Bt Research 2024, Vol.15, No.5, 215-222 http://microbescipublisher.com/index.php/bt 217 3.3 Role of gut microbiota in Bt toxin efficiency The efficiency of Bt toxins can be significantly influenced by the gut microbiota of the target insects. Studies have shown that the presence of specific gut bacteria can enhance the insecticidal activity of Bt toxins. The midgut histochemistry of Tuta absoluta populations with different susceptibility levels to Bt formulations revealed that the gut microbiota plays a role in the insect's response to the toxin. The presence of certain glycosylation patterns, as indicated by lectin labeling, correlates with higher susceptibility to Bt toxins, suggesting that gut microbiota may modulate the expression or availability of toxin receptors (Oliveira et al., 2018). This interaction between gut microbiota and Bt toxins highlights the complexity of the insecticidal process and the potential for leveraging microbiota to enhance Bt toxin efficacy. 4 Genetic Regulation of Bt Toxin Production 4.1 Key regulatory genes involved in toxin synthesis The production of Bacillus thuringiensis (Bt) toxins is a complex process regulated by various genetic and environmental factors. Several key regulatory genes have been identified that play significant roles in the synthesis of Bt toxins. For instance, the Forkhead box protein A (FOXA) transcription factor has been shown to up-regulate the expression of Cry1Ac toxin receptors ABCC2 and ABCC3 in Helicoverpa armigera and Spodoptera litura, thereby enhancing susceptibility to the toxin. Similarly, the GATAe transcription factor in Helicoverpa armigera has been found to increase the transcription of multiple Cry1Ac receptor genes, including cadherin (CAD), ABCC2, and alkaline phosphatase (ALP), across various insect cell lines, thereby inducing toxin susceptibility (Wei et al., 2019). These findings highlight the critical role of transcription factors in regulating the expression of genes essential for Bt toxin activity. 4.2 Environmental and host factors influencing gene expression Environmental and host factors also significantly influence the expression of Bt toxin-related genes. For example, exposure to sub-lethal doses of Cry1AcF toxin in Galleria mellonella larvae led to the upregulation of several Cry receptor genes, including ABC transporter, alkaline phosphatase, aminopeptidase N, and cadherin, in the midgut tissue (Dutta et al., 2022). Additionally, the MAPK signaling pathway has been implicated in the regulation of midgut ALPand ABCCgenes, which are associated with resistance to Cry1Ac toxin in Plutella xylostella (Guo et al., 2015). These studies suggest that both external environmental conditions and internal host regulatory mechanisms can modulate the expression of genes involved in Bt toxin susceptibility. 4.3 Post-Transcriptional Modifications and Toxin Efficiency 4.3.1 mRNA stability and toxin production The stability of mRNA transcripts encoding Bt toxin receptors is vital for sustained toxin production. For instance, the differential expression of ABCC2 and ABCC3 genes, regulated by transcription factors like FOXA and GATAe, can influence the stability and availability of mRNA for translation, thereby affecting the overall production of toxin receptors. 4.3.2 Translational regulation and toxin protein synthesis Translational regulation is another critical factor in Bt toxin production. The presence of specific regulatory elements in the mRNA can influence the efficiency of translation and the synthesis of toxin proteins (Feng, 2024). For example, the expression of Cry1Ac and Cry3A proteins in transgenic Populus × euramericana 'Neva' plants demonstrated differential protein levels, with Cry3A being expressed at much higher levels than Cry1Ac, indicating the role of translational regulation in determining protein abundance (Ren et al., 2021). 4.3.3 Folding and processing of toxin proteins The proper folding and processing of Bt toxin proteins are essential for their insecticidal activity. Proteomic analyses have revealed that the parasporal crystals of Bt strains, such as GR007, are composed of multiple Cry proteins, each requiring specific folding and processing mechanisms to achieve their active forms (Figure 1) (Pacheco et al., 2021). Additionally, the involvement of proteases like trypsin in the activation of Vip3Aa protoxin in Spodoptera litura larvae underscores the importance of post-translational modifications in the functional activation of Bt toxins (Song et al., 2016).
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