Identification of odorant-binding proteins and functional analysis of antenna-specific AplaOBP1 in the emerald ash borer, Agrilus planipennis

Olfaction-based strategies have been successfully applied to monitor the emerald ash borer Agrilus planipennis Fairmaire (Coleoptera: Buprestidae); however, roles of chemosensory-associated proteins in olfactory perception of A. planipennis are rarely reported. Here, we identified 11 odorant-binding proteins (OBPs) from the A. planipennis transcriptome and genome. Sequence and phylogenetic analyses revealed that A. planipennis OBPs (AplaOBPs) could be classified into four subfamilies. The reverse transcription PCR (RT-PCR) analysis showed that all three members of the ABPII, one plus-C OBP, and one minus-C OBP were specifically or predominantly expressed in the antennae of both sexes, indicating their possible involvement in chemoreception. Subsequently, an antenna-specific OBP, AplaOBP1, was investigated through in situ hybridization, immunocytochemistry, and fluorescent competitive binding assays. It was found that the AplaOBP1 gene was detected under the base of the antennal sensilla, and the AplaOBP1 protein was expressed in the lymph of multiparous sensilla basiconica types I and III. In vitro, recombinant AplaOBP1 exhibited high binding affinities (K-i = 3.38-9.25 mu M) with nine volatiles including five host-tree terpenes (e.g., myrcene, limonene, nerolidol, alpha-farnesene, and ocimene), which indicated that AplaOBP1 might be involved in the detection of host volatiles. The electrophysiological activities of all AplaOBP1 ligands were further confirmed using electroantennography, whose results imply that these volatiles may act as putative semiochemicals for A. planipennis. In conclusion, our results provide valuable insights into the molecular basis of olfaction in A. planipennis and help us use OBPs as targets to design novel olfactory regulators for management of A. planipennis.