Coordinative mediation of the response to alarm pheromones by three odorant binding proteins in the green peach aphid Myzus persicae

Odorant binding proteins (OBPs) play an essential role for insect chemosensation in insect peripheral nervous systems of antennae. Each antennal sensilla contains more than one OBP at high concentrations but the interactions and cooperation between co-localized OBPs are rarely reported. In present study, we cloned, expressed and purified eight OBPs of the green peach aphid Myzus persicae. The effects of knocking down the expression of these OBP genes by RNAi on the electrophysiological and behavioural responses of M. persicae to the aphid alarm pheromone, (E)-β-farnesene (EβF) were investigated. The results showed that the aphids could still be repelled by EβF when the expression of each of three OBP genes was individually knocked down. However, the simultaneous knockdown of MperOBP3/7/9 expression significantly reduced the electrophysiological response and the repellent behaviours of M. persicae to EβF than the single OBP gene knockdown (P < 0.05). Rather than a normal saturation binding curve of individual OBP, the binding curve of MperOBP3/7/9 is bell-shaped with a higher affinity for the fluorescent probe N-phenyl-1-naphthylamine (1-NPN). The competitive binding assays confirmed that MperOBP3, MperOBP7, MperOBP9 and MperOBP3/7/9 mixture exhibited a stronger binding affinity for EβF, than for sex pheromones and plant volatiles with a dissociation constant of 2.5 μM, 1.1 μM, 3.9 μM and 1.0 μM, respectively. The competitive binding curve of MperOBP3/7/9 mixture to EβF is shallow without bottom plateau, suggesting a conformational change and a rapid dissociation after the displacement of all 1-NPN (in vivo after the saturation binding of all OBPs by EβF). The interaction between OBPs and formation of a heterogeneous unit may facilitate the delivery EβF to the OR at electrophysiological and behavioural levels during insect odorant signal transduction thus mediate M. persicae response to the alarm pheromone EβF.