Current Insights on Vegetative Insecticidal Proteins (Vip) as Next Generation Pest Killers

In Vivo and In Vitro Interactions between Exopolysaccharides from Bacillus thuringensis HD270 and Vip3Aa11 Protein

Bacillus thuringiensis (Bt) secretes the nutritional insecticidal protein Vip3Aa11, which exhibits high toxicity against the fall armyworm (Spodoptera frugiperda). The Bt HD270 extracellular polysaccharide (EPS) enhances the toxicity of Vip3Aa11 protoxin against S. frugiperda by enhancing the attachment of brush border membrane vesicles (BBMVs). However, how EPS-HD270 interacts with Vip3Aa11 protoxin in vivo and the effect of EPS-HD270 on the toxicity of activated Vip3Aa11 toxin are not yet clear. Our results indicated that there is an interaction between mannose, a monosaccharide that composes EPS-HD270, and Vip3Aa11 protoxin, with a dissociation constant of Kd = 16.75 ± 0.95 mmol/L. When EPS-HD270 and Vip3Aa11 protoxin were simultaneously fed to third-instar larvae, laser confocal microscopy observations revealed the co-localization of the two compounds near the midgut wall, which aggravated the damage to BBMVs. EPS-HD270 did not have a synergistic insecticidal effect on the activated Vip3Aa11 protein against S. frugiperda. The activated Vip3Aa11 toxin demonstrated a significantly reduced binding capacity (548.73 ± 82.87 nmol/L) towards EPS-HD270 in comparison to the protoxin (34.96 ± 9.00 nmol/L). Furthermore, this activation diminished the affinity of EPS-HD270 for BBMVs. This study provides important evidence for further elucidating the synergistic insecticidal mechanism between extracellular polysaccharides and Vip3Aa11 protein both in vivo and in vitro.

Cross-pollination in seed-blended refuge and selection for Vip3A resistance in a lepidopteran pest as detected by genomic monitoring

The evolution of pest resistance to management tools reduces productivity and results in economic losses in agricultural systems. To slow its emergence and spread, monitoring and prevention practices are implemented in resistance management programs. Recent work suggests that genomic approaches can identify signs of emerging resistance to aid in resistance management. Here, we empirically examined the sensitivity of genomic monitoring for resistance management in transgenic Bt crops, a globally important agricultural innovation. Whole genome resequencing of wild North American Helicoverpa zea collected from non-expressing refuge and plants expressing Cry1Ab confirmed that resistance-associated signatures of selection were detectable after a single generation of exposure. Upon demonstrating its sensitivity, we applied genomic monitoring to wild H. zea that survived Vip3A exposure resulting from cross-pollination of refuge plants in seed-blended plots. Refuge seed interplanted with transgenic seed exposed H. zea to sublethal doses of Vip3A protein in corn ears and was associated with allele frequency divergence across the genome. Some of the greatest allele frequency divergence occurred in genomic regions adjacent to a previously described candidate gene for Vip3A resistance. Our work highlights the power of genomic monitoring to sensitively detect heritable changes associated with field exposure to Bt toxins and suggests that seed-blended refuge will likely hasten the evolution of resistance to Vip3A in lepidopteran pests.

Sortase-encoding genes, srtA and srtC, mediate Enterococcus faecalis OG1RF persistence in the Helicoverpa zea gastrointestinal tract

Enterococcus faecalis is a commensal and opportunistic pathogen in the gastrointestinal (GI) tract of mammals and insects. To investigate mechanisms of bacterial persistence in the gastrointestinal tract (GIT), we developed a non-destructive sampling model using Helicoverpa zea, a destructive agricultural pest, as host to study the role of bacterial sortase enzymes in mitigating persistence in the gastrointestinal tract. E. faecalis OG1RF ΔsrtA and E. faecalis OG1RF ΔsrtC, isogenic E. faecalis OG1RF sortase mutants grew similarly under planktonic growth conditions relative to a streptomycin-resistant E. faecalis OG1RFS WT in vitro but displayed impaired biofilm formation under, both, physiological and alkaline conditions. In the H. zea GI model, both mutants displayed impaired persistence relative to the WT. This represents one of the initial reports in which a non-destructive insect model has been used to characterize mechanisms of bacterial persistence in the Lepidopteran midgut and, furthermore, sheds light on new molecular mechanisms employed by diverse microorganisms to associate with invertebrate hosts.

The Prospect of Hydrolytic Enzymes from Bacillus Species in the Biological Control of Pests and Diseases in Forest and Fruit Tree Production

Plant diseases and insect pest damage cause tremendous losses in forestry and fruit tree production. Even though chemical pesticides have been effective in the control of plant diseases and insect pests for several decades, they are increasingly becoming undesirable due to their toxic residues that affect human life, animals, and the environment, as well as the growing challenge of pesticide resistance. In this study, we review the potential of hydrolytic enzymes from Bacillus species such as chitinases, β-1,3-glucanases, proteases, lipases, amylases, and cellulases in the biological control of phytopathogens and insect pests, which could be a more sustainable alternative to chemical pesticides. This study highlights the application potential of the hydrolytic enzymes from different Bacillus sp. as effective biocontrol alternatives against phytopathogens/insect pests through the degradation of cell wall/insect cuticles, which are mainly composed of structural polysaccharides like chitins, β-glucans, glycoproteins, and lipids. This study demonstrates the prospects for applying hydrolytic enzymes from Bacillus sp. as effective biopesticides in forest and fruit tree production, their mode of biocidal activity and dual antimicrobial/insecticidal potential, which indicates a great prospect for the simultaneous biocontrol of pests/diseases. Further research should focus on optimizing the production of hydrolytic enzymes, and the antimicrobial/insecticidal synergism of different Bacillus sp. which could facilitate the simultaneous biocontrol of pests and diseases in forest and fruit tree production.

Screening and characterization of Bacillus thuringiensis isolates for high production of Vip3A and Cry proteins and high thermostability to control Spodoptera spp

Bacillus thuringiensis (Bt) is an entomopathogenic bacterium that produces crystalline (Cry and Cyt) and soluble (vegetative insecticidal proteins or Vips) proteins during the sporulation and vegetative growth phases, respectively. Combining Cry and Vip proteins could delay insect resistance development and exhibit synergistic activity against various insect pests. This study aims to screen Bt isolates collected from Thailand for high Vip3A and Cry protein production levels and high thermostability to control Spodoptera spp. Among the selected Bt isolates with high target protein synthesis, Bt isolate 506 was found to be safe for further biopesticide formulation due to the absence of non-specific metabolite, as determined by the detection of thermo-stable β-exotoxin I based on biological assays and PCR analysis. Bt isolate 506 showed the presence of Cry1A, Cry2A, and Vip3A-type proteins identified as Cry1Aa45, Cry2Aa22, and Vip3A87, respectively. The insecticidal activity of whole culture extracts containing Vip3A and Cry mixtures and culture supernatants containing secreted Vip3A protein was evaluated against the second-instar larvae of S. exigua and S. frugiperda. The Bt isolate 506 showed high toxicity against both insects, and the insecticidal proteins produced by this isolate retained their activity after heating at 50 °C. This Bt isolate is a promising candidate for further development as a biopesticide against lepidopteran pests.

Utilization of a strong promoter combined with the knockout of protease genes to improve the yield of Vip3Aa in Bacillus thuringiensis BMB171

BACKGROUND

Vip3Aa is an insecticidal protein secreted by some Bacillus thuringiensis strains during vegetative growth. It has excellent insecticidal activity, its mechanism of action is different from that of Cry protein, and it can delay the development of pest resistance. To date, Vip3Aa has been widely used in genetically modified Bt crops. However, the secretion of Vip3Aa by industrial production strains is usually very low. Moreover, most of the Vip3Aa in the medium is degraded by proteases, limiting its application as a biopesticide.

RESULTS

We report a novel constitutive strong promoter from B. thuringiensis, P_rsi , which directs the abundant expression of vip3Aa in B. thuringiensis BMB171. Furthermore, to reduce the degradation of Vip3Aa caused by proteases, we constructed B. thuringiensis mutants in which different protease genes were knocked out. We found that the degradation of Vip3Aa was greatly inhibited and its yield was significantly improved in a mutant that lacked all three protease genes.

CONCLUSION

Our results provide a new strategy to enhance the production of Vip3Aa in B. thuringiensis and have reference value for the research and development of novel bioinsecticides. © 2023 Society of Chemical Industry.

Functional characterization of Vip3Aa from Bacillus thuringiensis reveals the contributions of specific domains to its insecticidal activity

Microbially derived, protein-based biopesticides offer a more sustainable pest management alternative to synthetic pesticides. Vegetative insecticidal proteins (Vip3), multidomain proteins secreted by Bacillus thuringiensis, represent a second-generation insecticidal toxin that has been preliminarily used in transgenic crops. However, the molecular mechanism underlying Vip3's toxicity is poorly understood. Here, we determine the distinct functions and contributions of the domains of the Vip3Aa protein to its toxicity against Spodoptera frugiperda larvae. We demonstrate that Vip3Aa domains II and III (DII-DIII) bind the midgut epithelium, while DI is essential for Vip3Aa's stability and toxicity inside the protease-enriched host insect midgut. DI-DIII can be activated by midgut proteases and exhibits cytotoxicity similar to full-length Vip3Aa. In addition, we determine that DV can bind the peritrophic matrix via its glycan-binding activity, which contributes to Vip3Aa insecticidal activity. In summary, this study provides multiple insights into Vip3Aa's mode-of-action which should significantly facilitate the clarification of its insecticidal mechanism and its further rational development.

Insecticidal activity of Bacillus thuringiensis towards Agrotis exclamationis larvae-A widespread and underestimated pest of the Palearctic zone

acillus thuringiensis is an entomopathogenic bacterium commonly used as a bioinsecticide against numerous invertebrate pests. However, the efficacy of this microbe has not yet been determined towards Agrotis exclamationis-a lepidopteran, polyphagous pest, widespread throughout the Palearctic zone. In this work we have detected very low susceptibility of A. exclamationis to B. thuringiensis commercial strains, used as microbial formulations in pest control. To investigate this matter, the biological activity of six selected (Cry1Aa, Cry1Ca, Cry1Ia, Cry2Ab, Cry9Ea and Vip3Aa), heterogously-expressed Bacillus thuringiensis insecticidal proteins has been assessed towards A. exclamationis. Only Cry9Ea and Vip3Aa caused significant mortality in the tested pest species, with LC50 values of 950 and 140 ng/cm2, respectively. The histopathological effects of Cry9Ea and Vip3Aa on A. exclamationis were determined. On the other hand, Cry1- and Cry2-type toxins, which are the main active molecules of the majority of currently-used B. thuringiensis-based biocontrol agents (including the commercial strains tested in this work), did not cause mortality in target insect, but only different levels of growth inhibition. Moreover, in the case of Cry1Ca and Cry1Ia hormesis has been observed-a phenomenon that may be disadvantageous in implementation of these proteins in pest management. The obtained results broaden the existing knowledge regarding B. thuringiensis insecticidal protein target range and depict variable susceptibility of A. exclamationis to different groups of Cry/Vip toxins. This work indicates Cry9Ea and Vip3Aa as good candidates for efficient biological control of A. exclamationis and possibly other Agrotinae and discusses the potential use of Vip3-type and Cry9-type insecticidal proteins as successful bioinsecticides.

Genome profiling of an indigenous Bacillus thuringiensis isolate, T405 toxic against the fall armyworm, Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae)

In this study, we present the molecular and insecticidal characteristics of an indigenous Bt isolate T405 toxic against the maize fall armyworm (FAW), Spodoptera frugiperda. The presence of cry1, cry2 (cry2Aa & cry2Ab) and vip3A1 genes in T405 was confirmed. The SDS-PAGE gel analysis confirmed the occurrence of Cry and Vip proteins with molecular masses of 130, ∼88 and 65 kDa in T405. LC₅₀ estimates of T405 and HD1 were 161.37 and 910.73 μg ml^-1 for neonates whereas, 412.29 and 1014.95 μg ml^-1 correspondingly for 2^nd instar FAW larvae. Scanning Electron Microscopy depicted the existence of bipyramidal, spherical and cubic crystals in T405 spore suspension. The whole genome sequencing and assembly of T405 produced a total of 563 scaffolds with a genome size of 6,673,691 bp. The BLAST similarity search showed that 12 plasmids were distributed in this genome. Genome annotation revealed the presence of 6174 protein coding genes, 13 rRNA and 98 tRNA, in which 6126 genes were completely annotated for their functions through sequence similarity search, domains/motifs identification and gene ontology studies. Further analysis of these genes identified the presence of many insecticidal toxin protein coding genes viz., cry1Ac32, cry1Ab9, cry1Aa6, cry1Ac5, cry1Aa18, cry1Ab8, cry1Ab11, cry2Aa9, cry1Ia40, cry2Aa9, cry1Ia40, cry2Ab35, cyt, vip3Aa7 and tpp80Aa and several additional virulence assisted factors viz., immune inhibitor A, phospholipase C, sphingomyelinase, cell wall hydrolases, chitinase, hemolysin XhlA and seven urease subunit coding genes (ureA, ureB, ureC, ureD, ureE, ureF, ureG) in the annotated genome.

Molecular and Toxicological Characterization of a Bacillus thuringiensis Strain Expressing a Vip3 Protein Highly Toxic to Spodoptera frugiperda (Lepidoptera: Noctuidae)

The characterization of the Bacillus thuringiensis (Berliner) LBIT-418 strain was based on a previous work which indicated its high insecticidal potential. Therefore, toxicological, molecular, and biochemical characterizations were conducted in this work to identify its unique features and its potential to be developed as a bioinsecticide. This strain, originally isolated from a healthy mosquito larva, was identified within the subspecies kenyae by sequencing of the hag gene and by the multilocus sequence typing (MLST) technique. Genes cry1Ac2, cry1Ea3, cry2Aa1 and cry2Ab4, and a cry1Ia were detected in its genome, in addition to a vip3Aa gene. In this research, the latter protein was successfully cloned, expressed, and purified and showed high toxicity towards the fall armyworm, Spodoptera frugiperda (J.E. Smith), fourth instar larvae in bioassays using the microdroplet ingestion technique, estimating an LD50 of 21.38 ng/larva. Additional bioassays were performed using the diet surface inoculation technique of the strain's spore-crystal complex against diamondback moth larvae, Plutella xylostella (Linnaeus), estimating an LC50 of 10.22 ng/cm2. Its inability to produce β-exotoxin was demonstrated by bioassays against the nematode Caenorhabditis elegans Maupas and by HPLC analysis. These results support the high potential of this strain to be developed as a bioinsecticide.

Responses to Bt toxin Vip3Aa by pink bollworm larvae resistant or susceptible to Cry toxins

BACKGROUND

Transgenic crops that make insecticidal proteins from Bacillus thuringiensis (Bt) have revolutionized management of some pests. However, evolution of resistance to Bt toxins by pests diminishes the efficacy of Bt crops. Resistance to crystalline (Cry) Bt toxins has spurred adoption of crops genetically engineered to produce the Bt vegetative insecticidal protein Vip3Aa. Here we used laboratory diet bioassays to evaluate responses to Vip3Aa by pink bollworm (Pectinophora gossypiella), one of the world's most damaging pests of cotton.

RESULTS

Against pink bollworm larvae susceptible to Cry toxins, Vip3Aa was less potent than Cry1Ac or Cry2Ab. Conversely, Vip3Aa was more potent than Cry1Ac or Cry2Ab against laboratory strains highly resistant to those Cry toxins. Five Cry-susceptible field populations were less susceptible to Vip3Aa than a Cry-susceptible laboratory strain (APHIS-S). Relative to APHIS-S, significant resistance to Vip3Aa did not occur in strains selected in the laboratory for > 700-fold resistance to Cry1Ac or both Cry1Ac and Cry2Ab.

CONCLUSIONS

Resistance to Cry1Ac and Cry2Ab did not cause strong cross-resistance to Vip3Aa in pink bollworm, which is consistent with predictions based on the lack of shared midgut receptors between these toxins and previous results from other lepidopterans. Comparison of the Bt toxin concentration in plants relative to the median lethal concentration (LC₅₀ ) from bioassays may be useful for estimating efficacy. The moderate potency of Vip3Aa against Cry1Ac- and Cry2Ab-resistant and susceptible pink bollworm larvae suggests that Bt cotton producing this toxin together with novel Cry toxins might be useful as one component of integrated pest management. © 2022 Society of Chemical Industry.

Coexpressing the Signal Peptide of Vip3A and the Trigger Factor of Bacillus thuringiensis Enhances the Production Yield and Solubility of eGFP in Escherichia coli

Many fusion tags have been developed to improve the expression of recombinant proteins. Besides the translocation of cargo proteins, the signal peptides (SPs) of some secretory proteins, such as the ssTorA and Iasp, have been used as an inclusion body tag (IB-tag) or the recombinant expression enhancer in the cytosol of E. coli. In this study, the approach to utilize the SP of Vip3A (Vasp) from Bacillus thuringiensis (Bt) as a fusion tag was investigated. The results showed that either the Vasp or its predicted N- (VN), H- (VH), and C-regions (VC), as well as their combinations (VNH, VNC, and VHC), were able to significantly enhance the production yield of eGFP. However, the hydrophobic region of the Vasp (VH and/or VC) made more than half of the eGFP molecules aggregated (VeGFP, VHeGFP, VCeGFP, VNHeGFP, VNCeGFP, and VHCeGFP). Interestingly, the addition of the Bt trigger factor (BtTF) led to the neutralization of the negative impact and solubilization of the fusion proteins. Therefore, the coexpression of Vasp or its derivates with the chaperone BtTF could be a novel dual-enhancement system for the production yield and solubility of recombinant proteins. Notably, EcTF was unable to impact the solubility of Vasp or its derivates guided proteins, suggesting its different specificities on the recognition or interaction. Additionally, this study also suggested that the translocation of Vip3 in the host cell would be regulated by the BtTF-involved model.

Vegetative Insecticidal Protein Vip3Aa Is Transported via Membrane Vesicles in Bacillus thuringiensis BMB171

Vegetative insecticidal protein Vip3Aa, secreted by many Bacillus thuringiensis (Bt) strains during the vegetative growth stage, represents the second-generation insecticidal toxin. In recent years, significant progress has been made on its structure and action mechanism. However, how it is translocated across the cytoplasmic membrane into the environment remains a mystery. This work demonstrates that Vip3Aa is not secreted by the General Secretion (Sec) System. To reveal the secretory pathway of Vip3A, we purified the membrane vesicles (MVs) of B. thuringiensis BMB171 and observed by TEM. The size of MVs was determined by the dynamic light scattering method, and their diameter was approximately 40–200 nm, which is consistent with the vesicles in Gram-negative bacteria. Moreover, Vip3A could be detected in the purified MVs by Western blot, and immunoelectron microscopy reveals Vip3A antibody-coated gold particles located in the MVs. After deleting its signal peptide, chitinase B (ChiB) failed to be secreted. However, the recombinant ChiB, whose signal peptide was substituted with the N-terminal 39 amino acids from Vip3A, was secreted successfully through MVs. Thus, this sequence is proposed as the signal region responsible for vesicle transport. Together, our results revealed for the first time that Vip3Aa is transported to the medium via MVs.

Sf-FGFR and Sf-SR-C Are Not the Receptors for Vip3Aa to Exert Insecticidal Toxicity in Spodoptera frugiperda

Simple Summary

The CRISPR/Cas9 gene editing and biochemical analysis show that knocking out the Sf-FGFR or Sf-SR-C gene will not change the sensitivity of Spodoptera frugiperda to Vip3Aa.

Abstract

Vip3Aa is a novel insecticidal protein secreted by Bacillus thuringiensis (Bt) during its vegetative growth stages. It has high insecticidal activity against lepidopteran pests such as Spodoptera frugiperda, and has no cross-resistance with Cry insecticidal proteins. As a new type of insecticide, it plays an important role in controlling agricultural pests. However, the insecticidal mechanism of the Vip3Aa toxin, especially its definite receptors, have not been fully revealed. In this study, the previously reported Vip3Aa receptor genes Sf-FGFR and Sf-SR-C were knocked out separately using the CRISPR/Cas9 system. Bioassay results showed that the sensitivity of these two knockout strains to Vip3Aa were not significantly changed compared to that of the normal strain. The current results are not consistent with the previously reports that Sf-SR-C and Sf-FGFR were the receptors of Vip3Aa in vitro. This suggests that the Sf-SR-C and Sf-FGFR genes we tested may not be critical in the mode of action of Vip3Aa in vivo in Spodoptera frugiperda.

Biotechnological Approaches for Host Plant Resistance to Insect Pests

Annually, the cost of insect pest control in agriculture crosses billions of dollars around the world. Until recently, broad-spectrum synthetic pesticides were considered as the most effective means of pest control in agriculture. However, over the years, the overreliance on pesticides has caused adverse effects on beneficial insects, human health and the environment, and has led to the development of pesticide resistant insects. There is a critical need for the development of alternative pest management strategies aiming for minimum use of pesticides and conservation of natural enemies for maintaining the ecological balance of the environment. Host plant resistance plays a vital role in integrated pest management but the development of insect-resistant varieties through conventional ways of host plant resistance takes time, and is challenging as it involves many quantitative traits positioned at various loci. Biotechnological approaches such as gene editing, gene transformation, marker-assisted selection etc. in this direction have recently opened up a new era of insect control options. These could contribute towards about exploring a much wider array of novel insecticidal genes that would otherwise be beyond the scope of conventional breeding. Biotechnological interventions can alter the gene expression level and pattern as well as the development of transgenic varieties with insecticidal genes and can improve pest management by providing access to novel molecules. This review will discuss the emerging biotechnological tools available to develop insect-resistant engineered crop genotypes with a better ability to resist the attack of insect pests.

Identification of Helicoverpa armigera promoters for biotechnological applications

Helicoverpa armigera and Helicoverpa zea are highly polyphagous major agricultural pests with a global distribution. Their control is based on insecticides, however, new, effective, and environmentally friendly control tools are required to be developed and validated. In an effort to facilitate the development of advanced biotechnological tools in these species that will take advantage of new powerful molecular biology techniques like CRISPR/Cas9, we used available transcriptomic data and literature resources, in order to identify RNA polymerase II and III promoters active in RP-HzGUT-AW1(MG), a midgut derived cell line from Helicoverpa zea. Following functional analysis in insect cell lines, four RNA polymerase II promoters from the genes HaLabial, HaTsp-2A, HaPtx-I and HaCaudal were found to exhibit high transcriptional activity in vitro. The HaTsp-2A promoter did not exhibit any activity in the non-midgut derived cell lines Sf-9 and Hi-5 despite high sequence conservation among Lepidoptera, suggesting that it may function in a gut specific manner. Furthermore, considering the utility of RNA polymerase III U6 promoters in methodologies such as RNAi and CRISPR/Cas9, we identified and evaluated four different U6 promoters of H. armigera. In vitro experiments based on luciferase and GFP reporter assays, as well as in vivo experiments targeting an essential gene of Helicoverpa, indicate that these U6 promoters are functional and can be used to experimentally silence or knockout target genes through the expression of shRNAs and sgRNAs respectively. Taking our findings together, we provide a set of promoters useful for the genetic manipulation of Helicoverpa species, that can be used in various applications in the context of agricultural biotechnology.

Critical domains in the specific binding of radiolabelled Vip3Af insecticidal protein to brush border membrane vesicles from Spodoptera spp. and cultured insect cells

Vegetative insecticidal proteins (Vip3) from Bacillus thuringiensis have been used, in combination with Cry proteins, to better control insect pests and as a strategy to delay the evolution of resistance to Cry proteins in Bt crops (crops protected from insect attack by the expression of proteins from B. thuringiensis). In this study, we have set up the conditions to analyze the specific binding of ¹²⁵I-Vip3Af to Spodoptera frugiperda and Spodoptera exigua brush border membrane vesicles (BBMV). Heterologous competition binding experiments revealed that Vip3Aa shares the same binding sites with Vip3Af, but that Vip3Ca does not recognize all of them. As expected, Cry1Ac and Cry1F did not compete for Vip3Af binding sites. By trypsin treatment of selected alanine-mutants, we were able to generate truncated versions of Vip3Af. Their use as competitors with ¹²⁵I-Vip3Af indicated that only those molecules containing domains I to III (DI-III and DI-IV) were able to compete with the trypsin-activated Vip3Af protein for binding, and that molecules only containing either domain IV or domains IV and V (DIV and DIV-V) were unable to compete with Vip3Af. These results were further confirmed with competition binding experiments using ¹²⁵I-DI-III. In addition, the truncated protein ¹²⁵I-DI-III also bound specifically to Sf21 cells. Cell viability assays showed that the truncated proteins DI-III and DI-IV were as toxic to Sf21 cells as the activated Vip3Af, suggesting that domains IV and V are not necessary for the toxicity to Sf21 cells, in contrast to their requirement in vivo. IMPORTANCE This study shows that Vip3Af binding sites are fully shared with Vip3Aa, only partially shared with Vip3Ca, and not shared with Cry1Ac and Cry1F in two Spodoptera spp. Truncated versions of Vip3Af revealed that only domains I to III were necessary for the specific binding, most likely because they can form the functional tetrameric oligomer and because domain III is supposed to contain the binding epitopes. In contrast to results obtained in vivo (bioassays against larvae), domains IV and V are not necessary for the ex vivo toxicity to Sf21 cells.

Early Warning of Resistance to Bt Toxin Vip3Aa in Helicoverpa zea

Evolution of resistance by pests can reduce the benefits of crops genetically engineered to produce insecticidal proteins from Bacillus thuringiensis (Bt). Because of the widespread resistance of Helicoverpa zea to crystalline (Cry) Bt toxins in the United States, the vegetative insecticidal protein Vip3Aa is the only Bt toxin produced by Bt corn and cotton that remains effective against some populations of this polyphagous lepidopteran pest. Here we evaluated H. zea resistance to Vip3Aa using diet bioassays to test 42,218 larvae from three lab strains and 71 strains derived from the field during 2016 to 2020 in Arkansas, Louisiana, Mississippi, Tennessee, and Texas. Relative to the least susceptible of the three lab strains tested (BZ), susceptibility to Vip3Aa of the field-derived strains decreased significantly from 2016 to 2020. Relative to another lab strain (TM), 7 of 16 strains derived from the field in 2019 were significantly resistant to Vip3Aa, with up to 13-fold resistance. Susceptibility to Vip3Aa was significantly lower for strains derived from Vip3Aa plants than non-Vip3Aa plants, providing direct evidence of resistance evolving in response to selection by Vip3Aa plants in the field. Together with previously reported data, the results here convey an early warning of field-evolved resistance to Vip3Aa in H. zea that supports calls for urgent action to preserve the efficacy of this toxin.

Microbial Toxins in Insect and Nematode Pest Biocontrol

Invertebrate pests, such as insects and nematodes, not only cause or transmit human and livestock diseases but also impose serious crop losses by direct injury as well as vectoring pathogenic microbes. The damage is global but greater in developing countries, where human health and food security are more at risk. Although synthetic pesticides have been in use, biological control measures offer advantages via their biodegradability, environmental safety and precise targeting. This is amply demonstrated by the successful and widespread use of Bacillusthuringiensis to control mosquitos and many plant pests, the latter by the transgenic expression of insecticidal proteins from B. thuringiensis in crop plants. Here, I discuss the prospects of using bacterial and fungal toxins for pest control, including the molecular basis of their biocidal activity.

Molecular characterization of a Bacillus thuringiensis strain from Argentina, toxic against Lepidoptera and Coleoptera, based on its whole-genome and Cry protein analysis

The insecticidal proteins of Bacillus thuringiensis are used in formulations of spore-crystal complexes and their genes have been incorporated into several crops, providing a model for genetic engineering in agriculture. Despite the variability of the Cry proteins described so far, it is still necessary to look for toxins with a broad spectrum of action, since a significant number of pests are not controlled with the available Cry proteins. It is also important to provide alternatives to address the problem of insect resistance, which has already appeared with the use of formulations and with transgenic plants that express cry genes that code for insecticidal proteins. The FCC 7 strain was characterized by the ultrastructural parasporal body under optical and electronic microscopy, and for the detection of Cry8-type proteins by genomic and proteomic approaches. The identity of the strain and the presence of putative toxin encoding genes, and virulence factors analyzed by Illumina Miseq 1500 platform genomic sequencing, was confirmed. The identity of the two Cry8 proteins that make up the parasporal body was confirmed by MALDI-TOF/TOF. To expand knowledge about the insecticidal activity of this strain, we conducted preliminary tests against the cotton boll weevil, Anthonomus grandis. Here we report the characterization of a novel B. thuringiensis isolate native to Argentina (FCC 7) toxic against A. grandis. The strain shows a rounded parasporal body harboring mainly a protein of about 140 kDa and two different types of Cry8 proteins. Through whole-genome sequencing, we identified the presence of two cry8-like crystal protein genes, one vpa-like and two vpb-like genes, and multiple virulence factors, deepening the knowledge of a strain that had already been described as toxic against some lepidopterans and coleopterans, including Spodoptera frugiperda, Anticarsia gemmatalis, Tenebrio molitor and Diabrotica speciosa.

Toxicity of insecticidal proteins from entomopathogenic bacteria to Galleria mellonella larvae

Entomopathogenic bacteria have great potential in insect control in the agricultural production because they produce a large variety of protein toxins that can kill their hosts by damaging the insect midgut. However, the mechanisms on how these toxins or specific insecticidal proteins act on insects are very diverse and elusive. Here we select Galleria mellonella larvae as the host to explore the effects of insecticidal proteins on the activities of three protective enzymes (SOD, POD, and CAT) and on the morphology of the midgut tissues. As a result, the activities of the three enzymes consistently increased and then decreased when the host was injected with the insecticidal proteins from the entomopathogenic bacterium Enterobacter cloacae. Moreover, the microscopy analysis showed that tissues, cells, and organelles of the host midgut are all diseased after uptake of the insecticidal proteins. Remarkably, the protein toxins contributed to the deformation of the midgut, blackening of the midgut surface, dissolution of cell membrane, shrinkage of cell nucleus, and chromatin condensation. Our findings will advance the explanation of G. mellonella pathogenesis caused by the insecticidal proteins.

Bacillus thuringiensis Toxins: Functional Characterization and Mechanism of Action

Bacillus thuringiensis (Bt)-based products are the most successful microbial insecticides to date [...].