Synergistic resistance of Helicoverpa armigera to Bt toxins linked to cadherin and ABC transporters mutations

Contribution of the transcription factor SfGATAe to Bt Cry toxin resistance in Spodoptera frugiperda through reduction of ABCC2 expression

Insect resistance evolution poses a significant threat to the advantages of biopesticides and transgenic crops utilizing insecticidal Cry-toxins from Bacillus thuringiensis (Bt). However, there is limited research on the relationship between transcriptional regulation of specific toxin receptors in lepidopteran insects and their resistance to Bt toxins. Here, we report the positive regulatory role of the SfGATAe transcription factor on the expression of the ABCC2 gene in Spodoptera frugiperda. DNA regions in the SfABCC2 promoter that are vital for regulation by SfGATAe, utilizing DAP-seq technology and promoter deletion mapping. Through yeast one-hybrid assays, DNA pull-down experiments, and site-directed mutagenesis, we confirmed that the transcription factor SfGATAe regulates the core control site PBS2 in the ABCC2 target gene. Tissue-specific expression analysis has revealed that SfGATAe is involved in the regulation and expression of midgut cells in the fall armyworm. Silencing SfGATAe in fall armyworm larvae resulted in reduced expression of SfABCC2 and decreased sensitivity to Cry1Ac toxin. Overall, this study elucidated the regulatory mechanism of the transcription factor SfGATAe on the expression of the toxin receptor gene SfABCC2 and this transcriptional control mechanism impacts the resistance of the fall armyworm to Bt toxins.

Improved effects of Helicoverpa armigera nucleopolyhedrovirus integrated with Campoletis chlorideae against H. armigera and impact of the virus on the parasitoid

BACKGROUND

Combined use can be an effective measure to improve pest control by viruses and parasitic wasps. However, not all combinations of natural enemies show improved effects. Helicoverpa armigera nucleopolyhedrovirus (HearNPV) and Campoletis chlorideae Uchida are two important natural enemies of Helicoverpa armigera. This study aimed to investigate the combined effects of C. chlorideae and HearNPV against H. armigera larvae and the impact of HearNPV on C. chlorideae.

RESULTS

The combination of HearNPV and C. chlorideae exerted increased mortality on H. armigera when C. chlorideae parasitized larvae one day after infection with HearNPV. C. chlorideae could distinguish between HearNPV-infected and noninfected larvae. Besides influencing host selection of C. chlorideae, HearNPV infection had negative effects on the development and reproduction of C. chlorideae. The developmental time of C. chlorideae was significantly prolonged and the percentage of emergence and adult eclosion of C. chlorideae was lower in infected hosts. The adult wasps were also smaller in body size, and female adults had fewer eggs when they developed in virus-infected hosts.

CONCLUSIONS

HearNPV combined with C. chlorideae could improve the efficacy of biological control against H. armigera. The results provided valuable information on the importance of timing in the combined use of HearNPV and C. chlorideae for the biological control of H. armigera. © 2023 Society of Chemical Industry.

Mutation in PgABCC2 confers low-level resistance to Cry1Ac in pink bollworm

BACKGROUND

With the increasing incidence of pest resistance to transgenic crops producing Bacillus thuringiensis (Bt) proteins in the field, elucidating the molecular basis of resistance is important for monitoring, delaying and countering pest resistance. Previous work revealed that mutation or down-regulated expression of the cadherin gene (PgCad1) is associated with pink bollworm (Pectinophora gossypiella) resistance to Cry1Ac, and 20 mutant PgCad1 alleles (r1-r20) were characterized. Here, we tested the hypothesis that the ABC transporter PgABCC2 is a functional receptor for the Bt toxin Cry1Ac and that a mutation is associated with resistance.

RESULTS

We identified and characterized the first resistance allele (rC2 ) of PgABCC2 in the laboratory-selected Cry1Ac-resistant strain AQ-C2 of pink bollworm. The rC2 allele had a one-base deletion in exon20, resulting in a frameshift and the introduction of a premature stop codon. This resulting PgABCC2 protein had a truncated C-terminus, including the loss of the NBD2 domain. AQ-C2 exhibited 20.2-fold greater resistance to Cry1Ac than the susceptible strain, and its inheritance of Cry1Ac resistance was recessive and genetically linked to PgABCC2. When produced in cultured insect cells, recombinant wild-type and rC2 mutant PgABCC2 proteins localized within the cell plasma membrane, although substantial cytoplasmic retention was also observed for the mutant protein, while the mutant PgABCC2 caused a 13.9-fold decrease in Cry1Ac toxicity versus the wild-type PgABCC2.

CONCLUSIONS

PgABCC2 is a functional receptor of Cry1Ac and the loss of its carboxyl terminus (including its NBD2 domain) confers low-level resistance to Cry1Ac in both larvae and in cultured cells. © 2024 Society of Chemical Industry.

Population genomics of Agrotis segetum provide insights into the local adaptive evolution of agricultural pests

BACKGROUND

The adaptive mechanisms of agricultural pests are the key to understanding the evolution of the pests and to developing new control strategies. However, there are few studies on the genetic basis of adaptations of agricultural pests. The turnip moth, Agrotis segetum (Lepidoptera: Noctuidae) is an important underground pest that affects a wide range of host plants and has a strong capacity to adapt to new environments. It is thus a good model for studying the adaptive evolution of pest species.

RESULTS

We assembled a high-quality reference genome of A. segetum using PacBio reads. Then, we constructed a variation map of A. segetum by resequencing 98 individuals collected from six natural populations in China. The analysis of the population structure showed that all individuals were divided into four well-differentiated populations, corresponding to their geographical distribution. Selective sweep analysis and environmental association studies showed that candidate genes associated with local adaptation were functionally correlated with detoxification metabolism and glucose metabolism.

CONCLUSIONS

Our study of A. segetum has provided insights into the genetic mechanisms of local adaptation and evolution; it has also produced genetic resources for developing new pest management strategies.

ATP Synthase Subunit α from Helicoverpa armigera Acts as a Receptor of Bacillus thuringiensis Cry1Ac and Synergizes Cry1Ac Toxicity

Insect resistance to Bacillus thuringiensis (Bt) toxins has led to an urgent need to explore the insecticidal mechanisms of Bt. Previous studies indicated that Helicoverpa armigera ATP synthase subunit α (HaATPs-α) is involved in Cry1Ac resistance. In this study, a real-time quantitative polymerase chain reaction (RT-PCR) confirmed that HaATPs-α expression was significantly reduced in the Cry1Ac-resistant strain (BtR). Cry1Ac feeding induced the downregulated expression of HaATPs-α in the susceptible strain, but not in the BtR strain. Furthermore, the interaction between HaATPs-α and Cry1Ac was verified by ligand blotting and homologous competition experiments. The in vitro gain and loss of function analyses showed HaATPs-α involved in Cry1Ac toxicity by expressing endogenous HaATPs-α and HaATPs-α double-stranded RNAs in Sf9 and midgut cells, respectively. Importantly, purified HaATPs-α synergized Cry1Ac toxicity to H. armigera larvae. These findings provide the first evidence that HaATPs-α is a potential receptor of Cry1Ac, it shows downregulated participation in Cry1Ac resistance, and it exhibits higher enhancement of Cry1Ac toxicity to H. armigera larvae.

Intraspecific diversity in the mechanisms underlying abamectin resistance in a cosmopolitan pest

Pesticide resistance relies on a myriad of mechanisms, ranging from single mutations to a complex and polygenic architecture, and it involves mechanisms such as target-site insensitivity, metabolic detoxification, or a combination of these, with either additive or synergistic effects. Several resistance mechanisms against abamectin, a macrocyclic lactone widely used in crop protection, have been reported in the cosmopolitan pest Tetranychus urticae. However, it has been shown that a single mechanism cannot account for the high levels of abamectin resistance found across different mite populations. Here, we used experimental evolution combined with bulked segregant analyses to map quantitative trait loci (QTL) associated with abamectin resistance in two genetically unrelated populations of T. urticae. In these two independent QTL mapping experiments, three and four QTLs were identified, of which three were shared between experiments. Shared QTLs contained genes encoding subunits of the glutamate-gated chloride channel (GluCl) and harboured previously reported mutations, including G314D in GluCl1 and G326E in GluCl3, but also novel resistance candidate loci, including DNA helicases and chemosensory receptors. Surprisingly, the fourth QTL, present only in only one of the experiments and thus unique for one resistant parental line, revealed a non-functional variant of GluCl2, suggesting gene knock-out as resistance mechanism. Our study uncovers the complex basis of abamectin resistance, and it highlights the intraspecific diversity of genetic mechanisms underlying resistance in a cosmopolitan pest.

Down-regulation of HaABCC3, potentially mediated by a cis-regulatory mechanism, is involved in resistance to Cry1Ac in the cotton bollworm, Helicoverpa armigera

Evolution of resistance to Cry proteins in multiple pest insects has been threatening the sustainable use of Bacillus thuringiensis (Bt)-transgenic crops. Better understanding about the mechanism of resistance to Cry proteins in insects is needed. Our preliminary study reported that the transcription of HaABCC3 was significantly decreased in a near-isogenic line (LFC2) of a Cry1Ac-resistant strain (LF60) of the global pest Helicoverpa armigera. However, the causality between HaABCC3 downregulation and resistance to Cry1Ac remains to be verified, and the regulatory mechanism underlying the HaABCC3 downregulation is still unclear. In this study, our data showed that both HaABCC3 and HaABCC3 downregulation were genetically linked to resistance to Cry1Ac in LF60. However, no InDels were observed in the coding sequence of HaABCC3 from LF60. Furthermore, F₁ offspring from the cross of LF60 and a HaABCC2/3-knockout mutant exhibited moderate resistance to Cry1Ac toxin; this indicated that the high resistance to Cry1Ac toxin in LF60 may have resulted from multiple genetic factors, including HaABCC2 mis-splicing and HaABCC3 downregulation. Results from luciferase reporter assays showed that promoter activity of HaABCC3 in LF60 was significantly lower than that in the susceptible strain, which indicated that HaABCC3 downregulation was likely mediated by promoter variation. Consistently, multiple variations of the GATA- or FoxA-binding sites in the promoter region of HaABCC3 were identified. Collectively, all results in this study suggested that the downregulation of HaABCC3 observed in the H. armigera LF60 strain, which is resistant to Cry1Ac, may be mediated by a cis-regulatory mechanism.

Transcriptional response and functional analysis of ATP-binding cassette transporters to tannic acid in pea aphid, Acyrthosiphon pisum (Harris)

Although tannins are widely distributed in broad beans and alfalfa, the pea aphid (Acyrthosiphon pisum) can still destroy them. The ATP binding cassette (ABC) transporters participate in the metabolism of plant secondary metabolites and pesticides in insects. However, whether ABC transporter genes play a role in the metabolism of tannins in the pea aphid is unclear. Here, we found that verapamil (an ABC transporter inhibitor) significantly increased the mortality of tannic acid to pea aphid, which indicated that ABC transporter gene was related to the metabolism of tannic acid by pea aphid. Then, we identified 54 putative ABC transporter genes from the genome database of A. pisum. These genes were divided into eight subfamilies, ApABCA to ApABCH, of which subfamily G has the largest number of genes with 19, followed by the subfamily C with 14. RT-qPCR results show that the expression levels of ApABCA2, ApABCC7, ApABCG2, and ApABCG3 were highly expressed in the first instar, while those of ApABCA3, ApABCG6, ApABCG7, ApABCH3, and ApABCH4 were highly expressed in adults. Furthermore, transcription levels of many ABC transporter genes were induced by tannic acid. Especially, ApABCG17 and ApABCH2 were obviously induced after being exposed to tannic acid. Meanwhile, knockdown of ApABCG17 by RNA interference resulted in increased sensitivity of pea aphid to tannic acid. These results suggest that ApABCG17 may be involved in tannic acid metabolism in pea aphid. This study will help us to understand the mechanism of tannic acid metabolism in pea aphid, and provides a basis for further research on the physiological function of ABC transporter genes in pea aphid.

Molecular and Kinetic Models for Pore Formation of Bacillus thuringiensis Cry Toxin

Cry proteins from Bacillus thuringiensis (Bt) and other bacteria are pesticidal pore-forming toxins. Since 2010, when the ABC transporter C2 (ABCC2) was identified as a Cry1Ac protein resistant gene, our understanding of the mode of action of Cry protein has progressed substantially. ABCC2 mediates high Cry1A toxicity because of its high activity for helping pore formation. With the discovery of ABCC2, the classical killing model based on pore formation and osmotic lysis became nearly conclusive. Nevertheless, we are still far from a complete understanding of how Cry proteins form pores in the cell membrane through interactions with their host gut membrane proteins, known as receptors. Why does ABCC2 mediate pore formation with high efficiency unlike other Cry1A-binding proteins? Is the “prepore” formation indispensable for pore formation? What is the mechanism underlying the synergism between ABCC2 and the 12-cadherin domain protein? We examine potential mechanisms of pore formation via receptor interactions in this paper by merging findings from prior studies on the Cry mode of action before and after the discovery of ABC transporters as Cry protein receptors. We also attempt to explain Cry toxicity using Cry–receptor binding affinities, which successfully predicts actual Cry toxicity toward cultured cells coexpressing ABC transporters and cadherin.

A versatile contribution of both aminopeptidases N and ABC transporters to Bt Cry1Ac toxicity in the diamondback moth

BACKGROUND

Biopesticides and transgenic crops based on Bacillus thuringiensis (Bt) toxins are extensively used to control insect pests, but the rapid evolution of insect resistance seriously threatens their effectiveness. Bt resistance is often polygenic and complex. Mutations that confer resistance occur in midgut proteins that act as cell surface receptors for the toxin, and it is thought they facilitate its assembly as a membrane-damaging pore. However, the mechanistic details of the action of Bt toxins remain controversial.

RESULTS

We have examined the contribution of two paralogous ABC transporters and two aminopeptidases N to Bt Cry1Ac toxicity in the diamondback moth, Plutella xylostella, using CRISPR/Cas9 to generate a series of homozygous polygenic knockout strains. A double-gene knockout strain, in which the two paralogous ABC transporters ABCC2 and ABCC3 were deleted, exhibited 4482-fold resistance to Cry1A toxin, significantly greater than that previously reported for single-gene knockouts and confirming the mutual functional redundancy of these ABC transporters in acting as toxin receptors in P. xylostella. A double-gene knockout strain in which APN1 and APN3a were deleted exhibited 1425-fold resistance to Cry1Ac toxin, providing the most direct evidence to date for these APN proteins acting as Cry1Ac toxin receptors, while also indicating their functional redundancy. Genetic crosses of the two double-gene knockouts yielded a hybrid strain in which all four receptor genes were deleted and this resulted in a > 34,000-fold resistance, indicating that while both types of receptor need to be present for the toxin to be fully effective, there is a level of functional redundancy between them. The highly resistant quadruple knockout strain was less fit than wild-type moths, but no fitness cost was detected in the double knockout strains.

CONCLUSION

Our results provide direct evidence that APN1 and APN3a are important for Cry1Ac toxicity. They support our overarching hypothesis of a versatile mode of action of Bt toxins, which can compensate for the absence of individual receptors, and are consistent with an interplay among diverse midgut receptors in the toxins' mechanism of action in a super pest.

Knockout of ABC Transporter ABCG4 Gene Confers Resistance to Cry1 Proteins in Ostrinia furnacalis

Ostrinia furnacalis is an important borer on maize. Long-term and large-scale planting of transgenic corn has led O. furnacalis evolving resistance and reducing the control effect. Recently, high levels of resistance to Bt Cry1 toxins have been reported to be genetically linked to the mutation or down-regulation of ABC transporter subfamily G gene ABCG4 in O. furnacalis. In order to further determine the relationship between ABCG4 gene and the resistance to Cry1 toxins in O. furnacalis, the novel CRISPR/Cas9 genome engineering system was utilized to successfully construct ABCG4-KO knockout homozygous strain. Bioassay results indicated that an ABCG4-KO strain had a higher resistance to Cry1 proteins compared with a susceptible strain (ACB-BtS). The result indicates that the ABCG4 gene may act as a receptor of the Bt Cry1 toxin in O. furnacalis. Furthermore, the development time was significantly changed in the early stage ABCG4-KO larvae, and the population parameters were also significantly changed. In summary, our CRISPR/Cas9-mediated genome editing study presents evidence that ABCG4 gene is a functional receptor for Bt Cry1 toxins, laying the foundation for further clarification of the Bt resistance mechanism.

Genetic Knockouts Indicate That the ABCC2 Protein in the Bollworm Helicoverpa zea Is Not a Major Receptor for the Cry1Ac Insecticidal Protein

Members of the insect ATP binding cassette transporter subfamily C2 (ABCC2) in several moth species are known as receptors for the Cry1Ac insecticidal protein from Bacillus thuringiensis (Bt). Mutations that abolish the functional domains of ABCC2 are known to cause resistance to Cry1Ac, although the reported levels of resistance vary widely depending on insect species. In this study, the function of the ABCC2 gene as a putative Cry1Ac receptor in Helicoverpa zea, a major pest of over 300 crops, was evaluated using CRISPR/Cas9 to progressively eliminate different functional ABCC2 domains. Results from bioassays with edited insect lines support that mutations in ABCC2 were associated with Cry1Ac resistance ratios (RR) ranging from 7.3- to 39.8-fold. No significant differences in susceptibility to Cry1Ac were detected between H. zea with partial or complete ABCC2 knockout, although the highest levels of tolerance were observed when knocking out half of ABCC2. Based on >500–1000-fold RRs reported in similar studies for closely related moth species, the low RRs observed in H. zea knockouts support that ABCC2 is not a major Cry1Ac receptor in this insect.