PI3K/Akt/CncC signaling pathway mediates the response to EPN-Bt infection in Holotrichia parallela larvae

BACKGROUND

Combining the entomopathogenic nematode (EPN), Heterorhabditis beicherriana LF strain, and Bacillus thuringiensis (Bt) HBF-18 strain is a practical strategy to manage the larvae of Holotrichia parallela Motschulsky (white grubs). However, the mechanisms underlying the larval defense response to this combined biocontrol strategy are unknown.

RESULTS

The activities of some antioxidant enzymes (SOD, POD, CAT) and some detoxifying enzymes (AChE, P-450, CarE, GST) in grubs showed an activation-inhibition trend throughout the EPN-Bt exposure time course. Eight potentially key antioxidant and detoxifying enzyme genes in response to EPN-Bt infection were identified from the midgut of grubs through RNA sequencing. After silencing CAT, CarE18, and GSTs1, the enzyme activities were significantly decreased by 30.29%, 68.80%, and 34.63%, respectively. Meanwhile, the mortality of grubs was increased by 18.40%, 46.30%, and 42.59% after exposure to EPN-Bt for 1 day. Interestingly, the PI3K/Akt signaling pathway was significantly enriched in KEGG enrichment analysis, and the expression levels of phosphatidylinositol 3-kinase (PI3K), protein kinase B (Akt), cap 'n' collar isoform-C (CncC), kelch-like ECH-associated protein 1 (Keap1), and CarE18 were all up-regulated when exposed to EPN-Bt for 1 day. Furthermore, RNAi-mediated PI3K silencing showed a similar down-regulated trend between PI3K/Akt/CncC and CarE18. Moreover, silencing PI3K rendered grubs more susceptible to EPN-Bt and accelerated symbiotic bacteria multiplication in grubs.

CONCLUSION

These results suggest that the PI3K/Akt/CncC pathway mediates the expression of CarE18 and participates in the defense response of H. parallela larvae against EPN-Bt infection. Our data provide valuable insights into the design of appropriate management strategies for this well-known agricultural pest. © 2022 Society of Chemical Industry.

Inheritance and Fitness Costs of Vip3Aa19 Resistance in Mythimna separata

The “high-dose/refuge” strategy is expected to work most effectively when resistance is inherited as a functionally recessive trait and the fitness costs associated with resistance are present. In the present study, a laboratory selected Mythimna separata strain that have evolved >634.5-fold resistance to Vip3Aa19 was used to determine the mode of inheritance. To determine if fitness costs were associated with the resistance, life history parameters (larva stage, pupa stage, pupal weight, adult longevity and fecundity) of resistant (RR), -susceptible (SS) and heterozygous (R♂S♀ and R♀S♂) strains on nontoxic diet were assayed. The LC50 values of R♀S♂ were significantly higher than that of R♂S♀ (254.58 μg/g vs. 14.75 μg/g), suggesting that maternal effects or sex linkage were present. The effective dominance h of F1 offspring decreased as concentration increased, suggesting the resistance was functionally dominant at low concentration and recessive at high concentration. The analysis of observed and expected mortality of the progeny from a backcross suggested that more than one locus is involved in conferring Vip3Aa19 resistance. The results showed that significant differences in many life history traits were observed among the four insect genotypes. In short, resistance to Vip3Aa19 in M. separata was inherited as maternal and multigene and the resistance in the strain was associated with significant fitness costs. The results described here provide useful information for understanding resistance evolution and for developing resistance management strategies.

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BACKGROUND

The fall armyworm Spodoptera frugiperda is a major agricultural pest that has invaded the East Hemisphere since 2016, generating a serious threat to food security worldwide including Africa and Asia. The Cry toxins produced by Bacillus thuringiensis (Bt) have been shown to be effective against this insect pest. In different insect ABC transporters (ABCC2 or ABCC3) have been shown to be involved as receptors of some Cry1 toxins. Here we analyzed the role of SfABCC2 and SfABCC3 in the toxicity of Cry1Fa and Cry1Ab toxins in this insect pest.

RESULTS

Two S. frugiperda SfABCC2 and SfABCC3 knockout strains, coding for potential functional Bt receptors, were created using CRISPR/Cas9 genome editing system. Both knockout strains showed resistance to both Cry1Fa and Cry1Ab toxins compared with the susceptible strain. SfABCC2 knockout strain showed higher resistance to both Cry toxins than SfABCC3 knockout strain, suggesting a major role of SfABCC2 in the mode of action of these Cry toxins. In addition, expression of SfABCC2 and SfABCC3 genes in Trichoplusia ni Hi5 cells also increased the susceptibility to Cry1Ab and Cry1Fa toxins, in agreement with the genome editing results. The double knockout of SfABCC2 and SfABCC3 strain was not viable in contrast to other lepidopteran species. Furthermore, we report here that SfABCC2 or SfABCC3 knockout strains increased their susceptibility to abamectin and spinosad insecticides.

CONCLUSION

We provide functional evidence that in S. frugiperda these two ABCC transporters serve as receptors of Bt Cry1Fa and Cry1Ab toxins. © 2020 Society of Chemical Industry.

Aminopeptidase-N from the Helicoverpa armigera (Hubner) brush border membrane vesicles as a receptor of Bacillus thuringiensis crylac delta-endotoxin

Brush border membrane vesicles (BBMVs) were prepared from the 2nd instar larvae of Helicoverpa armigera. Binding of the activated Cry1Ac of Bacillus thuringiensis (Bt) toxin was shown by immunoblot. A 120-kDa protein was identified as a receptor for the Cry1Ac type delta-endotoxin. The aminopeptidase-N activity of BBMVs was measured as the hydrolysis of L-leucine p-nitroanilide. The specific activity was 35 units/mg protein. The BBMV preparation also showed low level of alkaline phosphatase activity. Zn++ chelating agents 2,2'-dipyridyl and 1,10-phenanthroline inhibited aminopeptidase activity at 10 mM concentration, indicating the presence of zinc-dependent aminopeptidase in the brush border of H. armigera. The aminopeptidase activity was increased with increasing concentration of delta-endotoxin. The purified 120-kDa binding protein was N-terminally sequenced. The first 10-amino-acid sequence showed 60-77% similarity with human cysteine-rich secretory protein-1 precursor, inhibin alpha chain precursor. Salmonella flagellar hook protein and yeast carboxypeptidase S.

Comparison of Bacillus thuringiensis with other bacillus species based on enzymoserological examinations of their proteolytic enzymes

Investigated strains of Bacillus thuringiensis were all good producers of casein precipitating proteinases (CP-enzymes).

CP-enzymes produced by five strains of B. thuringiensis were serologically compared with enzymes produced by strains of B. cereus, B. licheniformis and B. subtilis. The technique used is a special enzymoserological procedure by which the proteolytic activity is neutralized by specific antisera. A close enzymoserological relationship was demonstrated between B. thuringiensis and B. cereus, while no interspecies cross-reactions occurred between enzymes of these species, and those of B. licheniformis and B. subtilis.

The possibility of undetected transference of B. thuringiensis from bacteriologically treated plant material to animals is discussed in relation to the high degree of similarity between B. cereus and B. thuringiensis.