Gut bacterium Acinetobacter sp. assists Camellia weevil with host plant adaptation by degrading tea saponin via the benzoate pathway.

Abstract

Background: The extent to which herbivorous insects depend on gut bacteria and the molecular mechanisms by which these microbes help overcome host plant chemical defenses remain controversial. This study explored how the gut symbiont Acinetobacter sp. AS23 of a Camellia weevil (Curculio chinensis) (CW) enhances the weevil's tolerance to toxic tea saponins in host plants.

Methods: We first conducted toxicity assays in which third-instar CW larvae were exposed to fermentation filtrates containing tea saponin degradation products from the weevil's gut bacterium, the AS23 strain. A combination of metabolomic and transcriptomic analyses was used to investigate the degradation pathway and key genes used by the AS23 strain in tea saponin metabolism. We then re-inoculated axenic larvae with bacterial mutants generated through CRISPR-Cas9 and verified gene functions in tea saponin degradation.

Results: Toxicity assays demonstrated that the AS23 strain exhibited time-dependent tea saponin degradation capabilities. The benzoate degradation pathway emerged as a core metabolic pathway enriched during tea saponin degradation, with the involvement of four key enzyme genes confirmed through qPCR and functional studies. Knockout strains exhibited a significantly reduced detoxification capacity and increased larval mortality when reintroduced into CWs' gut.

Conclusion: Our findings elucidated the key role of the AS23 strain in mediating CW larvae tolerance to tea saponins through the benzoate degradation pathway. This study highlights the potential of leveraging microbial saponin degradation pathways for developing environmentally friendly pest control strategies.