Nitrogen deficit decreases seed Cry1Ac endotoxin expression in Bt transgenic cotton

The spatial and temporal expression of insecticidal gene Cry1Ac in transgenic Bacillus thuringiensis (Bt) cotton varies with plant organs, plant age, as well as environmental conditions. The research was undertaken to study the impact of nitrogen deficit on Cry1Ac endotoxin concentration in seed in Yangtze River valley region in China in 2015 and 2016. To uncover the underlying mechanism, the nitrogen metabolism process including protein synthesis and degradation was characterized. Based on the results, nitrogen deficit decreased the boll weight, boll volume, boll number per plant, seed Cry1Ac endotoxin concentration, glutamate oxaloacetate transaminase (GOT) and glutamic pyruvic transaminase (GPT) activities, soluble protein concentration, but increased peptidase and protease activities, and free amino acid content in seed. Our results suggested that the decline of seed Cry1Ac endotoxin expression associated with decreased nitrogen was due to the enhanced protein decomposition and reduced protein synthesis, especially the altered activities of GOT and peptidase. There was a significant negative correlation between seed Cry1Ac endotoxin concentration and boll shell Cry1Ac endotoxin content under nitrogen deficiency. Therefore, seed Cry1Ac endotoxin concentration and boll shell Cry1Ac endotoxin concentration should be balanced to guarantee the insecticidal efficiency.