Bt Research 2025, Vol.16, No.4, 157-167 http://microbescipublisher.com/index.php/bt 162 toxic effects on three strains of Fallia meadow that had been resistant to Cry1Fa, Vip3Aa and Cry1Ab/Cry2Ab respectively. More importantly, this chimeric toxin does not show interaction resistance to the above-mentioned resistant insects, indicating that its mode of action may differ from existing toxins (Chae et al., 2022). This result proves that artificial design and synthesis of chimeric genes can create new functional toxins that are not present in natural toxin libraries, providing a new path to solve resistance. Figure 2 Microscope of B. thuringiensis HD-1 and HD-1-ΔhmgAunder oil lens (100 ×). (A) Crystal observation of strain HD-1. (B) Crystal observation of mutant HD-1-ΔhmgA. Both strains have no distinct difference in spore and crystal formation (Adopted from Zhu et al., 2022) 5.2 Research cases of using gene circuits to improve the efficiency of toxin expression In the development of Bt engineering bacteria, not only the type of toxin is important, but how to express it efficiently is also the key. The introduction of gene circuits has opened up new ways to improve toxin expression. A typical case is the group-induced toxin expression circuit designed by Chinese scientific researchers. They introduced the AIP (self-induced peptide) population sensing system element of B. subtilis in the Bt strain and coupled it with the expression of the target toxin gene. When the density of Bt cells increases, the accumulated signal peptide triggers transcriptional activation, thereby greatly increasing the expression of toxin genes. The Cry toxin yield in the middle and late stages of shake flask culture (high density stage) increased by about 30% compared with the control and did not affect early bacterial growth (Dubois et al., 2013; Slamti et al., 2014). This shows that it is feasible and effective to dynamically regulate toxin expression according to the strain growth stage through artificial gene circuits. 5.3 Effects of engineering Bt in field application in resistant pest control After years of research and experiments, several engineered Bt strains have entered the stage of field experiments and even commercial application. One of the representative cases is the Bt engineering strain G033A developed in China. The G033A strain is constructed through genetic engineering methods to integrate multiple insecticidal genes and is also the first Bt-engineering bacteria in China to obtain pesticide registration. This strain shows high toxicity against important Lepidopteran pests such as cotton bollworm and twillus, and is also the first Bt product that is highly effective against Coleopteran pests (such as sweet potato elephant arthros) (Zhan, 2024). Field test results show that the use of G033A preparation to prevent and treat sweet potato elephant aceta, the field correction and prevention effect is more than 85%, which is significantly better than traditional Bt strain preparations (Wang et al., 2020). What is more valuable is that after two generations of continuous application, the density of the target pest population did not rebound significantly, indicating that the broad spectrum and efficiency of the engineered strains have withstood the test under field conditions. Another example comes from the management of resistant bollworms. Some cotton bollworms in North China have become resistant to Cry1Ac, but a research team tested an engineering Bt strain, which not only expresses Cry1Ac, but also co-expresses
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