Mode of action of cadherin-based Bacillus thuringiensis Cry toxin synergists and toxin interactions with midgut receptors in lepidopteran pests of corn

Abstract

The Cry toxins produced by Bacillus thuringiensis (Bt) are the active component of the most widely used biopesticides effective against a range of crop pests and disease vectors. The high synergistic potential of recently discovered insect receptor (cadherin)-based Bt Cry toxin synergist provides a way to potentially surmount insecticidal activity of Cry toxins. However, a lack of understanding of cadherin synergist’s mode of action in the insect midgut presents a major challenge to fully realizing its potential. In the first part of my dissertation research I have elucidated the mechanism of cadherin-based Bt synergism by utilizing the differential Cry1Fa toxin enhancing properties of Spodoptera frugiperda (SfCad) and Manduca sexta (MsCad) cadherin fragments. I have shown that cadherin fragments enhance Bt toxicity by at least two different mechanisms or a combination thereof; increasing probability of toxin-receptor interactions by prolonging toxin stability in the insect gut and increasing the rate of pore formation in the midgut epithelium. The Cry1Ab and Cry1Fa toxins are two important Cry toxins that are widely used for lepidopteran pest control in corn and cotton. Co-expressing Cry1Ab and Cry1Fa toxins in corn is an approach being employed to broaden the insecticidal spectrum of transgenic Bt corn varieties. However, the long term utility of this Cry1Ab/Cry1Fa stacked corn variety will depend on whether these toxins share common binding sites in the targeted pests. Therefore, in the second part of my dissertation research, I have developed and used a fluorescence-based binding assay to study Cry1Fa and Cry1Ab toxin interaction with brush border membrane vesicles (BBMV) from three important pests of corn, Ostrinia nubilalis (European corn borer; ECB), Diatraea grandiosella (south western corn borer; SWCB), and Helicoverpa zea (corn earworm; CEW). Our results showed that Cry1Fa shares binding site(s) with Cry1Ab toxin in ECB, and SWCB for which Cry1Ab has higher affinity than Cry1Fa. Apart from the shared binding sites, Cry1Ab binds additional site(s) in ECB, and both toxins bind additional site(s) in SWCB. In CEW, Cry1Fa and Cry1Ab each have a high affinity binding site(s), which binds the heterologous toxin with low affinity.