A gustatory receptor tuned to the steroid plant hormone brassinolide in Plutella xylostella (Lepidoptera: Plutellidae)

Plutella xylostella is the most common Lepidopteran pest species and it causes an annual loss of US$ 4–5 billion per year. Due to its short life cycle, the pest has developed resistance to the common insecticides. It is believed that the presence of certain chemical components in the leaves of Brassica species is the reason it is usually selected as host plants by P. xylostella. Sinigrin and brassinolide (BL) are chemical compounds found in Brassica with a concentration that is higher than what is observed in any other plant species. Studies have shown that sinigrin is a feeding/oviposition stimulant for P. xylostella. P. xylostella larvae possesses gustatory receptors (GR) with a sensitivity to sinigrin and other glucosinolates. GRs are expressed in the dendrites of gustatory sensory neurons (GSNs) and they control the selective response of GSNs. Although studies have identified the functions of GRs in a few herbivorous insects, the functions of bitter GRs in most herbivorous insects is yet to be known. BL is a widespread plant hormone and its role in the growth and development of plants has been studied widely. However, not much is known about its influence on the behavior of phytophagous insects.

Researchers from the State Key Laboratory of Integrated Management of Pest Insects and Rodents, Institute of Zoology at the Chinese Academy of Sciences: Ke Yang, Xin-Lin Gong, Guo-Cheng Li, Ling-Qiao Huang, Chao Ning and Chen-Zhu Wang discovered possible stimulants or deterrents to feeding and oviposition in P. xylostella. Their findings showed that a bitter GR (PxylGr34) is tuned to BL as a deterrent to feeding and oviposition in P. xylostella. The research work is published in the Journal eLife.

The research team analyzed all the GRs in P. xylostella and discovered high levels of a bitter GR-PxylGr34 in the larval head, thoracic legs and gut. However, in adult P. xylostella, PxylGr34 was found to be restricted to the antennae. In the Xenopus oocytes expressing PxylGr34, BL and 24-epibrassinolide (EBL) were found to induce a strong response. The medial sensilla styloconica of the larvae had strong responses to BL and EBL. The authors also observed that increasing concentrations of BL and EBL resulted in a decrease in the feeding preference of larvae. In addition, increasing concentrations of brassinolide caused a decrease in oviposition preference. Knock-down of PxylGr34 by RNAi attenuates the taste responses of medial sensilla styloconica to BL, and abolishes BL-induced feeding inhibition. In summary, through this novel study, the authors have identified PxylGr34, a bitter GR that is highly expressed in the head of the larvae and antennae of the adult P. xylostella. It is tuned to the plant hormones BL and EBL, and it regulates the feeding and oviposition responses of P. xylostella to these chemical compounds. The findings have helped to increase the understanding of the deterrents of feeding/oviposition in phytophagous insects and the gustatory coding mechanism associated with it. This study has also suggested the potential use of plant hormones as agents that can suppress pest insects.