Functional analysis of a bitter gustatory receptor highly expressed in the larval maxillary galea of Helicoverpa armigera

Many plant secondary substances are feeding deterrents for insects and play a key role in the selection of host plants. The taste sensilla of phytophagous insects contain gustatory receptor neurons sensitive to deterrents but the molecular basis of deterrent chemoreception remains unknown. We investigated the function of Gr180, the most highly expressed bitter gustatory receptor in the maxillary galea of Helicoverpa armigera larvae. Functional analyses using the Xenopus oocyte expression system and two-electrode voltage clamp revealed that the oocytes expressing Gr180 responded to coumarin. Tip recording results showed that the medial sensilla styloconica of the maxilla of fifth instar larvae exhibited electrophysiological responses to coumarin. Two-choice feeding bioassays confirmed that coumarin inhibited larval feeding. A homozygous mutant strain of H. armigera with truncated Gr180 proteins (Gr180(-/-)) was established using the CRISPR-Cas9 system. The responses of the medial sensilla styloconica in Gr180(-/-) to coumarin were almost abolished, and the responses to sinigrin and strychnine, were also significantly decreased. Knockout of Gr180 alleviated the feeding deterrent effects of coumarin, sinigrin, and strychnine. Thus, we conclude that Gr180 is a receptor responding to coumarin,and also participates in sensing sinigrin and strychnine. These results enhance our understanding of the gustatory coding mechanisms of phytophagous insects to deterrents. Author summary Phytophagous insects avoid eating most plants because of the presence of distasteful secondary compounds, which introduces the question of how these compounds are perceived by insects. The cotton bollworm is a very serious crop pest in the Old World, and its caterpillars mainly feed on the reproductive organs of many host plants. Here, we study the function of a gustatory receptor, Gr180, highly expressed in the taste organ of Helicoverpa armigera caterpillars. The oocytes of frogs expressing Gr180 selectively responded to coumarin, a bitter compound widely exist in plants. When this gene was knocked out, the response of the deterrent cells in the medial styloconic sensilla to coumarin disappeared, and the responses to other two deterrents sinigrin and strychnine were also attenuated. However, the inhibitory effect of coumarin on the sucrose cells in the lateral styloconic sensilla was not affected. At the same time, the aversive feeding responses to these compounds were reduced in the knockout caterpillars. These findings are important for revealing the mechanisms of host selection in herbivorous insects and the use of feeding deterrents for pest control.

Automated summary

This study investigates the function of the gustatory receptor Gr180 in Helicoverpa armigera caterpillars. The results show that Gr180 responds to coumarin and is also involved in sensing sinigrin and strychnine. These findings enhance our understanding of the gustatory coding mechanisms of herbivorous insects to deterrents.