Bt Research 2024, Vol.15, No.5, 232-239 http://microbescipublisher.com/index.php/bt 233 families. Cry proteins, such as Cry1, Cry2, and Cry9, are well-known for their insecticidal properties and are widely used in transgenic crops to control pests. These proteins typically form parasporal crystals during sporulation and require activation by proteases in the insect midgut to exert their toxic effects (Chakroun et al., 2016). Cyt proteins, although less studied, also contribute to the insecticidal activity of Bt by forming pores in the midgut cells of insects. Vip proteins, or vegetative insecticidal proteins, are secreted during the vegetative growth phase of Bt and include Vip1, Vip2, and Vip3 families. Vip1 and Vip2 act as binary toxins against Coleoptera and Hemiptera, while Vip3 proteins are effective against Lepidoptera. Vip3 proteins share some mechanistic similarities with Cry proteins, such as binding to midgut receptors and forming pores, but they do not share binding sites, making them suitable for combination with Cry proteins to delay resistance development (Gupta et al., 2021). 2.2 Non-Bt insecticidal proteins In addition to Bt toxins, other insecticidal proteins have been identified and utilized for pest control. These include proteins from various bacterial, fungal, and plant sources. For instance, chaperone proteins like Hsp70 and Hsp90 from insects have been shown to enhance the toxicity of Bt Cry proteins by protecting them from degradation and facilitating their binding to midgut receptors (Garcia-Gomez et al., 2023). Proteins such as cadherin and ATP-binding cassette (ABC) transporters in insects play crucial roles in mediating the cytotoxic effects of Bt toxins, often exhibiting synergistic interactions that enhance overall toxicity (Chen et al., 2015). 2.3 General mechanisms of action in target insects The general mechanisms of action of Bt toxins and other insecticidal proteins involve several key steps. Initially, the toxins bind to specific receptors on the midgut epithelial cells of the target insect. For Cry proteins, this binding is often mediated by cadherin and ABC transporters, which facilitate the formation of pores in the cell membrane, leading to cell lysis and insect death. Vip proteins, particularly Vip3, follow a similar pathway but do not share binding sites with Cry proteins, allowing for complementary use in pest management strategies. Synergistic interactions between different Bt toxins, such as Cry and Vip proteins, have been observed, enhancing their insecticidal efficacy. For example, combinations of Cry1Ca and Vip3Aa have shown significant synergistic effects against pests like Spodoptera exiguaand Mythimna separata, leading to higher mortality rates compared to individual toxins (Yang et al., 2018; Baranek et al., 2021). These synergistic effects are crucial for developing new bioinsecticides and transgenic crops with improved pest control capabilities and resistance management. 3 Mechanisms of Synergistic Interactions 3.1 Synergism between Cry and Cyt proteins The synergistic interactions between Cry and Cyt proteins have been well-documented, particularly in the context of controlling mosquito populations. For instance, Cry10Aa and Cyt2Ba, when combined, exhibit a remarkable synergistic mosquitocidal activity against Aedes aegypti larvae, with a synergistic potentiation of 68.6-fold. This interaction is among the most powerful described so far with Bt toxins and is comparable to the synergy observed between Cyt1A and other Cry proteins such as Cry4Aa, Cry4Ba, or Cry11Aa (Valtierra-de-Luis et al., 2020; Wu, 2024). The higher insecticidal activity of the crystal compared to individual toxins is likely due to these synergistic interactions, particularly those involving Cyt1Aa. 3.2 Synergism with Vip and other Bt-associated proteins Vegetative insecticidal proteins (Vip) from Bacillus thuringiensis (Bt) also show significant synergistic interactions with Cry proteins. Vip3 proteins, for example, do not share binding sites with Cry proteins, making them excellent candidates for combination to prevent or delay insect resistance and broaden the insecticidal spectrum (Figure 1) (Chakroun et al., 2016). Studies have shown that combinations of Cry and Vip proteins, such as Cry1Ca and Vip3Aa, exhibit high synergistic activity against pests like Spodoptera exigua, with synergism factors greater than 4 (Baranek et al., 2021). The coexistence of Cry9Aa and Vip3Aa in the same plasmid in certain Bt strains has been shown to result in synergistic insecticidal activity against Chilo suppressalis.
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