Gut bacterial community and gene expression alterations induced by transgenic Bt maize contribute to insecticidal activity against Mythimna separata

Although the gut microbiota has been well reported to contribute to the pest insecticidal activity of Bacillus thuringiensis (Bt) toxin, the underlying mechanism remains largely unknown. Here, bioassay, RNA-seq, and 16S rRNA sequencing were employed to explore the potential mechanism by which gut microbiota enhance the virulence of Bt maize to Mythimna separata. Two groups of neonate larvae were fed with the leaves of conventional maize (CK group) and insect-resistant transgenic maize (Bt group), respectively. The results indicated that body weight, body length, and survival rate of M. separata in the Bt group were significantly reduced. Additionally, the induced growth retardation further led to normal mating failure and fecundity deprivation of M. separata in Bt group. The 16S rRNA sequencing results indicated that exposure to insect-resistant transgenic maize significantly decreased the diversity of the bacterial community and increased the relative abundance of Enterobacter cloacae and Enterococcus mundtii by 153.02-folds and 2.08-folds, respectively. Reintroduction of these two bacteria significantly increased the sensitivity of M. separata to Bt toxin. Transcriptome results revealed that numerous genes related to Notch, Hippo, Wnt, amino acid synthesis, and fatty acid metabolism pathways were significantly down-regulated in the Bt group, whereas almost all Bt toxin receptor aminopeptidase N genes were up-regulated. This study revealed that exposure to insect-resistant transgenic maize reduced gut bacterial diversity and altered the gene expression of M. separata. Furthermore, E. cloacae and E. mundtii enhanced the susceptibility of M. separata to Bt toxin. Our findings on the interaction between Bt toxins, gut microbiota, and the host insect will contribute to the development of pest management strategies and the promotion of transgenic crops in China.