Abstract
Insecticidal proteins from Bacillus thuringiensis (Bt) are used extensively in sprays and transgenic crops for pest control, but their efficacy is reduced when pests evolve resistance. Better understanding of the mode of action of Bt toxins and the mechanisms of insect resistance is needed to enhance the durability of these important alternatives to conventional insecticides. Mode of action models agree that binding of Bt toxins to midgut proteins such as cadherin is essential for toxicity, but some details remain unresolved, such as the role of toxin oligomers. In this study, we evaluated how Bt toxin Cry1Ac and its genetically engineered counterpart Cry1AcMod interact with brush border membrane vesicles (BBMV) from resistant and susceptible larvae of Pectinophora gossypiella (pink bollworm), a global pest of cotton. Compared with Cry1Ac, Cry1AcMod lacks 56 amino acids at the amino-terminus including helix α-1; previous work showed that Cry1AcMod formed oligomers in vitro without cadherin and killed P. gossypiella larvae harboring cadherin mutations linked with >1000-fold resistance to Cry1Ac. Here we found that resistance to Cry1Ac was associated with reduced oligomer formation and insertion. In contrast, Cry1AcMod formed oligomers in BBMV from resistant larvae. These results confirm the role of cadherin in oligomerization of Cry1Ac in susceptible larvae and imply that forming oligomers without cadherin promotes toxicity of Cry1AcMod against resistant P. gossypiella larvae that have cadherin mutations.
Highlights
The widespread bacterium Bacillus thuringiensis (Bt) produces insecticidal crystalline (Cry) proteins that have been used in sprays for pest control for more than 50 years [1]
We focused here on oligomer formation from Cry1Ac incubated with brush border membrane vesicles (BBMV) from resistant and susceptible larvae
With the optical density of bands from Cry1Ac oligomers in BBMV from APHIS-S standardized as 100%, the relative mean optical density of bands from AZP-R was 32%
Summary
The widespread bacterium Bacillus thuringiensis (Bt) produces insecticidal crystalline (Cry) proteins that have been used in sprays for pest control for more than 50 years [1]. Transgenic crops that produce Bt proteins to kill pests were first commercialized 20 years ago and were planted on 78 million hectares worldwide in 2014 [2, 3]. Cry toxins are effective against their target insect pests, but are not toxic to plants, vertebrates, or most non-target invertebrates [2, 4, 5, 6]. The evolution of resistance in insect pests is the primary threat to the PLOS ONE | DOI:10.1371/journal.pone.0144086. Cadherin Is Involved in Cry1Ac Toxin Oligomerization The evolution of resistance in insect pests is the primary threat to the PLOS ONE | DOI:10.1371/journal.pone.0144086 December 3, 2015
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