Mechanisms of CO2 sensing mediated by gustatory receptors in Holotrichia oblita larvae.

Carbon dioxide (CO2) is a critical cue for Holotrichia oblita larvae (white grubs), a widely distributed soil-dwelling pest, in locating host plant roots. However, the molecular mechanisms underlying the perception of CO2 in white grubs remain elusive. Here, our study demonstrated that CO2 could induce electrophysiological and behavioral responses in third-instar larvae. Twelve to fourteen S4 sensilla and one digitiform organ potentially involved in CO2 detection were observed on the antenna and maxillary palp of H. oblita larvae, respectively. Three gustatory receptor (GR) genes-HoblGR1, HoblGR2, and HoblGR3-were cloned and were predominantly expressed in the antennae and maxillary palps of third-instar larvae. The two-electrode voltage clamp (TEVC) recording system demonstrated that oocytes co-expressing HoblGR1 and HoblGR3, or HoblGR1, HoblGR2, and HoblGR3, gave robust responses to CO2. Additionally, silencing of HoblGR1 or HoblGR3 for 72 h significantly reduced the electrophysiological and behavioral responses of white grubs to commercial CO2 as well as root-emitted CO2. Our work reveals that HoblGR1 and HoblGR3 are indispensable and sufficient for CO2 sensing in H. oblita larvae, and this finding may aid in the development of CO2-based trapping strategies for white grubs.