Optimizing Bacillus thuringiensis (Bt) protein concentration in cotton: Coordinated regulation of exogenous amino acids and EDTA to reduce spatiotemporal variability in boll and leaf toxin

During the boll formation stage, cotton bolls exhibit the lowest expression of insecticidal proteins. Insect resistance varies notably among different organs, posing challenges for controlling cotton bollworms. Consequently, a strategy was designed in the 2020-2021 cotton growing season to coordinate the enhancement of protein synthesis and the attenuation of degradation. Two Bt cultivars of G. hirsutum, namely the hybrid Sikang 3 and the conventional Sikang 1, were used as test materials. Three treatments were implemented: CK (the control), T1 (amino acids), and T2 (amino acids and EDTA). These treatments were applied at the peak flowering period. The results show that, in comparison to the CK group, Bt protein content significantly increased both cotton bolls and their subtending leaves under T1 and T2 treatments. The maximum increase observed was 67.5% in cotton bolls and 21.7% in leaves. Moreover, the disparity in Bt protein content between cotton bolls and their subtending leaves notably decreased by 31.2%. Correlation analysis suggests that the primary physiological mechanisms for augmenting Bt protein content involve increased protein synthesis and decreased protein catabolism, independent of Bt gene expression levels. Stepwise regression and path analysis reveal that elevating soluble protein content and transaminase activity, while reducing catabolic enzyme activity, are instrumental in enhancing Bt protein content. Consequently, the coordinated regulation of amino acids and EDTA emerges as a strategy capable of improving the overall resistance of Bt cotton and mitigating spatiotemporal variations in Bt toxin concentrations in both cotton bolls and leaves.