Functional analysis of CYP6AE68, a cytochrome P450 gene associated with indoxacarb resistance in Spodoptera litura (Lepidoptera: Noctuidae)

Spodoptera litura (Fabricius) is a widely distributed, highly polyphagous pest that can cause severe damage to a variety of economically important crops. Various populations have developed resistance to different classes of insecticides. In this study, we report on two indoxacarb-resistant S. litura populations, namely Ind-R (resistance ratio = 18.37-fold) derived from an indoxacarb-susceptible (Ind-S) population and a population caught from a field (resistance ratio = 46.72-fold). A synergist experiment showed that piperonyl butoxide (PBO) combined with indoxacarb produced higher synergistic effects (synergist ratio = 5.29) in the Ind-R population as compared to Ind-S (synergist ratio = 3.08). Elevated enzyme activity of cytochrome P450 monooxygenases (P450s) was observed for Ind-R (2.15-fold) and the Field-caught population (4.03-fold) as compared to Ind-S, while only minor differences were noticed in the activities of esterases and glutathione S-transferases. Furthermore, expression levels of P450 genes of S. litura were determined by quantitative reverse transcription PCR to explore differences among the three populations. The results showed that the mRNA levels of CYP6AE68, a novel P450 gene belonging to the CYP6 family, were constitutively overexpressed in Ind-R (32.79-fold) and in the Field-caught population (68.11-fold). CYP6AE68 expression in S. litura was further analyzed for different developmental stages and in different tissues. Finally, we report that RNA interference-mediated silencing of CYP6AE68 increased the mortality of fourth-instar larvae exposed to indoxacarb at the LC₅₀ dose level (increase by 33.89%, 29.44% and 22.78% for Ind-S, Ind-R and the Field-caught population, respectively). In conclusion, the findings of this study indicate that expression levels of CYP6AE68 in S. litura larvae are associated with indoxacarb resistance and that CYP6AE68 may play a significant role in detoxification of indoxacarb.

Changes in both trans- and cis-regulatory elements mediate insecticide resistance in a lepidopteron pest, Spodoptera exigua

The evolution of insect resistance to insecticides is frequently associated with overexpression of one or more cytochrome P450 enzyme genes. Although overexpression of CYP450 genes is a well-known mechanism of insecticide resistance, the underlying regulatory mechanisms are poorly understood. Here we uncovered the mechanisms of overexpression of the P450 gene, CYP321A8 in a major pest insect, Spodoptera exigua that is resistant to multiple insecticides. CYP321A8 confers resistance to organophosphate (chlorpyrifos) and pyrethroid (cypermethrin and deltamethrin) insecticides in this insect. Constitutive upregulation of transcription factors CncC/Maf are partially responsible for upregulated expression of CYP321A8 in the resistant strain. Reporter gene assays and site-directed mutagenesis analyses demonstrated that CncC/Maf enhanced the expression of CYP321A8 by binding to specific sites in the promoter. Additional cis-regulatory elements resulting from a mutation in the CYP321A8 promoter in the resistant strain facilitates the binding of the orphan nuclear receptor, Knirps, and enhances the promoter activity. These results demonstrate that two independent mechanisms; overexpression of transcription factors and mutations in the promoter region resulting in a new cis-regulatory element that facilitates binding of the orphan nuclear receptor are involved in overexpression of CYP321A8 in insecticide-resistant S. exigua.

Insect pests developing resistance to insecticides used for their control is a major problem in agriculture. Many pests including the beet armyworm, Spodoptera exigua have developed resistance to insecticides used for their control. Information on the mechanisms of resistance would help in resistance management programs. Overexpression of detoxifying enzymes were associated with insecticide resistance, but their functions and regulatory mechanisms are still unidentified. The expression levels of P450 genes between susceptible and resistant strains of S. exigua were compared and CYP321A8 was identified as the major contributor to resistance to organophosphate and pyrethroid insecticides. Further studies uncovered two independent but synergistic mechanisms; constitutive upregulation of b-Zip transcription factors and mutations in the promoter that facilitates the binding of an orphan nuclear receptor, Knirps contributing to increase in the expression of CYP321A8 and resistance to multiple insecticides in S. exigua.

Xenobiotic transcription factors CncC and maf regulate expression of CYP321A16 and CYP332A1 that mediate chlorpyrifos resistance in Spodoptera exigua

Insect cytochrome P450 s (P450 s) are associated with the metabolic detoxification of toxic xenobiotics and their constitutive upregulation is often associated with resistance to natural and synthetic toxicants. The P450 s CYP321A16 and CYP332A1 are constitutively overexpressed in an insecticide-resistant strain of beet armyworm, Spodoptera exigua. However, the function and upstream regulation of these two P450 s remain unknown. Here, we investigated the function of CYP321A16 and CYP332A1 in resistance to the organophosphate insecticide, chlorpyrifos and their regulation by the transcription factors CncC and Maf. Transgenic strains of Drosophila melanogaster expressing CYP321A16 or CYP332A1 showed higher levels of tolerance to chlorpyrifos than the control flies with the same genetic background. Furthermore, recombinant CYP321A16 and CYP332A1 proteins metabolized chlorpyrifos. Analysis of the putative promoter sequences of the genes coding for CYP321A16 and CYP332A1 revealed conserved CncC/Maf binding sites. Transfection of luciferase reporter plasmids containing the promoter of CYP450 gene together with CncC and Maf expression plasmids significantly enhanced the activity of the reporter. Promoter truncation identified a site in the promoter of CYP321A16 that is critical for the CncC/Maf binding. These data demonstrate that resistance to chlorpyrifos in S. exigua is conferred by the combined action of CYP321A16 and CYP332A1 and uncovered their regulation by the transcription factors CncC and Maf.

Activiation of the nitric oxide cycle by citrulline and arginine restores susceptibility of resistant brown planthoppers to the insecticide imidacloprid

Pest management, which is critical for global crop productivity, is hampered by rapidly evolving insecticide resistance in insect pests. The ability to manage the development of insecticide resistance is thus vital. Nitric oxide (NO) is a ubiquitous signaling molecule with important functions in a variety of biological processes. Here we show that imidacloprid-resistant brown planthoppers (BPH) are deficient in citrulline and arginine, both of which are involved in NO production, but exogenous citrulline and arginine render resistant BPH vulnerable to imidacloprid. BPH insecticide resistance results from low NO production; exogenous arginine and citrulline augment the NO signaling in BPH, leading to downregulation of CYP6AY1 and CYP6ER1, the cytochrome P450 s that contribute to imidacloprid detoxification, thereby restoring susceptibility. Two amino acids that can be used to restore susceptibility in insecticide-resistant insects are identified, establishing a novel metabolome-based approach for killing insecticide-resistant pests and providing a useful template for managing insecticide resistance.

Silicon-mediated multiple interactions: Simultaneous induction of rice defense and inhibition of larval performance and insecticide tolerance of Chilo suppressalis by sodium silicate

The rice striped stem borer (SSB, Chilo suppressalis) is one of the most destructive pests of rice plants. Si-mediated rice defense against various pests has been widely reported, and sodium silicate (SS) has been used as an effective source of silicon for application to plants. However, there is quite limited information about the direct effects of Si application on herbivorous insects. SSB larval performance and their insecticide tolerance were examined after they had been reared either on rice plants cultivated in nutrient solution containing 0.5 and 2.0 mM SS or on artificial diets with 0.1% and 0.5% SS. SS amendment in either rice culture medium or artificial diets significantly suppressed the enzymatic activities of acetylcholinesterase, glutathione S-transferases, and levels of cytochrome P450 protein in the midgut of C. suppressalis larvae. Larvae fed on diets containing SS showed lower insecticide tolerance. Additionally, RNA-seq analysis showed that SS-mediated larval insecticide tolerance was closely associated with fatty acid biosynthesis and pyruvate metabolism pathways. Our results suggest that Si not only enhances plant resistance against insect herbivore, but also impairs the insect's capacity to detoxify the insecticides. This should be considered as another important aspect in Si-mediated plant-insect interaction and may provide a novel approach of pest management.

The Inducible CYP4C71 Can Metabolize Imidacloprid in Laodelphax striatellus (Hemiptera: Delphacidae)

Laodelphax striatellus (Fallén) is an important rice pest species which has developed high resistance to imidacloprid. Previous studies have demonstrated that CYP6AY3v2 and CYP353D1v2 were constitutively overexpressed in a imidacloprid resistant strain and can metabolize imidacloprid to mediated metabolic resistance. Further studies still needed to explore whether there are other L. striatellus P450 enzymes that can also metabolize imidacloprid. In this study, the expression level of L. striatellus CYP4C71 was significantly upregulated both in laboratory strains and field strains of L. striatellus after imidacloprid treatment for 4 h. The capability of CYP4C71 to metabolize imidacloprid was investigated. The full-length CYP4C71 was cloned, and its open reading frame was 1,515 bp with an enzyme estimated to be 505 amino acid residues in size. Furthermore, CYP4C71 was heterologously expressed along with L. striatellus cytochrome P450 reductase (CPR) in insect cells. A carbon monoxide difference spectra analysis confirmed the successful expression of CYP4C71. The recombinant CYP4C71 showed high P450 O-demethylation activity with PNP as a substrate. In vitro metabolism studies showed that recombinant CYP4C71 can metabolize imidacloprid to an easily excreted hydroxy-form. The rate of imidacloprid depletion in response to imidacloprid concentration revealed Michaelis Menten kinetics (R2 fitted curve = 0.99) with a relative low affinity: Kcat = 0.032 ± 0.009 pmol depleted imidacloprid/min/pmol P450 and Km=85.19 ± 2.93 μM. A relative big Km (85.19 ± 2.93 μM) indicated relative low imidacloprid's affinity for the CYP4C71 enzyme. In conclusion, CYP4C71 was another P450 enzyme that can metabolize imidacloprid with a relatively low affinity.

Identification and characterization of multiple dsRNases from a lepidopteran insect, the tobacco cutworm, Spodoptera litura (Lepidoptera: Noctuidae)

RNA interference (RNAi) efficiency varies among insects. RNAi is highly efficient and systemic in coleopteran insects but quite variable and inefficient in lepidopteran insects. Degradation of double-stranded RNA (dsRNA) by double-stranded ribonucleases (dsRNases) is thought to contribute to the variability in RNAi efficiency observed among insects. One or two dsRNases involved in dsRNA digestion have been identified in a few insects. To understand the contribution of dsRNases to reduced RNAi efficiency in lepidopteran insects, we searched the transcriptome of Spodoptera litura and identified six genes coding for DNA/RNA non-specific endonucleases. Phylogenetic analysis revealed the evolutionary expansion of dsRNase genes in insects. The mRNA levels of three midgut-specific dsRNases increased during the larval stage, and the highest dsRNA-degrading activity was detected in third-instar larvae. Proteins produced via the expression of three midgut-specific dsRNases, and the widely expressed dsRNase3, in a baculovirus system showed dsRNase activity for four out of five dsRNases tested. In addition, the increase in dsRNA-degrading activity and upregulation of dsRNase1 and 2 in larvae fed on cabbage leaves suggests that the diet of S. litura can influence dsRNase expression, dsRNA stability, and thus probably RNAi efficiency. This is the first report that multiple dsRNases function together in an RNAi-recalcitrant insect. The data included in this paper suggest that multiple dsRNases coded by the S. litura genome might contribute to the lower and variable RNAi efficiency reported in this and other lepidopteran insects.

Exposure to herbicides reduces larval sensitivity to insecticides in Spodoptera litura (Lepidoptera: Noctuidae)

Herbicides and insecticides are widely used in modern agriculture. It has been reported in various studies that application of insecticides can increase tolerance of herbivorous insects to insecticides. However, limited information exists on susceptibility to insecticides when insects are exposed to herbicides. This study was conducted to investigate the potential impact of the herbicides trifluralin and 2-methyl-4-chlorophenoxyacetic acid sodium salt (MCPA-Na) on the susceptibility of the nocturnal moth Spodoptera litura to the insecticides λ-cyhalothrin, phoxim and bifenthrin. We found that larvae exposed to trifluralin or MCPA-Na became significantly less susceptible to both insecticides than non-exposed control larvae. Herbicide-treated larvae did not show altered growth under the used test conditions. However, heads of herbicide-treated larvae showed increased expression of the acetylcholinesterase genes SlAce1 and SlAce2. Moreover, the fat body and midgut of herbicide-treated larvae displayed elevated expression of detoxification genes (the carboxylesterase gene SlCarE; the glutathione S-transferase genes SlGSTe2 and SlGSTe3; the cytochrome P450 monooxygenase genes CYP6B48, CYP9A40 and CYP321B1). The CYP6B48 gene exhibited highest inducibility. In conclusion, the data of this study suggest that exposure of S. litura larvae to herbicides may stimulate detoxification mechanisms that compromise the efficacy of insecticides.

Functional characterization of pheromone receptors in the moth Athetis dissimilis (Lepidoptera: Noctuidae)

Sex pheromones are crucial for communication between females and males in moths, and pheromone receptors (PRs) play a key role in peripheral coding of sex pheromones. During the last decade, many PR candidates have been identified based on transcriptome sequencing and bioinformatic analysis, but their detailed functions remain mostly unknown. Here, focusing on four PR candidates of Athetis dissimilis (AdisOR1, AdisOR6, AdisOR11 and AdisOR14) identified in a previous study, we first cloned the full-length cDNAs and determined the tissue expression profiles by quantitative real-time PCR (qPCR). The results revealed that expression of three of these genes were male antennae-specific, while AdisOR11 was similar in expression between male and female antennae. Furthermore, the expression level of AdisOR1 was much higher than those of the other three genes. Then, functional analysis was conducted using Xenopus oocyte system. AdisOR1 responded strongly to the sex pheromone component Z9-14:OH and the potential pheromone component Z9,E12-14:OH, suggesting its important role in the sex pheromone perception; AdisOR14 showed specificity for Z9,E12-14:OH; while AdisOR6 and AdisOR11 did not respond to any of the pheromone components and analogs tested. Taken together, this study contributes to elucidate the molecular mechanism of sex pheromone reception and provides potential targets for development of OR based pest control techniques in A. dissimilis.

Bioactivation of aflatoxin B1 by a cytochrome P450, CYP6AE19 induced by plant signaling methyl jasmonate in Helicoverpa armigra (Hübner)

Herbivore attack leads to enhanced production of defensive compounds to mount anti-herbivore defense in plants via activation of the jasmonate signaling pathway. On the other hand, some herbivores can eavesdrop on plants defense signaling and up-regulate their cytochrome P450 genes to increase detoxification of defensive compounds. However, the ecological risk of eavesdropping on plant defense signaling is largely unknown. In this study, we examined the induction of cytochrome P450s by methyl jasmonate (MeJA) and its consequence on the toxicity of aflatoxin B1 (AFB1) to Helicoverpa armigra larvae. The results show that MeJA applications either in a diet or volatile exposure enhanced the toxicity of AFB1 to the larvae. RNA sequences analysis revealed that cytochrome P450 CYP6AE19 was highly induced when MeJA was applied with AFB1. In addition, HaGST encoding glutathione-S-transferase that mainly transforms aflatoxin B1 exo-8,9-epoxide to aflatoxin B1 exo-8,9-glutathione was also induced. RNA interference of CYP6AE19 via injecting a double-stranded RNA decreased mortality of larvae exposed to AFB1; while injecting a double-stranded RNA of HaGST increased larval mortality. Furthermore, a protein model was generated and a subsequent docking simulation for AFB1 suggests the bioactivation as a major mechanism of AFB1. This study provides evidence that MeJA increased larval mortality of H. armigera via induction of CYP6AE19 that can bioactivate AFB1.

Mikania Micrantha Wilt Virus Alters Insect Vector's Host Preference to Enhance Its Own Spread

As an invasive weed, Mikaniamicrantha Kunth has caused serious damage to natural forest ecosystems in South China in recent years. Mikania micrantha wilt virus (MMWV), an isolate of the Gentian mosaic virus (GeMV), is transmitted by Myzuspersicae (Sulzer) in a non-persistent manner and can effectively inhibit the growth of M. micrantha. To explore the MMWV-M. micrantha-M. persicae interaction and its impact on the invasion of M. micrantha, volatile compounds (VOCs) emitted from healthy, mock-inoculated, and MMWV-infected plants were collected, and effects on host preference of the apterous and alate aphids were assessed with Y-shaped olfactometers. Gas chromatography-mass spectrometry (GC-MS) analysis indicated that MMWV infection changed the VOC profiles, rendering plants more attractive to aphids. Clip-cages were used to document the population growth rate of M.persicae fed on healthy, mock-inoculated, or MMWV-infected plants. Compared to those reared on healthy plants, the population growth of M. persicae drastically decreased on the MMWV-infected plants. Plant host choice tests based on visual and contact cues were also conducted using alate M.persicae. Interestingly, the initial attractiveness of MMWV-infected plants diminished, and more alate M. persicae moved to healthy plants. Taken together, MMWV appeared to be able to manipulate its plant host to first attract insect vectors to infected plants but then repel viruliferous vectors to promote its own dispersal. Its potential application for invasive weed management is discussed.

Plant allelochemicals affect tolerance of polyphagous lepidopteran pest Helicoverpa armigera (Hübner) against insecticides

Cotton bollworm, Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae), is a polyphagous lepidopteran pest distributed worldwide with a broad spectrum of host plants. However, the mechanism of H. armigera adaptation to various insecticides and defensive allelochemicals in its host plants is not fully understood. Therefore, this study examined the influence of consumption of plant allelochemicals on larval tolerance to methomyl and chlorpyrifos insecticides in H. armigera and its possible mechanism. Twelve plant allelochemicals were screened to evaluate their effects on larval sensitivity to methomyl. Of which flavone, coumarin, DIMBOA (2,4-Dihydroxy-7-methoxy-1,4-benzoxazin-3-one) and visnagin significantly reduced larval sensitivity to methomyl. Application of cytochrome P450 inhibitor piperonyl butoxide (PBO) significantly increased the mortality of methomyl-treated larvae. In contrast, PBO addition significantly decreased the mortality of chlorpyrifos-treated larvae. Moreover, allelochemical consumption enhanced the activities of glutathione S-transferase, carboxylesterase, cytochrome P450 and acetylcholinesterase in the midgut and fat body. The qRT-PCR analysis confirms that P450 genes, CYP6B2, CYP6B6 and CYP6B7 were induced by the four allelochemicals in the midguts and the fat bodies. In conclusion, the generalist H. armigera can take benefit of plant allelochemicals from its host plants to elaborate its defense against insecticides.

An overexpressed cytochrome P450 CYP439A1v3 confers deltamethrin resistance in Laodelphax striatellus Fallén (Hemiptera: Delphacidae)

Deltamethrin resistance in Laodelphax striatellus had been associated with its oxidative detoxification by overexpression of four cytochrome P450 monooxygenases like CYP353D1v2, CYP6FU1, CYP6AY3v2, and CYP439A1v3. The first three P450s have been validated for insecticide-metabolizing capability and only CYP6FU1 was found to degrade deltamethrin. In this study, an investigation was conducted to confirm the capability of CYP439A1v3 to degrade deltamethrin. The CYP439A1v3 was first expressed in Sf9 cell line and its recombinant enzyme was tested for metabolic activity against different insecticides using substrate depletion assay combined with metabolite identification. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and carbon monoxide (CO)-difference spectra analysis showed that the intact cytochrome P450 protein was successfully expressed. Tests with probe substrates proved its enzyme activity, as p-nitroanisole, ethoxycoumarin, and ethoxyresorufin were preferentially metabolized (specific activity 7.767 ± 1.22, 1.325 ± 0.37, and 0.355 ± 0.37 nmol/min per mg of protein, respectively) while only luciferin-HEGE was not. In vitro incubation of the recombinant CYP439A1v3 protein with deltamethrin revealed hydroxylation by producing hydroxydeltamethrin. On the contrary, no metabolite/metabolism was seen with nonpyrethroid insecticide, including imidacloprid, buprofezin, chlorpyrifos, and fipronil. To the best of our knowledge, this is the first study to link a CYP450 from family 439 to confer pyrethroid resistance to L. striatellus. This finding should help in the design of appropriate insecticide resistance management for control of this strain of L. striatellus.

Deltamethrin is metabolized by CYP6FU1, a cytochrome P450 associated with pyrethroid resistance, in Laodelphax striatellus

BACKGROUND

Cytochrome P450s (CYPs) are known to play a major role in metabolizing a wide range compounds. CYP6FU1 has been found to be over-expressed in a deltamethrin-resistant strain of Laodelphax striatellus. This study was conducted to express CYP6FU1 in Sf9 cells as a recombinant protein, to confirm its ability to degrade deltamethrin, chlorpyrifos, imidacloprid and traditional P450 probing substrates.

RESULTS

Carbon monoxide difference spectrum analysis indicated that the intact CYP6FU1 protein was expressed in insect Sf9 cells. Catalytic activity tests with four traditional P450 probing substrates revealed that the expressed CYP6FU1 preferentially metabolized p-nitroanisole and ethoxyresorufin, but not ethoxycoumarin and luciferin-HEGE. The enzyme kinetic parameters were tested using p-nitroanisole. The michaelis constant (K_m ) and catalytic constant (K_cat ) values were 17.51 ± 4.29 µm and 0.218 ± 0.001 pmol min^-1 mg^-1 protein, respectively. Furthermore, CYP6FU1 activity for degradation of insecticides was tested by measuring substrate depletion and metabolite formation. The chromatogram analysis showed obvious nicotinamide-adenine dinucleotide phosphate (NADPH)-dependent depletion of deltamethrin, and formation of the unknown metabolite. Mass spectra and the molecular docking model showed that the metabolite was 4-hydroxy-deltamethrin. However, the recombinant CYP6FU1 could not metabolize imidacloprid and chlorpyrifos.

CONCLUSION

These results confirmed that the over-expressed CYP6FU1 contributes to deltamethrin resistance in L. striatellus, and p-nitroanisole might be a potential diagnostic probe for deltamethrin metabolic resistance detection and monitoring. © 2017 Society of Chemical Industry.

Association of detoxification enzymes with butene-fipronil in larvae and adults of Drosophila melanogaster

Insecticide resistance is a major challenge in successful insect pest control as the insects have the ability to develop resistance to various widely used insecticides. Butene-fipronil is a novel compound with high toxicity to insects and less toxicity to the non-target organisms. In the present study, the effect of butene-fipronil alone and in combination with three enzyme inhibitors, piperonyl butoxide (PBO), diethyl maleate (DEM), and triphenyl phosphate (TPP), was carried out on larvae and adults of Drosophilia melanogaster. Our results indicated that the co-toxicity indices of butene-fipronil + PBO, butene-fipronil + TPP, and butene-fipronil + DEM mixtures were 437.3, 335.0, and 210.3, respectively, in the second-instar larvae, while 186.6, 256.2, and 238.5, respectively, in the adults, indicating synergistic effects. Interestingly, butene-fipronil increased the expression of CYP28A5 in the larvae; CYP9F2, CYP304A1, CYP28A5, and CYP318A1 in the female adults; and CYP303A1 and CYP28A5 in the male adults. Furthermore, high-level expression of Est-7 was observed in the female adults compared to larvae and male adults. Our results suggest that there is no difference in butene-fipronil metabolism in larvae and male and female adults of D. melanogaster.

Buprofezin Is Metabolized by CYP353D1v2, a Cytochrome P450 Associated with Imidacloprid Resistance in Laodelphax striatellus

CYP353D1v2 is a cytochrome P450 related to imidacloprid resistance in Laodelphax striatellus. This work was conducted to examine the ability of CYP353D1v2 to metabolize other insecticides. Carbon monoxide difference spectra analysis indicates that CYP353D1v2 was successfully expressed in insect cell Sf9. The catalytic activity of CYP353D1v2 relating to degrading buprofezin, chlorpyrifos, and deltamethrin was tested by measuring substrate depletion and analyzing the formation of metabolites. The results showed the nicotinamide-adenine dinucleotide phosphate (NADPH)-dependent depletion of buprofezin (eluting at 8.7 min) and parallel formation of an unknown metabolite (eluting 9.5 min). However, CYP353D1v2 is unable to metabolize deltamethrin and chlorpyrifos. The recombinant CYP353D1v2 protein efficiently catalyzed the model substrate p-nitroanisole with a maximum velocity of 9.24 nmol/min/mg of protein and a Michaelis constant of Km = 6.21 µM. In addition, imidacloprid was metabolized in vitro by the recombinant CYP353D1v2 microsomes (catalytic constant Kcat) 0.064 pmol/min/pmol P450, Km = 6.41 µM. The mass spectrum of UPLC-MS analysis shows that the metabolite was a product of buprofezin, which was buprofezin sulfone. This result provided direct evidence that L. striatellus cytochrome P450 CYP353D1v2 is capable of metabolizing imidacloprid and buprofezin.

Imidacloprid is hydroxylated by Laodelphax striatellus CYP6AY3v2

Laodelphax striatellus (Fallén) is one of the most destructive pests of rice, and has developed high resistance to imidacloprid. Our previous work indicated a strong association between imidacloprid resistance and the overexpression of a cytochrome P450 gene CYP6AY3v2 in a L. striatellus imidacloprid resistant strain (Imid-R). In this study, a transgenic Drosophila melanogaster line that overexpressed the L. striatellus CYP6AY3v2 gene was established and was found to confer increased levels of imidacloprid resistance. Furthermore, CYP6AY3v2 was co-expressed with D. melanogaster cytochrome P450 reductase (CPR) in Spodoptera frugiperda 9 (SF9) cells. A carbon monoxide difference spectra analysis indicated that CYP6AY3v2 was expressed predominately in its cytochrome P450 (P450) form, which is indicative of a good-quality functional enzyme. The recombinant CYP6AY3v2 protein efficiently catalysed the model substrate P-nitroanisole to p-nitrophenol with a maximum velocity (V_max ) of 60.78 ± 3.93 optical density (mOD)/min/mg protein. In addition, imidacloprid itself was metabolized by the recombinant CYP6AY3v2/nicotinamide adenine dinucleotide 2'-phosphate reduced tetrasodium salt (NADPH) CPR microsomes in in vitro assays (catalytic constant (K_cat ) = 0.34 pmol/min/pmol P450, michaelis constant (K_m ) = 41.98 μM), and imidacloprid depletion and metabolite peak formation were with a time dependence. The data provided direct evidence that CYP6AY3v2 is capable of hydroxylation of imidacloprid and conferring metabolic resistance in L. striatellus.

Imidacloprid is degraded by CYP353D1v2, a cytochrome P450 overexpressed in a resistant strain of Laodelphax striatellus

BACKGROUND

Cytochrome P450s are associated with the metabolising of a wide range of compounds, including insecticides. CYP353D1v2 has been found to be overexpressed in an imidacloprid-resistant strain of Laodelphax striatellus. Thus, this study was conducted to express CYP353D1v2 in Sf9 cells as a recombinant protein, to assess its ability to metabolise imidacloprid.

RESULTS

Western blot and carbon monoxide difference spectrum analysis indicated that the intact CYP353D1v2 protein had been successfully expressed in Sf9 insect cells. Catalytic activity tests with four traditional P450-activity-probing substrates found that the expressed CYP353D1v2 preferentially metabolised p-nitroanisole, ethoxycoumarin and ethoxyresorufin with specific activities of 32.70, 0.317 and 1.22 pmol min^-1 pmol^-1 protein respectively, but no activity to luciferin-H EGE. The enzyme activity for degrading imidacloprid was tested by measuring substrate depletion and formation of the metabolite. Kinetic parameters for imidacloprid were K_m 5.99 ± 0.95 µm and k_cat 0.03 ± 0.0004 min^-1 . The chromatogram analysis showed clearly the NADPH-dependent depletion of imidacloprid and the formation of an unknown metabolite. The UPLC-MS mass spectrum demonstrated that the metabolite was an oxidative product of imidacloprid, 5-hydroxy-imidacloprid.

CONCLUSION

These results suggest that CYP353D1v2 in L. striatellus is capable of degrading imidacloprid, and that enzyme activity can be evaluated well only by some traditional probing substrates. © 2017 Society of Chemical Industry.

Resistance irrelevant CYP417A2v2 was found degrading insecticide in Laodelphax striatellus

Cytochrome P450 monooxygenases (CYPs) usually overexpressed in resistant strain were found involved in oxidative detoxification of insecticides. In this study, an investigation was conducted to confirm if resistance irrelevant CYPs which were not overexpressed in resistant strain before, were capable of degrading insecticides. Three resistance irrelevant CYPs viz. CYP417A2v2, CYP425A1v2, and CYP4DJ1 from CYP4 family of Laodelphax striatellus were randomly selected for experiments. CYP417A2v2 and CYP425A1v2 were found expressed successfully in Sf9 cell line while CYP4DJ1 was not expressed successfully and out of two expressed CYPs, only CYP417A2v2 showed its efficient catalytic activity. For catalytic activity, three traditional model probe substrates and five insecticides were assayed. For the probe substrates screened, p-nitroanisole and ethoxycoumarin were preferentially metabolized by CYP417A2v2 (specific activity 3.76 ± 1.22 and 1.63 ± 0.37 nmol min-1 mg protein-1, respectively) and they may be potential diagnostic probes for this enzyme. Among insecticides, only imidacloprid was efficiently degraded by CYP417A2v2. Incubation of imidacloprid with CYP417A2v2 of L. striatellus and subsequent HPLC, LC-MS, and MS/MS analysis revealed the formation of imidacloprid metabolites, that is, 4' or 5'hydroxy-imidacloprid by hydroxylation. This result implies the exemption of CYPs character that it is not always, all the CYPs degrading insecticides being selected and overexpressed in resistant strains and the degrading CYPs without mutations to upregulate could be candidates during insecticide resistance evolution. This characterization of individual insect CYPs in insecticide degradation can provide insight for better understand of insecticide resistance development.

Constitutive overexpression of cytochrome P450 associated with imidacloprid resistance in Laodelphax striatellus (Fallén)

BACKGROUND

Imidacloprid is a principal insecticide for controlling rice planthoppers worldwide. Resistance to imidacloprid has been reported in a field population of Laodelphax striatellus. The present work was conducted to study the molecular mechanisms of imidacloprid resistance.

RESULTS

An imidacloprid-resistant strain was produced by selecting a field population with imidacloprid for 24 generations. Piperonyl butoxide (PBO) showed a 1.70-fold synergistic effect. Enzyme activity assays were conducted, and cytochrome P450 monooxygenase showed 1.88-fold activity. The mRNA expression levels of 57 P450 genes were compared. Four CYP genes were found to be overexpressed and significantly different to the susceptible strain. Four strains were selected with imidacloprid for a short period, and the expression levels of ten identified detoxification genes were then compared. Only CYP353D1v2 overexpressed and was significantly different to the susceptible strain. Strong correlation was found between CYP353D1v2 expression levels and imidacloprid treatments. Additionally, gene-silencing RNAi via dsRNA feeding showed that depressing the expression of CYP353D1v2 could significantly enhance the sensitivity of L. striatellus to imidacloprid.

CONCLUSION

Constitutive overexpression of four CYP genes was associated with imidacloprid resistance in long-term selection, and expression of CYP353D1v2 with imidacloprid resistance in short-term selection in L. striatellus.

Cytochrome P450 CYP4DE1 and CYP6CW3v2 contribute to ethiprole resistance in Laodelphax striatellus (Fallén)

Laodelphax striatellus Fallén (Hemiptera: Delphacidae), a destructive pest of rice, has developed high resistance to multiple insecticides, threatening the success of pest management programmes. The present study investigated ethiprole resistance mechanisms in a field population that is highly resistant to ethiprole. That population was used to establish a laboratory population that was subjected to further selection to produce a resistant strain. Target genes were cloned and compared between the resistant and the susceptible strains, the role of detoxification enzymes was examined, and the relative expression levels of 71 detoxification enzyme genes were tested using quantitative real time (RT)-PCR. The laboratory selection enhanced the resistance from 107-fold to 180-fold. The Rdl-type target site mutation seldom occurred in the resistant strain and is unlikely to represent the major mechanism underlying the observed resistance. Of the three important detoxification enzymes, only P450 monooxygenase was found to be associated with ethiprole resistance. Moreover, two genes, CYP4DE1 and CYP6CW3v2, were found to be overexpressed in the resistant strain. Furthermore, gene-silencing via a double-stranded RNA feeding test was carried out, and the results showed that the mRNA levels of CYP4DE1 and CYP6CW3v2 were reduced in the resistant strain, whereas ethiprole susceptibility was increased. These results suggest that CYP4DE1 and CYP6CW3v2 play an important role in ethiprole resistance in L. striatellus.