A recombinant bacterium expressing double-stranded RNA specific to integrin β1 subunit enhances Bt toxicity against a polyphagous insect pest, Spodoptera exigua

Oral toxicity of double-stranded RNA (dsRNA) specific to integrin β1 subunit (SeINT) was known in a polyphagous insect pest, Spodoptera exigua. For a practical purpose, this study prepared a recombinant Escherichia coli expressing dsRNA specific to SeINT. The dsRNA expression was driven by T7 RNA polymerase overexpressed by an inducer in the transformed E. coli. The produced dsRNA amount was proportional to the number of the cultured bacteria. The bacteria gave a significant oral feeding mortality to S. exigua larvae with significant reduction of the SeINT expression. The resulting insect mortality increased with the fed bacterial cell number. Pretreatment with a sonication significantly increased the insecticidal activity of the transformed bacteria. Compared to the control bacteria transformed by non-recombinant vector, the larvae fed the bacteria expressing dsRNA specific to SeINT suffered tissue damage in the midgut epithelium, which was characterized by a loose cell-cell contact and a significant cell death. The dsRNA-treated larvae were significantly more susceptible to a Cry toxin derived from Bacillus thuringinesis (Bt) than the larvae treated only with Cry toxin. This study demonstrates that a transformed bacterium expressing dsRNA specific to SeINT has a significant insecticidal activity by oral application against S. exigua and makes the target insects to be highly susceptible to Bt toxin.