Multiple cis-acting elements involved in up-regulation of a cytochrome P450 gene conferring resistance to deltamethrin in smal brown planthopper, Laodelphax striatellus (Fallén)

Two independent regulatory mechanisms synergistically contribute to P450-mediated insecticide resistance in a lepidopteran pest, Spodoptera exigua

BACKGROUND

Cytochrome P450 enzymes play a pivotal role in the detoxification of plant allelochemicals and insecticides. Overexpression of P450 genes has been proven to be involved in insecticide resistance in insects. However, the molecular mechanisms underlying the regulation of P450 genes in insects are poorly understood.

RESULTS

Here, we determine that upregulation of CYP321B1 confers resistance to organophosphate (chlorpyrifos) and pyrethroid (cypermethrin and deltamethrin) insecticides in the resistant Spodoptera exigua strain. Enhanced expression of transcription factors CncC/Maf contributes to the increase in the expression of CYP321B1 in the resistant strain. Reporter gene assays and site-directed mutagenesis analyses confirm that a specific binding site is crucial for binding CncC/Maf to activate the expression of CYP321B1. In addition, creation of a new binding site resulting from the cis-mutations in the promoter region of CYP321B1 in the resistant strain facilitates the binding of the POU/homeodomain transcription factor Nubbin, and further enhances the expression of this P450 gene. Furthermore, we authenticate that changes in both trans- and cis-regulatory elements in the promoter region of CYP321B1 act in combination to modulate the promoter activity in a synergistic manner.

CONCLUSIONS

Collectively, these results demonstrate that two distinct but synergistic mechanisms coordinately result in the overexpression of CYP321B1 involved in insecticide resistance in an agriculturally important insect pest, S. exigua. The information on mechanisms of metabolic resistance could help to understand the development of resistance to insecticides by other pests and contribute to designing effective integrated pest management strategies for the pest control.

New Insights into Expanding the Insecticidal Spectrum of dsRNA Mediated by the High Sequence Identity between dsRNA and Nontarget mRNA

RNA interference (RNAi) is being used to develop methods to control pests, yet its widespread application is limited by the high comprehensive application cost of dsRNAs. Here, we utilized the high identity matching between double-stranded RNA (dsRNA) and nontarget genes to achieve expanding the dsRNA insecticidal spectrum. First, we found that dsRNA was more likely to induce off-target effects in genes with higher transcript levels and higher sequence identity; the existence of either a completely contiguous matching sequence exceeding 15 nt or a partially contiguous matching sequence of 24 nt between genes can lead to off-target effects in Tribolium castaneum. Accordingly, we successfully interfered with T. castaneum and Laodelphax striatellus using dsRNA targeted against Nilaparvata lugens. Additionally, the use of dsRNA targeting L. striatellus effectively interfered with N. lugens, both instances resulting in lethal effects. Moreover, the dsRNA spray method proved to be more efficient than the rice seedling soaking method to deliver dsRNA. Our research offers new insights into expanding the insecticidal spectrum of dsRNA mediated by a high degree of sequence identity between genes.

Cis-regulation of the CYP6CS1 gene and its role in mediating cross-resistance in a pymetrozine-resistant strain of Nilaparvata lugens

Pymetrozine is currently one of the primary insecticides used to control the brown planthopper, Nilaparvata lugens Stål (Hemiptera: Delphacidae), but the long-term effectiveness of this chemical is threatened by growing issues of resistance. Previous studies in a laboratory selected strain of N. lugens, Pym-R, have shown that resistance to pymetrozine can evolve without target-site mutations. A key candidate gene identified is the cytochrome P450 gene CYP6CS1, which is overexpressed in the resistant Pym-R strain compared to the laboratory susceptible strain, Pym-S. In this study, we provide a deeper characterization of the regulatory mechanism and phenotypic effects of CYP6CS1 by comparing the resistant and susceptible variants of this gene. Using artificial constructs in Luciferase activity assays, we elucidate the role of indels in the overexpression of CYP6CS1 in the resistant strain. Additionally, transgenic Drosophila experiments also revealed that the CYP6CS1 gene not only contributes to resistance against pymetrozine, but is able to confer moderate to low cross-resistance to several other pesticides. This research provides vital insights into the possible genetic mechanisms that may contribute to pymetrozine resistance in field populations. Future work will aim to examine the relevance of CYP6CS1 variation in the field with the aim of developing diagnostic markers of resistance.

Inhibition of transcriptional regulation of detoxification genes contributes to insecticide resistance management in Spodoptera exigua

Synthetic insecticides have been widely used for the prevention and control of disease vectors and agricultural pests. However, frequent uses of insecticides have resulted in the development of insecticide resistance in these insect pests. The resistance adversely affects the efficacy of insecticides, and seriously reduces the lifespan of insecticides. Therefore, resistance management requires new strategies to suppress insecticide resistance. Here, we confirm that CncC/Maf are the key regulators of various detoxification genes involved in insecticide resistance in Spodoptera exigua. Then, we develop a cell screening platform to identify the natural compound inhibitors of CncC/Maf and determine that sofalcone can act as a CncC/Maf inhibitor in vitro and in vivo. Bioassay results showed that sofalcone significantly enhanced the toxicity (more than 3-fold) of chlorpyrifos and lambda-cyhalothrin against S. exigua larvae. Finally, we demonstrate that sofalcone can greatly improve the susceptibility of S. exigua larvae to insecticides by inhibiting the activity of the ROS/CncC-dependent detoxifying enzymes and downregulating the expression levels of detoxification genes. CncC/Maf inhibitors can be used as broad-spectrum synergists to overcome insecticide resistance in pest populations. Altogether, our results demonstrate that reduced expression of detoxification genes resulting from suppression of transcriptional regulation of these genes contributes to controlling insecticide resistance, which provides a very novel and high-efficiency green resistance management strategy.

Unveiling the Role of Two Rhodopsin-like GPCR Genes in Insecticide-Resistant House Flies, Musca domestica

Insecticide resistance in insects, driven by the overexpression of P450 enzymes, presents a significant challenge due to the enhanced metabolic detoxification of insecticides. Although the transcriptional regulation of P450 genes is not yet fully understood, G-protein-coupled receptor (GPCR) genes have emerged as key regulators in this process. This study is the first to associate GPCR genes with insecticide resistance in Musca domestica. We identified two key rhodopsin-like GPCR genes, ALHF_02706.g1581 and ALHF_04422.g2918, which were significantly overexpressed in the resistant ALHF strain compared to sensitive strains. Notably, both ALHF_02706.g1581 and ALHF_04422.g2918 were mapped to autosome 2, where critical but unidentified regulatory factors controlling resistance and P450 gene regulation are located. This supports our hypothesis that GPCRs function as trans-regulatory factors for P450-mediated resistance. Functional analysis using transgenic Drosophila demonstrated that overexpression of these rhodopsin-like GPCR genes increased permethrin resistance by approximately two-fold. Specifically, ALHF_02706.g1581 overexpression significantly upregulated the Drosophila resistance-related P450 genes CYP12D1, CYP6A2, and CYP6A8, while ALHF_04422.g2918 increased CYP6G1 and CYP6A2 expression, thereby enhancing insecticide detoxification in rhodopsin-like GPCR transgenic Drosophila lines. These findings suggest that these rhodopsin-like GPCR genes on autosome 2 may act as trans-regulatory factors for P450-mediated resistance, underscoring their critical role in insecticide detoxification and resistance development in M. domestica.

RNAseq-Based Carboxylesterase Nl-EST1 Gene Expression Plasticity Identification and Its Potential Involvement in Fenobucarb Resistance in the Brown Planthopper Nilaparvata lugens

Carbamate insecticides have been used for over four decades to control brown planthopper, Nilaparvata lugens, but resistance has been reported in many countries, including the Republic of Korea. The bioassay results on resistance to fenobucarb showed that the LC50 values were 3.08 for the susceptible strain, 10.06 for the 2015 strain, and 73.98 mg/L for the 2019 strain. Compared to the susceptible strain, the 2015 and 2019 strains exhibited resistance levels 3.27 and 24.02 times higher, respectively. To elucidate the reason for the varying levels of resistance to fenobucarb in these strains, mutations in the acetylcholinesterase 1 (ACE1) gene, the target gene of carbamate, were investigated, but no previously reported mutations were confirmed. Through RNA-seq analysis focusing on the expression of detoxification enzyme genes as an alternative resistance mechanism, it was found that the carboxylesterase gene Nl-EST1 was overexpressed 2.4 times in the 2015 strain and 4.7 times in the 2019 strain compared to the susceptible strain. This indicates a strong correlation between the level of resistance development in each strain and the expression level of Nl-EST1. Previously, Nl-EST1 was reported in an organophosphorus insecticide-resistant strain of Sri Lanka 2000. Thus, Nl-EST1 is crucial for developing resistance to organophosphorus and carbamate insecticides. Resistance-related genes such as Nl-EST1 could serve as expression markers for resistance diagnosis, and can apply to integrated resistance management of N. lugens.

Effects of Bacillus thuringiensis Treatment on Expression of Detoxification Genes in Chlorantraniliprole-Resistant Plutella xylostella

Detoxification genes are crucial to insect resistance against chemical pesticides, yet their expression may be altered by exposure to biopesticides such as spores and insecticidal proteins of Bacillus thuringiensis (Bt). Increased enzymatic levels of selected detoxification genes, including glutathione S-transferase (GST), cytochrome P450 (CYP450), and carboxylesterase (CarE), were detected in chlorantraniliprole (CAP)-resistant strains of the diamondback moth (DBM, Plutella xylostella) from China when compared to a reference susceptible strain. These CAP-resistant DBM strains displayed distinct expression patterns of GST 1, CYP6B7, and CarE-6 after treatment with CAP and a Bt pesticide (Bt-G033). In particular, the gene expression analysis demonstrated significant upregulation of the CYP6B7 gene in response to the CAP treatment, while the same gene was downregulated following the Bt-G033 treatment. Downregulation of CYP6B7 using RNAi resulted in increased susceptibility to CAP in resistant DBM strains, suggesting a role of this gene in the resistant phenotype. However, pretreatment with a sublethal dose of Bt-G033 inducing the downregulation of CYP6B7 did not significantly increase CAP potency against the resistant DBM strains. These results identify the DBM genes involved in the metabolic resistance to CAP and demonstrate how their expression is affected by exposure to Bt-G033.

New and emerging mechanisms of insecticide resistance

The continuous use of insecticides over the last eight decades has led to the development of resistance to these insecticides. Research in the last few decades showed that the mechanisms underlying resistance are diverse but can generally be classified under several modes of resistance such as target-site resistance, metabolic resistance, and penetration resistance. In this review, we highlight new discoveries in insecticide resistance research made over the past few years, including an emerging new mode of resistance, sequestration resistance, where the overexpression of olfactory proteins binds and sequesters insecticides in resistant strains, as well as recent research on how posttranscriptional regulation can impact resistance. Future research will determine the generality of these emerging mechanisms across insect species.

Two Point Mutations in CYP4CE1 Promoter Contributed to the Differential Regulation of CYP4CE1 Expression by FoxO between Susceptible and Nitenpyram-Resistant Nilaparvata lugens

Four P450s were reported to be important for imidacloprid resistance in Nilaparvata lugens, a major insect pest on rice, which was confirmed in this study in an imidacloprid-resistant strain (ImiR). Here we found that only two (CYP4CE1 and CYP6ER1) from these four P450 genes were overexpressed in a nitenpyram-resistant strain (NitR) when compared to a susceptible strain (SUS). CYP4CE1 RNAi reduced nitenpyram and imidacloprid resistance in NitR and ImiR strains, with a greater reduction in nitenpyram resistance. The transcription factor FoxO mediated nitenpyram resistance in NitR and ImiR strains, but it was not differentially expressed among strains. The potential reason for the differential regulation of FoxO on CYP4CE1 expression was mainly from sequence differences in the CYP4CE1 promoter between susceptible and resistant insects. In six FoxO response elements predicted in the CYP4CE1 promoter, the single-nucleotide polymorphisms were frequently detected in over 50% of NitR and ImiR individuals. The luciferase reporter assays showed that two mutations, -650T/G and -2205T/A in two response elements at the positions of -648 and -2200 bp, mainly contributed to the enhanced regulation on CYP4CE1 expression by FoxO in resistant insects. The frequency was over 69% for both -650T/G and -2205T/A detected in NitR and ImiR individuals but less than 20% in SUS insects. In conclusion, CYP4CE1 overexpression importantly contributed to nitenpyram resistance in N. lugens, and two mutations in the CYP4CE1 promoter of resistant insects led to an enhanced regulation on CYP4CE1 expression by FoxO.

Transcriptional regulation and overexpression of GST cluster enhances pesticide resistance in the cotton bollworm, Helicoverpa armigera (Lepidoptera: Noctuidae)

The rapid evolution of resistance in agricultural pest poses a serious threat to global food security. However, the mechanisms of resistance through metabolic regulation are largely unknown. Here, we found that a GST gene cluster was strongly selected in North China (NTC) population, and it was significantly genetically-linked to lambda-cyhalothrin resistance. Knockout of the GST cluster using CRISPR/Cas9 significantly increased the sensitivity of the knockout strain to lambda-cyhalothrin. Haplotype analysis revealed no non-synonymous mutations or structural variations in the GST cluster, whereas GST_119 and GST_121 were significantly overexpressed in the NTC population. Silencing of GST_119 or co-silencing of GST_119 and GST_121 with RNAi significantly increased larval sensitivity to lambda-cyhalothrin. We also identified additional GATAe transcription factor binding sites in the promoter of NTC_GST_119. Transient expression of GATAe in Hi5 cells activated NTC_GST_119 and Xinjiang (XJ)_GST_119 transcription, but the transcriptional activity of NTC_GST_119 was significantly higher than that of XJ_GST_119. These results demonstrate that variations in the regulatory region result in complex expression changes in the GST cluster, which enhances lambda-cyhalothrin resistance in field-populations. This study deepens our knowledge of the evolutionary mechanism of pest adaptation under environmental stress and provides potential targets for monitoring pest resistance and integrated management.

Multiple-P450 Gene Co-Up-Regulation in the Development of Permethrin Resistance in the House Fly, Musca domestica

This paper reports a study conducted at the whole transcriptome level to characterize the P450 genes involved in the development of pyrethroid resistance, utilizing expression profile analyses of 86 cytochrome P450 genes in house fly strains with different levels of resistance to pyrethroids/permethrin. Interactions among the up-regulated P450 genes and possible regulatory factors in different autosomes were examined in house fly lines with different combinations of autosomes from a resistant house fly strain, ALHF. Eleven P450 genes that were significantly up-regulated, with levels > 2-fold those in the resistant ALHF house flies, were in CYP families 4 and 6 and located on autosomes 1, 3 and 5. The expression of these P450 genes was regulated by trans- and/or cis-acting factors, especially on autosomes 1 and 2. An in vivo functional study indicated that the up-regulated P450 genes also conferred permethrin resistance in Drosophila melanogaster transgenic lines. An in vitro functional study confirmed that the up-regulated P450 genes are able to metabolize not only cis- and trans-permethrin, but also two metabolites of permethrin, PBalc and PBald. In silico homology modeling and the molecular docking methodology further support the metabolic capacity of these P450s for permethrin and substrates. Taken together, the findings of this study highlight the important function of multi-up-regulated P450 genes in the development of insecticide resistance in house flies.

A transposon-introduced G-quadruplex motif is selectively retained and constrained to downregulate CYP321A1

Insects utilize xenobiotic compounds to up- and downregulate cytochrome P450 monooxygenases (P450s) involved in detoxification of toxic xenobiotics including phytochemicals and pesticides. G-quadruplexes (G4)-forming DNA motifs are enriched in the promoter regions of transcription factors and function as cis-acting elements to regulate these genes. Whether and how P450s gain and keep G4 DNA motifs to regulate their expression still remain unexplored. Here, we show that CYP321A1, a xenobiotic-metabolizing P450 from Helicoverpa zea, a polyphagous insect of economic importance, has acquired and preserved a G4 DNA motif by selectively retaining a transposon known as HzIS1-3 that carries this G4 DNA motif in its promoter region. The HzIS1-3 G4 DNA motif acts as a silencer to suppress the constitutive and induced expression of CYP321A1 by plant allelochemicals flavone and xanthotoxin through folding into an intramolecular parallel or hybrid-1 conformation in the absence or presence of K⁺ . The G4 ligand N-methylmesoporphyrin IX (NMM) strengthens the silencing effect of HzIS1-3 G4 DNA motif by switching its structure from hybrid-1 to hybrid-2. The enrichment of transposons in P450s and other environment-adaptation genes implies that selective retention of G4 DNA motif-carrying transposons may be the main evolutionary route for these genes to obtain G4 DNA motifs.

Downregulation of the CsABCC2 gene is associated with Cry1C resistance in the striped stem borer Chilo suppressalis

Chilo suppressalis is a major target pest of transgenic rice expressing the Bacillus thuringiensis (Bt) Cry1C toxin in China. The evolution of resistance of this pest is a major threat to Bt rice. Since Bt functions by binding to receptors in the midgut (MG) of target insects, identification of Bt functional receptors in C. suppressalis is crucial for evaluating potential resistance mechanisms and developing effective management strategies. ATP-binding cassette (ABC) transporters have been vastly reported to interact with Cry1A toxins, as receptors and their mutations cause insect Bt resistance. However, the role of ABC transporters in Cry1C resistance to C. suppressalis remains unknown. Here, we measured CsABCC2 expression in C. suppressalis Cry1C-resistant (Cry1C-R) and Cry1C-susceptible strains (selected in the laboratory) via quantitative real-time PCR (qRT-PCR); the transcript level of CsABCC2 in the Cry1C-R strain was significantly lower than that in the Cry1C-susceptible strain. Furthermore, silencing CsABCC2 in C. suppressalis via RNA interference (RNAi) significantly decreased Cry1C susceptibility. Overall, CsABCC2 participates in Cry1C mode of action, and reduced expression of CsABCC2 is functionally associated with Cry1C resistance in C. suppressalis.

MAP4K4 controlled transcription factor POUM1 regulates PxABCG1 expression influencing Cry1Ac resistance in Plutella xylostella (L.)

Deciphering the molecular mechanisms of insect resistance to Bacillus thuringiensis (Bt) based biotechnology products including Bt sprays and Bt crops is critical for the long-term application of Bt technology. Previously, we established that down-regulation of the ABC transporter gene PxABCG1, trans-regulated by the MAPK signaling pathway, contributed to high-level resistance to Bt Cry1Ac toxin in diamondback moth, Plutella xylostella (L.). However, the underlying transcriptional regulatory mechanism was unknown. Herein, we identified putative binding sites (PBSs) of the transcription factor (TF) POUM1 in the PxABCG1 promoter and used a dual-luciferase reporter assay (DLRA) and yeast one-hybrid (Y1H) assay to reveal that POUM1 activates PxABCG1 via interaction with one of these sites. The expression of POUM1 was significantly decreased in the midgut tissue of Cry1Ac-resistant P. xylostella strains compared to a Cry1Ac-susceptible P. xylostella strain. Silencing of POUM1 expression resulted in reduced expression of the PxABCG1 gene and an increase in larval tolerance to Bt Cry1Ac toxin in the Cry1Ac-susceptible P. xylostella strain. Furthermore, silencing of PxMAP4K4 expression increased the expression of both POUM1 and PxABCG1 genes in the Cry1Ac-resistant P. xylostella strain. These results indicate that the POUM1 induces PxABCG1 expression, while the activated MAPK cascade represses PxABCG1 expression thus reducing Cry1Ac susceptibility in P. xylostella. This result deepens our understanding of the transcriptional regulatory mechanism of midgut Cry receptor genes and the molecular basis of the evolution of Bt resistance in insects.

MAPK-mediated transcription factor GATAd contributes to Cry1Ac resistance in diamondback moth by reducing PxmALP expression

The benefits of biopesticides and transgenic crops based on the insecticidal Cry-toxins from Bacillus thuringiensis (Bt) are considerably threatened by insect resistance evolution, thus, deciphering the molecular mechanisms underlying insect resistance to Bt products is of great significance to their sustainable utilization. Previously, we have demonstrated that the down-regulation of PxmALP in a strain of Plutella xylostella (L.) highly resistant to the Bt Cry1Ac toxin was due to a hormone-activated MAPK signaling pathway and contributed to the resistance phenotype. However, the underlying transcriptional regulatory mechanism remains enigmatic. Here, we report that the PxGATAd transcription factor (TF) is responsible for the differential expression of PxmALP observed between the Cry1Ac susceptible and resistant strains. We identified that PxGATAd directly activates PxmALP expression via interacting with a non-canonical but specific GATA-like cis-response element (CRE) located in the PxmALP promoter region. A six-nucleotide insertion mutation in this cis-acting element of the PxmALP promoter from the resistant strain resulted in repression of transcriptional activity, affecting the regulatory performance of PxGATAd. Furthermore, silencing of PxGATAd in susceptible larvae reduced the expression of PxmALP and susceptibility to Cry1Ac toxin. Suppressing PxMAP4K4 expression in the resistant larvae transiently recovered both the expression of PxGATAd and PxmALP, indicating that the PxGATAd is a positive responsive factor involved in the activation of PxmALP promoter and negatively regulated by the MAPK signaling pathway. Overall, this study deciphers an intricate regulatory mechanism of PxmALP gene expression and highlights the concurrent involvement of both trans-regulatory factors and cis-acting elements in Cry1Ac resistance development in lepidopteran insects.

Gene expression and regulation are associated with adaptive evolution in living organisms. The rapid evolution of insect resistance to Bt insecticidal Cry toxins is frequently associated with reduced expression of diverse midgut genes that code for Cry-toxin receptors. Nonetheless, our current knowledge about the regulation of gene expression of these pivotal receptor genes in insects is limited. Membrane-bound alkaline phosphatase (mALP) is a known receptor for Cry1Ac toxin in diverse insects and here, we report the transcriptional regulatory mechanism of the PxmALP gene related to Cry1Ac resistance in P. xylostella. We identified a MAPK signaling pathway that negatively regulates the PxGATAd transcriptional factor which is involved in the differential expression of PxmALP via interacting with the PxmALP promoter. Furthermore, a cis-acting element mutation repressing the regulatory activity of PxGATAd for PxmALP expression in the Cry1Ac resistant strain was identified. Our study provides an insight into the precise transcriptional regulatory mechanism that regulates PxmALP expression and is involved in the evolution of Bt Cry1Ac resistance in P. xylostella, which provides a paradigm for decoding the regulation landscape of midgut Cry-toxin receptor genes in insects.

The Role of Cytochrome P450s in Insect Toxicology and Resistance

Insect cytochrome P450 monooxygenases (P450s) perform a variety of important physiological functions, but it is their role in the detoxification of xenobiotics, such as natural and synthetic insecticides, that is the topic of this review. Recent advances in insect genomics and postgenomic functional approaches have provided an unprecedented opportunity to understand the evolution of insect P450s and their role in insect toxicology. These approaches have also been harnessed to provide new insights into the genomic alterations that lead to insecticide resistance, the mechanisms by which P450s are regulated, and the functional determinants of P450-mediated insecticide resistance. In parallel, an emerging body of work on the role of P450s in defining the sensitivity of beneficial insects to insecticides has been developed. The knowledge gained from these studies has applications for the management of P450-mediated resistance in insect pests and can be leveraged to safeguard the health of important beneficial insects.

A cis-Acting Mutation in the PxABCG1 Promoter Is Associated with Cry1Ac Resistance in Plutella xylostella (L.)

The molecular mechanisms of insect resistance to Cry toxins generated from the bacterium Bacillus thuringiensis (Bt) urgently need to be elucidated to enable the improvement and sustainability of Bt-based products. Although downregulation of the expression of midgut receptor genes is a pivotal mechanism of insect resistance to Bt Cry toxins, the underlying transcriptional regulation of these genes remains elusive. Herein, we unraveled the regulatory mechanism of the downregulation of the ABC transporter gene PxABCG1 (also called Pxwhite), a functional midgut receptor of the Bt Cry1Ac toxin in Plutella xylostella. The PxABCG1 promoters of Cry1Ac-susceptible and Cry1Ac-resistant strains were cloned and analyzed, and they showed clear differences in activity. Subsequently, a dual-luciferase reporter assay, a yeast one-hybrid (Y1H) assay, and RNA interference (RNAi) experiments demonstrated that a cis-mutation in a binding site of the Hox transcription factor Antennapedia (Antp) decreased the promoter activity of the resistant strain and eliminated the binding and regulation of Antp, thereby enhancing the resistance of P. xylostella to the Cry1Ac toxin. These results advance our knowledge of the roles of cis- and trans-regulatory variations in the regulation of midgut Cry receptor genes and the evolution of Bt resistance, contributing to a more complete understanding of the Bt resistance mechanism.

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.

Climate change and the genetics of insecticide resistance

Changes in global temperature and humidity as a result of climate change are producing rapid evolutionary changes in many animal species, including agricultural pests and disease vectors, leading to changes in allele frequencies of genes involved in thermotolerance and desiccation resistance. As some of these genes have pleiotropic effects on insecticide resistance, climate change is likely to affect insecticide resistance in the field. In this review, we discuss how the interactions between adaptation to climate change and resistance to insecticides can affect insecticide resistance in the field using examples in phytophagous and hematophagous pest insects, focusing on the effects of increased temperature and increased aridity. We then use detailed genetic and mechanistic studies in the model insect, Drosophila melanogaster, to explain the mechanisms underlying this phenomenon. We suggest that tradeoffs or facilitation between adaptation to climate change and resistance to insecticides can alter insecticide resistance allele frequencies in the field. The dynamics of these interactions will need to be considered when managing agricultural pests and disease vectors in a changing climate. © 2019 Society of Chemical Industry.

Transcription factors CncC/Maf and AhR/ARNT coordinately regulate the expression of multiple GSTs conferring resistance to chlorpyrifos and cypermethrin in Spodoptera exigua

BACKGROUND

Glutathione S-transferases (GSTs) are a superfamily of multifunctional dimeric proteins existing in both prokaryotic and eukaryotic organisms. They are involved in the detoxification of both endogenous and exogenous electrophiles, including insecticides. However, the molecular mechanisms underlying the regulation of GST genes in insects are poorly understood.

RESULTS

We first identified at least three GST genes involved in resistance to the insecticides chlorpyrifos and cypermethrin. Analysis of upstream sequences revealed that three GSTs (SeGSTo2, SeGSTe6 and SeGSTd3) harbor the same cap 'n' collar C/muscle aponeurosis fibromatosis (CncC/Maf) binding site, and SeGSTo2 and SeGSTe6 contain the aryl hydrocarbon receptor/aryl hydrocarbon receptor nuclear translocator (AhR/ARNT) binding site. Luciferase reporter assay showed co-transfection of reporter plasmid containing the SeGSTe6 promoter with CncC and/or Maf expressing constructs significantly boosted transcription. Similarly, AhR and/or ARNT expressing constructs also significantly increased the promoter activities. The co-transfection of mutated reporter plasmid with CncC/Maf or AhR/ARNT did not increase transcription activity anymore. Constitutive over-expression of CncC, Maf and AhR was also found in the HZ16 strain, which might be the molecular mechanism for up-regulated expression of multiple detoxification genes conferring resistance to insecticides.

CONCLUSION

These results suggest that CncC/Maf and AhR/ARNT coordinately regulate the expression of multiple GST genes involved in insecticide resistance in Spodoptera exigua. © 2019 Society of Chemical Industry.

Insecticides induce the co-expression of glutathione S-transferases through ROS/CncC pathway in Spodoptera exigua

Glutathione S-transferases (GSTs) are a family of multifunctional enzymes that are involved in detoxification of electrophilic toxic compounds. Although the co-induced expression of GST genes by insecticides in insects has been documented in recent years, the underlying regulatory mechanisms are not understood. In this study, a total of thirty-one cytosolic S. exigua GSTs (SeGSTs) was cloned and identified. The bioinformatics and gene expression patterns were also analyzed. Out of them, SeGSTe9, SeGSTs6, SeGSTe1, SeGSTe6, SeGSTe8, SeGSTe14, and SeGSTd1 were significantly co-expressed following exposure to three insecticides (lambda-cyhalothrin, chlorpyrifos and chlorantraniliprole). The analysis of upstream sequences revealed that all of these seven SeGSTs harbored CncC/Maf binding site. The luciferase reporter assay showed that the pGL3-SeGST promoter construct exhibited a significant increase in luciferase activities after exposure to insecticides, and mutation of CncC/Maf binding site diminish the induction effect. These data indicate that CncC/Maf pathway regulates the co-expression of GST genes in response to different insecticides in S. exigua. Insecticides significantly enhanced the ROS content and treatment with the ROS inhibitor N-acetylcysteine (NAC) decreased the insecticide-induced luciferase activities of the PGL3-GSTe6 promoter construct, but not the CncC-mutated construct. These results indicate that ROS mediates GST gene expression after exposure to insecticides through CncC/Maf pathway. Overall, these data show that insecticides induce the co-expression of glutathione S-transferases through the ROS/CncC pathway in S. exigua.

Mechanisms of resistance to thiamethoxam and dinotefuran compared to imidacloprid in the brown planthopper: Roles of cytochrome P450 monooxygenase and a P450 gene CYP6ER1

The brown planthopper (BPH, Nilaparvata lugens) has developed high resistance to the first-generation neonicotinoids (imidacloprid). With commercialization and widespread field use of the second-(thiamethoxam) and third-(dinotefuran) generation neonicotinoids, resistance to these insecticides is also reported. We investigated the cytochrome P450 monooxygenase-mediated detoxification in thiamethoxam- and dinotefuran- resistant in comparison to imidacloprid-resistant strains of BPH. In the three moderately resistant BPH strains selected separately with the three insecticides from a same susceptible strain, P450 activities were significantly enhanced over the susceptible control. Seven of 26 tested P450 genes were up-regulated and CYP6ER1 was a strongly over-expressed gene in all the three resistant strains. Knockdown of CYP6ER1 in the susceptible insects reduced P450 activity, retarded nymph growth and significantly increased sensitivity to each one of the three neonicotinoids. Taken together, we show that enhanced P450 activity and over-expression of CYP6ER1 gene are involved in BPH resistance to thiamethoxam and dinotefuran as to imidacloprid. These findings are of significance in management thiamethoxam and dinotefuran resistance in the BPH, especially in the management of potential cross-resistance to the three generations of neonicotinoids.

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.

Molecular characterization and evolution of a chemosensory receptor gene family in three notorious rice planthoppers, Nilaparvata lugens, Sogatella furcifera and Laodelphax striatellus, based on genome and transcriptome analyses

BACKGROUND

The white-backed planthopper (WBPH) Sogatella furcifera, the brown planthopper (BPH) Nilaparvata lugens, and the small brown planthopper (SBPH) Laodelphax striatellus (Hemiptera: Delphacidae) are rice pests that damage rice plants by sap-sucking and by transmitting viruses. Host-seeking behavior involves chemosensory receptor genes that include odorant receptors (ORs), ionotropic receptors (IRs) and gustatory receptors (GRs).

RESULTS

We used genome and transcriptome data to identify 141 ORs, 28 GRs and 25 IRs in BPH; 135 ORs, 18 GRs and 16 IRs in WBPH; and 37 ORs, 14 GRs and 6 IRs in SBPH. A phylogenetic analysis identified several specific OR clades of rice planthoppers, the results indicating that these OR members might be used to respond to specific host volatiles. OR co-receptor (Orco) is the most conserved and essential OR gene among these species and RNA interference (RNAi) can decrease their mRNA expression level to <50%. RNAi knockdown rice planthoppers were anosmia and were unable to seek or locate rice plants in behavioral tests.

CONCLUSION

The results demonstrate the importance of the planthopper Orco genes in locating rice plants. This information may aid in the development of RNAi-based transgenic rice and other pest management technologies. © 2018 Society of Chemical Industry.