A sublethal dose of neonicotinoid imidacloprid precisely sensed and detoxified by a C-minus odorant-binding protein 17 highly expressed in the legs of Apis cerana

Abstract

As a native honeybee species in East Asia, Apis cerana is essential for the stability of local agricultural and plant ecosystems by its' olfactory system for searching nectar and pollen sources. Odorant-binding proteins (OBPs) existing in the insect's olfactory system can recognize environmental semiochemicals. It was known that sublethal doses of neonicotinoid insecticides could still cause a variety of physiological and behavioral abnormalities in bees. However, the molecular mechanism of A. cerana sensing and response for insecticide has not been further investigated. In this study, we found an A. cerana OBP17 gene significantly up-regulated expressed after exposure to sublethal doses of imidacloprid based on the transcriptomics results. The spatiotemporal expression profiles showed that OBP17 was highly expressed in the legs. Competitive fluorescence binding assays showed that OBP17 had the special and high binding affinity to imidacloprid among the 24 candidate semiochemicals, and the K_A value of OBP17 binding with imidacloprid reached the maximum (6.94 × 10⁴ L/mol) at low-temperature. Thermodynamic analysis showed that the quenching mechanism changed from dynamic to static binding interaction with the increasing temperature. Meanwhile, the force changed from hydrogen bond and van der Waals force to hydrophobic interaction and electrostatic force, indicating the interaction exhibits variability and flexibility. Molecular docking showed that Phe107 contributed the most energy. RNA interference (RNAi) results showed that OBP17 knockdown significantly enhanced the electrophysiological response of the bees' forelegs to imidacloprid. Our study indicated that OBP17 could precisely touch and sense sublethal doses of neonicotinoid imidacloprid in the natural environment through its high expression in legs, and the upregulation expression of OBP17 exposure to imidacloprid probably implied that it participate in the detoxification processes of A. cerana. Also, our research enriches the theoretical knowledge of the sensing and detoxifying activities of non-target insects' olfactory sensory system to environmental sublethal doses of systemic insecticides.

Keywords: Apis cerana; Imidacloprid; Odorant-binding protein; RNAi; Thermodynamics.