Functional characterization of cis-acting elements mediating flavone-inducible expression of CYP321A1

Homology-based characterization of the cis-regulatory elements modulate flavone induction of CYP321A1 in Helicoverpa armigera

Xenobiotic response element (XRE) to flavone was the cis- regulatory elements that mediates the induction of the allelochemical-metabolizing CYP321A1 gene from Helicoverpa zea. However, it was unknown whether the XRE-Fla element existed in other species. Recently we have identified and cloned the CYP321A1 gene with promoter region in a related species, Helicoverpa armigera. Sequence similarity of two orthologous CYP321A1 genes was 97.27%, but the promoter sequence similarity was only 56.32%. Sequence alignment showed the XRE-Fla like element owns three mutations in H. armigera compared with H. zea. Progressive 5' deletions and internal mutation indicated that H. armigera XRE-Fla was the essential element of CYP321A1 gene in response to flavone. XRE-Fla mutations and EMSA analysis confirmed that the H. armigera XRE-Fla element binding factor was stronger than H. zea. The findings indicate the XRE element mutations mainly contribute to the differences between the flavone-induced expressions of two CYP321A1 genes, which improve the flexibility and adaptability for allelochemical response of H. armigera.

Keap1 Negatively Regulates Transcription of Three Counter-Defense Genes and Susceptibility to Plant Toxin Gossypol in Helicoverpa armigera

Expressions of a wide range of cytoprotective counter-defense genes are mainly regulated by the Keap1-Nrf2-ARE signaling pathway in response to oxidative stress from xenobiotics. Gossypol is the major antiherbivore secondary metabolite of cotton, but how the polyphagous pest Helicoverpa armigera copes with this phytochemical to utilize its favorite host plant cotton remains largely elusive. In this study, we first suppressed the Keap1 gene in newly hatched larvae of cotton bollworm by feeding them the siRNA diet for 4 days. All of the larvae were subsequently fed the artificial diet supplied with gossypol or the control diet for 5 days. We identified that the knockdown of the Keap1 gene significantly decreased larval mortality and significantly increased the percentages of larval survival, reaching the fourth instar, compared with ncsiRNA when exposed to a diet containing gossypol. Three counter-defense genes CYP9A17, CYP4L11 and UGT41B3, which were related to the induction or metabolism of gossypol according to the report before, were all significantly up-regulated after the knockdown of the Keap1 gene. The Antioxidant Response Elements (AREs) were also detected in the promoter regions of the three counter-defense genes above. These data indicate that the suppression of the Keap1 gene activates the Keap1-Nrf2-ARE signaling pathway, up-regulates the expressions of counter-defense genes involved in the resistance of oxidative stress and finally contributes to reducing the susceptibility of gossypol. Our results provide more knowledge about the transcriptional regulation mechanisms of counter-defense genes that enable the cotton bollworm to adapt to the diversity of host plants including cotton.

CYP321A Subfamily P450s Contribute to the Detoxification of Phytochemicals and Pyrethroids in Spodoptera litura

Although the induction of cytochrome P450 monooxygenases involved in insect detoxification has been well documented, the underlying regulatory mechanisms remain obscure. In Spodoptera litura, CYP321A subfamily members were effectively induced by exposure to flavone, xanthotoxin, curcumin, and λ-cyhalothrin, while knockdown of the CYP321A genes increased larval susceptibility to these xenobiotics. Homology modeling and molecular docking analyses showed that these four xenobiotics could stably bind to the CYP321A enzymes. Furthermore, two transcription factor genes, CncC and MafK, were significantly induced by the xenobiotics. Knockdown of CncC or MafK reduced the expression of four CYP321A genes and increased larval susceptibility to the xenobiotics. Dual-luciferase reporter assays showed that cotransfection of reporter plasmids carrying the CYP321A promoter with CncC and/or MafK-expressing constructs significantly magnified the promoter activity. These results indicate that the induction of CYP321A subfamily members conferring larval detoxification capability to xenobiotics is mediated by the activation of CncC and MafK.

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.

Identification and Functional Characterization of the Transcription Factors AhR/ARNT in Dendroctonus armandi

The aryl hydrocarbon receptor (AhR) and aryl hydrocarbon receptor nuclear translocator (ARNT) belong to the bHLH-PAS (basic Helix-Loop-Helix-Period/ARNT/Single-minded) family of transcription factors, which participate in the sensing and transmitting stimuli of exogenous and endogenous chemical substances, and subsequently activates genes transcription involved in various detoxification and physiological functions. However, they have not been identified in Dendroctonus armandi, and their roles in the detoxification metabolism are unclear. In the present study, AhR and ARNT of D. armandi were characterized. Spatiotemporal expression profiling indicated that DaAhR and DaARNT were highly expressed in the adult and larval stages of D. armandi and mainly expressed in the midgut and Malpighian tubules of adults. Additionally, the expression of DaAhR and DaARNT significantly increased after exposure to (-)-𝛽-pinene, (+)-3-carene, and (±)-limonene. Silencing DaAhR and DaARNT increased the susceptibility of D. armandi to (-)-𝛽-pinene, (+)-3-carene, and (±)-limonene, and the activities of detoxification enzyme were also remarkably reduced. Moreover, DaCYP6DF1 and DaGSTs2 were significantly down-regulated after injections of dsAhR and dsARNT in the male and female adults, with the expression of DaCYP6DF1 decreasing by higher than 70%. The present study revealed that the transcription factors AhR and ARNT of D. armandi were induced by terpenoids and participated in the regulation of DaCYP6DF1 expression, which was associated with D. armandi's susceptibility to (-)-𝛽-pinene and (±)-limonene. These results may provide a theoretical basis for the integrated control of D. armandi and improve our comprehension of insect toxicology.

Identification of cis-acting elements in response to fenvalerate in the CYP6B7 promoter of Helicoverpa armigera

Cytochrome P450-mediated detoxification plays an important role in the development of insecticide resistance. Previous studies have shown that cytochrome P450 CYP6B7 was induced by fenvalerate and involved in fenvalerate detoxification in Helicoverpa armigera. However, the transcriptional regulation of CYP6B7 induced by fenvalerate remains unclear. Here, a series of progressive 5' deletions of CYP6B7 promoter reporter genes were constructed, and the relative luciferase activities were detected. The results revealed that the relative luciferase activity of plasmid p (-655/-1) was significantly induced by fenvalerate. Further deletion of the region between -655 and -486 bp showed that the highest luciferase activity induced by fenvalerate was observed in plasmid p (-528/-1), while p (-485/-1) had the lowest fenvalerate-induced luciferase activity. Moreover, internal deletion and mutation in the region between -508 and -486 bp resulted in a significant reduction in fenvalerate-induced CYP6B7 promoter activity, suggesting that the cis-acting element responsible for fenvalerate in the CYP6B7 promoter was located between -508 and -486 bp. These results promote an understanding of the expression regulation mechanism of P450 genes that conferring resistance to insecticides.

Phytochemical Flavone Confers Broad-Spectrum Tolerance to Insecticides in Spodoptera litura by Activating ROS/CncC-Mediated Xenobiotic Detoxification Pathways

Tolerance to chemical insecticides can be driven by the necessity of herbivorous insects to defend against host plant-produced phytochemicals. However, how the phytochemicals are sensed and further transduced into a defense response associated with insecticide tolerance is poorly understood. Herein, we show that pre-exposure to flavone, a flavonoid phytochemical, effectively enhanced larval tolerance to multiple synthetic insecticides and elevated detoxification enzyme activities in Spodoptera litura. RNA-Seq analysis revealed that flavone induced a spectrum of genes spanning phase I and II detoxification enzyme families, as well as two transcription factors Cap "n" collar isoform C (CncC) and its partner small muscle aponeurosis fibromatosis (MafK). Knocking down of CncC by RNA interference suppressed flavone-induced detoxification gene expression and rendered the larvae more sensitive to the insecticides. Flavone exposure elicited a reactive oxygen species (ROS) burst, while scavenging of ROS inhibited CncC-mediated detoxification gene expression and suppressed flavone-induced detoxification enzyme activation. Metabolome analysis showed that the ingested flavone was mainly converted into three flavonoid metabolites, and only 3-hydroxyflavone was found to affect the ROS/CncC pathway-mediated metabolic detoxification. These results indicate that the ROS/CncC pathway is an important route driving detoxification gene expression responsible for insecticide tolerance after exposure to the phytochemical flavone.

Functional characterization of the transcription factors AhR and ARNT in Nilaparvata lugens

In the present study, the aryl hydrocarbon receptor (AhR) and aryl hydrocarbon receptor nuclear translocator (ARNT) of Nilaparvata lugens were cloned and identified. The NlAhR and NlARNT expression levels significantly increased after imidacloprid, etofenprox and isoprocarb treatments. Knockdowns of NlAhR and NlARNT increased the susceptibility of N. lugens to imidacloprid, etofenprox and isoprocarb, and the detoxification enzyme activities were also significantly decreased. In addition, NlCYP301A1, NlGSTt1 and NlCarE7 were significantly down-regulated after injections of dsNlAhR and dsNlARNT, with the NlCarE7 expression decreasing by greater than 80%. Moreover, after knocking down NlCarE7, the susceptibility of N. lugens to etofenprox and isoprocarb significantly increased. Both NlAhR and NlARNT bound the NlCarE7 promoter and significantly enhanced the transcriptional activity. Our research revealed the functional roles of transcription factors NlAhR and NlARNT in the detoxification metabolism of N. lugens. The results provide a theoretical basis for the pest management and comprehensive control of N. lugens and increase our knowledge of insect toxicology.

Transcriptional regulation of xenobiotic detoxification genes in insects - An overview

Arthropods have well adapted to the vast array of chemicals they encounter in their environment. Whether these xenobiotics are plant allelochemicals or anthropogenic insecticides one of the strategies they have developed to defend themselves is the induction of detoxification enzymes. Although upregulation of detoxification enzymes and efflux transporters in response to specific inducers has been well described, in insects, yet, little is known on the transcriptional regulation of these genes. Over the past twenty years, an increasing number of studies with insects have used advanced genetic tools such as RNAi, CRISPR/Cas9 and reporter gene assays to dissect the genomic grounds of their xenobiotic response and hence contributed substantially in improving our knowledge on the players involved. Xenobiotics are partly recognized by various "xenobiotic sensors" such as membrane-bound or nuclear receptors. This initiates a molecular reaction cascade ultimately leading to the translocation of a transcription factor to the nucleus that recognizes and binds to short sequences located upstream their target genes to activate transcription. To date, a number of signaling pathways were shown to mediate the upregulation of detoxification enzymes in arthropods and to play a role in either metabolic resistance to insecticides or host-plant adaptation. These include nuclear receptors AhR/ARNT and HR96, GPCRs, CncC and MAPK/CREB. Recent work reveals that upregulation and activation of some components of these pathways as well as polymorphism in the binding motifs of transcription factors are linked to insects' adaptive processes. The aim of this mini-review is to summarize and describe recent work that shed some light on the main regulatory routes of detoxification gene expression in insects.

Regulation of insect P450s in response to phytochemicals

Insect herbivores use phytochemicals as signals to induce expression of their phytochemical-detoxifying cytochrome P450 monooxygenases (P450s). The regulatory cascades that transduce phytochemical signals to enhanced expression of P450s are the focus of this review. At least seven signaling pathways, including RTK/MAPK, GPCR/CREB, GPCR/NFκB, ROS/CncC/Keap1, AhR/ARNT, cytosol NR, and nucleus-located NR, may be involved in phytochemical induction of P450s. Constitutive overexpression, overphosphorylation, and/or activation of one or more effectors in the corresponding pathway are common causes of P450 overexpression that lead to phytochemical or insecticide resistance. Future research should pay more attentions to the starting point of each pathway, the number of pathways and their cross talk for a given phytochemical, and the pathways for downregulation of P450s.

Identification of Wild-Type CYP321A2 and Comparison of Allelochemical-Induced Expression Profiles of CYP321A2 with Its Paralog CYP321A1 in Helicoverpa zea

One possible way to overcome the diversity of toxic plant allelochemicals idiosyncratically distributed among potential host plants is to have more counterdefense genes via gene duplication or fewer gene losses. Cytochrome P450 is the most important gene family responsible for detoxification of the diversity of plant allelochemicals. We have recently reported the identification and cloning of the transposon (HzSINE1)-disrupted non-functional CYP321A2, a duplicated paralog of the xenobiotic-metabolizing P450 CYP321A1 from a laboratory colony of Helicoverpa zea. Here we report the identification of the wild-type intact allele of CYP321A2 from another H. zea colony. This CYP321A2 allele encodes a deduced protein of 498 amino acids and has the P450 signature motifs. Quantitative RT-PCR experiments showed that this CYP321A2 allele was highly expressed in midgut and fat body and achieved the highest expression level in the developmental stage of 5th and 3rd instar larvae. CYP321A2 and CYP321A1 were constitutively expressed in low levels but can be differentially and significantly induced by a range of the plant allelochemicals and plant signal molecules, among which xanthotoxin, flavone, and coumarin were the most prominent inducers of CYP321A2 both in midgut and fat body, whereas flavone, coumarin, and indole-3-carbinol were the prominent inducers of CYP321A1 in midgut and fat body. Moreover, xanthotoxin- and flavone-responsive regulatory elements of CYP321A1 were also detected in the promoter region of CYP321A2. Our results enrich the P450 inventory by identifying an allelochemical broadly induced CYP321A2, a paralog of CYP321A1 in H. zea. Our data also suggest that the CYP321A2/CYP321A1 paralogs are a pair of duplicated genes of multigene families and CYP321A2 could potentially be involved in the detoxification of plant allelochemicals and adaptation of H. zea to its chemical environment.

Transcription Factors AhR/ARNT Regulate the Expression of CYP6CY3 and CYP6CY4 Switch Conferring Nicotine Adaptation

Nicotine is one of the most toxic secondary plant metabolites in nature and it is highly toxic to herbivorous insects. The overexpression of CYP6CY3 and its homologous isozyme CYP6CY4 in Myzus persicae nicotianae is correlated with nicotine tolerance. The expanded (AC)n repeat in promoter is the cis element for CYP6CY3 transcription. These repeat sequences are conserved in the CYP6CY3 gene from Aphis gossypii and the homologous P450 genes in Acyrthosiphon pisum. The potential transcriptional factors that may regulate CYP6CY3 were isolated by DNA pulldown and sequenced in order to investigate the underlying transcriptional regulation mechanism of CYP6CY3. These identified transcriptional factors, AhR and ARNT, whose abundance was highly correlated with an abundance of the CYP6CY3 gene, were validated. RNAi and co-transfection results further confirm that AhR and ARNT play a major role in the transcriptional regulation of the CYP6CY3 gene. When the CYP6CY3 transcript is destabilized by AhR/ARNT RNAi, the transcription of the CYP6CY4 is dramatically up-regulated, indicating a compensatory mechanism between the CYP6CY3 and CYP6CY4 genes. Our present study sheds light on the CYP6CY3 and CYP6CY4 mediated nicotine adaption of M. persicae nicotianae to tobacco. The current studies shed light on the molecular mechanisms that underlie the genotypic and phenotypic changes that are involved in insect host shifts and we conclude that AhR/ARNT regulate the expression of CYP6CY3 and CYP6CY4 cooperatively, conferring the nicotine adaption of M. persicae nicotianae to tobacco.

Copper-induced H2O2 accumulation confers larval tolerance to xanthotoxin by modulating CYP6B50 expression in Spodoptera litura

In the plant-insect arms race, plants synthesize toxic compounds to defend against herbivorous insects, whereas insects employ cytochrome P450 monooxygenases (P450s) to detoxify these phytotoxins. As ubiquitous environmental contaminants, heavy metals can be easily absorbed by plants and further accumulated in herbivorous insects through the food chains, resulting in tangible consequences for plant-insect interactions. However, whether heavy metals can influence P450 activities and thereby cause further effects on larval tolerance to phytotoxins remains unknown. In this study, we shown that prior exposure to copper (Cu) enhanced larval tolerance to xanthotoxin in Spodoptera litura, a major polyphagous pest of agriculture. P450 activities were induced in larvae exposed to Cu or xanthotoxin, and a midgut specific expressed P450 gene, CYP6B50 was cross-induced after exposure to these two toxic xenobiotics. Knocking down CYP6B50 by RNA interference (RNAi) rendered the larvae more sensitive to xanthotoxin. As defense against oxidative stress following metal exposure has been demonstrated to affect insecticide resistance, the reactive oxygen species (ROS) generation and antioxidant enzyme activities were assessed. Cu exposure caused the accumulation of hydrogen peroxide (H₂O₂) and enhanced the activities of superoxide dismutase (SOD) and peroxidase (POD) in larval midgut. In addition, two antioxidant response elements (AREs) were identified from the CYP6B50 promoter, indicating that Cu-induced CYP6B50 expression may be related to the ROS burst. Application of ROS scavenger N-acetylcysteine (NAC) effectively suppressed CYP6B50 expression, inhibited P450 activities and impaired larval tolerance to xanthotoxin that had been induced by Cu. These results indicate that the increase in CYP6B50 expression regulated by Cu-induced H₂O₂ generation contributed to the enhancement of larval tolerance to xanthotoxin in S. litura. Ingestion of heavy metals from their host plants can inadvertently boost the counter-defense system of herbivorous insects to protect themselves against plant defensive toxins.

CrgA Protein Represses AlkB2 Monooxygenase and Regulates the Degradation of Medium-to-Long-Chain n-Alkanes in Pseudomonas aeruginosa SJTD-1

AlkB monooxygenases in bacteria are responsible for the hydroxylation of medium- and long-chain n-alkanes. In this study, one CrgA protein of Pseudomonas aeruginosa SJTD-1, a member of LysR family, was proved to regulate AlkB2 monooxygenase and the degradation of medium-to-long-chain n-alkanes (C₁₄-C₂₀) by directly binding to the upstream of alkB2 gene. Two specific sites for CrgA binding were found in the promoter region of alkB2 gene, and the imperfect mirror repeat (IIR) structure was proved critical for CrgA recognition and binding. Hexadecyl CoA and octadecyl CoA could effectively release the CrgA binding and start the transcription of alkB2 gene, implying a positive regulation of metabolic intermediate. In the presence of medium-to-long-chain n-alkanes (C₁₄-C₂₀), deletion of crgA gene could enhance the transcription and expression of AlkB2 monooxygenase significantly; and in n-octadecane culture, strain S1_ΔalkB1&crgA grew more vigorously than strain S1 _{ΔalkB1 &crgA} . Almost no regulation of CrgA protein was observed to alkB1 gene in vitro and in vivo. Therefore, CrgA acted as a negative regulator for the medium-to-long-chain n-alkane utilization in P. aeruginosa SJTD-1. The work will promote the regulation mechanism study of n-alkane degradation in bacteria and help the bioremediation method development for petroleum pollution.

CYP4CJ1-mediated gossypol and tannic acid tolerance in Aphis gossypii Glover

Cytochrome P450 monooxygenases play a key role in herbivorous pest adaptation to host plants by the detoxification against plant allelochemicals. A new P450 gene (CYP4CJ1) was identified from Aphis gossypii, which displayed a positive response to plant allelochemicals. The transcript levels of CYP4CJ1 could be significantly induced by both gossypol and tannic acid. Knockdown of CYP4CJ1 increased the sensitivity of A. gossypii to these two plant allelochemicals. These results suggest that CYP4CJ1 could be involved in the tolerance of A. gossypii to some plant allelochemicals. Subsequently, we examined the regulatory mechanism of CYP4CJ1 based on the transcriptional and post-transcriptional level. A promoter region from -1422 to -1166 of CYP4CJ1 was identified, which was an essential plant allelochemical responsive region. In addition, miR-4133-3p was found to participate in the regulation of CYP4CJ1 post-transcriptionally. Our results suggest that the transcript abundance of CYP4CJ1, following the exposure of A. gossypii to gossypol and tannic acid can be attributed to both the transcriptional and post-transcriptional regulation mechanisms. These results are important for understanding the roles of P450s in the plant allelochemical tolerance of A. gossypii.

Identification of the 2-tridecanone cis-acting element in the promoter of cytochrome P450 CYP6B7 in Helicoverpa armigera

The expression level of cytochrome P450 genes in insects can be induced by plant allelochemicals, which is important for insects to adapt to host plants. Cytochrome P450 CYP6B7 has been reported to be involved in pyrethroid insecticide resistance in Helicoverpa armigera, and its transcription level was induced by some inducers. Currently, the regulatory mechanism of the induced expression of CYP6B7 remains unknown, although it is very important for understanding the detoxification mechanism to allelochemicals in host plants. The objective of the present study was to investigate the cis-acting element in the promoter of CYP6B7 mediating the inducible up-regulation of CYP6B7 in H. armigera by 2-tridecanone. The promoter region of CYP6B7 was cloned by genome walking technique and analyzed by transient transfection assay. Progressive 5' deletion of the promoter region of CYP6B7 revealed that the relative luciferase activity of construct -320/+232 could be significantly induced by 2-tridecanone. Further stepwise deletion between -320 and -238 bp found that construct -292/+232 could also be significantly induced by 2-tridecanone, but the adjacent construct -256/+232 could not, suggesting the essential role of the sequence between -292 and -257 bp for 2-tridecanone induction. Nucleotide mutations between -292 and -281 bp had no influence on the induction effect by 2-tridecanone, but nucleotide mutations between -280 and -257 bp significantly decreased the induction effect. These results demonstrated that the cis-acting element for 2-tridecanone induction was between -280 and -257 bp in the promoter of CYP6B7.

Transcription factor aryl hydrocarbon receptor/aryl hydrocarbon receptor nuclear translocator is involved in regulation of the xenobiotic tolerance-related cytochrome P450 CYP6DA2 in Aphis gossypii Glover

The cotton aphid, Aphis gossypii, is one of the most economically important agricultural pests worldwide as it is polyphagous and resistant to many classes of insecticides. Overexpression of the cytochrome P450 monooxygenase (P450) CYP6DA2 has previously been found to be associated with gossypol and spirotetramat tolerance in the cotton aphid. In the present study, the elements located in the promoter region (-357:-343; -250:-241; -113:-104) of CYP6DA2 were shown to control promoter activity, and gossypol induction was observed. We hypothesized that the expression of CYP6DA2 is subject to transcriptional regulation. To investigate the underlying mechanism, we assessed two transcription factors, aryl hydrocarbon receptor (AhR) and aryl hydrocarbon receptor nuclear translocator (ARNT), and found that the abundance of AhR was highly correlated with CYP6DA2 abundance. RNA interference of AhR or ARNT significantly decreased the levels of the target gene as well as those of its counterpart, and both dramatically repressed CYP6DA2 expression. Cotransfection of the ARNT, AhR, or AhR plus ARNT and CYP6DA2 promoter constructs elevated CYP6DA2 promoter activity, with the AhR plus ARNT cotransfection being the most effective. Thus, these elements located in the promoter were responsible for CYP6DA2 transcription, and CYP6DA2 expression was regulated by the transcription factors AhR and ARNT.

Cytochrome P450 CYP6DA2 regulated by cap 'n'collar isoform C (CncC) is associated with gossypol tolerance in Aphis gossypii Glover

Cotton plants accumulate phytotoxins, such as gossypol and related sesquiterpene aldehydes, to resist insect herbivores. The survival of insects exposed to toxic secondary metabolites depends on the detoxification metabolism mediated by limited groups of cytochrome P450. Gossypol has an antibiotic effect on Aphis gossypii, and as the concentrations of gossypol were increased in the present study, the mortality of cotton aphids increased from 4 to 28%. The fecundity of the cotton aphids exposed to gossypol was also significantly reduced compared with the control. The transcriptional levels of CYP6DA2 in cotton aphids were significantly induced when exposed to gossypol, and knockdown of the CYP6DA2 transcripts by RNA interference (RNAi) significantly increased the toxicity of gossypol to cotton aphids. To further understand the gossypol regulatory cascade, the 5'-flanking promoter sequences of CYP6DA2 were isolated with a genome walker, and the promoter was very active and was inducible by gossypol. Co-transfection of the cap 'n' collar isoform C (CncC) and CYP6DA2 promoters dramatically increased the expression of CYP6DA2, and suppression of the CncC transcripts by RNAi significantly decreased the expression levels of CYP6DA2, and significantly increased the toxicity of gossypol to cotton aphids. Thus, the transcriptional regulation of CYP6DA2 involved the transcriptional factor CncC.

Only a minority of broad-range detoxification genes respond to a variety of phytotoxins in generalist Bemisia tabaci species

Generalist insect can utilize two different modes for regulating their detoxification genes, the constitutive mode and the induced mode. Here, we used the Bemisia tabaci sibling species MEAM1 and MED, as a model system for studying constitutive and induced detoxification resistance and their associated tradeoffs. B. tabaci adults were allowed to feed through membranes for 24 h on diet containing only sucrose or sucrose with various phytotoxins. Quantitative real-time PCR analyses of 18 detoxification genes, indicated that relatively few transcripts were changed in both the MEAM1 and MED species, in response to the addition of phytotoxins to the diet. Induced transcription of detoxification genes only in the MED species, in response to the presence of indole-3-carbinol in the insect’s diet, was correlated with maintenance of reproductive performance in comparison to significant reduction in performance of the MEAM1 species. Three genes, COE2, CYP6-like 5 and BtGST2, responded to more than one compound and were highly transcribed in the insect gut. Furthermore, functional assays showed that the BtGST2 gene encodes a protein capable of interacting with both flavonoids and glucosinolates. In conclusion, several detoxification genes were identified that could potentially be involved in the adaptation of B. tabaci to its host plants.

Cytochrome P450s from the fall armyworm (Spodoptera frugiperda): responses to plant allelochemicals and pesticides

Spodoptera frugiperda is a polyphagous lepidopteran pest that encounters a wide range of toxic plant metabolites in its diet. The ability of this insect to adapt to its chemical environment might be explained by the action of major detoxification enzymes such as cytochrome P450s (or CYP). Forty-two sequences coding for P450s were identified and most of the transcripts were found to be expressed in the midgut, Malpighian tubules and fat body of S. frugiperda larvae. Relatively few P450s were expressed in the established cell line Sf9. In order to gain information on how these genes respond to different chemical compounds, larvae and Sf9 cells were exposed to plant secondary metabolites (indole, indole-3-carbinol, quercetin, 2-tridecanone and xanthotoxin), insecticides (deltamethrin, fipronil, methoprene, methoxyfenozide) or model inducers (clofibrate and phenobarbital). Several genes were induced by plant chemicals such as P450s from the 6B, 321A and 9A subfamilies. Only a few genes responded to insecticides, belonging principally to the CYP9A family. There was little overlap between the response in vivo measured in the midgut and the response in vitro in Sf9 cells. In addition, regulatory elements were detected in the promoter region of these genes. In conclusion, several P450s were identified that could potentially be involved in the adaptation of S. frugiperda to its chemical environment.

Expression analysis of two P450 monooxygenase genes of the tobacco cutworm moth (Spodoptera litura) at different developmental stages and in response to plant allelochemicals

Cytochrome P450 monooxygenases (P450s) of insects are known to be involved in the metabolism or detoxification of plant allelochemicals and insecticides. Spodoptera litura (Lepidoptera, Noctuidae) is a polyphagous moth responsible for severe yield losses in many crops. In this study, two full-length P450 genes, CYP6B48 and CYP6B58, were cloned from S. litura. The cDNA sequences encode proteins with 503 and 504 amino acids, respectively. Phylogenetic analysis revealed that CYP6B48 and CYP6B58 belong to the CYP6B subfamily of P450s. Quantitative real-time PCR analyses showed that CYP6B48 and CYP6B58 were expressed only at larval stage, but not at pupal and adult stages. The highest levels of transcripts were found in the midguts and fat bodies of the larvae. No expression was detected in the ovary or hemolymph. Feeding with diets containing cinnamic acid, quercetin, or coumarin did not affect expression of CYP6B48. In contrast, diet supplemented with xanthotoxin dramatically increased the levels of CYP6B48 transcript in the midgut and fat bodies. Larvae fed with flavone had high levels of transcript of CYP6B48 in the midgut, whereas only slightly elevated levels were found in the fat bodies. Effects of the tested allelochemicals on CYP6B58 expression were minor. Hence, our findings show that S. litura responds to specific allelochemicals such as xanthotoxin with the accumulation of CYP6B48 transcripts, suggesting that specific signals in the food control the insect's ability to convert toxic allelochemicals to less harmful forms at the transcriptional level.

Role of G-protein-coupled receptor-related genes in insecticide resistance of the mosquito, Culex quinquefasciatus

G-protein-coupled receptors regulate signal transduction pathways and play diverse and pivotal roles in the physiology of insects, however, the precise function of GPCRs in insecticide resistance remains unclear. Using quantitative RT-PCR and functional genomic methods, we, for the first time, explored the function of GPCRs and GPCR-related genes in insecticide resistance of mosquitoes, Culex quinquefasciatus. A comparison of the expression of 115 GPCR-related genes at a whole genome level between resistant and susceptible Culex mosquitoes identified one and three GPCR-related genes that were up-regulated in highly resistant Culex mosquito strains, HAmCq^G8 and MAmCq^G6, respectively. To characterize the function of these up-regulated GPCR-related genes in resistance, the up-regulated GPCR-related genes were knockdown in HAmCq^G8 and MAmCq^G6 using RNAi technique. Knockdown of these four GPCR-related genes not only decreased resistance of the mosquitoes to permethrin but also repressed the expression of four insecticide resistance-related P450 genes, suggesting the role of GPCR-related genes in resistance is involved in the regulation of resistance P450 gene expression. This results help in understanding of molecular regulation of resistance development in Cx. quinquefasciatus.

Common and unique cis-acting elements mediate xanthotoxin and flavone induction of the generalist P450 CYP321A1

How polyphagous herbivores up-regulate their counterdefense genes in response to a broad range of structurally different allelochemicals remains largely unknown. To test whether this is accomplished by having more allelochemical-response elements or the similar number of functionally more diverse elements, we mapped out the cis-acting elements mediating the induction of the allelochemical-metabolizing CYP321A1 from the generalist Helicoverpa zea by xanthotoxin and flavone, two structurally distinct allelochemicals with very different encounter rate by this species. Seven xanthotoxin-responsive elements were localized by analyzing promoter activities of varying length of CYP321A1 promoter in H. zea fatbody cells. Compared with the 5 flavone-responsive elements mapped out previously, there are four common elements (1 essential element, 2 enhancers, and 1 negative element) mediating induction of CYP321A1 by both of the two allelochemicals. The remaining four elements (3 enhancers and 1 negative element), however, only regulate induction of CYP321A1 by either of the two allelochemicals. Co-administration of the two allelochemicals resulted in an induction fold that is significantly lower than the expected additive value of the two allelochemicals. These results indicate that xanthotoxin- and flavone-induced expressions of CYP321A1 are mediated mainly by the functionally more diverse common elements although the allelochemical-unique elements also play a role.

Natural history and intragenomic dynamics of the Transib transposon Hztransib in the cotton bollworm Helicoverpa zea

Hztransib, recently identified from Helicoverpa zea, represents the first intact and transcriptionally active Transib element. Its open reading frame was detected in Helicoverpa armigera, from which H. zea evolved, and in Helicoverpa assulta, the common ancestor of H. zea and H. armigera, but its remaining parts were found only in H. armigera. Thirty-nine Hztransib insertion sites, all of which are polymorphic, were detected from eight populations of H. zea. Out of the 39 insertion sites, 35 were not frequently occupied, with 1-33 occurrences in a total of 128 individuals from the eight populations (16 larvae per population). Its copy number ranged from 5.8 to 14.2 per individual, with putative intact copies always more abundant than internally deleted ones. Taking this evidence together, Hztransib probably transferred to H. zea from H. armigera and most likely still retains its capacity to maintain structural integrity, increase copy number and remobilize in H. zea.