Bt Research 2024, Vol.15, No.2, 76-86 http://microbescipublisher.com/index.php/bt 78 2.3 Plasmid diversity in Bt strains The diversity of plasmids in Bt strains is extensive, with each strain potentially harboring multiple plasmids of varying sizes and genetic compositions. The Bt isolate T414, for example, contains a chromosome and 15 different types of plasmids, each carrying various insecticidal genes such as cry1Aa, cry1Ab, cry1Ac, and vip3Aa (Reyaz et al., 2019). Similarly, the Bt strain Bt-UNVM_94 has a plasmid that encodes Cry7Ga2 and Mpp2Aa3 toxins, providing dual insecticidal activity against lepidopteran and coleopteran pests (Peralta et al., 2021). This diversity is further exemplified by the presence of cryptic plasmids in Bt strain YBT-1520, which contain putative toxin-antitoxin systems that contribute to plasmid stability and maintenance (Liu et al., 2008). 3 Types of Plasmid-encoded Toxins Bacillus thuringiensis (Bt) is renowned for its production of various plasmid-encoded toxins that are pivotal in its role as a bioinsecticide. These toxins are categorized based on their structure and target specificity. The primary types of plasmid-encoded toxins include Cry, Cyt, Vip, and other less common toxins. 3.1 Cry toxins Cry toxins are the most extensively studied and utilized group of Bt toxins. These proteins form parasporal crystals during sporulation and exhibit insecticidal activity by binding to specific receptors in the insect midgut, leading to cell lysis and death. Cry toxins are highly diverse, with numerous subtypes targeting different insect orders. Cry1A toxins bind specifically to cadherin receptors in the midgut of Lepidoptera, such as Manduca sexta, and are crucial for the entomopathogenicity of Bt (Dorsch et al., 2002). Cry4Aa and Cry4Ba toxins are mosquito-active toxins that form pores in the target membranes, with specific residues like His180 playing a critical role in their biotoxicity (Bourchookarn et al., 2021). The Cry34Ab1/Cry35Ab1 binary toxin is a novel binary toxin specific for the western corn rootworm, forming ion channels in lipid membranes and demonstrating a unique mode of action (Masson et al., 2004). Cry19B toxin, a newly identified class of delta-endotoxin from Bt serovar higo, shows specificity towards Culex pipiens molestus. 3.2 Cyt toxins Cyt toxins, such as Cyt1Aa and Cyt2Ba, are another important group of Bt toxins. These proteins synergize with Cry toxins, enhancing their insecticidal activity and overcoming resistance mechanisms. Cyt1Aa toxin functions as a membrane-bound receptor for Cry11Aa, enhancing its binding and toxicity. This synergistic interaction is crucial for the effectiveness of Bt subsp. israelensis against mosquito larvae (Pérez et al., 2005). Another example of synergy is the coexpression of CytA and CryIVD genes in Bt, which results in parasporal inclusions that are highly toxic to mosquito larvae, with CytA enhancing the toxicity of CryIVD. 3.3 Vip toxins Vip (Vegetative Insecticidal Proteins) toxins are secreted during the vegetative growth phase of Bt and have a different mode of action compared to Cry and Cyt toxins. They target a broader range of insect pests and are valuable in integrated pest management strategies. Vip3A toxins are effective against a wide range of Lepidoptera and are used in transgenic crops to provide protection against pests that are resistant to Cry toxins (Figure 2) (Pardo-López et al., 2013). 3.4 Other plasmid-encoded toxins In addition to Cry, Cyt, and Vip toxins, Bt produces other plasmid-encoded toxins that contribute to its insecticidal properties. Chitinase and cellulase, enzymes encoded on large plasmids, enhance the bioactive properties of Bt strains by degrading the insect cuticle and facilitating toxin penetration (Fayad et al., 2020). Bacitracin, an antibiotic produced by some Bt strains, also contributes to their overall bioactivity and potential use in biocontro (Fayad et al., 2020). In summary, the diverse array of plasmid-encoded toxins in Bacillus thuringiensis, including Cry, Cyt, Vip, and other bioactive molecules, underscores its versatility and effectiveness as a bioinsecticide. Understanding the specific roles and interactions of these toxins is crucial for developing new strategies to combat insect resistance and enhance the efficacy of Bt-based products.
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