Midgut juice components affect pore formation by the Bacillus thuringiensis insecticidal toxin Cry9Ca

Processing Properties and Potency of Bacillus thuringiensis Cry Toxins in the Rice Leaffolder Cnaphalocrocis medinalis (Guenée)

Different Cry toxins derived from Bacillus thuringiensis (Bt) possess different insecticidal spectra, whereas insects show variations in their susceptibilities to different Cry toxins. Degradation of Cry toxins by insect midgut extracts was involved in the action of toxins. In this study, we explored the processing patterns of different Cry toxins in Cnaphalocrocis medinalis (Lepidoptera: Crambidae) midgut extracts and evaluated the impact of Cry toxins degradation on their potency against C. medinalis to better understand the function of midgut extracts in the action of different Cry toxins. The results indicated that Cry1Ac, Cry1Aa, and Cry1C toxins could be degraded by C. medinalis midgut extracts, and degradation of Cry toxins by midgut extracts differed among time or concentration effects. Bioassays demonstrated that the toxicity of Cry1Ac, Cry1Aa, and Cry1C toxins decreased after digestion by midgut extracts of C. medinalis. Our findings in this study suggested that midgut extracts play an important role in the action of Cry toxins against C. medinalis, and the degradation of Cry toxins by C. medinalis midgut extracts could reduce their toxicities to C. medinalis. They will provide insights into the action of Cry toxins and the application of Cry toxins in C. medinalis management in paddy fields.

Midgut transcriptomal response of the rice leaffolder, Cnaphalocrocis medinalis (Guenée) to Cry1C toxin

Cnaphalocrocis medinalis (Guenée) is one of the important insect pests in rice field. Bt agents were recommended in the C. medinalis control and Bt rice is bred as a tactic to control this insect. However, the tolerance or resistance of insect to Bt protein is a main threat to the application of Bt protein. In order to investigate the response of C. medinalis transcriptome in defending a Cry1C toxin, high-through RNA-sequencing was carried in the C. medinalis larvae treated with and without Cry1C toxin. A total of 35,586 high-quality unigenes was annotated in the transcriptome of C. medinalis midgut. The comparative analysis identified 6,966 differently expressed unigenes (DEGs) between the two treatments. GO analysis showed that these genes involved in proteolysis and extracellular region. Among these DEGs, carboxylesterase, glutathione S-transferase and P450 were differently expressed in the treated C. medinalis midgut. Furthermore, trypsin, chymotrypsin, and carboxypeptidase were identified in DEGs, and most of them up-regulated. In addition, thirteen ABC transporters were downregulated and three upregulated in Cry1C-treated C. medinalis midgut. Based on the pathway analysis, antigen processing and presentation pathway, and chronic myeloid leukemia pathway were significant in C. medinalis treated with Cry1C toxin. These results indicated that serine protease, detoxification enzymes and ABC transporter, antigen processing and presentation pathway, and chronic myeloid leukemia pathway may involved in the response of C. medinalis to Cry1C toxin. This study provides a transcriptomal foundation for the identification and functional characterization of genes involved in the toxicity of Bt Cry protein against C. medinalis, and provides potential clues to the studies on the tolerance or resistance of an agriculturally important insect pest C. medinalis to Cry1C toxin.

The distinct properties of natural and GM cry insecticidal proteins

The Cry toxins are a family of crystal-forming proteins produced by the bacterium Bacillus thuringiensis. Their mode of action is thought to be to create pores that disrupt the gut epithelial membranes of juvenile insects. These pores allow pathogen entry into the hemocoel, thereby killing the insect. Genes encoding a spectrum of Cry toxins, including Cry mutants, Cry chimaeras and other Cry derivatives, are used commercially to enhance insect resistance in genetically modified (GM) crops. In most countries of the world, such GM crops are regulated and must be assessed for human and environmental safety. However, such risk assessments often do not test the GM crop or its tissues directly. Instead, assessments rely primarily on historical information from naturally occurring Cry proteins and on data collected on Cry proteins (called 'surrogates') purified from laboratory strains of bacteria engineered to express Cry protein. However, neither surrogates nor naturally occurring Cry proteins are identical to the proteins to which humans or other nontarget organisms are exposed by the production and consumption of GM plants. To-date there has been no systematic survey of these differences. This review fills this knowledge gap with respect to the most commonly grown GM Cry-containing crops approved for international use. Having described the specific differences between natural, surrogate and GM Cry proteins this review assesses these differences for their potential to undermine the reliability of risk assessments. Lastly, we make specific recommendations for improving risk assessments.

Effects of Periplocoside P from Periploca sepium on the Midgut Transmembrane Potential of Mythimna separata Larvae

Periplocoside P (PSP) isolated from the root bark of Periploca sepium contains a pregnane glycoside skeleton and possesses high insecticidal properties. Preliminary studies indicated that PSP disrupts epithelial functions in the midgut of lepidopteran larvae. In the present study, we examined the effects of PSP on the apical and basolateral membrane voltages, V_a and V_bl, respectively, of cells from (1) midguts isolated from the larvae of the oriental armyworm Mythimna separata that were in vitro incubated with toxins and (2) midguts isolated from M. separata larvae force-fed with PSP. We compared the effects of PSP with the effects of the Bacillus thuringiensis toxin Cry1Ab and inactive periplocoside E (PSE) on the midgut epithelial cells. The results showed that V_a rapidly decreased in the presence of PSP in a time- and dose-dependent manner, similar to the effects of Cry1Ab. By contrast, PSE did not affect the V_a and V_bl. Additionally, PSP did not influence the V_bl. Given these results, we speculate that PSP may modulate transport mechanisms at the apical membrane of the midgut epithelial cells by inhibiting the V-type H⁺ ATPase.

Different Effects of Bacillus thuringiensis Toxin Cry1Ab on Midgut Cell Transmembrane Potential of Mythimna separata and Agrotis ipsilon Larvae

Bacillus thuringiensis (Bt) Cry toxins from the Cry1A family demonstrate significantly different toxicities against members of the family Noctuidae for unknown reasons. In this study, membrane potential was measured and analyzed in freshly isolated midgut samples from Mythimna separata and Agrotis ipsilon larvae under oral administration and in vitro incubation with Bt toxin Cry1Ab to elucidate the mechanism of action for further control of these pests. Bioassay results showed that the larvae of M. separata achieved a LD50 of 258.84 ng/larva at 24 h after ingestion; M. separata larvae were at least eightfold more sensitive than A. ipsilon larvae to Cry1Ab. Force-feeding showed that the observed midgut apical-membrane potential (V(am)) of M. separata larvae was significantly depolarized from -82.9 ± 6.6 mV to -19.9 ± 7.2 mV at 8 h after ingestion of 1 μg activated Cry1Ab, whereas no obvious changes were detected in A. ipsilon larvae with dosage of 5 μg Cry1Ab. The activated Cry1Ab caused a distinct concentration-dependent depolarization of the apical membrane; V(am) was reduced by 50% after 14.7 ± 0.2, 9.8 ± 0.4, and 7.6 ± 0.6 min of treatment with 1, 5, and 10 μg/mL Cry1Ab, respectively. Cry1Ab showed a minimal effect on A. ipsilon larvae even at 20 μg/mL, and V(am) decreased by 26.3% ± 2.3% after 15 min. The concentrations of Cry1Ab displayed no significant effect on the basolateral side of the epithelium. The V(am) of A. ipsilon (-33.19 ± 6.29 mV, n = 51) was only half that of M. separata (-80.94 ± 6.95 mV, n = 75). The different degrees of sensitivity to Cry1Ab were speculatively associated with various habits, as well as the diverse physiological or biochemical characteristics of the midgut cell membranes.