated Bacillus thuringiensis Cry pesticidal proteins

Abstract

Abstract The entomopathogenic bacteria Bacillus thuringiensis (Bt) produce parasporal-crystal inclusions composed of different pesticidal proteins such as Cry, that show insecticidal activity against insect pests. Cry toxins are highly susceptible to degradation when exposed to adverse temperature and continuous sun-UV-light. Thus, encapsulation techniques are designed to improve their biopesticide performance and shelf-life. However, the effects of polymeric matrix encapsulation on the mechanism of action of Cry toxins produced by Bt kurstaki and Bt aizawai has not been evaluated. Here, we analyzed the solubilization, activation and the binding of Bt insecticidal Cry proteins to their receptors after microencapsulation and compared with commercial non-encapsulated Bt biopesticides. We show that solubilization is one step in the mechanism of action of these proteins that could limit Cry toxin action, the microencapsulation of Bt biopesticides did not alter protein profiles solubilization compared to those non-encapsulated, showing a 130 kDa (corresponding to Cry1 protoxin) and 70 kDa (corresponding to Cry2 protoxin) proteins. Activation with trypsin, chymotrypsin and larval midgut juice was analyzed, showing that this step is highly efficient, and proteins were cleaved producing similar ~ 55 to 65 kDa activated toxins in microencapsulated and non-encapsulated formulations. Binding assays with receptors that are present in brush border membrane vesicles (BBMV) of Manduca sexta and Spodoptera frugiperda larvae showed similar binding curves for conventional and microencapsulated formulations. Finally, LC50 bioassays against these pests, showed no significant differences among the treatments. However, when these formulations were subjected to UV radiation, we observed for the microencapsulated Bt formulations provided higher mortality against S. frugiperda larvae, supporting a higher protective effect against degradation. Overall, our results show that microencapsulation of Bt biopesticides did not affect the mechanism of action of their pesticidal proteins while enhanced protection to UV radiation. These data will contribute to the development of more efficient Bt biopesticide formulations.