Identification of Aminopeptidase-N2 as a Cry2Ab binding protein in Manduca sexta

V-ATPase E mediates Cry2Ab binding and toxicity in Helicoverpa armigera

Cry2Ab is one of the important alternative Bt proteins that can be used to manage insect pests resistant to Cry1A toxins and to expand the insecticidal spectrum of pyramided Bt crops. Previous studies have showed that vacuolar H+-ATPase subunits A and B (V-ATPase A and B) may be involved in Bt insecticidal activities. The present study investigated the role of V-ATPases subunit E in the toxicity of Cry2Ab in Helicoverpa amigera. RT-PCR analysis revealed that oral exposure of H. amigera larvae to Cry2Ab led to a significant reduction in the expression of H. armigera V-ATPase E (HaV-ATPase E). Ligand blot, homologous and heterologous competition experiments confirmed that HaV-ATPases E physically and specifically bound to activated Cry2Ab toxin. Heterologous expressing of HaV-ATPase E in Sf9 cells made the cell line more susceptible to Cry2Ab, whereas knockdown of the endogenous V-ATPase E in H. zea midgut cells decreased Cry2Ab's cytotoxicity against this cell line. Further in vivo bioassay showed that H. armigera larvae fed a diet overlaid with both Cry2Ab and E. coli-expressed HaV-ATPase E protein suffered significantly higher mortality than those fed Cry2Ab alone. These results support that V-ATPases E is a putative receptor of Cry2Ab and can be used to improve Cry2Ab toxicity and manage Cry2Ab resistance at least in H. armigera.

Functional analysis of transgenic cry1Ah-1 maize

Maize is an important food crop in the world, but the yield and quality of maize have been significantly reduced due to the impact of insect pests. In order to address this issue, the cry1Ah gene was subjected to error-prone PCR for mutagenesis, and subsequently, the mutant cry1Ah-1 gene was introduced into maize inbred line GSH9901 callus using the Agrobacterium-mediated method. The T2 generation transformed plants were obtained by subculture, and 9 transgenic positive plants were obtained by molecular detection which was carried out by PCR, qRT-PCR, Bt gold-labeled immunoassay test strips, Western blot and ELISA. It was found that the Cry1Ah-1 gene could be transcribed normally in maize leaves, of which OE1 and OE3 had higher relative expression levels and could successfully express proteins of 71.94 KD size. They were expressed in different tissues at the 6-leaf stage, heading stage and grain-filling stage, and could ensure the protection of maize from corn borer throughout the growth period. The biological activities of OE1 and OE3 were tested indoors and in the field, and the results showed that in indoors, the corn borer that fed on OE1 and OE3 corn leaves had a mortality rate of 100 % after 3 days; in the field, OE1 and OE3 had strong insecticidal activity against corn borer, reaching a high resistance level. In conclusion, the transgenic cry1Ah-1 maize has a strong insecticidal effect on corn borer, and has a good prospect of commercialization.

Cadherin Is a Binding Protein but Not a Functional Receptor of Bacillus thuringiensis Cry2Ab in Helicoverpa armigera

Midgut receptors play a critical role in the specificity of Cry toxins for individual insect species. Cadherin proteins are essential putative receptors of Cry1A toxins in lepidopteran larvae. Cry2A family members share common binding sites in Helicoverpa armigera, and one of them, Cry2Aa, has been widely reported to interact with midgut cadherin. Here, we studied the binding interaction and functional role of H. armigera cadherin in the mechanism of Cry2Ab toxicity. A region spanning from cadherin repeat 6 (CR6) to the membrane-proximal region (MPR) of cadherin protein was produced as six overlapping peptides to identify the specific binding regions of Cry2Ab. Binding assays showed that Cry2Ab binds nonspecifically to peptides containing CR7 and CR11 regions in a denatured state but binds specifically only to CR7-containing peptides in the native state. The peptides CR6-11 and CR6-8 were transiently expressed in Sf9 cells to assess the functional role of cadherin. Cytotoxicity assays showed that Cry2Ab is not toxic to the cells expressing any of the cadherin peptides. However, ABCA2-expressing cells showed high sensitivity to Cry2Ab toxin. Neither increased nor decreased sensitivity to Cry2Ab was observed when the peptide CR6-11 was coexpressed with the ABCA2 gene in Sf9 cells. Instead, treating ABCA2-expressing cells with a mixture of Cry2Ab and CR6-8 peptides resulted in significantly reduced cell death compared with treatment with Cry2Ab alone. Moreover, silencing of the cadherin gene in H. armigera larvae showed no significant effect on Cry2Ab toxicity, in contrast to the reduced mortality in ABCA2-silenced larvae. IMPORTANCE To improve the efficiency of production of a single toxin in crops and to delay the evolution of insect resistance to the toxin, the second generation of Bt cotton, expressing Cry1Ac and Cry2Ab, was introduced. Understanding the mode action of the Cry proteins in the insect midgut and the mechanisms insects use to overcome these toxins plays a crucial role in developing measures to counter them. Extensive studies have been conducted on the receptors of Cry1A toxins, but relatively little has been done about those of Cry2Ab. By showing the nonfunctional binding of cadherin protein with Cry2Ab, we have furthered the understanding of Cry2Ab receptors.

Anti-idiotypic single-chain variable fragment antibody partially mimic the functionally spatial structure of Cry2Aa toxin

The anti-idiotypic antibody is widely used in the field of immunology to simulate structural features or even induce the biological activity of antigens. In this study, we obtained seven anti-idiotypic single-chain variable fragments (scFv) antibodies of Cry2Aa toxin from a phage-displayed mutant library constructed using error-prone PCR technique. A mutant designated 2-12B showed the best binding ability amongst all anti-idiotypic scFv isolates to Plutella xylostella brush border membrane vesicles (BBMVs). 2-12B and Cry2Aa toxin shared a potential receptor of polycalin in P. xylostella BBMVs. Homology modeling and molecular docking demonstrated that 2-12B and Cry2Aa toxin have seven common binding amino acid residues in polycalin. Insect bioassay results suggested that 2-12 had insecticidal efficacy against P. xylostella larvae. These results indicated that the Cry2Aa anti-idiotypic scFv antibody 2-12B partially mimicked the structure and function of Cry2Aa toxin. The anti-idiotypic scFv antibody provides the basic material for the future study of surrogate molecules or new insecticidal materials.

Comparative gut transcriptome analysis of Diatraea saccharalis in response to the dietary source

The sugarcane borer (Diatraea saccharalis, Fabricius, 1794) is a devastating pest that causes millions of dollars of losses each year to sugarcane producers by reducing sugar and ethanol yields. The control of this pest is difficult due to its endophytic behavior and rapid development. Pest management through biotechnological approaches has emerged in recent years as an alternative to currently applied methods. Genetic information about the target pests is often required to perform biotechnology-based management. The genomic and transcriptomic data for D. saccharalis are very limited. Herein, we report a tissue-specific transcriptome of D. saccharalis larvae and a differential expression analysis highlighting the physiological characteristics of this pest in response to two different diets: sugarcane and an artificial diet. Sequencing was performed on the Illumina HiSeq 2000 platform, and a de novo assembly was generated. A total of 27,626 protein-coding unigenes were identified, among which 1,934 sequences were differentially expressed between treatments. Processes such as defence, digestion, detoxification, signaling, and transport were highly represented among the differentially expressed genes (DEGs). Furthermore, seven aminopeptidase genes were identified as candidates to encode receptors of Cry proteins, which are toxins of Bacillus thuringiensis used to control lepidopteran pests. Since plant-insect interactions have produced a considerable number of adaptive responses in hosts and herbivorous insects, the success of phytophagous insects relies on their ability to overcome challenges such as the response to plant defences and the intake of nutrients. In this study, we identified metabolic pathways and specific genes involved in these processes. Thus, our data strongly contribute to the knowledge advancement of insect transcripts, which can be a source of target genes for pest management.

Bacillus thuringiensis Cry1Da_7 and Cry1B.868 Protein Interactions with Novel Receptors Allow Control of Resistant Fall Armyworms, Spodoptera frugiperda (J.E. Smith)

Two new modified Bacillus thuringiensis (Bt) proteins, Cry1Da_7 and Cry1B.868, with activity against fall armyworms (FAW), Spodoptera frugiperda (J.E. Smith), were evaluated for their potential to bind new insect receptors compared to proteins currently deployed as plant-incorporated protectants (PIPs) in row crops. Results from resistant insect bioassays, disabled insecticidal protein (DIP) bioassays, and cell-based assays using insect cells expressing individual receptors demonstrate that receptor utilizations of the newly modified Cry1Da_7 and Cry1B.868 proteins are distinct from each other and from those of commercially available Bt proteins such as Cry1F, Cry1A.105, Cry2Ab, and Vip3A. Accordingly, these two proteins target different insect proteins in FAW midgut cells and when pyramided together should provide durability in the field against this economically important pest.IMPORTANCE There is increased concern with the development of resistance to insecticidal proteins currently expressed in crop plants, especially against high-resistance-risk pests such as fall armyworm (FAW), Spodoptera frugiperda, a maize pest that already has developed resistance to Bacillus thuringiensis (Bt) proteins such as Cry1F. Lepidopteran-specific proteins that bind new insect receptors will be critical in managing current Cry1F-resistant FAW and delaying future resistance development. Results from resistant insect assays, disabled insecticidal protein (DIP) bioassays, and cell-based assays using insect cells expressing individual receptors demonstrate that target receptors of the Cry1Da_7 and Cry1B.868 proteins are different from each other and from those of commercially available Bt proteins such as Cry1F, Cry1A.105, Cry2Ab, and Vip3A. Therefore, pyramiding these two new proteins in maize will provide durable control of this economically important pest in production agriculture.

High-affinity phage-displayed peptide as a recognition probe for the detection of Cry2Ad2-3

Cry2A is widely used in transgenic crops in combination with Cry1A toxins. The sensitive and robust detection of Cry2A toxin in food and the environment is necessary to monitor the safety of biopesticides. Here, we describe an approach that involves the use of phage-displayed peptide for the detection of Cry2Ad2-3-the main area of Cry2Ad2 insecticidal activity. After four rounds of panning, six positive monoclonal phage particles were obtained. Pep5 with a sequence of ACSYNHNSKCGGG displayed low cross-reactivity with other Cry toxins. The working range of detection for Cry2Ad2-3 toxin standards in the brush border membrane vesicle (BBMV)-peptide sandwich ELISA was 10-50.625 ng mL^-1 and the detection limit (LOD) was 8 ng mL^-1. Molecular insight into the interaction of pep5 with Cry2Ad2-3 was gleaned using homology modeling and docking. Molecular docking results showed that high-affinity peptide tended to dock in the groove between the two domains of Cry2Ad2-3. The interactions within the toxin-pep5 complex were due to hydrogen bond and hydrophobic interaction. Pep5 also lead us to trap the binding region. Therefore, peptides may be a cost-efficient alternative for detecting Cry toxins and studying their mechanisms.

Disabled insecticidal proteins: A novel tool to understand differences in insect receptor utilization

The development of insect resistance to pesticides via natural selection is an acknowledged agricultural issue. Likewise, resistance development in target insect populations is a significant challenge to the durability of crop traits conferring insect protection and has driven the need for novel insecticidal proteins (IPs) with alternative mechanism of action (MOA) mediated by different insect receptors. The combination or "stacking" of transgenes encoding different insecticidal proteins in a single crop plant can greatly delay the development of insect resistance, but requires sufficient knowledge of MOA to identify proteins with different receptor preferences. Accordingly, a rapid technique for differentiating the receptor binding preferences of insecticidal proteins is a critical need. This article introduces the Disabled Insecticidal Protein (DIP) method as applied to the well-known family of three-domain insecticidal proteins from Bacillus thuringiensis and related bacteria. These DIP's contain amino acid substitutions in domain 1 that render the proteins non-toxic but still capable of competing with active proteins in insect feeding assays, resulting in a suppression of the expected insecticidal activity. A set of insecticidal proteins with known differences in receptor binding (Cry1Ab3, Cry1Ac.107, Cry2Ab2, Cry1Ca, Cry1A.105, and Cry1A.1088) has been studied using the DIP method, yielding results that are consistent with previous MOA studies. When a native IP and an excess of DIP are co-administered to insects in a feeding assay, the outcome depends on the overlap between their MOAs: if receptors are shared, then the DIP saturates the receptors to which the native protein would ordinarily bind, and acts as an antidote whereas, if there is no shared receptor, the toxicity of the native insecticidal protein is not inhibited. These results suggest that the DIP methodology, employing standard insect feeding assays, is a robust and effective method for rapid MOA differentiation among insecticidal proteins.

Specific Binding Protein ABCC1 Is Associated With Cry2Ab Toxicity in Helicoverpa armigera

A pyramid strategy combining the crystal (Cry) 1A and 2A toxins in Bacillus thuringiensis (Bt) crops are active against many species of insects and nematode larvae. It has been widely used to delay pest adaption to genetically modified plants and broaden the insecticidal spectrum in many countries. Unfortunately, Cry2A can also bind with the specific receptor proteins of Cry1A. ATP-binding cassette (ABC) transporters can interact with Cry1A toxins as receptors in the insect midgut, and ABC transporter mutations result in resistance to Bt proteins. However, there is limited knowledge of the ABC transporters that specifically bind to Cry2Ab. Here, we cloned the ABCC1 gene in Helicoverpa armigera, which expressed at all larval stages and in nine different tissues. Expression levels were particularly high in fifth-instar larvae and Malpighian tubules. The two heterologously expressed HaABCC1 transmembrane domain peptides could specifically bind to Cry2Ab with high affinity levels. Moreover, transfecting HaABCC1 into the Spodoptera frugiperda nine insect cell significantly increased its mortality when exposed to Cry2Ab in vitro, and silencing HaABCC1 in H. armigera by RNA interference significantly reduced the mortality of larvae exposed to Cry2Ab in vivo. Altogether current results suggest that HaABCC1 serves as a functional receptor for Cry2Ab.

Genetically modified soybean expressing insecticidal protein (Cry1Ac): Management risk and perspectives

A Bt soybean has been recently developed, thus, efficiently regulating the populations of major lepidopteran pests. However, in other cases, these benefits have been reduced or lost because of the rapid evolution of pest resistance to the Bt toxins in transgenic crops. When pest populations are exposed to Bt crops and to refuges (non- Bt plants), the evolution of resistance is governed by the fitness of resistant individuals relative to susceptible individuals for both the presence and absence of Bt toxins. One major ecological concern regarding the biosafety of Bt crops on the environment is their potential effects on non-target organisms, especially predators and parasitoids that play an important role in pest control. This information is important for supporting insect resistance management (IRM) programs and for improving agricultural practices in a crop production system with Bt plants. Before the use of Bt plants for insect pest control in Brazil is adopted, IRM programs should be established to ensure the sustainability of this technology for integrated pest management (IPM). This review presents data on Bt soybean and lepidopteran pests as well as on the importance of natural enemies as a form of biological control, and applications for IPM and IRM.