Countering pest resistance with genetically modified Bt toxins.

Abstract Insecticidal crystalline (Cry) toxins from the bacterium Bacillus thuringiensis (Bt) used in sprays and transgenic crop plants have provided major benefits for pest control, including decreased reliance on broad-spectrum chemical insecticides. However, extensive use of Bt toxins has selected for resistance, thus reducing or eliminating these benefits against some populations of at least seven species of major crop pests. This chapter reviews efforts to counter pest resistance to native Bt toxins with genetically engineered toxins called Cry1AbMod and Cry1AcMod. We generated these modified toxins by trimming the genes encoding the native toxins Cry1Ab and Cry1Ac so they lack the nucleotides that code for a portion of the amino-terminal end of the protein, including helix α-1 and part of helix α-2. Consistent with the sequential binding model for the mode of action of the toxins, the Cry1AMod toxins formed oligomers without binding to cadherin, but the native toxins required cadherin for oligomer formation. The modified toxins were more potent than the corresponding native toxins in 13 of 19 pairwise comparisons with 12 resistant strains of nine species of Lepidoptera, including field-selected strains of Plutella xylostella, Trichoplusia ni and Spodoptera frugiperda. The potency of modified toxins relative to native toxins in these resistant strains did not depend on the resistance mechanism. Against susceptible strains, the modified toxins were less effective than their native counterparts in nearly all cases. Transgenic tobacco plants producing Cry1AbMod killed larvae of Manduca sexta, which represents progress towards commercial use of the modified toxins for pest management.