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The team of Professor Wu Yidong from the school of plant protection, Nanjing Agricultural University has made important progress in the study of Bt Insecticidal Mechanism, and found a new "double channel" Insecticidal Mechanism of Bt insecticidal protein to cotton bollworm.
On April 1, the results were officially published in PLoS patterns magazine, with Dr. Wang Jing as the first author and Professor Wu Yidong as the corresponding author.
Bt toxin is a kind of insecticidal protein with significant activity to cotton bollworm. Transgenic Bt cotton has been planted in China since 1997. In recent years, the frequency of individuals resistant to Bt insecticidal protein Cry1Ac increased gradually. Therefore, it is an important basis for early warning of Bt resistance, formulation of resistance control strategies and development of new techniques to overcome Bt resistance to clarify Bt Insecticidal Mechanism and Bt resistance mechanism of cotton bollworm.
After ingestion of Bt toxin, Bt toxin recognizes receptors on the microvilli of midgut epithelial cells of cotton bollworm and interacts with a series of receptor proteins to form a permeable pore on the midgut cell membrane, causing midgut cells to break and fall off, and the larvae stop feeding and die. One of the important mechanisms of cotton bollworm to deal with Bt protein is the loss of receptor function, which makes the efficiency of Bt toxin perforation decrease or can not be perforated, leading to the loss of Bt toxin insecticidal activity.
According to the latest research of Professor Wu Yidong's team, both ABCC2 and ABCC3 are BT receptors, which can't get Bt resistance by knocking out the two genes respectively with CRISPR gene editing technology; meanwhile, after knocking out the two genes at the same time, they can get a very high level of resistance of more than 15000 times. This means that knocking out the two genes at the same time will make the Bt toxin completely ineffective in attacking the cotton bollworm.
Wu Yidong explained that ABCC2 and ABCC3 are a pair of BT receptors with highly similar structure and overlapping functions. When Bt toxin seeks for receptors to launch an attack, it is equivalent to acquiring a "double channel" deep into the enemy camp. Therefore, the absence of any receptor in ABCC2 and ABCC3 does not affect the insecticidal effect of BT, which limits the resistance evolution ability of cotton bollworm in ABCC2 and ABCC3 pathways.
There is a complex relationship of mutual adaptation and coevolution between the attack and defense of cotton bollworm and Bt toxin. Under the suppression of Bt toxin to the "double channel" killing mechanism of cotton bollworm, cotton bollworm can avoid its edge, and develop a new resistance mechanism in the weak link of Bt toxin attack. In the early research of Professor Wu Yidong's team, two resistance mechanisms have been found to weaken Bt Insecticidal ability of Helicoverpa armigera: one is that the BT receptor hacad (a cadherin) of Helicoverpa armigera is mutated through gene deletion, the other is that the tetratransmembrane protein tspan1 is mutated through l31s point. In these two cases, Helicoverpa armigera has either lost the receptor function of hacad or enhanced intestinal repair ability, The Bt resistance was significantly enhanced.
The team's research also found that the resistance gene carried by field resistant individuals of cotton bollworm in China was mainly hacad mutation before 2010 and tspan1 point mutation after 2013, and ABCC2 and ABCC3 mutations had not been detected in the field. The reason may be that this research revealed that ABCC2 and ABCC3, a pair of functional redundant receptors, provided parallel for the attack of Bt toxin "Double channel", therefore, binds the "hands and feet" of the cotton bollworm that use the mutation of the two receptors to escape the attack.
The discovery of Bt toxin "double channel" insecticidal mechanism not only provides a new perspective for the study of BT action mechanism and pest resistance mechanism, but also provides enlightenment for the development of new resistance control strategies, which means that the new generation of Bt cotton can form a "multi-channel" Insecticidal Mechanism for cotton bollworm by aggregating multiple Bt Insecticidal Genes, thus effectively delaying the progress of Bt resistance of cotton bollworm Rate of change.
Links to published papers:
Functional redundancy of two ABC transporter proteins in mediating toxicity of Bacillus thuringiensis to cotton bollworm. PLoS Pathogens 2020, 16: e1008427
https://doi.org/10.1371/journal.ppat.1008427
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