Protein reduces chlorpyrifos toxicity in Spodoptera exigua

Researchers have identified a resistance mechanism to chlorpyrifos in Spodoptera exigua. The study indicates a direct link between this protein and the insecticide, mediated by the SeOBP11 protein. This interaction reduced the bioavailability of the product and decreased caterpillar mortality in an in vivo assay.

The study evaluated 36 genes for odorant-binding proteins, known as OBPs, in Spodoptera exigua. These proteins are traditionally associated with olfaction. The study showed their expression in tissues without olfactory function, such as hemolymph, fat body, and midgut. This pattern indicates involvement in the transport of hydrophobic molecules, including insecticides.

Lineages compared

The research compared a susceptible laboratory strain with a multi-resistant field strain. The resistant strain came from scallion (Allium fistulosum) crops in Huizhou, Guangdong province, China. It showed 935 times greater resistance to chlorpyrifos and 3.449 times greater resistance to lambda-cyhalothrin.

Among the genes evaluated, SeOBP11 stood out the most. The transcript level was 20,4 times higher in the resistant strain compared to the susceptible strain. Other genes also showed a significant increase, such as SeOBP28, SeOBP9, SeOBP24, and SeOBP23. SeOBP2 and SeOBP13 showed reduced expression in the resistant strain.

The researchers produced recombinant proteins for six SeOBPs: SeOBP2, SeOBP9, SeOBP11, SeOBP13, SeOBP23, and SeOBP24. They then measured the affinity of these proteins for 34 insecticides using competitive fluorescence binding assays. Chlorpyrifos bound to all six proteins with high affinity. SeOBP11 showed the strongest binding, with a Ki of 5,0 micromolar.

Specificity of interactions

The specificity of the interactions was noteworthy. Twenty of the 34 insecticides tested showed no detectable binding, or only weak interaction, with the six proteins evaluated. The study also found no simple relationship between chemical class and binding. Among the organophosphates, chlorpyrifos and foxim showed strong binding with several SeOBPs. Profenofos, methyl-pirimiphos, diazinon and malathion did not follow the same pattern.

The functional assay focused on the SeOBP11 and chlorpyrifos pair. The researchers pre-incubated chlorpyrifos with recombinant SeOBP11 and injected the mixture into caterpillars of Spodoptera exigua. The application of chlorpyrifos alone, at a concentration of 140 micromolar, caused mortality close to 80% within 24 hours. The mixture with SeOBP11 reduced mortality to approximately 40%. This effect remained in the 48-hour and 72-hour evaluations.

Insecticide sequestration

The result indicates sequestration of the insecticide by the protein. The binding reduces the free fraction of chlorpyrifos available to reach its targets in the insect. Researchers classify this process as sequestration-based resistance. The mechanism differs from classic resistance models, such as metabolic detoxification, alteration in the target site, and reduced cuticular penetration.

Molecular docking analysis reinforced this interpretation. Chlorpyrifos, thiodicarb, and dimethoate occupied the same SeOBP11 binding cavity. Hydrophobic interactions predominated in the complexes with the highest affinity. Chlorpyrifos showed the most favorable profile. Thiodicarb occupied an intermediate position. Dimethoate had a weak interaction.

Researchers identify SeOBP11 as a potential molecular marker for monitoring resistance in Spodoptera exigua. The study also suggests new approaches to resistance management, focusing on the expression or function of SeOBPs.

Further information at doi.org/10.1016/j.pestbp.2026.107211