Odorant receptor 75 is essential for attractive response to plant volatile p-anisaldehyde in Western flower thrips

The Western flower thrip (WFT), Frankliniella occidentalis, is a major pest of many vegetable crops and also a vector for the tomato spotted wilt virus, causing devastating damage worldwide. Odorant receptors (ORs) play an important role in host plant searching, however, specific functions of those ORs in WFT remain unclear. In this study, the attractive activity of four plant volatiles ((S)-(-)-verbenone, p-anisaldehyde, methyl isonicotinate, and benzaldehyde) to WFT was confirmed using a Y-tube olfactometer. Then, the specific receptor, OR75, was screened out as the candidate OR for these odorants, as its expression was significantly upregulated upon exposure to these odorants. Further in vitro functional assays with Xenopus oocyte expression system confirmed sensitivity of OR75 to p-anisaldehyde and three other odorants (β-ionone, undecanal and cinnamaldehyde). Of the three odorants, β-ionone was also attractive to WFT. Further, in vivo RNA interference experiments showed that the dsOR75 treated thrips lost their attractive response to p-anisaldehyde, but retained response to β-ionone. Finally, 3-D structures prediction and molecular docking showed that OR75 formed a hydrogen bond with p-anisaldehyde at His150 residue, while no hydrogen bond formed with β-ionone, undecanal or cinnamaldehyde. Taken together, OR75 plays a crucial role in perception of p-anisaldehyde, which helps us understand the host-seeking mechanisms of WFT, and provides a basis for development of olfactory based pest control strategies. This is the first report of an OR playing roles in sensing p-anisaldehyde in thrips.