Transcriptome profiling and RNA interference reveals relevant detoxification genes in Monochamus alternatus response to (+)-α-pinene

Understanding the mechanism of insect resistance to toxic effects of phytochemicals can provide an insight into plant–herbivore interactions. Monochamus alternatus Hope, a main vector of pine wood nematode ( Bursaphelenchus xylophilus ), prefers to infest masson pine ( Pinus massoniana Lamb) and causes huge economic and environmental losses. α-Pinene is the primary component of resin produced by masson pine and plays a crucial role in defending against herbivorous insects, but its defensive effects on M . alternatus larvae are barely known. Here, we explored the physiological metabolism in M . alternatus larvae that were fumigated with (+)-α-pinene, (-)-α-pinene, β-pinene, (R)-(+)-limonene, myrcene and (-)-α-phellandrene. (+)-α-Pinene showed the highest fumigant toxicity to M . alternatus larvae among these monoterpenes, and the fourth instar larvae possessed the highest resistance to this compound at the larval stage. Comparative transcriptome analysis detected 423 up-regulated and 311 down-regulated genes in the larvae fumigated with (+)-α-pinene, which were mainly related to detoxification, energy and protein metabolism. Among the differentially expressed genes encoding for detoxification enzymes, 14 P450s, 13 UGTs, 1GSTs and 3ABCs were dramatically up-regulated in the larvae fumigated with (+)-α-pinene. Silencing of P450s via RNA interference led to an increased mortality in the larval fumigated with (+)-α-pinene. Piperonyl butoxide exposure significantly increased the mortality and inhibited the expressions of P450s, UGTs and ABCs in the larvae fumigated with (+)-α-pinene. These results suggested an important role of P450s in the resistance of M . alternatus larvae to (+)-α-pinene fumigation, which may facilitate the understanding of terpenoid detoxification in M . alternatus larvae.