Independent and Synergistic Effects of Knocking out Two ABC Transporter Genes on Resistance to Bacillus thuringiensis Toxins Cry1Ac and Cry1Fa in Diamondback Moth

Throwing Brazilian strains into the melting pot of P. xylostella resistance to Bacillus thuringiensis

The resistance of pest insects to biopesticides based on the bacterium Bacillus thuringiensis (Bt) is normally associated with changes to the receptors involved in the mechanism of action of the pesticidal proteins produced by Bt. In some strains of Plutella xylostella (the diamondback moth) resistance has evolved through a signalling mechanism in which the genes encoding the receptor proteins are downregulated whereas in others it has been linked to structural changes in the receptors themselves. One such well characterized mutation is in the ABCC2 gene indicating that changes to this protein can result in resistance. However other studies have found that knocking out this protein does not result in a significant level of resistance. In this study we wanted to test the hypothesis that constitutive receptor downregulation is the major cause of Bt resistance in P. xylostella and that mutations in the now poorly expressed receptor genes may not contribute significantly to the phenotype. To that end we investigated the expression of a receptor (ABCC2) and the major regulator of the signalling pathway (MAP4K4) in two resistant and four susceptible strains. No correlation was found between expression levels and susceptibility; however, a frameshift mutation was identified in the ABCC2 receptor in a newly characterized resistant strain.

Key role of the ryanodine receptor I4790K mutation in mediating diamide resistance in Plutella xylostella

The diamondback moth Plutella xylostella, a global insect pest of cruciferous vegetables, has evolved resistance to many classes of insecticides including diamides. Three point mutations (I4790M, I4790K, and G4946E) in the ryanodine receptor of P. xylostella (PxRyR) have been identified to associate with varying levels of resistance. In this study, we generated a knockin strain (I4790K-KI) of P. xylostella, using CRISPR/Cas9 to introduce the I4790K mutation into PxRyR of the susceptible IPP-S strain. Compared to IPP-S, the edited I4790K-KI strain exhibited high levels of resistance to both anthranilic diamides (chlorantraniliprole 1857-fold, cyantraniliprole 1433-fold) and the phthalic acid diamide flubendiamide (>2272-fold). Resistance to chlorantraniliprole in the I4790K-KI strain was inherited in an autosomal and recessive mode, and genetically linked with the I4790K knockin mutation. Computational modeling suggests the I4790K mutation reduces the binding of diamides to PxRyR by disrupting key hydrogen bonding interactions within the binding cavity. The approximate frequencies of the 4790M, 4790K, and 4946E alleles were assessed in ten geographical field populations of P. xylostella collected in China in 2021. The levels of chlorantraniliprole resistance (2.3- to 1444-fold) in these populations were significantly correlated with the frequencies (0.017-0.917) of the 4790K allele, but not with either 4790M (0-0.183) or 4946E (0.017-0.450) alleles. This demonstrates that the PxRyR I4790K mutation is currently the major contributing factor to chlorantraniliprole resistance in P. xylostella field populations within China. Our findings provide in vivo functional evidence for the causality of the I4790K mutation in PxRyR with high levels of diamide resistance in P. xylostella, and suggest that tracking the frequency of the I4790K allele is crucial for optimizing the monitoring and management of diamide resistance in this crop pest.

CRISPR/Cas9-mediated silencing of CD44: unveiling the role of hyaluronic acid-mediated interactions in cancer drug resistance

A comprehensive overview of CD44 (CD44 Molecule (Indian Blood Group)), a cell surface glycoprotein, and its interaction with hyaluronic acid (HA) in drug resistance mechanisms across various types of cancer is provided, where CRISPR/Cas9 gene editing was utilized to silence CD44 expression and examine its impact on cancer cell behavior, migration, invasion, proliferation, and drug sensitivity. The significance of the HA-CD44 axis in tumor microenvironment (TME) delivery and its implications in specific cancer types, the influence of CD44 variants and the KHDRBS3 (KH RNA Binding Domain Containing, Signal Transduction Associated 3) gene on cancer progression and drug resistance, and the potential of targeting HA-mediated pathways using CRISPR/Cas9 gene editing technology to overcome drug resistance in cancer were also highlighted.

Utilization of Diverse Molecules as Receptors by Cry Toxin and the Promiscuous Nature of Receptor-Binding Sites Which Accounts for the Diversity

By 2013, it had been shown that the genes cadherin-like receptor (Cad) and ATP-binding cassette transporter subfamily C2 (ABCC2) were responsible for insect resistance to several Cry1A toxins, acting as susceptibility-determining receptors, and many review articles have been published. Therefore, this review focuses on information about receptors and receptor-binding sites that have been revealed since 2014. Since 2014, studies have revealed that the receptors involved in determining susceptibility vary depending on the Cry toxin subfamily, and that binding affinity between Cry toxins and receptors plays a crucial role. Consequently, models have demonstrated that ABCC2, ABCC3, and Cad interact with Cry1Aa; ABCC2 and Cad with Cry1Ab and Cry1Ac; ABCC2 and ABCC3 with Cry1Fa; ABCB1 with Cry1Ba, Cry1Ia, Cry9Da, and Cry3Aa; and ABCA2 with Cry2Aa and Cry2Ba, primarily in the silkworm, Bombyx mori. Furthermore, since 2017, it has been suggested that the binding sites of BmCad and BmABCC2 on Cry1Aa toxin overlap in the loop region of domain II, indicating that Cry toxins use various molecules as receptors due to their ability to bind promiscuously in this region. Additionally, since 2017, several ABC transporters have been identified as low-efficiency receptors that poorly induce cell swelling in heterologously expressing cultured cells. In 2024, research suggested that multiple molecules from the ABC transporter subfamily, including ABCC1, ABCC2, ABCC3, ABCC4, ABCC10, and ABCC11, act as low-efficiency receptors for a single Cry toxin in the midgut of silkworm larvae. This observation led to the hypothesis that the presence of such low-efficiency receptors contributes to the evolution of Cry toxins towards the generation of highly functional receptors that determine the susceptibility of individual insects. Moreover, this evolutionary process is considered to offer valuable insights for the engineering of Cry toxins to overcome resistance and develop countermeasures against resistance.

ABC transporter subfamily B1 as a susceptibility determinant of Bombyx mori larvae to Cry1Ba, Cry1Ia and Cry9Da toxins

ATP binding cassette (ABC) transporters are a diverse family of transmembrane proteins. Specific subfamily members expressed in the lepidopteran midgut can act as susceptibility determinants for several insecticidal Bt Cry proteins. However, the susceptibility determinants to many Cry toxins still remain unclear. Therefore, we knocked out a series of ABC transporters that are highly expressed in the midgut of Bombyx mori larvae by transcription activator-like effector nuclease (TALEN)-mediated gene editing, and the lineages that became resistant to Cry toxins were searched by toxin overlay bioassay. As a result, the B. mori ABC transporter subfamily B1 (BmABCB1) knockout lineage showed 19.17-fold resistance to Cry1Ba, 876.2-fold resistance to Cry1Ia, and 29.1-fold resistance to Cry9Da, suggesting that BmABCB1 is the determinant of susceptibility to these toxins. BmABCC2 and BmABCC3 have been shown to be susceptibility determinants based on their function as receptors. Therefore, we next heterologously expressed these ABC transporters in HEK293T cells and performed a cell swelling assay to examine whether these molecules could exert receptor functions. As a result, BmABCB1-expressing cells showed swelling response to Cry1Ia and Cry9Da, and cells expressing PxABCB1, which is the Plutella xylostella ortholog of BmABCB1, showed swelling for Cry1Ba, suggesting that ABCB1 is a susceptibility determinant by functioning as a receptor to these toxins. Furthermore, in order to clarify how high binding affinity is based on receptor function, we performed surface plasmon resonance analysis and found that each KD of Cry1Ba, Cry1Ia, and Cry9Da to BmABCB1 were 7.69 × 10-8 M, 2.19 × 10-9 M, and 4.17 × 10-6 M respectively.

Genome-scale analysis of ABC transporter genes and characterization of the ABCC type transporter genes in the oriental armyworm, Mythimna separata (Walker)

The oriental armyworm Mythimna separata is a polyphagous, migratory corn pest in China and other Asian countries. Transgenic Bacillus thuringiensis (Bt) corn may effectively control this insect pest. Several reports have suggested that ATP-binding cassette (ABC) transporter proteins may act as receptors that bind Bt toxins. However, our knowledge about ABC transporter proteins in M. separata is limited. We identified 43 ABC transporter genes in the M. separata genome by bioinformatics prediction. Evolutionary tree analysis grouped these 43 genes into 8 subfamilies, ABCA to ABCH. Among the 13 ABCC subfamily genes, the transcript levels of MsABCC2 and MsABCC3 were upregulated. In addition, RT-qPCR analyses of these two potentials showed that both were predominantly expressed in the midgut tissue. Knock-down of MsABCC2, but not MsABCC3, decreased Cry1Ac susceptibility as indicated by increased larval weight and reduced larval mortality. This suggested that MsABCC2 might play a more important role in Cry1Ac toxicity and that it is a putative Cry1Ac receptor in M. separata. Together, these findings provide unique and valuable information for future elucidating of the role of ABC transporter genes in M. separata, which is highly valuable and important for the long-term application of Bt insecticidal protein.

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.

Resistance of Cabbage Loopers to Bacillus thuringiensis (Bt) Toxin Cry1F and to Dual-Bt Toxin WideStrike Cotton Plants

The Cry proteins from Bacillus thuringiensis (Bt) are major insecticidal toxins in formulated Bt sprays and are expressed in genetically engineered Bt crops for insect pest control. However, the widespread application of Bt toxins in the field imposes strong selection pressure on target insects, leading to the evolution of insect resistance to the Bt toxins. Identification and understanding of mechanisms of insect resistance to Bt toxins are an important approach for dissecting the modes of action of Bt toxins and providing knowledge necessary for the development of resistance management technologies. In this study, cabbage looper (Trichoplusia ni) strains resistant to the transgenic dual-Bt toxin WideStrike cotton plants, which express Bt toxins Cry1Ac and Cry1F, were selected from T. ni strains resistant to the Bt formulation Bt-DiPel. The WideStrike-resistant T. ni larvae were confirmed to be resistant to both Bt toxins Cry1Ac and Cry1F. From the WideStrike-resistant T. ni, the Cry1F resistance trait was further isolated to establish a T. ni strain resistant to Cry1F only. The levels of Cry1F resistance in the WideStrike-resistant and the Cry1F-resistant strains were determined, and the inheritance of the Cry1F-resistant trait in the two strains was characterized. Genetic association analysis of the Cry1F resistance trait indicated that the Cry1F resistance in T. ni isolated in this study is not shared with the Cry1Ac resistance mechanism nor is it associated with a mutation in the ABCC2 gene, as has so far been reported in Cry1F-resistant insects. IMPORTANCE Insecticidal toxins from Bacillus thuringiensis (Bt) are highly effective for insect control in agriculture. However, the widespread application of Bt toxins exerts strong selection for Bt resistance in insect populations. The continuing success of Bt biotechnology for pest control requires the identification of resistance and understanding of the mechanisms of resistance to Bt toxins. Cry1F is an important Bt toxin used in transgenic cotton, maize, and soybean varieties adopted widely for insect control. To understand the mode of action of Cry1F and mechanisms of Cry1F resistance in insects, it is important to identify Cry1F-specific resistance and the resistance mechanisms. In this study, Trichoplusia ni strains resistant to commercial "WideStrike" cotton plants that express Bt toxins Cry1Ac and Cry1F were selected, and a Cry1F-specific resistant strain was isolated. The isolation of the novel Cry1F-specific resistance in the T. ni provided an invaluable biological system to discover a Cry1F-specific novel resistance mechanism.

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.

Functional Diversity of the Lepidopteran ATP-Binding Cassette Transporters

The ATP-binding cassette (ABC) transporter gene family is ubiquitous in the living world. ABC proteins bind and hydrolyze ATP to transport a myriad of molecules across various lipid-containing membrane systems. They have been studied well in plants for transport of a variety of compounds and particularly, in vertebrates due to their direct involvement in resistance mechanisms against several toxic molecules/metabolites. ABC transporters in insects are found within large multigene families involved in the efflux of chemical insecticides and toxic/undesired metabolites originating from food and endogenous metabolism. This review deals with ABC transporter subfamilies of few agronomically important Lepidopteran pests. The transcriptional dynamics and regulation of ABC transporters during insect development emphasizes their functional diversity against insecticides, Cry toxins, and plant specialized metabolites. To generate insights about molecular function and physiological roles of ABCs, functional and structural characterization is necessary. Also, expansion and divergence of ABC transporter gene subfamilies in Lepidopteran insects needs more systematic investigation. We anticipate that newer methods of insect control in agriculture can benefit from an understanding of ABC transporter interactions with a vast range of natural specialized molecules and synthetic compounds.

A feasibility trial of genomics-based diagnosis detecting insecticide resistance of the diamondback moth

BACKGROUND

Insecticide resistance management has been key for crop protection for over 70 years and is increasingly important because the development of new active ingredients has decreased in recent years. By monitoring the development of resistance in a timely manner, we can effectively prolong insecticide efficacy. Genomic-based diagnosis can reliably predict resistance development if information on resistant mutations against major pesticides is available. Here, we developed a feasibility trial of genomics-based diagnosis of insecticide resistance in diamondback moth (Plutella xylostella) populations in Nagano Prefecture, Japan. Amplicon sequencing analyses using a next-generation sequencer (Illumina MiSeq) for major insecticides, including diamides, pyrethroids, Bacillus thuringiensis (Bt) toxin (Cry1Ac), organophosphates, and spinosyns, were conducted.

RESULTS

Mutations related to the resistance of pyrethroids, organophosphates, and diamides (flubendiamide and chlorantraniliprole) prevailed, while those of a diamide (cyantraniliprole), Bt (Cry1Ac), and spinosyns were scanty, suggesting that they are still effective. The results of the genomics-based diagnosis were generally concordant with the results of bioassays. Resistance development tendencies were generally uniform across Nagano.

CONCLUSION

An insecticide-resistance management campaign can be conducted in Nagano Prefecture with a quick genomic-based diagnosis in early spring while bioassay is the only option for monitoring resistances whose mutations are unavailable. Our study is the first step in the future management of insecticide resistance in all significant pests. © 2022 Society of Chemical Industry.

Novel genetic basis of resistance to Bt toxin Cry1Ac in Helicoverpa zea

Crops genetically engineered to produce insecticidal proteins from the bacterium Bacillus thuringiensis have advanced pest management, but their benefits are diminished when pests evolve resistance. Elucidating the genetic basis of pest resistance to Bacillus thuringiensis toxins can improve resistance monitoring, resistance management, and the design of new insecticides. Here, we investigated the genetic basis of resistance to Bacillus thuringiensis toxin Cry1Ac in the lepidopteran Helicoverpa zea, one of the most damaging crop pests in the United States. To facilitate this research, we built the first chromosome-level genome assembly for this species, which has 31 chromosomes containing 375 Mb and 15,482 predicted proteins. Using a genome-wide association study, fine-scale mapping, and RNA-seq, we identified a 250-kb quantitative trait locus on chromosome 13 that was strongly associated with resistance in a strain of Helicoverpa zea that had been selected for resistance in the field and lab. The mutation in this quantitative trait locus contributed to but was not sufficient for resistance, which implies alleles in more than one gene contributed to resistance. This quantitative trait locus contains no genes with a previously reported role in resistance or susceptibility to Bacillus thuringiensis toxins. However, in resistant insects, this quantitative trait locus has a premature stop codon in a kinesin gene, which is a primary candidate as a mutation contributing to resistance. We found no changes in gene sequence or expression consistently associated with resistance for 11 genes previously implicated in lepidopteran resistance to Cry1Ac. Thus, the results reveal a novel and polygenic basis of resistance.

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.

Bt Cry1Ac resistance in Trichoplusia ni is conferred by multi-gene mutations

The three-domain Cry toxin Cry1Ac from Bacillus thuringiensis （Bt） is an important insecticidal toxin in Bt sprays and has been used in transgenic Bt-crops to confer insect resistance. The cabbage looper, Trichoplusia ni, has developed resistance to Bt sprays in commercial greenhouses, and the resistance to Cry1Ac has been previously identified to be associated with altered expression of the APN1 and APN6 genes and be genetically linked to a locus on chromosome 15. In this study, the Cry1Ac resistance locus in T. ni was further finely mapped, and the specific Cry1Ac resistance-conferring mutation in the resistance locus was identified to be a 4 bp frameshift insertion in the ABCC2 gene by whole genome resequencing, midgut transcriptome analysis, candidate gene cDNA sequencing and mutation site genomic DNA sequencing. By CRISPR/Cas9 mutagenesis, a series of ABCC2 and ABCC3 mutant T. ni strains were generated, and the role of ABCC2 in the toxicity of Cry1Ac in T. ni was confirmed. The results from this study also showed that knockout of ABCC2 in T. ni conferred resistance to Cry1Ac at a level lower than that in the greenhouse-derived resistant T. ni strain and that the Cry1Ac resistance-associated alteration of APN1 and APN6 expression was independent of ABCC2 gene mutations, indicating that the altered expression of APN1 and APN6 was controlled by another gene mutation in Cry1Ac resistant T. ni. Furthermore, T. ni larval bioassays showed that the level of Cry1Ac resistance in F₁ families from reciprocal crosses of the Cry1Ac resistant strain with an ABCC2 knockout CRISPR strain was significantly higher than that in ABCC2 knockout strain, indicating the presence of additional Cry1Ac resistance-conferring mutation(s) in the Cry1Ac resistant strain. Therefore, the resistance to Cry1Ac in T. ni is conferred by a mutation in ABCC2 and an additional mutation (or mutations) which leads to altered expression of APN1 and APN6. The additional Cry1Ac resistance mutation or mutations remain to be identified.

ATP-binding cassette transporter subfamily C members 2, 3 and cadherin protein are susceptibility-determining factors in Bombyx mori for multiple Bacillus thuringiensis Cry1 toxins

Field-evolved resistance of insect pests to Bacillus thuringiensis (Bt) toxins (Cry toxins) is a threat to the efficacy of Bt-based bio-insecticides and transgenic crops. Recent reports have suggested that ATP-binding cassette transporter subfamily C2 (ABCC2) and cadherin-like receptor play important roles in conferring susceptibility to Cry1 toxins. However, the receptors involved in Bt susceptibility in each insect remain unclear. To determine the receptors that are involved in the susceptibility of Bombyx mori to Cry1 toxins (1Ab, 1Ac and 1Fa), we conducted diet overlay bioassay using B. mori strains disrupted with one or two receptor (s) among BmABCC2, BmABCC3, and cadherin-like receptor (BtR175) generated by transcription activator-like effector nuclease (TALEN)-mediated gene editing. The single-knockout strains for BmABCC2 showed resistance to Cry1Ab and Cry1Ac, whereas only strains with double knockout of BmABCC2 and BmABCC3 exhibited high resistance to Cry1Fa. Progeny populations generated from the crossing of heterozygotes for BtR175 knockout allele included 25% theoretical homozygotes for the BtR175 knockout allele and they showed resistance to Cry1Ab and Cry1Ac. Then, through a cell swelling assay using Sf9 cells ectopically expressing the receptor, we analyzed the mechanisms underlying the different contributions of BmABCC2, BmABCC3, and BtR175 to larval susceptibility. The receptor activity of BmABCC2 for Cry1Ab and Cry1Ac was far higher than that of BmABCC3, and BtR175 synergistically enhanced the receptor activity of BmABCC2. This result well explained the important involvement of BmABCC2 and BtR175 in the larval susceptibility to Cry1A toxins. By contrast, the receptor activities of BmABCC2 and BmABCC3 for Cry1Fa were observed at a similar level and synergistic effect of BtR175 was small. This finding explains the equal importance of BmABCC2 and BmABCC3 and very small contribution of BtR175 on larval susceptibility to Cry1Fa. Thus, we demonstrated the different importance of BmABCC2, BmABCC3, and BtR175 to various Cry1 toxins as susceptibility-determining factors in B. mori larvae and the underlying basis for the observed differences. Furthermore, a weak correlation was indicated between the binding affinity and receptor activities of BmABCC2 and BmABCC3 to Cry1 toxins.

Neuropeptide F from endocrine cells in Plutella xylostella midgut modulates feeding and synergizes Cry1Ac action

With the wide cultivation of transgenic plants throughout the world and the rising risk of resistance to Bacillus thuringiensis crystal (Cry) toxins, it is essential to design an adaptive resistance management strategy for continued use. Neuropeptide F (NPF) of insects has proven to be valuable for the production of novel-type transgenic plants via its important role in the control of feeding behavior. In this study, the gene encoding NPF was cloned from the diamondback moth, Plutella xylostella, an important agricultural pest. Real-time quantitative reverse transcription-polymerase chain reaction and in situ hybridization showed a relatively high expression of P. xylostella-npf (P. x-npf) in endocrine cells of the midgut of fourth instar larvae, and it was found to participate in P. xylostella feeding behavior and Cry1Ac-induced feeding inhibition. Prokaryotic expression and purification provided structure unfolded P. x-npf from inclusion bodies for diet surface overlay bioassays and the results demonstrated a significant synergistic effect of P. x-npf on Cry1Ac toxicity by increasing intake of noxious food which contains Cry toxins, especially quick death at an early stage of feeding. Our findings provided a potential new way to efficiently control pests by increasing intake of lower dose Cry toxins and a novel hint for the complex Cry toxin mechanism.

Synergism of Bacillus thuringiensis Toxin Cry1Ac by a Fragment of Toxin-Binding Polycalin from Plutella xylostella

The continued success of pest control using insecticidal crystal (Cry) proteins from Bacillus thuringiensis (Bt) in transgenic plants was threatened by the evolution of resistance. Previous studies suggested that polycalin from Plutella xylostella could bind to Cry1Ac toxin as a potential receptor. In this study, a fragment of P. xylostella polycalin (Pxpolycalinf, G2209-A2942) containing a carboxyl-terminal GPI-anchored signal peptide was cloned and expressed. Purified Pxpolycalinf retained the binding ability to Cry1Ac and synergized Cry1Ac toxicity to the third larvae of P. xylostella in bioassays. Moreover, the polyclonal antibody of Pxpolycalinf decreased the Cry1Ac activity after being fed together with normal food. Further, the ELISA results showed the concentration-dependent binding of Pxpolycalinf to P. xylostella brush border membrane vesicles (BBMV). Spodoptera frugiperda 9 (Sf9) cells expressing Pxpolycalinf were not susceptive to Cry1Ac, whereas Pxpolycalinf increased Cry1Ac cytotoxicity to Sf9 cells expressing P. xylostella ATP-dependent binding cassette transporter C2 (PxABCC2). Immunolocalization presented the binding of Pxpolycalinf to the Sf9 cell membrane, and ELISA showed the concentration-dependent binding of Pxpolycalinf to Sf9 cell extraction. These results here provide the first evidence that a fragment of P. xylostella polycalin, a potential receptor of Cry1Ac, synergizes Cry1Ac toxicity to P. xylostella larvae and Sf9 cells expressing PxABCC2.

Evaluating Cross-Resistance to Cry and Vip Toxins in Four Strains of Helicoverpa armigera With Different Genetic Mechanisms of Resistance to Bt Toxin Cry1Ac

Evolution of resistance by pests has diminished the efficacy of transgenic crops producing insecticidal proteins from Bacillus thuringiensis (Bt). In China, where transgenic cotton producing Bt toxin Cry1Ac has been planted since 1997, field control failures have not been reported but the frequency of resistance to Cry1Ac has increased in the cotton bollworm, Helicoverpa armigera. This provides incentive to switch to multi-toxin Bt cotton, which is grown in many other countries. Previous work created four laboratory strains of H. armigera with >100-fold resistance to Cry1Ac, with the genetic basis of resistance known in all but the LF256 strain. Here, we analyzed the genetic basis of resistance in Cry1Ac in LF256 and evaluated cross-resistance of all four strains to three toxins produced by widely planted multi-toxin Bt cotton: Cry1Fa, Cry2Ab, and Vip3Aa. DNA sequencing revealed that LF256 lacked the mutations in three genes (HaTSPAN1, HaABCC2, and HaABCC3) that confer resistance to Cry1Ac in two other strains of H. armigera we analyzed. Together with previous results, the data reported here show that each of the four strains examined has a different genetic basis of resistance to Cry1Ac. Significant positive cross-resistance occurred to Cry1Fa in three of the four strains tested but not to Cry2Ab or Vip3Aa in any strain. Thus, Cry2Ab and Vip3Aa are likely to be especially valuable for increasing the efficacy and durability of Bt cotton against H. armigera populations that have some resistance to Cry1Ac.

CRISPR-mediated mutations in the ABC transporter gene ABCA2 confer pink bollworm resistance to Bt toxin Cry2Ab

Crops genetically engineered to produce insecticidal proteins from Bacillus thuringiensis (Bt) have many benefits and are important globally for managing insect pests. However, the evolution of pest resistance to Bt crops reduces their benefits. Understanding the genetic basis of such resistance is needed to better monitor, manage, and counter pest resistance to Bt crops. Previous work shows that resistance to Bt toxin Cry2Ab is associated with mutations in the gene encoding the ATP-binding cassette protein ABCA2 in lab- and field-selected populations of the pink bollworm (Pectinophora gossypiella), one of the world’s most destructive pests of cotton. Here we used CRISPR/Cas9 gene editing to test the hypothesis that mutations in the pink bollworm gene encoding ABCA2 (PgABCA2) can cause resistance to Cry2Ab. Consistent with this hypothesis, introduction of disruptive mutations in PgABCA2 in a susceptible strain of pink bollworm increased the frequency of resistance to Cry2Ab and facilitated creation of a Cry2Ab-resistant strain. All Cry2Ab-resistant individuals tested in this study had disruptive mutations in PgABCA2. Overall, we found 17 different disruptive mutations in PgABCA2 gDNA and 26 in PgABCA2 cDNA, including novel mutations corresponding precisely to single-guide (sgRNA) sites used for CRISPR/Cas9. Together with previous results, these findings provide the first case of practical resistance to Cry2Ab where evidence identifies a specific gene in which disruptive mutations can cause resistance and are associated with resistance in field-selected populations.