Insecticides based on the parasporal Cry proteins synthesized by the bacterium Bacillus thuringiensis (Bt) are considered the most important alternative to traditional synthetic insecticides. These Cry toxins posses a unique mode of action. After ingestion the toxins are solubilized and activated by midgut enzymes. Active toxins bind and insert on the membrane of the columnar cells of the midgut epithelium forming pores that result in impaired permeability and cell lysis due to osmotic shock.
One of the major concerns regarding the use of Bt insecticides is the observation that target insects can become resistant. Although several mechanisms of resistance have been proposed, the alteration of binding to the specific receptors in the midgut is the most common.
Heliothis virescens is an important pest in the US, causing extensive damage to cotton and other crops. To control this insect, in 1996 transgenic cotton expressing Bt Cry1Ac toxin was introduced in the US.
A Heliothis virescens strain (YHD2) under laboratory selection against Cry1Ac (the most active Bt toxin against this insect) developed high levels of resistance to Cry1Ac and high levels of cross-resistance to other toxins including Cry1Aa, Cry1Ab and Cry1Fa. This resistant strain has been continuously selected with Cry1Ac toxin since 1994, and the levels of resistance have increased through this time.
Previously, we reported that a dramatic decrease in toxin binding and pore formation in brush border membrane vesicles (BBMV) from the resistant insects are responsible for the high levels of resistance and cross-resistance observed in this strain. However, ligand blots and affinity purification of Cry1Ac-binding molecules from solubilized BBMV indicated that the toxin binding molecules are still present in BBMV from resistant larvae. Soybean agglutinin lectin blots detected differences in BBMV protein glycosylation between vesicles from susceptible and resistant insects.
The molecular mechanism involved in the reduction of toxin binding is being investigated in this work.
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