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

Abstract

High-strength ammonium (NH₄⁺), 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 NH₄⁺. 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 NH₄⁺. Herein, the responses of M. aquaticum to GS inhibition by 0.1 mM methionine sulfoximine (MSX) under 15 mM NH₄⁺ were investigated. After 5 days, visible NH₄⁺ toxicity symptoms were observed in MSX-treated plants. Compared with the control, the NH₄⁺ 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 NH₄⁺ accumulation via promoting NH₄⁺ uptake and decreasing NH₄⁺ assimilation in M. aquaticum. This study revealed for the first time that GS inhibition under high NH₄⁺ condition can lead to phytotoxicity in M. aquaticum due to NH₄⁺ accumulation. The physiological and molecular responses of the leaves, stems, and roots confirmed the importance of GS in the high NH₄⁺ tolerance of M. aquaticum. These findings provide new insights into NH₄⁺ tolerance mechanisms in M. aquaticum and a theoretical foundation for the phytoremediation of high NH₄⁺-loaded swine wastewater.

Keywords: Amino acids reprogramming; Ammonium accumulation; Glutamine synthetase; Methionine sulfoximine; Myriophyllum aquaticum; Phytoremediation.