Bt Research 2025, Vol.16, No.4, 157-167 http://microbescipublisher.com/index.php/bt 159 including crystal toxin proteins (Cry) and cytolytic toxin proteins (Cyt), as well as the plant sterile proteins (Vip) secreted during the vegetative phase. Among them, Cry toxin is the most studied and most diverse category, and is usually present in an insoluble prototoxin form of about 130 kDa. When the insects feed on the preparation containing Bt spore crystals, the Cry prototoxin is dissolved in the alkaline environment of the pest's midgut and is cleaved by protease to activate mature toxin fragments of about 60 kDa to 70 kDa. Activated Cry toxin first binds with high affinity to the receptor protein on the surface of the cell membrane of the midgut epithelial cell, and then inserts into the cell membrane and forms holes, resulting in cell lysis and worm death (Figure 1) (Mendoza-Almanza et al., 2020). On the other hand, some studies have proposed a "signaling pathway model", believing that Cry toxin binds to cadherin receptors to trigger apoptosis signaling cascade, leading to programmed cell death. These two mechanisms may coexist in different insect species or different toxins, reflecting the complexity of the Cry toxin action. In terms of structure, a typical three-domain Cry toxin contains three functional domains: Domain I at the N-terminal is a bundle-like structure composed of 7 to 8 alpha helixes, which is the functional domain of the toxin insertion membrane to form a channel; Domain II is formed by three antiparallel β plates folded with multiple variable loop structures, responsible for determining the specific identification of the toxin for receptors; Domain III is a β-sheet interlayer of the Sander structure, which is involved in strengthening binding to the receptor and stabilizing the entire toxin structure (Bravo et al., 2017). Figure 1 Different morphologies of Bacillus thuringiensis crystals (Adopted from Mendoza-Almanza et al., 2020) Image caption: (A) image observed at 40× in an optical microscope, the crystal was stained with malachite green. (B) Image observed at SEM with different magnifications from the left to the right (1) 20,000×, (2) 15,000×, (3) 15,000×, and (4) 50,000×. VC: vegetative cell; CC: cubic crystal; S: spore; BC: bipyramidal crystal; OC: ovoid crystal; SC: spherical crystal (Adopted from Mendoza-Almanza et al., 2020) 3.2 Molecular basis of pest resistance The mechanism by which pests develop resistance to Bt toxins is one of the key scientific issues that currently affect the sustained effectiveness of Bt strains and Bt crops. Studies have shown that the main mechanisms of pests' anti-Bt are concentrated in the following aspects: receptor alteration: This is the most common resistance mechanism. Target insects can weaken the binding efficiency of toxins to receptors by mutation or downregulating Bt toxin receptor genes on the membrane of the midgut epithelial cell. For example, mutations in the cadherin receptor gene were detected in the resistant population of rhodoptera anti-Cry1Ac, resulting in the inability to
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