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

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.