Glutamine synthetase plays an important role in ammonium tolerance of Myriophyllum aquaticum.

High-strength ammonium (NH), the main characteristic of swine wastewater, poses a significant threat to the rural ecological environment. As a novel phytoremediation technology, Myriophyllum aquaticum wetlands have high tolerance and removal rate of NH. Glutamine synthetase (GS), a pivotal enzyme in nitrogen (N) metabolism, is hypothesized to play an important role in the tolerance of M. aquaticum to high NH. Herein, the responses of M. aquaticum to GS inhibition by 0.1 mM methionine sulfoximine (MSX) under 15 mM NH were investigated. After 5 days, visible NH toxicity symptoms were observed in MSX-treated plants. Compared with the control, the NH accumulation in the leaves increased by 20.99 times, while that of stems and roots increased by 3.27 times and 47.76 %, suggesting that GS inhibition had a greater impact on the leaves. GS inhibition decreased pigments in the leaves by 8.64 %-41.06 %, triggered oxidative stress, and affected ions concentrations in M. aquaticum. The concentrations of glutamine (Gln) and asparagine decreased by 63.46 %-97.43 % and 12.37 %-76.41 %, respectively, while the concentrations of most other amino acids increased after 5 days of MSX treatment, showing that GS inhibition reprogrammed the amino acids synthesis. A decrease in Gln explains the regulations of N-related genes, including increased expression of AMT in roots and decreased expression of GS, GOGAT, GDH, and AS, which would cause further NH accumulation via promoting NH uptake and decreasing NH assimilation in M. aquaticum. This study revealed for the first time that GS inhibition under high NH condition can lead to phytotoxicity in M. aquaticum due to NH accumulation. The physiological and molecular responses of the leaves, stems, and roots confirmed the importance of GS in the high NH tolerance of M. aquaticum. These findings provide new insights into NH tolerance mechanisms in M. aquaticum and a theoretical foundation for the phytoremediation of high NH-loaded swine wastewater.