Bt Research 2024, Vol.15, No.5, 215-222 http://microbescipublisher.com/index.php/bt 216 specificity towards different insect orders, including Lepidoptera, Diptera, and Coleoptera (Benfarhat-Touzri et al., 2019; Li et al., 2020; Arsov et al., 2023). These toxins typically form parasporal crystals during sporulation, which dissolve in the insect midgut, releasing the active toxin. The structure of Cry toxins generally includes three domains: a receptor-binding domain, a pore-forming domain, and a protease-resistant core (Liu et al., 2022). Cyt toxins, on the other hand, have a different structure and mode of action, often working synergistically with Cry toxins to enhance insecticidal activity (Valtierra-de-Luis et al., 2020). 2.2 Genomic identification of toxin genes The identification of Bt toxin genes has been greatly facilitated by genomic sequencing and PCR-based screening. For instance, the genome of Bt strain BTG was sequenced, revealing the presence of multiple cry genes, including cry1Ab35, cry1Db, cry1Fb, cry1Ib, cry2Ab, cry8Ea1, and cry9Ba, which contribute to its broad-spectrum insecticidal activity. Similarly, the cry1D-250 gene was identified and cloned from a Bt strain with high toxicity against Spodoptera littoralis, highlighting the importance of genomic tools in discovering new insecticidal proteins (Benfarhat-Touzri et al., 2019). These genomic approaches not only help in identifying the presence of toxin genes but also in understanding their distribution and potential for pest control. 2.3 Functional insights intoCry andCyt genes Functional studies have provided significant insights into the roles of Cry and Cyt genes in insecticidal activity. For example, the Cry2A toxin's interaction with the ATP-binding cassette subfamily A member 2 (ABCA2) receptor in Bombyx mori has been elucidated, showing that mutations in ABCA2 confer resistance specifically to Cry2A without affecting susceptibility to other Cry toxins. Additionally, the Cry1Ac toxin's binding to the cadherin receptor BT-R1 in Manduca sexta triggers a signaling cascade leading to insect death, demonstrating the critical role of receptor interactions in Cry toxin function (Chen et al., 2021). The synergistic activity between Cry10Aa and Cyt2Ba toxins against Aedes aegypti larvae underscores the potential of combining different Bt toxins to enhance insecticidal efficacy. These functional insights are crucial for developing new strategies to manage insect resistance and improve the effectiveness of Bt-based biopesticides. 3 Mechanisms of Insecticidal Activity 3.1 Bt toxin interaction with insect midgut receptors The interaction between Bacillus thuringiensis (Bt) toxins and insect midgut receptors is a critical step in the insecticidal process. The Cry1A toxins, including Cry1Aa, Cry1Ab, and Cry1Ac, bind tightly to the cadherin-like receptor BT-R1 in the midgut of the moth Manduca sexta, initiating a signaling cascade that leads to insect death (Liu et al., 2022). This binding is highly specific, with the toxins competing for the same binding site on BT-R1, localized in the 12th cadherin repeat. The C-terminal region of Cry1Ab protoxin has been shown to provide additional binding sites for alkaline phosphatase (ALP) and aminopeptidase N (APN) receptors, enhancing the toxin's binding affinity and toxicity (Peña-Cardeña et al., 2018). The ATP-binding cassette (ABC) transporters, such as ABCC2 and ABCC3, also play a significant role as receptors for Cry toxins, with their expression regulated by the transcription factor FOXA, which modulates insect susceptibility to Cry1Ac toxin (Li et al., 2017). 3.2 Mode of action of different Bt toxins Different Bt toxins exhibit distinct modes of action, which are influenced by their structural domains and receptor interactions. For instance, the Cry4Ba toxin, specific to mosquito larvae, utilizes its C-terminal domain to interact with the membrane-bound alkaline phosphatase receptor in Aedes aegypti, with specific residues like Leu615 playing a crucial role in this interaction (Thammasittirong et al., 2021). The Cry1Ac toxin's mode of action involves binding to multiple receptors, including cadherin and ABC transporters, with synergistic effects observed when both receptor types are present (Chen et al., 2015). Furthermore, chimeric Bt proteins like Cry1A.2 and Cry1B.2 have been engineered to combine domains from different toxins, broadening their insecticidal spectrum and minimizing receptor overlap, thus enhancing their effectiveness against various lepidopteran pests.
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