Identification of midgut membrane proteins from different instars of Helicoverpa armigera (Lepidoptera: Noctuidae) that bind to Cry1Ac toxin

Influence of Cry1Ab protein on growth and development of a predatory spider, Pardosa pseudoannulata, from protective perspectives

The expression of Cry proteins in genetically modified rice varieties safeguards the crop from lepidopteran pests. These proteins have the potential to be transferred through the food chain to arthropods like planthoppers and predatory spiders, triggering defensive responses in these unintended organisms. Hence, we hypothesized that Cry protein might influence the growth and development of spiders by altering protective enzyme activities. The results showed that Cry1Ab protein could accumulate in tissues and subcellular organelles of Pardosa pseudoannulata from Nilaparvata lugens. Cry1Ab protein exposure prolonged the developmental duration in the 5th and 7th instar spiderlings but induced no alterations of other growth indicators, such as body length, median ocular area, and survival rate. In addition, Cry1Ab protein exerted no adverse impacts on several detoxifying enzymes (i.e., superoxide dismutase, catalase, glutathione peroxidase, and acetylcholine esterase) in muscle, midgut, ganglia, and hemolymph at subcellular components (i.e., microsome and cytoplasm). To further explore the effects of Cry1Ab protein on the spiderlings, we performed an integrated transcriptome analysis on spiderlings exposed to Cry1Ab protein. The results showed that Cry1Ab protein might prolong the development duration of P. pseudoannulata via the altered cuticle metabolism (e.g., chitin metabolic process and structural constituent of cuticle). In addition, the gene expression profile associated with detoxifying enzymes and three stress-responsive pathways (JAK/STAT, JNK/SAPK, and Hippo pathways) also displayed no significant alterations under Cry1Ab exposure. Collectively, this integrated analysis generates multidimensional insights to assess the effects of Cry1Ab protein on non-target spiders and demonstrates that Cry1Ab protein exerts no toxicity in P. pseudoannulata.

Bacillus thuringiensis-based bioinsecticides affect predation of Euborellia annulipes on diamondback moth larvae

Interactions between earwigs and entomopathogens, such as Bacillus thuringiensis (Bt), are still poorly understood. This study tested whether Bt-based bioinsecticides have any effect on the predation of Euborellia annulipes (Lucas) (Dermaptera: Anisolabididae) on Plutella xylostella (L.) (Lepidoptera: Plutellidae), one of the pests with the largest number of cases of use and resistance to Bt. Fourth instar larvae were Bt infected by feeding on collard green leaves treated with Dipel®WG and XenTari®WG at the manufacturer-recommended doses. We used one no-choice condition, in which the predator had access to uninfected or Bt-infected larvae separately, and four free-choice conditions: uninfected vs Dipel®-infected larvae, uninfected vs XenTari®-infected larvae, Dipel®-infected vs XenTari®-infected larvae, and uninfected vs Bt-infected larvae with both bioinsecticides. Uninfected larvae were less consumed than those infected by both Bt-bioinsecticides in the no-choice condition. There was a higher consumption of uninfected over Dipel®-infected larvae in the free-choice condition. Overall, uninfected larvae were preferred over both Bt-based bioinsecticides infected larvae. We also used six different prey densities. The ringlegged earwig's predation rate enhanced as the prey population density increased, but the functional response was not affected by Bt-infection, being type II. The predator invested a low amount of handling time on Bt-fed prey and increased the maximum predation rate. Bt-based bioinsecticides cause effects on E. annulipes predation by altering their feeding preference and some aspects of its predatory behavior. The results of our study provide an important background for understanding interactions between earwigs and Bt. In addition, they can be used for decision making during approaches to integrated P. xylostella management.

RNAi-mediated knockdown of gut receptor-like genes prohibitin and α-amylase altered the susceptibility of Galleria mellonella to Cry1AcF toxin

Background

Due to the prolonged usage of Bt-based biopesticides and Bt-transgenic crops worldwide, insects are continually developing resistance against Cry toxins. This resistance may occur if any mechanistic step in the insecticidal process is disrupted possibly because of the alteration in Cry-receptor binding affinity due to mutation in receptor genes. Compared to other lepidopteran insects, Cry receptor-related research has made asymmetric progress in the model insect Galleria mellonella.

Results

Present study describes the molecular characterization and functional analysis of five Cry toxin receptor-related genes (prohibitin, GLTP, α-amylase, ADAM and UDP-GT) and a gut repair gene (arylphorin) from the gut tissues of G. mellonella. Protein–protein docking analysis revealed that Cry1AcF putatively binds with all the five candidate proteins, suggesting their receptor-like function. These receptor-like genes were significantly overexpressed in the gut tissues of fourth-instar G. mellonella larvae upon early exposure to a sub-lethal dose of Cry1AcF toxin. However, targeted knockdown (by using bacterially-expressed dsRNAs) of these genes led to variable effect on insect susceptibility to Cry1AcF toxin. Insects pre-treated with prohibitin and α-amylase dsRNA exhibited significant reduction in Cry1AcF-induced mortality, suggesting their probable role as Cry receptor. By contrast, insects pre-treated with GLTP, ADAM and UDP-GT dsRNA exhibited no significant decline in mortality. This maybe explained by the possibility of RNAi feedback regulation (as few of the receptors belong to multigene family) or redundant role of GLTP, ADAM and UDP-GT in Cry intoxication process.

Conclusion

Since the laboratory culture of G. mellonella develop Bt resistance quite rapidly, findings of the current investigation may provide some useful information for future Cry receptor-related research in the model insect.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08843-8.

Role of Lectin in the Response of Aedes aegypti Against Bt Toxin

Aedes aegypti is one of the world’s most dangerous mosquitoes, and a vector of diseases such as dengue fever, chikungunya virus, yellow fever, and Zika virus disease. Currently, a major global challenge is the scarcity of antiviral medicine and vaccine for arboviruses. Bacillus thuringiensis var israelensis (Bti) toxins are used as biological mosquito control agents. Endotoxins, including Cry4Aa, Cry4Ba, Cry10Aa, Cry11Aa, and Cyt1Aa, are toxic to mosquitoes. Insect eradication by Cry toxin relies primarily on the interaction of cry toxins with key toxin receptors, such as aminopeptidase (APN), alkaline phosphatase (ALP), cadherin (CAD), and ATP-binding cassette transporters. The carbohydrate recognition domains (CRDs) of lectins and domains II and III of Cry toxins share similar structural folds, suggesting that midgut proteins, such as C-type lectins (CTLs), may interfere with interactions among Cry toxins and receptors by binding to both and alter Cry toxicity. In the present review, we summarize the functional role of C-type lectins in Ae. aegypti mosquitoes and the mechanism underlying the alteration of Cry toxin activity by CTLs. Furthermore, we outline future research directions on elucidating the Bti resistance mechanism. This study provides a basis for understanding Bti resistance, which can be used to develop novel insecticides.

Genome evolution in an agricultural pest following adoption of transgenic crops

Evolution of resistance to management approaches in agricultural landscapes is common and results in economic losses. Early detection of pest resistance prior to significant crop damage would benefit the agricultural community. It has been hypothesized that new genomic approaches could track molecular signals of emerging resistance and trigger efforts to preempt widespread damage. We tested this hypothesis by quantifying genomic changes in the pest Helicoverpa zea over a 15-y period concurrent with commercialization of transgenic Bacillus thuringiensis–expressing crops and their subsequent loss of efficacy. Our results demonstrate the complex nature of evolution in agricultural ecosystems and provide insight into the potential and pitfalls of using genomic approaches for resistance monitoring.

Replacing synthetic insecticides with transgenic crops for pest management has been economically and environmentally beneficial, but these benefits erode as pests evolve resistance. It has been proposed that novel genomic approaches could track molecular signals of emerging resistance to aid in resistance management. To test this, we quantified patterns of genomic change in Helicoverpa zea, a major lepidopteran pest and target of transgenic Bacillus thuringiensis (Bt) crops, between 2002 and 2017 as both Bt crop adoption and resistance increased in North America. Genomic scans of wild H. zea were paired with quantitative trait locus (QTL) analyses and showed the genomic architecture of field-evolved Cry1Ab resistance was polygenic, likely arising from standing genetic variation. Resistance to pyramided Cry1A.105 and Cry2Ab2 toxins was controlled by fewer loci. Of the 11 previously described Bt resistance genes, 9 showed no significant change over time or major effects on resistance. We were unable to rule out a contribution of aminopeptidases (apns), as a cluster of apn genes were found within a Cry-associated QTL. Molecular signals of emerging Bt resistance were detectable as early as 2012 in our samples, and we discuss the potential and pitfalls of whole-genome analysis for resistance monitoring based on our findings. This first study of Bt resistance evolution using whole-genome analysis of field-collected specimens demonstrates the need for a more holistic approach to examining rapid adaptation to novel selection pressures in agricultural ecosystems.

Bacillus thuringiensis Cry1Ab Domain III β-16 Is Involved in Binding to Prohibitin, Which Correlates with Toxicity against Helicoverpa armigera (Lepidoptera: Noctuidae)

Helicoverpa armigera is a major insect pest of several crops worldwide. This insect is susceptible to some Bacillus thuringiensis (Bt) Cry insecticidal proteins expressed in transgenic crops or used in biopesticides. Previously, we identified H. armigera prohibitin (HaPHB) as a Cry1Ac-binding protein. Here, we further analyzed the potential role of PHB as a Cry toxin receptor in comparison to cadherin (CAD), well recognized as a Cry1Ac receptor. HaPHB-2 midgut protein and HaCAD toxin-binding region (TBR) fragment from H. armigera were expressed in Escherichia coli cells, and binding assays with different Cry1 toxins were performed. We demonstrated that Cry1Ab, Cry1Ac, and Cry1Fa toxins bound to HaPHB-2 in a manner similar to that seen with HaCAD-TBR. Different Cry1Ab mutant toxins located in domain II (Cry1AbF371A and Cry1AbG439D) or domain III (Cry1AbL511A and Cry1AbN514A), which were previously characterized and found to be affected in receptor binding, were analyzed regarding their binding interaction with HaPHB-2 and toxicity against H. armigera One β-16 mutant (Cry1AbN514A) showed increased binding to HaPHB-2 that correlated with 6-fold-higher toxicity against H. armigera, whereas the other β-16 mutant (Cry1AbL511A) was affected in binding to HaPHB-2 and lost toxicity against H. armigera Our data indicate that β-16 from domain III of Cry1Ab is involved in interactions with HaPHB-2 and in toxicity. This report identifies a region of Cry1Ab involved in binding to HaPHB-2 from a Lepidoptera insect, suggesting that this protein may participate as a novel receptor in the mechanism of action of the Cry1 toxins in H. armigera IMPORTANCE Helicoverpa armigera is a polyphagous pest that feeds on important crops worldwide. This insect pest is sensitive to different Cry1 toxins from Bacillus thuringiensis In this study, we analyzed the potential role of PHB-2 as a Cry1 toxin receptor in comparison to CAD. We show that different Cry1 toxins bound to HaPHB-2 and HaCAD-TBR similarly and identify β-16 from domain III of Cry1Ab as a binding region involved in the interaction with HaPHB-2 and in toxicity. This report characterized HaPHB-Cry1 binding interaction, providing novel insights into potential target sites for improving Cry1 toxicity against H. armigera.

Functional validation of DvABCB1 as a receptor of Cry3 toxins in western corn rootworm, Diabrotica virgifera virgifera

Western corn rootworm (WCR), Diabrotica virgifera virgifera (Coleoptera: Chrysomelidae), is a serious insect pest in the major corn growing areas of North America and in parts of Europe. WCR populations with resistance to Bacillus thuringiensis (Bt) toxins utilized in commercial transgenic traits have been reported, raising concerns over their continued efficacy in WCR management. Understanding the modes of action of Bt toxins is important for WCR control and resistance management. Although different classes of proteins have been identified as Bt receptors for lepidopteran insects, identification of receptors in WCR has been limited with no reports of functional validation. Our results demonstrate that heterologous expression of DvABCB1 in Sf9 and HEK293 cells conferred sensitivity to the cytotoxic effects of Cry3A toxins. The result was further validated using knockdown of DvABCB1 by RNAi which rendered WCR larvae insensitive to a Cry3A toxin. However, silencing of DvABCB2 which is highly homologous to DvABCB1 at the amino acid level, did not reduce the sensitivity of WCR larvae to a Cry3A toxin. Furthermore, our functional studies corroborate different mode-of-actions for other insecticidal proteins including Cry34Ab1/35Ab1, Cry6Aa1, and IPD072Aa against WCR. Finally, reduced expression and alternatively spliced transcripts of DvABCB1 were identified in a mCry3A-resistant strain of WCR. Our results provide the first clear demonstration of a functional receptor in the molecular mechanism of Cry3A toxicity in WCR and confirmed its role in the mechanism of resistance in a mCry3A resistant strain of WCR.

Comparative Mortality and Adaptation of a Smurf Assay in two Species of Tenebrionid Beetles Exposed to Bacillus thuringiensis

Bacillus thuringiensis is a spore-forming bacterium which infects insect larvae naturally via the oral route. Its virulence factors interact with the epithelium of the digestive tract of insect larvae, disrupting its function and eventually leading to the death of susceptible hosts. The most cited B. thuringiensis killing mechanism is the extensive damage caused to the insect midgut, leading to its leakage. The mortality caused by B. thuringiensis has been shown to vary between serovars and isolates, as well as between host life stages. Moreover, whether susceptibility to B. thuringiensis-induced gut leakage is generalized to all host species and whether there is individual variation within species is unclear. In this study, we adapted a non-invasive “Smurf” assay from Drosophila melanogaster to two species of tenebrionid beetles: The mealworm beetle Tenebrio molitor and the red flour beetle Tribolium castaneum, during exposure to B. thuringiensis. We highlight a differential mortality between two age/size classes of T. molitor larvae, as well as different killing dynamics between B. thuringiensis var. tenebrionis and var. tolworthi in T. castaneum. The Smurf assay did not reveal a high occurrence of extensive gut disintegration in both host species upon ingestion during B. thuringiensis exposure.