Relationship Between Glutathione S-Transferase Genes CfGSTe1 and CfGSTd1 and Ethyl Formate Tolerance in Cryptolestes ferrugineus

Glutathione S-transferases (GSTs) are key detoxification enzymes in insects and play important roles in the development of insect tolerance to chemical insecticides. Ethyl formate (EF), characterized by high efficacy, low toxicity, and low residue, is regarded as a promising green fumigant for stored-grain pest control.

To elucidate the molecular mechanisms underlying pest tolerance to EF, this study focuses on the important stored-grain pest Cryolestes ferrugineus, aiming to analyze the relationship between GST genes (CfGSTe1 and CfGSTd) and EF tolerance.

The present study conducted bioassays to determine the susceptibility of C. ferrugineus with three different levels of phosphine resistance to EF. Through synergistic assays with diethyl maleate (DEM), the potential enhancement of EF fumigation efficacy was evaluated, and the effects of EF treatment on GST activity were analyzed. According to the previous transcriptome data of C. ferrugineus, two key GST genes (CfGSTe1 and CfGSTd1) were identified and subjected to amino acid sequence and phylogenetic analyses. The temporal and spatial expression patterns of these two genes, as well as their transcriptional responses to EF fumigation stress, were further analyzed by using real-time quantitative PCR (RT-qPCR). Finally, the effects of CfGSTe1 and CfGSTd1 on EF tolerance were analyzed by individually silencing these genes using RNA interference (RNAi) technology.

Bioassay results showed that C. ferrugineus with varying levels of phosphine resistance exhibited no significant differences in sensitivity to EF, confirming the absence of cross-resistance between the two fumigants. The synergist DEM significantly enhanced the fumigant toxicity of EF, and the GST activity in insects was markedly increased under EF stress, suggesting that GSTs play an important role in the detoxification metabolism of EF. Sequence and phylogenetic analyses indicated that CfGSTe1 and CfGSTd1 encode 216 and 215 amino acids, respectively, both containing conserved GST catalytic sites and belonging to the Epsilon and Delta families. The RT-qPCR results indicated that both genes were highly expressed at the adult stage, primarily in the midgut, fat body, and Malpighian tubules, and could be significantly induced by EF exposure. After effectively silencing CfGSTe1 and CfGSTd1 via RNAi, the tolerance of C. ferrugineus to EF was significantly reduced, as evidenced by markedly increased adult mortality following fumigation.

The CfGSTe1 and CfGSTd1 may play important roles in the detoxification metabolism of EF in C. ferrugineus, suggesting a close association with insect tolerance to this fumigant.