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Gamma-Aminobutyric Acid (GABA) Modulates Nitrate Concentrations and Metabolism in the Leaves of Pakchoi (Brassica campestris ssp. chinensis Makino) Treated with a Nitrogen-Rich Solution

Li, Jingrui, Tian, Zhen, Wu, Xiaolei, Lv, Guiyun, Ma, Wanli, Zhang, Ying, Gao, Hongbo
Plant molecular biology reporter 2018 v.36 no.3 pp. 530-542
Brassica rapa subsp. chinensis, ammonium, enzyme activity, gamma-aminobutyric acid, genes, glutamate decarboxylase, glutamate-ammonia ligase, glutamic acid, green leafy vegetables, harvesting, leaves, metabolites, nitrate reductase, nitrate reduction, nitrates, nitrite reductase, nitrites, nitrogen, nutrient solutions, plant growth, protein synthesis
Pakchoi plants were grown in 32 mM NO₃⁻ nutrient solution with or without 2.5 mM γ-aminobutyric acid (GABA) to investigate metabolite changes, gene and protein expression levels, and the activities of key enzymes related to nitrate metabolism in the leaves over a period of 0–12 days. High-nitrogen treatment enhanced plant growth and the NO₃⁻, NO₂⁻, NH₄⁺, Gln, and Glu contents in the leaves; promoted the gene and protein expression of nitrate reductase (NR) and glutamate decarboxylase (GAD); and increased the activities of NR, nitrite reductase (NiR), glutamine synthetase (GS), glutamate synthase (GOGAT), and GAD. The endogenous GABA concentration in the leaves was enhanced in parallel with the increase in GAD activity. The GABA-treated leaves displayed the greatest increases in the gene and protein expression levels of NR and GAD and in the activities of NR, NiR, GS, GOGAT, and GAD. In addition, accelerated rates of nitrate reduction and assimilation were detected, and these changes occurred concurrently with the observed increases in gene or protein expression and enzyme activity. As a result, the concentrations of NH₄⁺, Gln, Glu, and endogenous GABA were significantly elevated, and the NO₃⁻ and NO₂⁻ contents were significantly decreased, in GABA-treated leaves compared with plants exposed to nitrogen-rich conditions. Our results reveal a potential positive that GABA may act as a nitrogen source to improve the plant growth and the most prominent effect of decreasing nitrate contents by accelerating NO₃⁻ reduction and assimilation. Exogenous GABA plays an important role in reducing the NO₃⁻ content of leaves, and thereby improves the ability to harvest leafy vegetables containing higher levels of endogenous GABA.