Bt Research 2025, Vol.16, No.5, 194-203 http://microbescipublisher.com/index.php/bt 199 have lower SpoOA activity, delayed or incomplete spore program initiation, which may lead to the inability to express the toxin gene on time. There are also studies that show that there are differences in the level of main σ factor and nutritional response regulation in low-virulence strains, such as CodY is in a state of continuous activation, which inhibits the secondary metabolic pathway (Qi et al., 2015). Third, low-virulent strains may invest more metabolic resources in growth rather than synthesis of virulent products. Comparing the metabolites, it was found that low-virulent strains tend to accumulate more amino acids and biofilm-related metabolites, while high-virulent strains synthesize large quantities of toxin-related precursors. 6 Gene Regulation in Bt Interaction with Host Plants 6.1 Gene regulation mode of Bt survival and expression in plants In agricultural ecosystems, Bt bacteria agents are often applied to the plant surface or soil, and some Bt strains can colonize on the rhizosphere, body surface and even within the tissues of the plant, becoming a member of the plant microbial community. The survival and gene expression of Bt in plant environments is different from that in laboratory cultures or in insects. Plants secrete a variety of compounds that can serve as signals to influence Bt's regulatory network. Studies have shown that after exposure to plant tissue, some Bt strains will upregulate the expression of flagellar and adhesion-related genes, enhancing motility and adhesion in order to colonize the plant surface and roots. The survival of Bt in plants also requires its virility to be controlled to avoid adversely affecting the plant itself. Studies have found that some endogenous Bts reduce the expression of certain strong proteases or toxins, and improve resource utilization and competition for the expression of related genes. This regulation adjustment may be accomplished through molecular sensing pathways of plant signaling, such as sensing signals such as jasmonic acid in plants through a two-component system and regulating downregulation of toxin genes (Dezhabad et al., 2018). 6.2 Case analysis: differences in Bt gene expression regulation in corn and cotton Corn and cotton are two widely grown genetically modified Bt crops. They express insecticidal proteins derived from Bt to resist pests such as corn borer and cotton bollworm. The Bt gene and expression regulation strategies used in these two crops are different, resulting in the different expression characteristics of Bt toxins in plants. Taking insect-resistant corn as an example, many commercial Bt corn varieties introduce the cry1Ab or cry1Ac gene and are usually driven by strong promoters to continuously express the toxins in most tissues of the plant. This ensures that all parts of the corn (leaf, stem, ear) have sufficient toxin content to kill the feeding pests (Figure 2) (Noor et al., 2022). Early insect-resistant cottons mostly used ovary-specific or inducible promoters to drive the Bt gene to reduce expression in cotton fibers and other sites, thereby saving energy and reducing the potential impact on cotton growth. This difference leads to different expression patterns of Bt toxin genes in corn and cotton. The Bt toxin in corn tends to be consistently expressed in the whole plant, while cotton may be highly expressed in the reproductive organs and relatively lower in the nutritional organs. On the other hand, the physiological differences and growth environment of the two crops will also affect the regulation of Bt genes. Some studies have compared the insect-resistant effects of insect-resistant corn and insect-resistant cotton field planting, and found that due to the different types and contents of Bt proteins expressed in corn and cotton, the control effect and resistance development speed of target pests are also different (Machado et al., 2020; Yu et al., 2022a). Cry1Ab is commonly expressed in corn, and there is a cross resistance problem with Cry1Ac expressed in cotton. Cotton bollworms eat Cry1Ac on cotton for a long time, which may produce cross resistance to Cry1Ab on corn, thereby weakening the insect-proof effect of corn. 6.3 The effect of host factors on Bt gene network Factors in the host plant itself will also affect the gene regulatory network of Bt. When Bt interacts with plants, plants release multiple signal molecules that can be sensed by Bt and trigger corresponding regulatory responses. The defensive hormones such as salicylic acid and jasmonic acid produced when plants suffer from harm not only
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