Molecular characterization of glutamate-gated chloride channel and its possible roles in development and abamectin susceptibility in the rice stem borer, Chilo suppressalis

Comparative Insecticidal Activity and Mechanism of Isocycloseram versus Other GABAergic Insecticides against the Fall Armyworm

The fall armyworm (FAW) is a serious agricultural pest and has developed resistance to multiple insecticides. It is necessary to introduce novel insecticide(s) for controlling FAW. Isocycloseram is a completely novel isoxazoline insecticide. However, its activity and mode of action against FAW have not been reported. In this study, isocycloseram exhibited a higher insecticidal activity (LC50 = 0.26 mg/kg) than fipronil (LC50 = 7.72 mg/kg) against FAW. The median inhibitory concentration (IC50) of isocycloseram (IC50 = 8.52 nM) was almost equal to that of the desmethyl-broflanilide (IC50 = 7.32 nM) to the SfrRDL1 receptor. The IC50 of isocycloseram to the SfrRDL2 receptor was 11.13 nM, which was obviously less than that of desmethyl-broflanilide, dieldrin, fipronil, fluxametamide. Compared with the SfrRDL2 receptor, the SfrRDL1 receptor exhibited higher sensitivity to GABAergic insecticides. The recombinant SfrGluCl receptor was successfully stimulated by l-glutamate; however, the currents were low and weakly inhibited by isocycloseram at 10 μM. In conclusion, our results provided the theoretical basis for usage of GABAergic insecticides for controlling FAW.

Mitigation of avermectin exposure-induced brain tissue damage in carp by quercetin

Avermectin is widely used as an important insecticide in agricultural production, but it also shows strong toxicity to non-target organisms. Quercetin is a natural flavonoid that is widely used due to its good anti-inflammatory and antioxidant effects. We believe that quercetin may have a potential therapeutic effect on avermectin poisoning. This experiment was proposed to observe the effect of quercetin on the toxic response to avermectin by observing the toxic response caused by avermectin in the brain of carp. In this project, 60 carp were studied as control group (Control), quercetin administration group (QUE), avermectin exposure group (AVM) and quercetin treatment avermectin exposure group (QUE + AVM) with different interventions to study the effect of quercetin on avermectin. The carp brain tissues were stained and simultaneously analyzed for blood-brain barrier (BBB), oxidative stress indicators, inflammatory factors, and apoptosis using qPCR technique. The results of the study indicate that avermectin exhibits a neurotoxic mechanism of action in fish by decreasing the transcript levels of tight junction protein-related genes, which in turn leads to the rupture of the BBB in the carp brain tissue. Avermectin induced apoptosis in carp brain tissue by increasing oxidative stress response and promoting inflammatory cell infiltration. Quercetin could reduce the accumulation of reactive oxygen species (ROS) in the brain tissue of carp caused by avermectin exposure toxicity, maintain redox homeostasis, reduce inflammatory response, and protect brain tissue cells from apoptosis. The present study confirmed the therapeutic and protective effects of quercetin on neurotoxicity in carp caused by avermectin exposure.

Knockdown of the glutamate-gated chloride channel gene decreases emamectin benzoate susceptibility in the fall armyworm, Spodoptera frugiperda

Emamectin benzoate (EB), a derivative of avermectin, is the primary insecticide used to control the fall armyworm (FAW) in China. However, the specific molecular targets of EB against FAW remain unclear. In this study, we cloned the glutamate-gated chloride channel (GluCl) gene, which is known to be a primary molecular target for avermectin. We first investigated the transcript levels of SfGluCl in FAW and found that the expression level of SfGluCl in the head and nerve cord was significantly higher than that in other tissues. Furthermore, we found that the expression level of SfGluCl was significantly higher in eggs than that in other developmental stages, including larvae, pupae, and adults. Additionally, we identified three variable splice forms of SfGluCl in exons 3 and 9 and found that their splice frequencies remained unaffected by treatment with the LC50 of EB. RNAi mediated knockdown of SfGluCl showed a significant reduction of 42% and 65% after 48 and 72 h of dsRNA feeding, respectively. Importantly, knockdown of SfGluCl sifgnificantly reduced LC50 and LC90 EB treatment induced mortality of FAW larvae by 15% and 44%, respectively, compared to the control group feeding by dsEGFP. In contrast, there were no significant changes in the mortality of FAW larvae treated with the control insecticides chlorantraniliprole and spinetoram. Finally, molecular docking simulations revealed that EB bound to the large amino-terminal extracellular domain of SfGluCl by forming five hydrogen bonds, two alkyl hydrophobic interactions and one salt bridge. These findings strongly suggest that GluCl may serve as one of the molecular targets of EB in FAW, shedding light on the mode of action of this important insecticide.

PxRdl2 dsRNA increased the insecticidal activities of GABAR-targeting compounds against Plutella xylostella

The utilization of RNA interference (RNAi) for pest management has garnered global interest. The bioassay results suggested the knockout of the PxRdl2 gene significantly increased the insecticidal activities of the γ-aminobutyric acid receptor (GABAR)-targeting compounds (fipronil, two pyrazoloquinazolines, and two isoxazolines), thereby presenting a viable target gene for RNAi-mediated pest control. Consequently, we suggest enhancing the insecticidal activities of GABAR-targeting compounds by knockdown the transcript level of PxRdl2. Furthermore, PxRdl2 dsRNA was expressed in HT115 Escherichia coli to reduce costs and protect dsRNA against degradation. In comparison to in vitro synthesized dsRNA, the recombinant bacteria (ds-B) exhibited superior interference efficiency and greater stability when exposed to UV irradiation. Collectively, our results provide a strategy for insecticide spray that combines synergistically with insecticidal activities by suppressing PxRdl2 using ds-B and may be beneficial for reducing the usage of insecticide and slowing pest resistance.

Characterization of Glutamate-Gated Chloride Channel in Tribolium castaneum

The glutamate-gated chloride channels (GluCls) play essential roles in signal transduction by regulating fast inhibitory synaptic transmission in the nervous system of invertebrates. While there is only one GluCl subunit in the insect, the diversity of insect GluCls is broadened by alternative splicing. In the present study, three TcGluCl variant genes were cloned from the red flour beetle Tribolium castaneum. Analysis of the characteristics of TcGluCls including sequence features, genomic structures, and alternative splicing revealed that TcGluCls had the typical structural features of GluCls and showed high homologies with the GluCls from other insect orders. The TcGluCl-encoding gene consists of nine exons and three variants (TcGluCl-3a, TcGluCl-3b, and TcGluCl-3c) were generated by the alternative splicing of exon 3, which was a highly conserved alternative splicing site in insect GluCls. Homology modeling of TcGluCl-3a showed that the exon 3 coding protein located at the N-terminal extracellular domain, and there were no steric clashes encountered between the exon 3 coding region and ivermectin/glutamate binding pocket, which indicated that the alternative splicing of exon 3 might have no impact on the binding of GluCls to glutamate and insecticide. In addition to the head tissue, TcGluCl-3a and TcGluCl-3c also had high expressions in the ovary and testis of T. castaneum, whereas TcGluCl-3b showed high expression in the midgut, suggesting the diverse physiological functions of TcGluCl variants in T. castaneum. The total TcGluCl and three variants showed the highest expression levels in the early stage larvae. The expressions of TcGluCl, TcGluCl-3b, and TcGluCl-3c were significantly increased from the late-stage larvae to the early stage pupae and indicated that the TcGluCl might be involved in the growth and development of T. castaneum. These results are helpful to further understand the molecular characteristics of insect GluCls and provide foundations for studying the specific function of the GluCl variant.

Mode of Action of Novel Pyrazoloquinazoline on Diamondback Moth (Plutella xylostella) Ligand-Gated Chloride Channels

In our previous study, a series of novel pyrazoloquinazolines were synthesized. Pyrazoloquinazoline 5a showed high insecticidal activity against the diamondback moth (Plutella xylostella) and no cross-resistance to fipronil. Patch clamp electrophysiology performed on P. xylostella pupae brains and two-electrode voltage clamp electrophysiology performed on Xenopus Laevis oocytes indicated that 5a might act on the ionotropic γ-aminobutyric acid (GABA) receptor (GABAR) and glutamate-gated chloride channel (GluCl). Moreover, 5a's potency on PxGluCl was about 15-fold higher than on fipronil, which may explain why there was no cross-resistance between 5a and fipronil. Downregulation of the PxGluCl transcription level significantly enhanced the insecticidal activity of 5a on P. xylostella. These findings shed light on the mode of action of 5a and provide important insights into the development of new insecticides for agricultural applications.

Preparation and Characterization of Insecticide/Calix[4]arene Complexes and Their Enhanced Insecticidal Activities against Plutella xylostella

Applications of supramolecular materials in plant protection have attracted significant interest in recent years. To develop a feasible method to improve the efficacy and reduce the usage of chemical pesticides, the effect of calix[4]arene (C4A) inclusion on enhancing the insecticidal activity of commercial insecticides was investigated. Results showed that all three tested insecticides (chlorfenapyr, indoxacarb, and abamectin) with distinct molecular sizes and modes of action were able to form stable 1:1 host-guest complexes with C4A through simple preparation steps. The insecticidal activities of the complexes against Plutella xylostella were effectively enhanced compared to the guest molecule, with the synergism ratio being up to 3.05 (for indoxacarb). An obvious correlation was found between the enhanced insecticidal activity and the high binding affinity between insecticide and C4A, while the improvement in water solubility may not be a determining factor. The work would provide hints for the further development of functional supramolecular hosts as synergists in pesticide formulations.

Overexpression of glutamate-gated chloride channel in the integument is mainly responsible for emamectin benzoate resistance in the western flower thrips Frankliniella occidentalis

BACKGROUND

Western flower thrips Frankliniella occidentalis is a serious polyphagous pest worldwide. In this study, we investigated the potential mechanisms of resistance including enhanced metabolism and target site insensitivity in an emamectin benzoate (EB)-resistant (EB-R) strain.

RESULTS

The EB-R strain of F. occidentalis showed 356-fold increased resistance compared to a susceptible RDA strain. Analysis of cross-resistance to four other insecticides confirmed that EB resistance is highly specific to the contact toxicity of EB. Synergistic bioassay and quantitative PCR of cytochrome P450 monooxygenase (CYP) genes revealed that three overexpressed Cyps were likely involved in resistance. Among three putative glutamate-gated chloride channel (GluCl) genes identified, FoGluClc showed four radical amino acid substitutions and 3.8-fold and 31-fold transcription level in the head and integument in the EB-R strain when compared to the RDA strain. Backcrossing analysis and RNA interference confirmed that both amino acid substitution and overexpression of FoGluClc are responsible for EB resistance. In situ hybridization revealed that FoGluClc is mainly distributed in the integument in the EB-R strain. Cross-comparison of known genomes and transcriptomes of thrips species revealed that FoGluClc is unique to the Frankliniella genus.

CONCLUSION

While mutations and overexpression of FoGluClc play major roles in EB resistance, the overexpressed Cyps are partially involved as metabolic factors. Higher expression of FoGluClc in the integument may suggest its role in the first-line defense against EB in the EB-R strain. Unique distribution of FoGluClc in the Frankliniella genus but not in other thrips species further suggests that FoGluClc may be a surplus channel not having an essential endogenous function and is thus recruited as a defense barrier against xenobiotics. © 2022 Society of Chemical Industry.

Polylactic acid nanoparticles for co-delivery of dinotefuran and avermectin against pear tree pests with improved effective period and enhanced bioactivity

Pesticide compounding technology for disease and pest control emerges as an effective way to increase the effectiveness of pesticides while reducing pesticides resistance. Nanomaterials and encapsulation technology have offered a new insight into preparing efficient pesticide formulations, especially constructing a co-delivery nanoparticle for synergistic pesticides. In this study, a dinotefuran/avermectin co-delivery nanoparticles (DACNPs) against pear tree pests with polylactic acid (PLA) as the wall material were constructed by double-emulsion method combined with high-pressure homogenization technique. The drug content of the DACNPs was 39.1% with an average size of 245.7 ± 4.2 nm and the mean polymer dispersity index (PDI) value was 0.123. The DACNPs showed high foliar retention and good spread performance on target leaves due to the nanoscale effect. The obtained DACNPs showed a better control effect on Grapholitha molesta Busck and Psylla chinensis Yang et Li compared with the commercial formulations, which could significantly prolong the effective duration and enhance the bioactivity with lower amounts and application frequency of pesticides. This study may provide new insights into developing novel pesticide formulations to improve the utilization rate of pesticides, reduce environmental pollution and minimize the cost of farming.

Functional Characteristics of the Lepidopteran Ionotropic GABA Receptor 8916 Subunit Interacting with the LCCH3 or the RDL Subunit

The ionotropic γ-aminobutyric acid (iGABA) receptor is commonly considered as a fast inhibitory channel and is an important insecticide target. Since 1990, RDL, LCCH3, and GRD have been successively isolated and found to be potential subunits of the insect iGABA receptor. More recently, one orphan gene named 8916 was found and considered to be another potential iGABA receptor subunit according to its amino acid sequence. However, little information about 8916 has been reported. Here, the 8916 subunit from Chilo suppressalis was studied to determine whether it can form part of a functional iGABA receptor by co-expressing this subunit with CsRDL1 or CsLCCH3 in the Xenopus oocyte system. Cs8916 or CsLCCH3 did not form functional ion channels when expressed alone. However, Cs8916 was able to form heteromeric ion channels when expressed with either CsLCCH3 or CsRDL1. The recombinant heteromeric Cs8916/LCCH3 channel was a cation-selective channel, which was sensitive to GABA or β-alanine. The current of the Cs8916/LCCH3 channel was inhibited by dieldrin, endosulfan, fipronil, or ethiprole. In contrast, fluralaner, broflanilide, and avermectin showed little effect on the Cs8916/LCCH3 channel (IC₅₀s > 10 000 nM). The Cs8916/RDL1 channel was sensitive to GABA, but was significantly different in EC₅₀ and I_max for GABA to those of homomeric CsRDL1. Fluralaner, fipronil, or dieldrin showed antagonistic actions on Cs8916/RDL1. In conclusion, Cs8916 is a potential iGABA receptor subunit, which can interact with CsLCCH3 to generate a cation-selective channel that is sensitive to several insecticides. Also, as Cs8916/RDL1 has a higher EC₅₀ than homomeric CsRDL1, Cs8916 may affect the physiological functions of CsRDL1 and therefore play a role in fine-tuning GABAergic signaling.

Flonicamid and knockdown of inward rectifier potassium channel gene CsKir2B adversely affect the feeding and development of Chilo suppressalis

BACKGROUND

The selective insecticide flonicamid shows highly insecticidal activities against piercing-sucking insects and has been widely used for the control of Hemipteran insect pests, whereas its effects on Lepidopteran insect pests remain largely unknown. Recently, inward rectifier potassium (Kir) channel has been verified to be a target of flonicamid, however, functional characterization of Lepidopteran Kir genes is still lacking.

RESULTS

Flonicamid shows no insecticidal toxicity against Chilo suppressalis larvae. However, the feeding and growth of larvae were reversibly inhibited by flonicamid (50-1200 mg L^-1 ). Flonicamid treatment also remarkably reduced and delayed the pupation and eclosion of Chilo suppressalis. Additionally, five distinct Kir channel genes (CsKir1, CsKir2A, CsKir2B, CsKir3A and CsKir3B) were cloned from Chilo suppressalis. Expression profiles analysis revealed that CsKir2A was predominately expressed in the hindgut of larvae, whereas CsKir2B had high expressions in the Malpighian tubules and hindgut. RNA interference (RNAi)-mediated knockdown of CsKir2B significantly reduced the growth and increased the mortalities of larvae, whereas silencing of CsKir2A had no obvious effects on Chilo suppressalis.

CONCLUSION

Flonicamid exhibits adverse effects on the growth and development of Chilo suppressalis. CsKir2B might be involved in the feeding behavior of Chilo suppressalis. These results provide valuable information on the effects of flonicamid on non-target insects as well as the function of insect Kir channels, and are helpful in developing new insecticide targeting insect Kir channels. © 2020 Society of Chemical Industry.

Mechanisms for multiple resistances in field populations of rice stem borer, Chilo suppressalis (Lepidoptera: Crambidae) from Sichuan Province, China

Chilo suppressalis Walker (Lepidoptera: Crambidae) is a widely destructive pest occurring in rice, particularly in the rice-growing regions of Asia. In recent years, C. suppressalis has developed resistance to several insecticides because of the extensive use of insecticides. The resistance levels to four insecticides were determined among populations from different regions of Sichuan Province, China, using a drop-method bioassay. Based on LC₅₀ values of a laboratory susceptible strain, all field populations showed moderate level of resistance to triazophos (23.9- to 83.5-fold) and were either susceptible or had a low level of resistance to abamectin (2.1- to 5.8-fold). All field-collected populations had a low or moderate level of resistance to chlorpyrifos (1.7- to 47.1-fold) and monosultap (2.7- to 13.5-fold). The synergism experiment indicated that the resistance of the XW19 to triazophos may be associated with cytochrome P450 monooxygenases (P450s), with the highest synergistic ratio (SR) of 3.05-fold and increased ratio (IR) of 2.28-fold for piperonylbutoxide (PBO). The P450 activity of the TJ19 population was the greatest among the six field populations. Moreover, the relative expression levels of four resistance-related P450 genes were detected with qRT-PCR, and the results indicated that CYP324A12, CYP321F3 and CYP9A68 were overexpressed in the resistant population, especially in the XW19 population (by 1.2-, 3.4 -, and 18.0-fold, respectively). In addition, the relative expression levels of CYP9A68 among the CZ19 and TJ19 populations were also enhanced 10.5- and 24.9-fold, respectively. These results suggested that CYP324A12, CYP321F3 and CYP9A68 may be related to the resistance development of C. suppressalis to triazophos.

Identification and transcriptional response of ATP-binding cassette transporters to chlorantraniliprole in the rice striped stem borer, Chilo suppressalis

BACKGROUND

As the largest transporter gene family in metazoans, ATP-binding cassette (ABC) transporters regulate the efflux of a broad spectrum of substrates from the cytoplasm to the outside of the cell. In arthropods, ABC transporters are involved in phase III of the detoxification process, and play important roles in the metabolism and transport of insecticides.

RESULTS

We identified 54 ABC transporters from the genome and transcriptome of Chilo suppressalis, one of the most damaging pests of rice in China. The identified ABC transporters were classified into eight subfamilies (ABCA to ABCH) based on NCBI BLAST and phylogenetic analysis. Synergism studies showed that treatment with verapamil, a potent inhibitor of ABC transporters, resulted in significantly increased toxicity of chlorantraniliprole against C. suppressalis larvae. Among the 21 tested ABC genes, three ABC transporter genes including CsABCC8, CsABCG1C and CsABCH1 were significantly upregulated after chlorantraniliprole treatment.

CONCLUSION

ABC transporters play important roles in the detoxification and transport of chlorantraniliprole in C. suppressalis. The results from our study provide valuable information on C. suppressalis ABC transporters, and are helpful in understanding the roles of ABC transporters in chlorantraniliprole resistance mechanisms in C. suppressalis and other insect pests. © 2020 Society of Chemical Industry.

Involvement of Two Paralogous Methoprene-Tolerant Genes in the Regulation of Vitellogenin and Vitellogenin Receptor Expression in the Rice Stem Borer, Chilo suppressalis

Besides the function of preventing metamorphosis in insects, the juvenile hormone (JH) plays a role in female reproduction; however, the underlying mechanism is largely unknown. The methoprene-tolerant (Met) protein belongs to a family of basic helix-loop-helix–Per-Arnt-Sim (bHLH-PAS) transcription factors and functions as the JH intracellular receptor. In this study, two full length cDNAs encoding Met (CsMet1 and CsMet2) were isolated from the rice stem borer, Chilo suppressalis. Structural analysis revealed that both CsMet1 and CsMet2 exhibited typical bHLH, PAS-A, PAS-B, and PAC (PAS C terminal motif) domains. Comparative analysis of transcript level using reverse transcription-quantitative PCR (RT-qPCR) revealed that CsMet1 was predominant in almost all examined developmental stages and tissues. Treatment with methoprene in vivo induces the transcription of both CsMet1 and CsMet2. Notably, injection of dsCsMet1 and dsCsMet2 suppressed the expression levels of vitellogenin (CsVg) and Vg receptor (CsVgR). These findings revealed the potential JH signaling mechanism regulating C. suppressalis reproduction, and provided evidence that RNAi-mediated knockdown of Met holds great potential as a control strategy of C. suppressalis.

Molecular characterization and functional analysis of the vitellogenin receptor in the rice stem borer, Chilo suppressalis

As a member of the low-density lipoprotein receptor (LDLR) superfamily, vitellogenin (Vg) receptor (VgR) is responsible for the uptake of Vg into developing oocytes and is a potential target for pest control. Here, a full-length VgR complementary DNA (named as CsVgR) was isolated and characterized in the rice stem borer, Chilo suppressalis. The composite CsVgR gene contained an open reading frame of 5,484 bp encoding a protein of 1,827 amino acid residues. Structural analysis revealed that CsVgR contained two ligand-binding domains (LBDs) with four Class A (LDLR_A ) repeats in LBD1 and seven in LBD2, which was structurally different from most non-Lepidopteran insect VgRs having five repeats in LBD1 and eight in LBD2. The developmental expression analysis showed that CsVgR messenger RNA expression was first detectable in 3-day-old pupae, sharply increased in newly emerged female adults, and reached a peak in 2-day-old female adults. Consistent with most other insects VgRs, CsVgR was exclusively expressed in the ovary. Notably, injection of dsCsVgR into late pupae resulted in fewer follicles in the ovarioles as well as reduced fecundity, suggesting a critical role of CsVgR in female reproduction. These results may contribute to the development of RNA interference-mediated disruption of reproduction as a control strategy of C. suppressalis.