Inhibition of protein degradation increased Bt protein concentration in Bt cotton

Abstract

Bt cotton production is mainly challenged by two problems, i.e., the low concentration of Bacillus thuringiensis (Bt) protein at boll setting stage and its lowest insect resistance in bolls of all organs. Therefore, increasing Bt protein concentration at boll stage, especially in bolls, has become the main goal in increasing insect resistance in cotton. In this study, two protein degradation inhibitors (EDTA and Leupeptin) were sprayed on the bolls, subtending leaves, and whole cotton plants respectively at peak flowering stage of two Bt cultivars (medium maturation Sikang 1 (SK1) and early maturation Zhongmian 425 (Z425)) in 2019 and 2020, respectively. The Bt protein content and protein degradation metabolism were investigated. The results showed that the Bt protein concentration enhanced by 21.3 to 38.8%, 25.0 to 38.6% in treated bolls of SK1 and ZM425 respectively, while decreasing in the subtending leaves of these treated bolls. In the treated leaves, the Bt protein concentration increased by 7.6 to 23.5%, 11.2 to 14.9% for SK1 and Z425, respectively. The combined application of EDTA and Leupeptin to the whole cotton plants increased the Bt protein concentration in both bolls and subtending leaves. The Bt protein concentration in bolls was higher, increasing by 22.5 to 31.0%, 19.6 to 32.5% for SK1 and ZM425 respectively. The organs treated with EDTA or/and Leupeptin showed reduced free amino acid content, protease and peptidase activities and a significant enhancing in soluble protein content. These results indicated that inhibiting of protein degradation could improve protein content, thus increasing the Bt protein concentration in bolls or/and leaves of cotton plants. Therefore, the increase in the Bt protein concentration without yield reduction suggested that the two protein degradation inhibitors may applicable for insect resistance in cotton production.