Bt Research 2025, Vol.16, No.2, 55-62 http://microbescipublisher.com/index.php/bt 58 4.3 Chimeric toxin design for overcoming resistance Chimeric toxins are another strategy. It combates pest resistance by fusing the functional regions of different Bt toxins or combining with other proteins, such as synthetic peptides or non-Bt toxin fragments. For instance, when the non-toxic ricin B chain was fused with Cry1Ac, the killing power of the toxin against various resistant pests was significantly enhanced (Deist et al., 2014). In addition, after removing a certain α -helical region in Cry1A toxin, it can form oligomers and maintain toxicity even in receptor-deficient resistant pests, thereby breaking the dependence on specific receptors (Soberón et al., 2007; Abdullah et al., 2009). These chimeric strategies provide new molecular tools for addressing pest resistance in the field. 5 Synergistic Approaches with Other Technologies 5.1 Bt toxins combined with RNA interference (RNAi) strategies The combined use of Bt toxins and RNAi is regarded as an important direction for addressing pest resistance. RNAi can target the key genes of pests and weaken their defense ability against Bt toxins. This can enhance the insecticidal effect of Bt toxin and also slow down the development of resistance. Although there are no direct experimental data at present, the literature generally holds that combining Bt toxin engineering with other molecular tools is expected to expand the insecticidal range and improve the effect of resistance management (Then, 2009; Deist et al., 2014). 5.2 Integration with beneficial microbes and endophytes for enhanced delivery Beneficial microorganisms and endophytes can also serve as delivery tools to help Bt toxins become more stable in plants and enhance their efficacy in the intestines of insects. Some studies have found that Bt toxins have a synergistic effect with some exogenous factors such as microorganisms and enzymes in the environment, which can enhance the selectivity and efficacy of the toxins. Meanwhile, the combination of Bt toxins and other biological factors also helps to expand the insecticidal spectrum and make field applications more durable (Then, 2009; Deist et al., 2014). 5.3 Stacking of Bt genes with other resistance traits in transgenic crops The stacking of multiple genes is currently the main trend in the improvement of Bt crops. By expressing multiple Bt genes in the same crop or co-expressing Bt genes with other resistance genes (such as protease inhibitors, RNAi elements), the insecticidal range can be significantly expanded and the resistance of pests can also be delayed (Deist et al., 2014). For instance, the commercial genetically modified crop SmartStax is superimposed with multiple Bt toxins. By taking advantage of their synergistic effects, it has achieved efficient control over various types of pests. However, it should also be noted that the synergy may sometimes enhance the toxicity to non-target organisms, so risk assessment is very important (Then, 2009). 6 Case Study: Bt Toxin Engineering for Resistance Management 6.1 Selection of engineered Bt variants tested against resistant insect strains To address the resistance of pests to Bt toxins, researchers have employed a variety of molecular engineering methods to design and screen new variants of Bt toxins. For instance, by using directed evolution and protein engineering, scientists have obtained some novel variants of Cry1Ac. These variants can efficiently bind to novel receptors of resistant insects, such as the cadherin-like receptor of Trichoplusia ni. Among resistant insects that were originally insensitive to Cry1Ac, these variants demonstrated significant insecticidal activity (Badran et al., 2016; Dovrat and Aharoni, 2016). In addition, by removing specific α -helical structures or fusing new peptides, Bt toxins can also bypass the resistance caused by receptor mutations and resume their insecticidal effects (Deist et al., 2014). 6.2 Laboratory and field validation of improved efficacy Both laboratory and field trials have demonstrated that the engineered Bt toxin has a better effect in resistant pest populations. For instance, the Cry1Ac variant obtained through continuous directed evolution has a 335-fold increase in insecticidal efficacy in Cry1AC-resistant Spodoptera litura, and its toxicity to non-resistant insects is similar to that of the wild type (Badran et al., 2016; Dovrat and Aharoni, 2016). Similarly, Cry1A toxin with α
RkJQdWJsaXNoZXIy MjQ4ODYzNA==