Bioactivities of synthetic salicylate-substituted carboxyl (E)-β-Farnesene derivatives as ecofriendly agrochemicals and their binding mechanism with potential targets in aphid olfactory system

Background: The aphid alarm pheromone, (E)-β-farnesene (EβF), is a natural product secreted from the aphid cornicle as a signal to warn companions of danger. Odorant binding proteins (OBPs) are the vital targets in insect signal transduction pathways. To improve bioactivity of EβF as more economic and stable aphid control agents, EβF derivatives containing an active substructure, salicylic acid moiety, were designed, synthesized, and evaluated for their bioactivities in a structure-function study under laboratory conditions.

Results: EβF derivatives, (E)-3,7-dimethylocta-2,6-dien-1-yl-2-hydroxy-3-methylbenzoate and (E)-3,7-dimethylocta-2,6-dien-1-yl-2-hydroxy-3-methoxybenzoate showed outstanding aphid-repellent activity at a dose of 5 μg against Acyrthosiphon pisum (repellency proportions of 67.3% and 71.2%, respectively) and Myzus persicae (repellency proportions of 80.0% and 74.4%, respectively) in laboratory. EβF and most of its derivatives bound strongly to ApisOBP9 with a higher affinity than those of the reported potential targets AphisOBP3 and ApisOBP7. The binding affinities to these three ApisOBPs were generally consistent with the in vivo aphid-repellent activity. A molecular docking study suggested that the hydrophobic effect was crucial for the interactions between the derivatives and the OBPs.

Conclusion: New EβF derivatives containing salicylic acid moiety and their repellent activity, binding mechanism with three potential OBPs are presented. A new OBP, ApisOBP9, was characterized as a potential EβF and EβF derivatives binding protein for the first time. The hydrophobic nature of these analogues is responsible for their activity. Two analogues 3b and 3e with outstanding aphid-repellent activity could be new leads for aphid control agents.