Exploring the binding affinity and characteristics of DcitOBP9 in citrus psyllids.

Odorant-binding proteins (OBPs) are crucial in insect olfaction. The most abundant expressed OBP of citrus psyllids, DcitOBP9 encodes 148 amino acids. DcitOBP9 lacks a transmembrane structure and possesses a 17-amino acid signal peptide at the N-terminus. Characterized by the six conserved cysteine sites, DcitOBP9 is classified as the Classical-OBP family. RT-qPCR experiments revealed ubiquitous expression of DcitOBP9 across all developmental stages of the citrus psyllid, with predominant expression in adults antennae. Fluorescence competitive binding assays demonstrated DcitOBP9's strong affinity for ocimene, linalool, dodecanoic acid, and citral, and moderate affinity for dimethyl trisulfide. Additionally, it binds to myrcia, (-)-trans-caryophyllene, (±)-Citronellal, nonanal, and (+)-α-pinene. Among them, ocimene, linalool, and dodecanoic acid were dynamically bound to DcitOBP9, while citral was statically bound to DcitOBP9. Molecular docking simulations with the top five ligands indicated that amino acid residues V92, S72, P128, L91, L75, and A76 are pivotal in the interaction between DcitOBP9 and these odorants. These findings suggest DcitOBP9's involvement in the citrus psyllid's host plant recognition and selection behaviors, thereby laying a foundation for elucidating the potential physiological and biological functions of DcitOBP9 and developing attractants.