Toxicity and Mode of Action of Bacillus thuringiensis Cry Proteins in the Mediterranean Corn Borer, Sesamia nonagrioides (Lefebvre)

Abstract

Sesamia nonagrioides is one of the most damaging pests of corn in Spain and other Mediterranean countries. Bt corn expressing the Bacillus thuringiensis Cry1Ab toxin is being grown on about 58,000 ha in Spain. Here we studied the mode of action of this Cry protein on S. nonagrioides (binding to specific receptors, stability of binding, and pore formation) and the modes of action of other Cry proteins that were found to be active in this work (Cry1Ac, Cry1Ca, and Cry1Fa). Binding assays were performed with 125 I-or biotin-labeled toxins and larval brush border membrane vesicles (BBMV). Competition experiments indicated that these toxins bind specifically and that Cry1Aa, Cry1Ab, and Cry1Ac share a binding site. Cry1Ca and Cry1Fa bind to different sites. In addition, Cry1Fa binds to Cry1A's binding site with very low affinity and vice versa. Binding of Cry1Ab and Cry1Ac was found to be stable over time, which indicates that the observed binding is irreversible. The pore-forming activity of Cry proteins on BBMV was determined using the voltage-sensitive fluorescent dye DiSC 3 (5). Membrane permeability increased in the presence of the active toxins Cry1Ab and Cry1Fa but not in the presence of the nonactive toxin Cry1Da. In terms of resistance management, based on our results and the fact that Cry1Ca is not toxic to Ostrinia nubilalis, we recommend pyramiding of Cry1Ab with Cry1Fa in the same Bt corn plant for better long-term control of corn borers.

Bacillus thuringiensis Cry1Ac Toxin-Binding and Pore-Forming Activity in Brush Border Membrane Vesicles Prepared from Anterior and Posterior Midgut Regions of Lepidopteran Larvae

“…It is known that in the presence of a transmembrane electrical potential, the fluorescence quenching of the voltagesensitive dye DiSC 3 (5) can be used to monitor the diffusional K ϩ movements across the vesicular membrane in insect midgut BBMV (9,21,37,38). In the present study, we measured the ability of Cry1Ac to induce the formation of pores with K ϩ channel properties by recording the fluorescence signal variations of the dye DiSC 3 (5) with increasing inwardly directed K ϩ gradients.…”

Midgut microbiota and host immunocompetence underlieBacillus thuringiensiskilling mechanism

“…Indeed, field monitoring of pest susceptibility, isolation of new toxins, and the study of resistance mechanisms have been widely pursued by the scientific community (10,15). The growing evidence that the alteration of toxin binding is the most common resistance mechanism (6) emphasizes the importance of binding models to predict the effective toxin combinations for successful implementation of gene pyramiding strategies (12,(16)(17)(18)(19).…”

Biotechnological Approaches for the Control of Insect Pests in Crop Plants

“…However, Bt toxins are not toxic to people, wildlife, or most beneficial insects [102,103] and therefore the opportunities for biological control are great. The effect of Bt toxins on a range of lepidopteran insects has been studied including: Bombyx mori [104], Helicoverpa armigera [105], Heliothis virescens [106,107], Manduca sexta [108,109], Ostrinia nubilalis [110][111][112][113], Plutella xylostella [114,115], Sesamia nonagrioides [115], Spodoptera exigua [116], Spodoptera frugiperda [117] and Spodoptera littoralis [118]. The Cry toxins produced in Bt crops generally target lepidopteran pests, although some also target coleopteran pests [100].…”