Characterization and functional analysis of a carboxylesterase gene associated with chlorpyrifos resistance in Nilaparvata lugens (Stål)

Resistance selection of triflumezopyrim in Laodelphax striatellus (fallén): Resistance risk, cross-resistance and metabolic mechanism

The risk assessment and resistance mechanisms of insecticide resistance are critical for resistance management strategy before a new insecticide is widely used. Triflumezopyrim (TFM) is the first commercialized mesoionic insecticide, which can inhibit nicotinic acetylcholine receptor with high-performance against the small brown planthopper (SBPH), Laodelphax striatellus (Fallén). In our study, the resistance of SBPH to TFM increased 26.29-fold, and the actual heritability of resistance was 0.09 after 21 generations of continuous selection by TFM. After five generations of constant feeding under insecticide-free conditions from F16 generation, the resistance level decreased 2.05-fold, and the average resistance decline rate per generation was 0.01, but there were no statistical decline. The TFM resistant strains had no cross-resistance to imidacloprid, nitenpyram, thiamethoxam, dinotefuran, flonicamid, pymetrozine, and chlorfenapyr. The third and fifth nymphal stage duration, pre-adult stage, adult preoviposition period, longevity, emergence rate, and hatchability of the resistant strain were significantly lower than those of the susceptible strain, while the female-male ratio was considerably increased. The fitness cost was 0.89. Further, cytochrome P450 monooxygenase (P450) and carboxylesterase (CarE) activities were markedly increased, but only the enzyme inhibitor piperonyl butoxide (PBO) had a significant synergistic effect on the resistant strain. The expression of CYP303A1, CYP4CE2, and CYP419A1v2 of P450 genes was significantly increased. SBPH has a certain risk of resistance to TFM with continuous application. The TFM resistance may be due to the increased activity of P450 enzyme regulated by the overexpression of P450 genes.

RNAi Mediated Gene Silencing of Detoxification Related Genes in the Ectropis oblique

Ectropis oblique is one of the main pests that feed on tea leaves. At present, the main control method is chemical control, but the long-term use of insecticides has been related to the development of insect resistance. One of the resistance mechanisms is the upregulation of relevant detoxification enzymes for defense. In this study, four genes with increased expression were screened from the gene sequences annotated from the transcriptome data of deltamethrin-treated larvae of E. oblique, which are acid phosphatase EoACP138, and cytochrome P450 EoCYP316, carboxylesterase EoCarE592 and acetylcholine esterase EoAchE989, respectively. The fourth instar larvae of E. oblique were stimulated by deltamethrin, chlorpyrifos and fenpropathrin respectively, and the expression levels of the genes were detected by qRT-PCR. The result showed that all four genes’ expression had significantly increased under the stimulation of three insecticides. RNAi technology was used to silence the expression of genes of EoACP138, EoCYP316, EoCarE592 and EoAchE989 in the fourth instar larvae of E. oblique. The change in the expression levels of the above genes in the larvae treated with dsRNA and stimulated with pesticides was determined by qRT-PCR. The target genes have been effectively silenced after feeding on dsRNA and higher sensitivity with higher mortality to pesticides was observed in the larvae interfered with dsRNA. The above genes are related to the detoxification and metabolism of resistance of E. oblique, which lays a foundation for further study on the mechanism of insecticide resistance in E. oblique.

Decreased carboxylesterase expression associated with increased susceptibility to insecticide in Mythimna separata

Carboxylesterases are one of the three major types of detoxification enzyme in insects. In this study, we screened 12 full-length carboxylesterase cDNA sequences from the oriental armyworm Mythimna separata; they were named MsCarE1-MsCarE12 and registered in GenBank with accession numbers MK440541-MK440552. Treatment of fourth instar larvae of M. separata with the LD₅₀ of the insecticide chlorantraniliprole increased the expression levels of MsCarE3 and MsCarE4, while treatment with the LD₅₀ of lambda-cyhalothrin significantly increased the expression levels of MsCarE5 and MsCarE10. Spatiotemporal expression detection showed that MsCarE3, MsCarE4, MsCarE5, and MsCarE10 were expressed at different developmental stages and in different tissues of M. separata and their expression levels were different. Induction using a high dose of chlorantraniliprole resulted in lower expression of MsCarE3 and MsCarE4. LD₅₀ of lambda-cyhalothrin induced higher expression of MsCarE5 and MsCarE10, while LD₇₀ induced higher MsCarE10 expression at 3, 6, and 12 h after treatment. RNA interference successfully inhibited the expression of MsCarE3, MsCarE4, MsCarE5, and MsCarE10, to different degrees at different time points. Silencing of MsCarE5, or MsCarE5 and MsCarE10 simultaneously changed carboxylesterase activity and increased the susceptibility of M. separata larvae to lambda-cyhalothrin. This study provides a new method to increase the insect susceptibility to insecticide.

A review of physiological resistance to insecticide stress in Nilaparvata lugens

Insecticides are widely used in agriculture as effective means to control pests. However, pests have not been completely mitigated with the increased use of insecticides. Instead, many side effects have arisen, especially the '3Rs' (resistance, resurgence, and residue). The brown planthopper, Nilaparvata lugens, is one of the most threatening rice pests. The main insecticides for controlling N. lugens belong to organochlorine, organophosphorus, carbamate, neonicotinoid and pyrethroid groups. However, metabolic enzymes, including cytochrome P450s, esterases, glutathione-S-transferases, and ATP-binding cassette transporters, effectively promote the detoxification of insecticides. Besides, mutations of neurological target sites, such as acetylcholinesterase, nicotinic acetylcholine, γ-aminobutyric acid receptor, and ryanodine receptor, result in insensitivity to insecticides. Here, we review the physiological metabolic resistance in N. lugens under insecticide stress to provide a theoretical basis for identifying and developing more effective and harmless insecticides.

Overexpression of ATP-binding cassette transporter Mdr49-like confers resistance to imidacloprid in the field populations of brown planthopper, Nilaparvata lugens

BACKGROUND

The brown planthopper (BPH), Nilaparvata lugens (Stål), is the most severe pest attacking rice crops using sucking mouthparts. It causes significant damages to rice growth and food production worldwide. With the long-term and wide use of insecticides, field populations of BPH have developed resistance to many insecticides.

RESULTS

Here, we showed that upregulation of an ATP-binding cassette transporter gene NlMdr49-like contributes to imidacloprid resistance in field populations of BPH. A comparative transcriptome analysis was performed to evaluate the gene expression in two field populations (JXSG18 and YNTC18). Compared with a susceptible strain (Sus), 202 upregulated genes and 170 downregulated genes were identified in both field populations. Functional enrichment analysis revealed that the differentially expressed genes (DEGs) are mainly linked to metabolic process and transmembrane transport. Among the candidate DEGs, NlMdr49-like was significantly upregulated in both field populations. Based on the genome and transcriptome of BPH, the full-length complementary DNA of NlMdr49-like was sequenced and its molecular characteristics were analyzed. Expression pattern analysis of various tissues showed that NlMdr49-like was predominantly expressed in midgut and Malpighian tubules which are important excretion organs. Knocking down NlMdr49-like reduced BPH resistance to imidacloprid, but did not affect its resistance to the other nine insecticides (chlorpyrifos, thiamethoxam, nitenpyram, dinotefuran, sulfoxaflor, triflumezopyrim, ethiprole, buprofezin and pymetrozine). Furthermore, a transgenic strain of Drosophila melanogaster overexpressing NlMdr49-like was less susceptible to imidacloprid.

CONCLUSIONS

Our findings indicate that upregulation of NlMdr49-like is another mechanism contributing to imidacloprid resistance in N. lugens. This result is helpful to further understand the resistance mechanism of N. lugens to imidacloprid. © 2021 Society of Chemical Industry.

The KNRL nuclear receptor controls hydrolase-mediated vitellin breakdown during embryogenesis in the brown planthopper, Nilaparvata lugens

Vitellin (Vn) homeostasis is central to the fecundity of oviparous insects. Most studies have focused on the synthesis and transportation of Vn as a building block for developing eggs during vitellogenesis; however, less is known about how the utilization of this nutrient reserve affects embryonic development. Here, we show that the single ortholog of the knirps and knirps-like nuclear receptors, KNRL, negatively regulates Vn breakdown by suppressing the expression of hydrolase genes in the brown planthopper, Nilaparvata lugens. KNRL was highly expressed in the ovary of adult females, and knockdown of KNRL by RNA interference resulted in the acceleration of Vn breakdown and the inhibition of embryonic development. Transcriptome sequencing analysis revealed that numerous hydrolase genes, including cathepsins and trypsins were up-regulated after KNRL knockdown. At least eight of the nine significantly enriched Gene Ontology terms for the up-regulated genes were in proteolysis-related categories. The expression levels of five selected trypsin genes and the enzymatic activities of trypsin in the embryos were significantly increased after KNRL knockdown. Moreover, trypsin injection prolonged egg duration, delayed embryonic development, accelerated Vn breakdown and severely reduced egg hatchability, a pattern similar to that observed in KNRL-silenced N. lugens. These observations suggest that KNRL controls Vn breakdown in embryos via the transcriptional inhibition of hydrolases. Generally, this study provides a foundation for understanding how embryo nutrient reserves are mobilized during embryogenesis and identifies several genes and pathways that may prove valuable targets for pest control.

Mechanism of metabolic resistance to pymetrozine in Nilaparvata lugens: over-expression of cytochrome P450 CYP6CS1 confers pymetrozine resistance

BACKGROUND

Pymetrozine is commonly used for the control of Nilaparvata lugens, and resistance to pymetrozine has been frequently reported in the field populations in recent years. However, the mechanism of brown planthopper resistance to pymetrozine is still unknown.

RESULTS

In this study, a pymetrozine-resistant strain (PMR) was established, and the potential biochemical resistance mechanism of N. lugens to pymetrozine was investigated. Pymetrozine was synergized by the inhibitor piperonyl butoxide (PBO) in the PMR with 2.83-fold relative synergistic ratios compared with the susceptible strain (Sus). Compared with the Sus, the cytochrome P450 monooxygenase activity of PMR was increased by 1.7 times, and two P450 genes (NlCYP6CS1 and NlCYP301B1) were found to be significantly overexpressed more than 6.0-fold in the PMR. Pymetrozine exposure induced upregulation of NlCYP6CS1 expression in the Sus, but the expression of NlCYP301B1 did not change significantly. In addition, RNA interference (RNAi)-mediated suppression of NlCYP6CS1 gene expression dramatically increased the toxicity of pymetrozine against N. lugens. Moreover, transgenic lines of Drosophila melanogaster expressing NlCYP6CS1 were less susceptible to pymetrozine, and had a stronger ability to metabolize pymetrozine.

CONCLUSIONS

Taken together, our findings indicate that the overexpression of NlCYP6CS1 is one of the key factors contributing to pymetrozine resistance in N. lugens. And this result is helpful in proposing a management strategy for pymetrozine resistance.

Carboxylesterase genes in nitenpyram-resistant brown planthoppers, Nilaparvata lugens

Carboxylesterases (CarEs) represent one of the major detoxification enzyme families involved in insecticide resistance. However, the function of specific CarE genes in insecticide resistance is still unclear in the insect Nilaparvata lugens (Stål), a notorious rice crop pest in Asia. In this study, a total of 29 putative CarE genes in N. lugens were identified, and they were divided into seven clades; further, the β-esterase clade was significantly expanded. Tissue-specific expression analysis found that 17 CarE genes were abundantly distributed in the midgut and fat body, while 12 CarE genes were highly expressed in the head. The expression of most CarE genes was significantly induced in response to the challenge of nitenpyram, triflumezopyrim, chlorpyrifos, isoprocarb and etofenprox. Among these, the expression levels of NlCarE2, NlCarE4, NlCarE9, NlCarE17 and NlCarE24 were increased by each insecticide. Real-time quantitative polymerase chain reaction and RNA interference assays revealed the NlCarE1 gene to be a candidate gene mainly involved in nitenpyram resistance, while simultaneously silencing NlCarE1 and NlCarE19 produced a stronger effect than silencing either one individually, suggesting a cooperative relationship in resistance formation. These findings lay the foundation for further clarification of insecticide resistance mediated by CarE in N. lugens.

Resistance development characteristics of reared German cockroach (Blattodea: Blattellidae) to chlorpyrifos

Understanding the process of resistance development of German cockroach, Blattella germanica (L.), in detail is necessary to potentially delay the development of insecticides resistance by rotation or discontinuation of insecticides at the right time. In this study, we investigated the resistance development of the reared German cockroach to chlorpyrifos (CPF) for 23 generations from susceptible cockroaches. CPF 50% lethal dose (LD₅₀) and resistance ratio of each generation cockroaches were determined. The CPF LD₅₀ to each generation cockroaches was used as the insecticide selection pressure of this generation by topical application. The resistance development curve was depicted according to the CPF LD₅₀ to all 23 generations of cockroaches. As a result, a highly resistant German cockroach cohort to CPF, which the resistance ratio was 21.63, was obtained after 23 generations’ selection. During the selection, the cockroaches developed low resistance from F1 to F5, moderate resistance from F6 to F12, and high resistance from F13 to F23. There was a rapid resistance increase every 5–7 generations. The resistance growing showed relatively slow from F1 to F11. The fastest growing phase of the resistance was from F12 to F20, in which accounted for more than 80% of the total resistance increase in 23 generations. The development of resistance to CPF tended to slow down from F21 to F23. These findings may provide a basis for the rational use of insecticides, delaying the development of resistance by rotation or discontinuation.

Adipokinetic hormone enhances CarE-mediated chlorpyrifos resistance in the brown planthopper, Nilaparvata lugens

Adipokinetic hormone (AKH), the principal stress-responsive neurohormone in insects, has been implicated in insect responses to insecticides. However, the functionality of AKH and its mode of signalling in insecticide resistance are unknown. Herein, we demonstrated that the enhanced activity of carboxylesterases (CarEs) is involved in the chlorpyrifos resistance in Nilaparvata lugens [brown planthopper (BPH)]. Chlorpyrifos exposure significantly induced the expression of AKH and its receptor AKHR in the susceptible BPH (Sus), and these two AKH signalling genes were over-expressed in the chlorpyrifos-resistant strain (Res) compared to Sus. RNA interference (RNAi) against AKH or AKHR decreased the CarE activity and suppressed the BPH's resistance to chlorpyrifos in Res. Conversely, AKH peptide injection elevated the CarE activity and enhanced the BPH's survival against chlorpyrifos in Sus. Furthermore, five CarE genes were identified to be positively affected by the AKH pathway using RNAi and AKH injection. Among these CarE genes, CarE and Esterase E4-1 were found to be over-expressed in Res compared to Sus, and knockdown of either gene decreased the BPH's resistance to chlorpyrifos. In conclusion, AKH plays a role in enhancing chlorpyrifos resistance in the BPH through positive influence on the expression of CarE genes and CarE enzyme activity.

Characterization of a novel hyper-thermostable and chlorpyrifos-hydrolyzing carboxylesterase EstC: A representative of the new esterase family XIX

Carboxylesterases have widely been used in a series of industrial applications, especially, the detoxification of pesticide residues. In the present study, EstC, a novel carboxylesterase from Streptomyces lividans TK24, was successfully heterogeneously expressed, purified and characterized. Phylogenetic analysis showed that EstC can be assigned as the first member of a novel family XIX. Multiple sequence alignment indicated that EstC has highly conserved structural features, including a catalytic triad formed by Ser155, Asp248 and His278, as well as a canonical Gly-His-Ser-Ala-Gly pentapeptide. Biochemical characterization indicated that EstC exhibited maximal activity at pH 9.0 (Tris-HCl buffer) and 55 °C. It also showed higher activity towards short-chain substrates, with the highest activity for p-nitrophenyl acetate (pNPA2) (K_m = 0.31 ± 0.02 mM, k_cat/K_m = 1923.35 ± 9.62 s^-1 mM^-1) compared to other pNP esters used in this experiment. Notably, EstC showed hyper-thermostability and good alkali stability. The activity of EstC had no significant changes when it was incubated under 55 °C for 100 h and reached half-life after incubation at 100 °C for 8 h. Beyond that, EstC also showed stability at pH ranging from 6.0 to 11.0 and about 90% residual activity still reserved after treatment at pH 8.0 or 9.0 for 26 h, especially. Furthermore, EstC had outstanding potential for bioremediation of chlorpyrifos-contaminated environment. The recombinant enzyme (0.5 U mL^-1) could hydrolyze 79.89% chlorpyrifos (5 mg L^-1) at 37 °C within 80 min. These properties will make EstC have a potential application value in various industrial productions and detoxification of chlorpyrifos residues.

Functional Characterization of an α-Esterase Gene Associated with Malathion Detoxification in Bradysia odoriphaga

Carboxylesterases (CarEs) are a multigene superfamily of metabolic enzymes involved in metabolic detoxification of xenobiotics. In this study, an α-esterase gene (BoαE1) was identified from Bradysia odoriphaga. Phylogenetic analysis classified BoαE1 into the α-esterase clade. Developmental expression analysis indicated that BoαE1 was significantly expressed in the second to fourth larval stages. Tissue-specific expression analysis indicated that BoαE1 was highly expressed in the larval midgut. After exposure to LC₃₀ of malathion, the CarE activity of B. odoriphaga was induced and the transcriptional level of BoαE1 was significantly up-regulated. Silencing of BoαE1 significantly increased the susceptibility of B. odoriphaga larvae to malathion. Inhibition assays in vitro indicated that malathion significantly inhibited BoαE1 activity. GC-MS assay showed that BoαE1 possesses hydrolase activity toward malathion and participates in the detoxification of malathion. These results strongly suggest that BoαE1 plays a crucial role in detoxification of malathion in B. odoriphaga.

Activation of CncC pathway by ROS burst regulates cytochrome P450 CYP6AB12 responsible for λ-cyhalothrin tolerance in Spodoptera litura

Frequent insecticide use poses an environmental hazard and also selects for insecticide tolerance. Increased metabolic detoxification by cytochrome P450 monooxygenases (P450s) is the most common mechanism of insecticide tolerance. However, the underlying regulatory mechanisms remain unknown. We studied the midgut-specific P450 gene, CYP6AB12, associated with λ-cyhalothrin tolerance. Its regulatory pathway was investigated in the tobacco cutworm, Spodoptera litura (Fabricius). P450 activities and CYP6AB12 transcript levels increased after λ-cyhalothrin exposure. Inhibiting P450 activities with piperonyl butoxide and silencing CYP6AB12 by double-stranded RNA (dsRNA) injection decreased larval tolerance to λ-cyhalothrin. λ-Cyhalothrin exposure induced the expression of the cap 'n' collar isoform C (CncC) and muscle aponeurosis fibromatosis (Maf), increased hydrogen peroxide (H₂O₂) contents and elevated antioxidant enzyme activities. CncC knockdown by dsRNA feeding suppressed CYP6AB12 expression and decreased larval tolerance to λ-cyhalothrin. In contrast, application of the CncC agonist curcumin induced CYP6AB12 expression and enhanced insecticide tolerance. Ingestion of the reactive oxygen species (ROS) scavenger N-acetylcysteine reduced H₂O₂ accumulation, suppressed the expression of CncC, Maf and CYP6AB12 and led to increased larval susceptibility to λ-cyhalothrin. The results demonstrate that in S. litura, λ-cyhalothrin induces cytochrome P450 CYP6AB12 via elicitation of the ROS burst and activation of the CncC pathway.

Copper exposure enhances Spodoptera litura larval tolerance to β-cypermethrin

Environmental xenobiotics can influence the tolerance of insects to chemical insecticides. Heavy metals are widespread distributed, can be easily bio-accumulated in plants and subsequently within phytophagous insects via the food chains. However, less attention has been paid to the effect of heavy metal exposure on their insecticide tolerance. In this study, pre-exposure of copper (Cu, 25-100 mg kg^-1) significantly enhanced the subsequent tolerance of Spodoptera litura to β-cypermethrin, a widely used pyrethroid insecticide in crop field. Cytochrome P450 monooxygenases (CYPs) activities were cross-induced in larvae exposed to Cu and β-cypermethrin, while the activities of glutathione S-transferase (GST) and carboxylesterase (CarE) were not affected. Application of piperonyl butoxide (PBO), a P450 synergist, effectively impaired the tolerance to β-cypermethrin in Cu-exposed S. litura larvae with a synergistic ratio of 1.72, indicating that P450s contribute to larval tolerance to β-cypermethrin induced by Cu exposure. Among the four CYP6AB family genes examined, only larval midgut-specific CYP6AB12 was found to be cross-induced by Cu and β-cypermethrin. RNA interference (RNAi)-mediated silencing of CYP6AB12 effectively decreased the mRNA levels of the target gene, and significantly reduced the larval tolerance to β-cypermethrin following exposure to Cu. These results showed that pre-exposure of heavy metal Cu enhanced larval tolerance to β-cypermethrin in S. litura, possibly through the cross-induction of P450s. Our findings provide new insights on the relationship between heavy metals and chemical insecticides that may benefit both the risk evaluation of heavy metal contamination and development of pest management strategies.