Abstract
In this article, we review the latest works on the insecticidal mechanisms of Bacillus thuringiensis Cry toxins and the resistance mechanisms of insects against Cry toxins. Currently, there are two models of insecticidal mechanisms for Cry toxins, namely, the sequential binding model and the signaling pathway model. In the sequential binding model, Cry toxins are activated to bind to their cognate receptors in the mid-intestinal epithelial cell membrane, such as the glycophosphatidylinositol (GPI)-anchored aminopeptidases-N (APNs), alkaline phosphatases (ALPs), cadherins, and ABC transporters, to form pores that elicit cell lysis, while in the signaling pathway model, the activated Cry toxins first bind to the cadherin receptor, triggering an extensive cell signaling cascade to induce cell apoptosis. However, these two models cannot seem to fully describe the complexity of the insecticidal process of Cry toxins, and new models are required. Regarding the resistance mechanism against Cry toxins, the main method insects employed is to reduce the effective binding of Cry toxins to their cognate cell membrane receptors by gene mutations, or to reduce the expression levels of the corresponding receptors by trans-regulation. Moreover, the epigenetic mechanisms, host intestinal microbiota, and detoxification enzymes also play significant roles in the insects’ resistance against Cry toxins. Today, high-throughput sequencing technologies like transcriptomics, proteomics, and metagenomics are powerful weapons for studying the insecticidal mechanisms of Cry toxins and the resistance mechanisms of insects. We believe that this review shall shed some light on the interactions between Cry toxins and insects, which can further facilitate the development and utilization of Cry toxins.
Highlights
Bacillus thuringiensis (Bt) is a spore-producing Gram-positive bacterium (Li Z. et al, 2020)
The Cry toxins undergoing mutations that affect oligomerization or pore formation can result in insufficient binding to cadherins, which reduces the insecticidal activity of Cry toxins to produce only partial resistance, indicating that the signaling pathway model may be affected by the sequential binding of Cry toxins with the receptors (Xu et al, 2014; Melo et al, 2016)
Compared with the traditional chemical insecticides, Bt preparations and Bt-transgenic crops are distinguished by their high specificity and environmental safety
Summary
Bacillus thuringiensis (Bt) is a spore-producing Gram-positive bacterium (Li Z. et al, 2020). The middle domain II is uncommon, as it is composed of three antiparallel β-sheets arranged in a circular mode to form a hydrophobic core with some highly variable and exposed loop regions (Figure 1C), which are often suggested to confer the binding specificity of the Cry toxin with the mid-intestinal epithelial cell membrane receptors of target insects (Bravo et al, 2007; Evdokimov et al, 2014).
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