Glutathione S-Transferase May Contribute to the Detoxification of (S)-(−)-Palasonin in Plutella xylostella (L.) via Direct Metabolism

SIMPLE SUMMARY: Plutella xylostella is one of the most devastating pests worldwide due to resistance to a variety of chemical pesticides. Therefore, there is an urgent need for control alternatives. Our previous studies have found a plant-derived active substance (S)-(−)-palasonin (PLN) has a competent toxic effect on P. xylostella. However, we noticed cross-resistance between (S)-(−)-palasonin and other insecticides. We therefore hypothesized that metabolic resistance may be more important in (S)-(−)-palasonin resistance than target-site resistance. We investigated the contribution of detoxification enzymes in the metabolism of (S)-(−)-palasonin in P. xylostella. We found that the GST enzyme activity in the field strain of P. xylostella changed most significantly compared with the sensitive strain. We then evaluated the role of GSTs in detoxifying (S)-(−)-palasonin. The results show that the GST activities of P. xylostella increased significantly after exposure to (S)-(−)-palasonin. RT-qPCR shows that 19 of 20 GSTs genes were up-regulated after exposure to (S)-(−)-palasonin. Furthermore, the results of in vitro inhibition and metabolic experiments show that (S)-(−)-palasonin can competitively bind to GST, and that GSTd1, GSTd2, GSTs1 and GSTs2 have a capability to metabolize (S)-(−)-palasonin. This study contributes to the application of (S)-(−)-palasonin in the control of P. xylostella and to the resistance evaluation of botanical insecticides. ABSTRACT: The control of P. xylostella primarily involves chemical insecticides, but overuse has brought about many negative effects. Our previous study reported that (S)-(−)-palasonin (PLN) is a plant-derived active substance with significant insecticidal activity against P. xylostella. However, we noticed a possible cross-resistance between (S)-(−)-palasonin and other insecticides which may be related to metabolic detoxification. In order to further explore the detoxification effect of detoxification enzymes on (S)-(−)-palasonin in P. xylostella, the effects of (S)-(−)-palasonin on enzyme activity and transcription level were determined, and the detoxification and metabolism of GSTs on (S)-(−)-palasonin were studied by in vitro inhibition and metabolism experiments. During this study, GST enzyme activity was significantly increased in P. xylostella after (S)-(−)-palasonin treatment. The expression levels of 19 GSTs genes were significantly increased whereas the expression levels of 1 gene decreased. Furthermore, (S)-(−)-palasonin is shown to be stabilized with GSTs and metabolized GSTs (GSTd1, GSTd2, GSTs1 and GSTs2) in vitro, with the highest metabolic rate of 80.59% for GSTs1. This study advances the beneficial utilization of (S)-(−)-palasonin as a botanical pesticide to control P. xylostella in the field.