Bt Research 2025, Vol.16, No.2, 47-54 http://microbescipublisher.com/index.php/bt 48 pass insecticidal genes to other strains, which can accelerate the genetic diversity and adaptability of Bt (Pacheco et al., 2021). Cryptic plasmids generally do not carry known functional genes, but may help strains survive in the environment or ensure the stable existence of plasmids (Nair et al., 2018). 2.2 Molecular mechanisms of plasmid replication, transfer, and maintenance The replication, transfer and maintenance of Bt plasmids rely on multiple molecular mechanisms. During replication, the plasmid has dedicated replication starting points and regulatory elements to ensure that it can be replicated and distributed during cell division. Conjugative plasmids can also encode transfer proteins, such as Tra proteins, to facilitate the propagation of plasmids between different strains (Pacheco et al., 2021). In addition, the stability of plasmids also relies on the "virus-antitoxin" system and chaperone proteins, so that plasmids are less likely to be lost. Studies have shown that Bt strains often obtain new plasmids through horizontal gene transfer, which makes them more adaptable to different environments and host insects. 2.3 Plasmidome studies: comparative genomics and plasmid evolution in Bt Plasmidome research has found that there are a great many types of plasmids in Bt strains, and the evolutionary process is very complex. Whole genome sequencing and comparative analysis indicated that Bt strains could carry multiple large plasmids, which contained abundant insecticidal protein genes and virulence factors (Pacheco et al., 2021). Plasmid recombination, fusion or gene loss often occurs among different strains, which promotes the expansion of the insecticidal spectrum and also brings new toxins. For instance, some plasmids of Bt strains were recombined to obtain new gene combinations, resulting in stronger effects on various pests (Wang et al., 2020; Pacheco et al., 2021). The diversity of plasmids is closely related to the adaptability of Bt in different environments, and these plasmids also provide rich genetic resources for the development of new biopesticides (Nair et al., 2018). 3 Plasmid-Encoded Traits Relevant to Bt Adaptation 3.1 Insecticidal crystal (Cry/Cyt) proteins and their genetic organization The insecticidal effect of Bt mainly relies on the Cry protein and Cyt protein on the plasmid. Research has found that cry9A and vip3A genes are often in the same plasmid and they are put together through a recombination mechanism. This combination can produce a synergistic insecticidal effect. Not only is the insecticidal effect stronger, but it can also slow down the emergence of pest resistance (Wang et al., 2020). In addition, on the giant plasmid poh1 of the Bt ATCC 10792 strain, there are not only multiple insecticidal genes, but also a gene of a novel crystal protein kinase, which makes the insecticidal methods more diverse (Chelliah et al., 2019). The arrangement and coexistence of these genes provide Bt strains with more efficient insecticidal capabilities and also serve as the genetic basis for their adaptation to the environment and application in biological control. 3.2 Plasmid-borne antibiotic resistance and stress tolerance genes Plasmids are also important vectors of antibiotic resistance genes (ARGs) and tolerance genes. In the plasmids of Bt and other bacteria, there are often genes that can resist the stress of antibiotics, heavy metals, ultraviolet rays, etc. These genes are not only abundant in hospitals or human-related environments, but also common in natural environments such as soil and wastewater (Figure 1) (Palomino et al., 2022; Finks and Martiny, 2023; Gomathinayagam and Muthukaliannan, 2024). For instance, the poh1 plasmid simultaneously contains tetracycline resistance protein and multiple antimicrobial peptide genes, which enables the strain to survive more easily in complex environments (Chelliah et al., 2019). In addition, metabolic and stress-related genes on plasmids can regulate energy utilization and cellular protection mechanisms, thereby indirectly improving antibiotic tolerance. The study also found that when antibiotics and oxidative stress coexist, they promote plasmid conjugated transfer and accelerate the diffusion of resistance genes (Chen et al., 2025). 3.3 Synergistic interactions between chromosomal and plasmid genes There is also a close connection between plasmids and genes on chromosomes. Genes on plasmids and those on chromosomes can jointly regulate cell functions and enhance adaptability through protein-protein interactions (Downing and Rahm, 2022). The global regulatory systems of chromosomes, such as CCR and ArcAB, if mutated,
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