Abstract:

Aminopeptidase N (APN) and cadherin are key receptors of Bacillus thuringiensis Cry1A toxins in brush border membrane vesicles (BBMVs) of lepidopteran insects. Effects of RNAi-mediated silencing of an APN gene Haapn1 and a cadherin gene Ha_BtR on Cry1Ac toxicity were investigated by injecting dsRNAs of these two genes into the 4th instar larvae of Helicoverpa armigera in this experiment. Reduction of mRNA expression of Haapn1 (30%-49%) and Ha_BtR (30%-37%) was observed in the larvae injected respectively with Haapn1 dsRNA and Ha_BtR dsRNA (1 μg/larva) compared with the control larvae injected with elution solution (ES) only. Mortality of larvae injected with Haapn1 dsRNA was significantly lower than that of the control larvae injected with ES in treatments of 40 and 70 μg/cm² of activated Cry1Ac, but there was no difference in mortality of larvae injected with either Haapn1 dsRNA or ES in treatments of 100 and 170 μg/cm² of Cry1Ac protoxin. RNAi-mediated gene silencing by injecting Ha_BtR dsRNA had no effect on toxicity of both activated Cry1Ac and Cry1Ac protoxin. However, toxicity of both activated Cry1Ac and Cry1Ac protoxin against the 4th instar larvae injected with a mixture of Haapn1 dsRNA and Ha_BtR dsRNA was significantly reduced. These results further confirm that both Haapn1 and Ha_BtR are functional receptors of Cry1Ac in H. armigera, and both of them are involved in intoxication of Cry1Ac. Our results also suggest that mutations occurring in either Haapn1 or Ha_BtR may result in resistance to Cry1Ac in H. armigera.

Key words: Helicoverpa armigera, Bt toxin receptor, protoxin, RNAi, gene silencing, aminopeptidase N, cadherin