Involvement of the processing step in the susceptibility/tolerance of two lepidopteran larvae to Bacillus thuringiensis Cry1Aa toxin

• Cry1Aa toxin is over ten times more toxic to Tuta absoluta than to Spodoptera littoralis.
• Activation of Cry1Aa by Tuta absoluta gut juice generates a 65 kDa active toxin.
• Spodoptera littoralis gut juice leads to a complete degradation of Cry1Aa.
• Cry1Aa binds to proteins with different sizes in the BBMVs of the two larvae.
• Cry1Aa causes brush border membrane lysis in the midgut of the two insects.

Bacillus thuringiensis (Bt) Cry1A toxins are known for their effectiveness against lepidopteran insects. In this study, the entomopathogenic activity of Cry1Aa was investigated against two lepidopteran larvae causing serious threat to various crops, Spodoptera littoralis and Tuta absoluta. Contrarily to S. littoralis, which showed low susceptibility to Cry1Aa (40% mortality with 1 μg/cm2), T. absoluta was very sensitive to this delta-endotoxin (LC50 of 95.8 ng/cm2). The different steps in the mode of action of this toxin on the two larvae were studied with the aim to understand the origin of their difference of susceptibility. Activation of the 130 kDa Cry1Aa protein by T. absoluta larvae juice generated a 65 kDa active toxin, whereas S. littoralis gut juice led to a complete degradation of the protoxin. The study of the interaction of the brush border membrane vesicles (BBMV) with purified biotinylated Cry1Aa toxin revealed six and seven toxin binding proteins in T. absoluta and S. littoralis BBMV, respectively. Midgut histopathology of Cry1Aa fed larvae demonstrated approximately similar damage caused by the toxin in the two larvae midguts. These results suggest that the activation step was strongly involved in the difference of susceptibility of the two larvae to Cry1Aa.

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