Performance insights into spray-dryer microencapsulated Bacillus thuringiensis cry pesticidal proteins with gum arabic and maltodextrin for effective pest control

Abstract

Bacillus thuringiensis (Bt) produces crystals composed mainly of Cry pesticidal proteins with insecticidal activity against pests but are highly susceptible to degradation by abiotic factors. In this sense, encapsulation techniques are designed to improve their performance and lifetime. However, the effects of polymeric matrix encapsulation such as gum arabic and maltodextrin by spray-dryer in the mechanisms of action of Bt kurstaki and Bt aizawai are unknown. We analyzed crystal solubilization, protoxin activation, and receptor binding after microencapsulation and compared them with commercial non-encapsulated products. Microencapsulation did not alter protein crystal solubilization, providing 130 kDa (Cry1 protoxin) and 70 kDa (Cry2 protoxin). Activation with trypsin, chymotrypsin, and larval midgut juice was analyzed, showing that this step is highly efficient, and the protoxins were cleaved producing similar ~ 55 to 65 kDa activated proteins for both formulations. Binding assays with brush border membrane vesicles of Manduca sexta and Spodoptera frugiperda larvae provided a similar binding for both formulations. LC₅₀ bioassays showed no significant differences between treatments but the microencapsulated treatment provided higher mortality against S. frugiperda when subjected to UV radiation. Microencapsulation did not affect the mechanism of action of Cry pesticidal proteins while enhancing protection against UV radiation. These data will contribute to the development of more efficient Bt biopesticide formulations. KEY POINTS: • Microencapsulation did not affect the mechanisms of action of Cry pesticidal proteins produced by Bt. • Microencapsulation provided protection against UV radiation for Bt-based biopesticides. • The study's findings can contribute to the development of more efficient Bt biopesticide formulations.

Keywords: Bacillus thuringiensis; Manduca sexta; Mechanism of action; Microencapsulation; Spodoptera frugiperda.

Fig. 1

A Protein profile of the supernatant from formulations after solubilization in 50 mM NaOH buffer and 0.2% β-mercaptoethanol; B BSA control curve. Formulations: 1, Xentari^®; 2, Dipel^®; 3, Dipel^®/Xentari^®; 4, MP_Bta; 5, MP_Btk; and 6, MP_Btk/Bta

Fig. 2

Evaluation of the protein profiles on SDS-PAGE of different formulations after solubilization in midgut juice (30% v/v) isolated from S. frugiperda larvae. A Protein profile of formulations incubated in midgut juice pH 9.5 for 24 h; B protein profile of formulations incubated in midgut juice pH 10.5 for 24 h. Formulations: 1, Xentari^®; 2, Dipel^®; 3, Dipel®/Xentari^®; 4, MP_Bta; 5, MP_Btk; and 6, MP_Btk/Bta

Fig. 3

Binding interaction of activated toxin samples from different formulations to BBMV from S. frugiperda (A, B) and M. sexta (C, D). A sample of 10 µg BBMV was incubated with different concentrations of activated toxins present in different formulations. Plots were constructed after densitometry analysis of the 65 kDa band shown in Figure S3

Fig. 4

Effect of UV radiation on the toxicity of different Bt formulations against S. frugiperda larvae. A Xentari^® formulation and mixtures with different proportions of MP_Bta; B Dipel^® formulation and mixtures with different proportions of MP_Btk; C Dipel®/Xentari^® formulation and mixtures with different proportions of MP_Btk/Bta