A glutathione S-transferase (BdGSTd9) participates in malathion resistance via directly depleting malathion and its toxic oxide malaoxon in Bactrocera dorsalis (Hendel)

Background: The oriental fruit fly, Bactrocera dorsalis (Hendel), is a widespread agricultural pest that has evolved resistance to many commonly used insecticides including malathion. Glutathione S-transferases (GSTs) are multifunctional enzymes that metabolize insecticides directly or indirectly. The specific mechanism used by GSTs to confer malathion resistance in B. dorsalis is unclear.

Results: BdGSTd9 was identified from B. dorsalis and was expressed at twice the level in a malathion-resistant strain (MR) than in a susceptible strain (MS). By using RNAi of BdGSTd9, the toxicity of malathion against MR was increased. Protein modelling and docking of BdGSTd9 with malathion and malaoxon indicated key amino acid residues for direct binding in the active site. In vitro assays with engineered Sf9 cells overexpressing BdGSTd9 demonstrated lower cytotoxicity of malathion. High performance liquid chromatography (HPLC) analysis indicated that malathion could be broken down significantly by BdGSTd9, and it also could deplete the malathion metabolite malaoxon, which possesses a higher toxicity to B. dorsalis. Taken together, the BdGSTd9 of B. dorsalis could not only deplete malathion, but also react with malaoxon and therefore enhance malathion resistance.

Conclusion: BdGSTd9 is a component of malathion resistance in B. dorsalis. It acts by depleting both malathion and malaoxon. © 2020 Society of Chemical Industry.