Abstract:  【Aim】 This study aims to reveal the mechanism of action of FTZ-F1, a key nuclear receptor factor in insect ecdysone signaling pathway, in response to the stress of chlorfenapyr and phoxim in Spodoptera litura. 【Methods】 The FTZ-F1 gene of S. litura was identified by bioinformatics methods, sequence alignment was conducted and phylogenetic tree was constructed. The mulberry leaves treated with LC₃₀ of chlorfenapyr and phoxim by leaf dipping method were fed to the 3rd instar larvae of S. litura, and the survived larvae were collected at 1, 12, 24, 36 and 48 h, respectively, after feeding on the leaves, and the expression levels of SlFTZ-F1 in the larvae were detected by qRT-PCR. SlFTZ-F1 gene was silenced by RNAi technology, and the expression level of SlFTZ-F1 was detected by qRT-PCR after dsRNA injection. The mulberry leaves treated with LC₃₀ of chlorfenapyr and phoxim by leaf dipping method were fed to the 3rd instar SlFTZ-F1-silenced larvae, and the mortality rates of S. litura larvae were recorded at 24 and 48 h after feeding. Eight glutathione S-transferase (SlGST) genes of S. litura were selected and qRT-PCR was used to detect the expression levels of these SlGST genes in the SlFTZ-F1-silenced S. litura larvae. 【Results】 The open reading frame of SLFTZ-F1 of S. litura is 1 665 bp in length, encoding 555 amino acids with the isoelectric point of 6.39 and the theoretical molecular weight of 61.77 kD. SlFTZ-F1 contains DNA-binding domain, FTZ-F1 box and ligand-binding domain. The phylogenetic tree showed that SlFTZ-F1 and SfFTZ-F1 in Spodoptera frugiperda were clustered into a subbranch. The expression levels of SlFTZ-F1 in the 3rd instar larvae of S. litura at 1, 24 and 36 h after feeding on the mulberry leaves treated with LC₃₀ of chlorfenapyr, and at 24 and 36 h after feeding on the mulberry leaves treated with LC₃₀ of phoxim were significantly up-regulated as compared to those in the control feeding on the mulberry leaves treated with ddH₂O. The expression level of SlFTZ-F1 in the 3rd instar larvae of S. litura at 48 h after dsSlFTZ-F1 injection was significantly decreased as compared with that in the control group injected with dsGFP. When the 3rd instar SlFTZ-F1-silenced larvae of S. litura had been fed on the mulberry leaves treated with LC₃₀ of chlorfenapyr and phoxim, respectively, for 48 h, their mortality rates were significantly increased by 22% and 28%, respectively. The expression levels of the eight SlGST genes in the SlFTZ-F1-silenced larvae of S. litura were significantly down-regulated as compared to those in the control. 【Conclusion】 The expression of SlFTZ-F1 gene of S. litura larvae is significantly induced by chlorfenapyr and phoxim. After silencing SlFTZ-F1, the sensitivity of S. litura larvae to chlorfenapyr and phoxim significantly increases, and the expression of detoxification enzyme SlGST genes is significantly inhibited, suggesting that the development-related transcription factor FTZ-F1 plays an essential role in response to the stress of commonly used insecticides in S. litura.

Key words: Spodoptera litura, FTZ-F1, chlorfenapyr, phoxim, insecticide tolerance, RNAi