Effects of Nitrogen Fertilizer on the Bt Protein Content in Transgenic Rice and Nitrogen Metabolism Mechanism

Transgenic rice with Bacillus thuringiensis (Bt) genes has been successfully cultivated in recent years. The stable and sustainable expression of Bt protein is the guarantee of effective resistance within these strains against target pests. It is well known that the synthesis of Bt protein in Bt crops needs nitrogen (N). Therefore, how is the efficacy of transgenic Bt rice against target pests in response to N application? In our study, we found that enhanced nitrogen inputs increased soluble protein content (SPC) and glutamine synthetase (GS), but decreased amino acid content (AAC); it is evident that the protein synthesis was enhanced, and degradation was reduced with increasing nitrogen application. Thus, both increased protein synthesis and decreased protein degradation contributed to the increased Bt protein content (BTC) in transgenic Bt rice. Responses of the three Bt rice lines to N proportions and N sources were different. All these differences can be explained by the changes of nitrogen metabolism of different genotypes. The BTC in leaves was higher than that in other organs, and the fully expanded green leaf had higher BTC than the senescent leaf and tender leaf. Despite the difference of Bt protein expression, all transgenic rice lines showed 100% resistance to target pests under different N conditions. Our results indicated that the response of Bt protein expression in transgenic Bt rice with different genotypes to N fertilizer was inconsistent; moreover, there exist obvious spatial-temporal differences. Therefore, in order to guarantee the stability and sustainability resistance of Bt rice, it is necessary to adopt appropriate N management practice according to the expression pattern of Bt protein of transgenic Bt rice with different genotypes.