Functional Characterization of Two Carboxylesterase Genes Involved in Pyrethroid Detoxification in Helicoverpa armigera

SlCarE054 in Spodoptera litura (Lepidoptera: Noctuidae) showed direct metabolic activity to β-cypermethrin with stereoselectivity

Carboxylesterases (CarEs) is an important detoxification enzyme system in phase Ⅰ participating in insecticides resistance. In our previous study, SlCarE054, a CarEs gene from lepidoptera class, was screened out to be upregulated in a pyrethroids and organophosphates resistant population. Its overexpression was verified in two field-collected populations of Spodoptera litura (Lepidoptera: Noctuidae) resistant to pyrethroids and organophosphates by qRT-PCR. Spatiotemporal expression results showed that SlCarE054 was highly expressed in the pupae stage and the digestive tissue midgut. To further explore its role in pyrethroids and organophosphates resistance, its metabolism activity to insecticides was determined by UPLC. Its recombinant protein showed significant metabolism activity to cyhalothrin and fenvalerate, but not to phoxim or chlorpyrifos. The metabolic activity of SlCarE054 to β-cypermethrin showed stereoselectivity, with higher metabolic activity to θ-cypermethrin than the enantiomer α-cypermethrin. The metabolite of β-cypermethrin was identified as 3-phenoxybenzaldehyde. Further modelling and docking analysis indicated that β-cypermethrin, cyhalothrin and fenvalerate could bind with the catalytic triad of the 3D structure of SlCarE054. The interaction of β-cypermethrin with SlCarE054 also showed the lowest binding energy. Our work provides evidence that SlCarE054 play roles in β-cypermethrin resistance in S. litura.

Two cuticle-enriched chemosensory proteins confer multi-insecticide resistance in Spodoptera frugiperda

Recent studies revealed that insect chemosensory proteins (CSPs) both play essential roles in insect olfaction and insect resistance. However, functional evidence supporting the crosslink between CSP and insecticide resistance remains unexplored. In the present study, 22 SfruCSP transcripts were identified from the fall armyworm (FAW) and SfruCSP1 and SfruCSP2 are enriched in the larval cuticle and could be induced by multiple insecticides. Both SfruCSP1 and SfruCSP2 are highly expressed in the larval inner endocuticle and outer epicuticle, and these two proteins exhibited high binding affinities with three insecticides (chlorfenapyr, chlorpyrifos and indoxacarb). The knockdown of SfruCSP1 and SfruCSP2 increased the susceptibility of FAW larvae to the above three insecticides, and significantly increased the penetration ratios of these insecticides. Our in vitro and in vivo evidence suggests that SfruCSP1 and SfruCSP2 are insecticide binding proteins and confer FAW larval resistance to chlorfenapyr, chlorpyrifos and indoxacarb by an insecticide sequestration mechanism. The study should aid in the exploration of larval cuticle-enriched CSPs for insect resistance management.

Identification and biochemical characterization of a carboxylesterase gene associated with β-cypermethrin resistance in Dermanyssus gallinae

Dermanyssus gallinae is a major hematophagous ectoparasite in layer hens. Although the acaricide β-cypermethrin has been used to control mites worldwide, D. gallinae has developed resistance to this compound. Carboxylesterases (CarEs) are important detoxification enzymes that confer resistance to β-cypermethrin in arthropods. However, CarEs associated with β-cypermethrin resistance in D. gallinae have not yet been functionally characterized. Here, we isolated a CarE gene (Deg-CarE) from D. gallinae and assayed its activity. The results revealed significantly higher expression of Deg-CarE in the β-cypermethrin-resistant strain (RS) than in the susceptible strain (SS) toward α-naphthyl acetate (α-NA) and β-naphthyl acetate (β-NA). These findings suggest that enhanced esterase activities might have contributed to β-cypermethrin resistance in D. gallinae. Quantitative real-time PCR analysis revealed that Deg-CarE expression levels were significantly higher in adults than in other life stages. Although Deg-CarE was upregulated in the RS, significant differences in gene copy numbers were not observed. Additionally, Deg-CarE expression was significantly induced by β-cypermethrin in both the SS and RS. Moreover, silencing Deg-CarE via RNA interference decreased the enzyme activity and increased the susceptibility of the RS to β-cypermethrin, confirming that Deg-CarE is crucial for β-cypermethrin detoxification. Finally, recombinant Deg-CarE (rDeg-CarE) expressed in Escherichia coli displayed high enzymatic activity toward α/β-NA. However, metabolic analysis indicated that rDeg-CarE did not directly metabolize β-cypermethrin. The collective findings indicate that D. gallinae resistance to β-cypermethrin is associated with elevated CarEs protein activity and increased Deg-CarE expression levels. These findings provide insights into the metabolic resistance of D. gallinae and offer scientific guidance for the management and control of D. gallinae.

Heterologous expression, biochemical characterization and prospects for insecticide biosensing potential of carboxylesterase Ha006a from Helicoverpa armigera

Enzymes have attracted considerable scientific attention for their crucial role in detoxifying a wide range of harmful compounds. In today's global context, the extensive use of insecticides has emerged as a significant threat to the environment, sparking substantial concern. Insects, including economically important pests like Helicoverpa armigera, have developed resistance to conventional pest control methods through enzymes like carboxyl/cholinesterases. This study specifically focuses on a notable carboxyl/cholinesterase enzyme from Helicoverpa armigera (Ha006a), with the goal of harnessing its potential to combat environmental toxins. A total of six insecticides belonging to two different classes displayed varying inhibitory responses towards Ha006a, thereby rendering it effective in detoxifying a broader spectrum of insecticides. The significance of this research lies in discovering the bioremediation property of Ha006a, as it hydrolyzes synthetic pyrethroids (fenvalerate, λ-cyhalothrin and deltamethrin) and sequesters organophosphate (paraoxon ethyl, profenofos, and chlorpyrifos) insecticides. Additionally, the interaction studies between organophosphate insecticides and Ha006a helped in the fabrication of a novel electroanalytical sensor using a modified carbon paste electrode (MCPE). This sensor boasts impressive sensitivity, with detection limits of 0.019 μM, 0.15 μM, and 0.025 μM for paraoxon ethyl, profenofos, and chlorpyrifos, respectively. This study provides a comprehensive biochemical and biophysical characterization of the purified esterase Ha006a, showcasing its potential to remediate different classes of insecticides.

Oxidative stress and mitochondrial damage in lambda-cyhalothrin toxicity: A comprehensive review of antioxidant mechanisms

Lambda-cyhalothrin, also known as cyhalothrin, is an efficient, broad-spectrum, quick-acting pyrethroid insecticide and acaricide and the most powerful pyrethroid insecticide in the world. However, there is increasing evidence that lambda-cyhalothrin is closely related to a variety of toxicity drawbacks (hepatotoxicity, nephrotoxicity, neurotoxicity and reproductive toxicity, among others) in non-target organisms, and oxidative stress seems to be the main mechanism of toxicity. This manuscript reviews the oxidative and mitochondrial damage induced by lambda-cyhalothrin and the signalling pathways involved in this process, indicating that oxidative stress occupies an important position in lambda-cyhalothrin toxicity. The mechanism of antioxidants to alleviate the toxicity of lambda-cyhalothrin is also discussed. In addition, the metabolites of lambda-cyhalothrin and the major metabolic enzymes involved in metabolic reactions are summarized. This review article reveals a key mechanism of lambda-cyhalothrin toxicity-oxidative damage and suggests that the use of antioxidants seems to be an effective method for preventing toxicity.

Effects of different insecticides on transcripts of key genes in CncC pathway and detoxification genes in Helicoverpa armigera

The CncC pathway regulates the expression of multiple detoxification genes and contributes to the detoxification and antioxidation in insects. Many studies have focused on the impacts of plant allelochemicals on the CncC pathway, whereas studies on the effects of pesticides on key genes involved in this pathway are very limited. In this study, the effects of different types of commonly used insecticides on the transcripts of CncC, Keap1, and Maf and multiple detoxification genes of Helicoverpa armigera were evaluated using real-time quantitative polymerase chain reaction. The results showed that 8 insecticides (bifenthrin, λ-cyhalothrin, chlorantraniliprole, cyantraniliprole, spinosad, indoxacarb, chlorfenapyr, tolfenpyrad, and thiacloprid) significantly induced the expression of CncC and 4 insecticides (cypermethrin, acetamiprid, thiacloprid, and indoxacarb) suppressed the expression of Keap1 both at 24 h and 48 h; meanwhile, the expression levels of Maf were induced by 5 insecticides (fenvalerate, chlorantraniliprole, cyantraniliprole, lufenuron, and tolfenpyrad) at 24 h or 48 h. Multiple detoxification genes, especially cytochrome P450s genes, showed different up-regulation after bifenthrin, λ-cyhalothrin, chlorantraniliprole, cyantraniliprole, indoxacarb, and spinosad treatment for 48 h. Our results suggest that the CncC pathway and detoxification genes can be activated by different insecticides in H. armigera. These results establish a foundation for further studies on the relationship between the CncC pathway and the detoxification genes in H. armigera.

Effect of melatonin and luzindole antagonist on fipronil toxicity, detoxification and antioxidant enzyme system in different tissues of Helicoverpa armigera (Lepidoptera: Noctuidae)

Studies have investigating the detoxification and antioxidant enzymes with melatonin under pesticide stress in many vertebrates, whereas no reports produced in invertebrates. In this study possible role of melatonin and luzindole effect on fipronil toxicity and the detoxification, antioxidant enzymes in H. armigera has been reported. Result showed high toxicity of fipronil treatment (LC50 4.24 ppm), followed by increased LC50 value with melatonin pretreatment (6.44 ppm). Whereas decreased toxicity was observed with melatonin and luzindole combination (3.72 ppm). The detoxification enzymes AChE, esterase and P450 were increased in larval head and whole body with exogenous melatonin level compared to control 1-1.5 μmol/mg of protein. The antioxidant levels of CAT, SOD and GST in whole body and head tissue had been increased by melatonin and fipronil combination 1.1-1.4 unit/mg of protein followed by GPx and GR in larval head (1-1.2 μmol/mg of protein). Mean while the luzindole antagonist inhibits CAT, SOD, GST and GR oxidative enzyme level (1-1.5 fold) in most of the tissue compared to melatonin and fipronil treatment (p < 0.01). Hence this study concludes that the melatonin pretreatment can reduce the fipronil toxicity by enhanced detoxification and antioxidant enzyme system in H. armigera.

Sublethal effects of halofenozide on larval development and detoxification in Phaedon brassicae (Coleoptera: Chrysomelidae)

The brassica leaf beetle, Phaedon brassicae, is a serious defoliator of cruciferous crops. Halofenozide (Hal), an ecdysone agonist, is a new class of insect growth-regulating insecticide. Our preliminary experiment revealed the outstanding larval toxicity of Hal against P. brassicae. However, the metabolic degradation of this compound in insects remains unclear. In this study, oral administration of Hal at LC10 and LC25 caused severe separation of the cuticle and epidermis, leading to larval molting failure. Sublethal dose exposure also significantly reduced the larval respiration rate as well as their pupation rates and pupal weights. Conversely, the activities of the multifunctional oxidase, carboxylesterase (CarE), and glutathione S-transferase (GST) were significantly enhanced in Hal-treated larvae. Further analysis using RNA sequencing identified 64 differentially expressed detoxifying enzyme genes, including 31 P450s, 13 GSTs, and 20 CarEs. Among the 25 upregulated P450s, 22 genes were clustered into the CYP3 clan, and the other 3 genes belonged to the CYP4 clan. Meanwhile, 3 sigma class GSTs and 7 epsilon class GSTs were dramatically increased, accounting for the majority of the upregulated GSTs. Moreover, 16 of the 18 overexpressed CarEs were clustered into the coleopteran xenobiotic-metabolizing group. These results showed the augmented expression of detoxification genes in P. brassicae after exposed to sublethal dose of Hal, and helped to better understand the potential metabolic pathways that could contribute to the reduced sensitivity to Hal in this pest. Overall, a deep insight into the detoxification mechanisms would provide practical guidance for the field management of P. brassicae.

Identification of the Flavone-Inducible Counter-Defense Genes and Their cis-Elements in Helicoverpa armigera

Flavone is widely found in plants and plays an important role in plant defense against pests. Many pests, such as Helicoverpa armigera, use flavone as a cue to upregulate counter-defense genes for detoxification of flavone. Yet the spectrum of the flavone-inducible genes and their linked cis-regulatory elements remains unclear. In this study, 48 differentially expressed genes (DEGs) were found by RNA-seq. These DEGs were mainly concentrated in the retinol metabolism and drug metabolism-cytochrome P450 pathways. Further in silico analysis of the promoter regions of 24 upregulated genes predicted two motifs through MEME and five previously characterized cis-elements including CRE, TRE, EcRE, XRE-AhR and ARE. Functional analysis of the two predicted motifs and two different versions of ARE (named ARE1 and ARE2) in the promoter region of the flavone-inducible carboxylesterase gene CCE001j verified that the two motifs and ARE2 are not responsible for flavone induction of H. armigera counter-defense genes, whereas ARE1 is a new xenobiotic response element to flavone (XRE-Fla) and plays a decisive role in flavone induction of CCE001j. This study is of great significance for further understanding the antagonistic interaction between plants and herbivorous insects.

Inducible Gut-Specific Carboxylesterase SlCOE030 in Polyphagous Pests of Spodoptera litura Conferring Tolerance between Nicotine and Cyantraniliprole

Insecticides tolerance in herbivorous arthropods is associated with preadaptation to host plant allelochemicals. However, how plant secondary metabolites activate detoxifying metabolic genes to develop tolerance remains unclear. Herein, the tolerance of Spodoptera litura larvae to cyantraniliprole was increased after nicotine exposure. An S. litura α esterase, SlCOE030, was predominantly expressed in the midgut and induced after exposure to cyantraniliprole, nicotine, and cyantraniliprole plus nicotine. Drosophila melanogaster with ectopically overexpressed SlCOE030 enhanced cyantraniliprole and nicotine tolerance by 4.91- and 2.12-fold, respectively. Compared to UAS-SlCOE030 and Esg-GAL4 lines, the Esg > SlCOE030 line laid more eggs after nicotine exposure. SlCOE030 knockdown decreased the sensitivity of nicotine-treated S. litura larvae to cyantraniliprole. Metabolism assays indicated that recombinant SlCOE030 protein metabolizes cyantraniliprole. Homology modeling and molecular docking analysis demonstrated that SlCOE030 exhibits effective affinities for cyantraniliprole and nicotine. Thus, insect CarEs may result in the development of cross-tolerance between synthetic insecticides and plant secondary metabolites.

Effects of azadirachtin on detoxification-related gene expression in the fat bodies of the fall armyworm, Spodoptera frugiperda

The fall armyworm, Spodoptera frugiperda, has become a worldwide pest and threatens world food production. A previous study indicated that azadirachtin, the most effective botanical insecticide for S. frugiperda, inhibits larval growth of the insect. The effect of azadirachtin on the tissues of the larvae, however, remains to be determined. In this study, the effects of azadirachtin on the structure of fat bodies were analyzed. Comparative transcriptomic analysis was conducted between controls and samples treated with 0.1 μg/g azadirachtin for 7 days to explore potential relevant mechanisms. The expression of 5356 genes was significantly affected after azadirachtin treatment, with 3020 up-regulated and 2336 down-regulated. Among them, 137 encode detoxification enzymes, including 53 P450s, 20 GSTs, 27 CarEs, 16 UGTs, and 12 ABC transporters. Our results indicated that azadirachtin could destroy fat body structure and change the mRNA levels of detoxification-related genes. The up-regulated genes encoding detoxification enzymes might be related to detoxifying azadirachtin. Our results elucidate a preliminary mechanism of azadirachtin detoxification in the fat bodies of S. frugiperda larvae.

Increasing ecological heterogeneity can constrain biopesticide resistance evolution

Microbial biopesticides containing living parasites are valuable emerging crop protection technologies against insect pests, but they are vulnerable to resistance evolution. Fortunately, the fitness of alleles that provide resistance, including to parasites used in biopesticides, frequently depends on parasite identity and environmental conditions. This context-specificity suggests a sustainable approach to biopesticide resistance management through landscape diversification. To mitigate resistance risks, we advocate increasing the range of biopesticides available to farmers, whilst simultaneously encouraging other aspects of landscape-wide crop heterogeneity that can generate variable selection on resistance alleles. This approach requires agricultural stakeholders to prioritize diversity as well as efficiency, both within agricultural landscapes and the biocontrol marketplace.

Effects of Methyl Jasmonate Fumigation on the Growth and Detoxification Ability of Spodoptera litura to Xanthotoxin

Methyl jasmonate (MeJA) is a volatile substance derived from jasmonic acid (JA), and it responds to interbiotic and abiotic stresses by participating in interplant communication. Despite its function in interplant communication, the specific role of MeJA in insect defense responses is poorly understood. In this study, we found that carboxylesterase (CarE) activities, glutathione-S-transferase (GSTs) activities, and cytochrome mono-oxygenases (P450s) content increased more after the feeding of diets containing xanthotoxin, while larvae exposed to MeJA fumigation also showed higher enzyme activity in a dose-dependent manner: lower and medium concentrations of MeJA induced higher detoxification enzyme activities than higher concentrations of MeJA. Moreover, MeJA improved the growth of larvae fed on the control diet without toxins and diets with lower concentrations of xanthotoxin (0.05%); however, MeJA could not protect the larvae against higher concentrations of xanthotoxin (0.1%, 0.2%). In summary, we demonstrated that MeJA is effective at inducing S. litura defense response, but the enhanced detoxifying ability could not overcome the strong toxins.

Microbial elimination of pyrethroids: specific strains and involved enzymes

Pyrethroids, which are synthetic organic insecticides, are widely used in agriculture and households to resist pests and control disease transmission. However, pyrethroids have inevitably caused environmental pollution, leading to concerns for food safety and human health. Bioremediation has emerged as one of the most promising methods to eliminate pyrethroids compounds. Pyrethroid-degrading microorganisms and the relevant enzymes have shown an efficient ability in degrading pyrethroids by hydrolyzing the ester linkage. In this review, a wide variety of pyrethroid-degrading strains were presented and classified from different sources, such as wastewater, soils, and oceans. In addition, the recombinant expression, enzyme identification, and molecular modification of these microbial pyrethroid-degrading enzymes were also compared and discussed in detail. Moreover, the potential applications of pyrethroid-degrading enzymes, including immobilization and biodegradation towards a series of pyrethroids, were also presented. All of the positive results obtained from this review could be a good guideline for the other research in this field. KEY POINTS: • Distribution of pyrethroid-degrading strains in different sources was summarized. • Enzymatic properties including pH, temperature, and substrate specificity were compared. • Promising molecular modification and immobilization of hydrolases were present.

Integrated miRNA and transcriptome profiling to explore the molecular mechanism of Spodoptera frugiperda larval midgut in response to azadirachtin exposure

As a destructive agricultural pest, Spodoptera frugiperda has spread worldwide in the past few years. Azadirachtin, an environmentally friendly and most promising compound, showed adverse effects, including mortality and growth inhibition, against S. frugiperda. While the effects of azadirachtin on the midgut of this pest remain to be determined. In this study, structural damage was observed in the larval midguts of S. frugiperda with azadirachtin exposure. RNA-seq on the larval midguts with different azadirachtin treatments was performed. Compared to the control group, a total of 3344 and 4759 differentially expressed genes (DEGs) were identified in the midguts with 0.1 and 0.5 μg/g azadirachtin exposure, respectively. Among them, the DEGs encoding detoxification enzymes/proteins, immune-related proteins, digestion and absorption-related proteins, and transcript factors were further analyzed. High-throughput sequencing was also used for the identification of differentially expressed microRNAs in different treatments. A total of 153 conserved miRNAs and 147 novel miRNAs were identified, of which 11 and 29 miRNAs were affected by 0.1 and 0.5 μg/g azadirachtin treatments, respectively. The integrated analysis found that 13 and 178 miRNA versus mRNA pairs were acquired in the samples with 0.1 and 0.5 μg/g azadirachtin treatments, respectively. The results of high-throughput sequencing were confirmed by real-time quantitative polymerase chain reaction (RT-qPCR). These results provide useful information for revealing the molecular mechanism of S. frugiperda larval midgut in response to azadirachtin.

Overexpression of PxαE14 Contributing to Detoxification of Multiple Insecticides in Plutella xylostella (L.)

The diamondback moth, Plutella xylostella (L.), has evolved with varying degrees of resistance to almost all major classes of insecticides and has become the most resistant pest worldwide. The multiresistance to different types of insecticides has been frequently reported in P. xylostella, but little is known about the mechanism. In this study, a carboxylesterase (CarE) gene, PxαE14, was found significantly overexpressed in a field-evolved multiresistant P. xylostella population and can be dramatically induced by eight of nine tested insecticides. Results of the real-time quantitative polymerase chain reaction (RT-qPCR) showed that PxαE14 was predominantly expressed in the midgut and malpighian tubule of larvae. Knockdown of PxαE14 dramatically increased the susceptibility of the larvae to β-cypermethrin, bifenthrin, chlorpyrifos, fenvalerate, malathion, and phoxim, while overexpression of PxαE14 in Drosophila melanogaster increased the tolerance of the fruit flies to these insecticides obviously. More importantly, gas chromatography-mass spectrometry (GC-MS) and liquid chromatography-tandem mass spectrometry (LC-MS/MS) assay showed that the recombinant PxαE14 expressed in Escherichia coli exhibited metabolic activity against the six insecticides. The homology modeling, molecular docking, and molecular dynamics simulation analyses showed that these six insecticides could stably bind to PxαE14. Taken together, these results demonstrate that constitutive and inductive overexpression of PxαE14 contributes to detoxification of multiple insecticides involved in multiresistance in P. xylostella. Our findings provide evidence for understanding the molecular mechanisms underlying the multiresistance in insect pests.

Characterization of the insecticide detoxification carboxylesterase Boest1 from Bradysia odoriphaga Yang et Zhang (Diptera: Sciaridae)

BACKGROUND

In insects, carboxylesterases (CarEs) are enzymes involved in the detoxification of insecticides. However, the molecular mechanism of CarE-mediated insecticide metabolism in Bradysia odoriphaga, a serious agricultural pest, remains unclear. The aim of this study is to investigate the detoxification process of malathion, bifenthrin, and imidacloprid by B. odoriphaga carboxylesterase (Boest1).

RESULTS

An alpha class CarE gene Boest1 was cloned from B. odoriphaga. The results of real-time quantitative polymerase chain reaction showed that Boest1 is up-regulated with age during the larval stage, and the level of transcription of Boest1 is higher in the midgut and Malpighian tubule than in other tissues. The expression level of Boest1 was significantly increased after exposure to malathion and bifenthrin. Recombinant BoEST1 expressed in vitro showed high catalytic activity toward α-naphthyl acetate, which was substantially inhibited by malathion and triphenyl phosphate. The in vitro metabolism assays showed that BoEST1 demonstrates hydrolytic capacity toward malathion and bifenthrin but not imidacloprid. The binding free energy analysis indicates that BoEST1 has a higher affinity for malathion and bifenthrin than imidacloprid.

CONCLUSION

These results suggest that BoEST1 plays a role in the breakdown of insecticides and may be involved in the development of resistance in the Chinese chive pest B. odoriphaga; our findings also provide data for better pest management and perspectives for new pesticides development. © 2021 Society of Chemical Industry.

Effects of camptothecin on histological structures and gene expression profiles of fat bodies in Spodoptera frugiperda

Spodoptera frugiperda is a serious threat to global food production. Our previous study demonstrated that Camptothecin (CPT), a bioactive secondary metabolite from Camptotheca acuminata (Decne: Nyssaceae), exhibits adverse impact on the larval midgut of S. frugiperda and inhibits insect growth. However, effects of CPT on fat bodies of S. frugiperda larvae have not been examined yet. In the present study, we found that histological structures of fat bodies of S. frugiperda larvae were damaged in insects treated with CPT. Comparative transcriptomic analyses among different fat body samples from controls and insects treated with 1.0 and 5.0 μg/g CPT were performed. A total of 4212 and 5044 differentially expressed genes (DEGs) were identified in the samples treated with 1.0 and 5.0 μg/g CPT, respectively. Our data indicated that the pathways of detoxification, immune response, fatty acids, chitin, and hormone biosynthesis in fat bodies were affected by CPT treatments based on DEGs. These results provided a comprehensive view of the damage and gene expression changes in fat bodies of S. frugiperda after CPT exposure, which shall be useful to reveal the mechanism of CPT toxicity against S. frugiperda in future.

Two single mutations in carboxylesterase 001C improve fenvalerate hydrolase activity in Helicoverpa armigera

Carboxylesterases (CarEs) usually play critical roles in the detoxification of toxic chemicals and therefore may be involved in insecticide resistance in agricultural pests. Previous work has shown that CarE 001C from Helicoverpa armigera was able to metabolize the isomers of cypermethrin and fenvalerate. In this study, seven mutants of CarE 001C with single amino acid substitution were produced and expressed in the Escherichia coli. Enzyme kinetic analysis indicated that all seven mutations dramatically reduced enzymatic activities toward the generic substrate α-naphthyl acetate, but in vitro metabolism assay showed that two of the mutations, H423I and R322L, significantly improved hydrolase activities toward fenvalerate, with their recorded specific activities being 3.5 and 5.1 nM·s^-1·mg ^-1 proteins, respectively. Further, thermostability assay showed that the stability of one mutant enzyme was enhanced. This study will help us better understand the potential of CarEs in insecticide detoxification and resistance in H. armigera.

The effects of quercetin combined with nucleopolyhedrovirus on the growth and immune response in the silkworm (Bombyx mori)

Flavonoids are secondary metabolites that help plants resist insect attack. It can resist insect attack by inhibiting insect immune defense, and pathogens can also inhibit insect immune defense. It is speculated that the combination of flavonoids and pathogens may inhibit the immune defense and have stronger toxicity to silkworm. In this study, the combined treatment of quercetin with Bombyx mori nuclear polyhedrosis virus (BmNPV) had significant negative effects on the growth and survival of silkworm compared with BmNPV group. The detoxifying enzyme activity of BmNPV group was significantly increased at 96 h, while the activity of the combined treatment group was significantly decreased with the increase of quercetin exposure time (72 or 96 h). The activity of antioxidant enzymes also showed a similar trend, that was, the activity of antioxidant enzymes in the combined treatment group also decreased significantly with the increase of quercetin exposure time, which led to the increase of reactive oxygen species content. The silkworm cells would produce lipid peroxidation, malondialdehyde content was significantly increased, so that the expression of immune-related genes (the antimicrobial peptide, Toll pathway, IMD pathway, JAK-STAT pathway, and melanin genes) were decreased, leading to the damage of the immune system of silkworm. These results indicated that quercetin combined with BmNPV could inhibit the activities of protective enzymes and lead to oxidative damage to silkworm. It can also affect the immune response of the silkworm, and thus resulting in abnormal growth. This study provides the novel conclusion that quercetin accumulation will increase the susceptibility of silkworm to pathogens.

Identification of key residues of carboxylesterase PxEst-6 involved in pyrethroid metabolism in Plutella xylostella (L.)

The long-term and excessive use of insecticides has led to severe environmental problems and the evolution of insecticide resistance in insects. Carboxylesterases (CarEs) are important detoxification enzymes conferring insecticide resistance on insects. Herein, the detoxification process of Plutella xylostella (L.) carboxylesterase 6 (PxEst-6), one representative P. xylostella carboxylesterase, is investigated with cypermethrin, bifenthrin, cyfluthrin and λ-cyhalothrin. RT-qPCR shows that PxEst-6 is highly expressed in the midgut and cuticles of the third instar larvae. Exposure to pyrethroid insecticides resulted in PxEst-6 up-regulation in a short time. Metabolic assays indicate that PxEst-6 has the capacity to metabolize these pyrethroid insecticides. The combination of molecular docking, binding mode analyses and alanine mutations demonstrated that His451, Lys458 and Gln431 were key residues of PxEst-6 for metabolizing pyrethroids and the acetate groups derived from pyrethroids were key sites for being metabolized by PxEst-6. H451- and K458-derived hydrogen bond (H-bond) interactions with the pyrethroid acetate groups and the polar interactions with the pyrethroid acetate group provided by the Q431 sidechain were crucial to the pyrethroids' metabolism by PxEst-6. Our study contributes to revealing the reasons for pyrethroid resistance in P. xylostella, and provides a fundamental basis for the development of novel pyrethroid insecticides.

Functional analysis of a carboxylesterase gene involved in beta-cypermethrin and phoxim resistance in Plutella xylostella (L.)

BACKGROUND

Carboxylesterases (CarEs) are associated with detoxification of xenobiotics, including insecticides, in organism bodies. Overexpression of CarE genes is considered to have an important role in insecticide resistance in insects, however its involvement in multi-insecticide resistance has rarely been reported. This study aimed to assess the function of a CarE gene (PxαE8) in resistance to five insecticides in Plutella xylostella.

RESULTS

Relative expression of PxαE8 in three multi-insecticide-resistant Plutella xylostella populations, GD-2017, GD-2019 and HN, was14.8-, 19.5- and 28.0-fold higher than that in the susceptible population. Exposure to lethal concentrations associated with 25% mortality (LC₂₅ ) of beta-cypermethrin, chlorantraniliprole, metaflumizone, phoxim and tebufenozide could induce the specific activity of CarEs and increase the relative expression of PxαE8. By contrast, knockdown of PxαE8 expression dramatically reduced the activity of CarEs and increased the resistance of P. xylostella (GD-2019) larvae to beta-cypermethrin and phoxim by 47.4% and 45.5%, respectively. Further, a transgenic line of Drosophila melanogaster overexpressing PxαE8 was constructed and the bioassay results showed that the tolerance of transgenic Drosophila to beta-cypermethrin and phoxim was 3.93- and 3.98-fold higher than that of the untransgenic line.

CONCLUSION

These results provide evidence that overexpression of PxαE8 is involved in resistance, at least to beta-cypermethrin and phoxim, in multi-insecticide-resistant P. xylostella populations, which could help in further understanding the molecular mechanisms of multi-insecticide resistance in this pest. © 2020 Society of Chemical Industry.

Enhanced hydrolysis of β-cypermethrin caused by deletions in the glycin-rich region of carboxylesterase 001G from Helicoverpa armigera

BACKGROUND

Carboxylesterase (CarE) is a major class of enzyme involved in the detoxification of toxic xenobiotics in various insect species. Previous work has shown that the carboxylesterase gene CarE001G found in Helicoverpa armigera is more active and can metabolize synthesized pyrethroids, such as β-cypermethrin, one of the commonly used commercial insecticides for lepidopteran pest control. In addition, CarE001G is very special as it has a very specific glycine-rich region located adjacent to its C-terminal. But whether mutations in this unique sequence can change the biochemistry and function of CarE001G are unknown.

RESULTS

In this study, four variants of CarE001G with different deletions in the glycine-rich region were obtained and functionally expressed in Escherichia coli. The recombinant proteins were purified and confirmed by Western blot and mass spectrometry analyses. These mutant enzymes showed high catalytic efficiency toward the model substrate α-naphthyl acetate. Inhibition study showed that β-cypermethrin had relatively strong inhibition on CarE activities. In vitro metabolism assay showed that the mutant enzymes significantly enhanced their metabolic activities toward β-cypermethrin with specific activities between 4.0 and 5.6 nmol L^-1 min^-1 mg^-1 protein. Molecular docking analyses consistently demonstrated that deletion mutations in the glycine-rich region may facilitate the anchoring of the β-cypermethrin molecule in the active binding pocket of the mutant enzymes.

CONCLUSION

The data show that deletion mutations can cause qualitative change in the capacity of CarEs in the detoxification of β-cypermethrin. This indicates that deletion mutations in the glycine-rich region may have the potential to cause synthesized pyrethroid (SP) resistance in H. armigera in the future. © 2020 Society of Chemical Industry.