Enhancing seed yield and nitrogen use efficiency of Brassica napus L. under low nitrogen by overexpression of G-proteins from Arabidopsis

Heterotrimeric G-proteins, composed of G alpha, G beta, and G gamma subunits, are involved in the regulation of multiple signaling pathways in plants. OsDEP1 (a G gamma subunit) of rice and TaNBP1 (a G beta subunit) of wheat are homologs of Arabidopsis AGG3 and AGB1, respectively, which are regulators of grain size and also involved in nitrogen responses. However, the function of Arabidopsis G-proteins in nitrogen utilization under different nitrogen conditions has not been fully investigated. In this study, to evaluate the role of Arabidopsis G-proteins in yield and nitrogen use efficiency (NUE), overexpression transgenic lines AtGPA1, AtAGB1 together with AtAGG1 (AGB1-AGG1), AtAGB1 together with AtAGG2 (AGB1-AGG2), and AtAGB1 together with AtAGG3 (AGB1-AGG3) were created in Brassica napus 'K407'. Analysis of multiple transgenic B. napus lines showed that overexpression of GPA1, AGB1-AGG1, AGB1-AGG2, or AGB1-AGG3 led to increased biomass of seedling plants, including a well-developed root system, and increased nitrogen uptake under low and high nitrogen conditions. The activity of glutamine synthetase, a key nitrogen assimilating enzyme, and the expression levels of genes that are involved in nitrogen uptake and assimilation were significantly increased in overexpression plants under the low nitrogen condition. These properties enabled overexpression plants to increase the number of seeds per silique by 12-27% only under the low nitrogen condition, effectively improving yield per plant by 9-69% and NUE by 7-49%. These results reveal roles of G-proteins in regulating seed traits and NUE, and provide a strategy that can substantially improve crop yield and NUE. Overexpression in Brassica napus of Arabidopsis G-protein genes increases biomass of seedling plants under low and high nitrogen conditions, and increases nitrogen use efficiency and yield under low nitrogen condition.