γ-Aminobutyric acid addition alleviates ammonium toxicity by limiting ammonium accumulation in rice (Oryza sativa) seedlings.

Excessive use of nitrogen (N) fertilizer has increased ammonium (NH ) accumulation in many paddy soils to levels that reduce rice vegetative biomass and yield. Based on studies of NH toxicity in rice (Oryza sativa, Nanjing 44) seedlings cultured in agar medium, we found that NH concentrations above 0.75 mM inhibited the growth of rice and caused NH accumulation in both shoots and roots. Use of excessive NH also induced rhizosphere acidification and inhibited the absorption of K, Ca, Mg, Fe and Zn in rice seedlings. Under excessive NH conditions, exogenous γ-aminobutyric acid (GABA) treatment limited NH accumulation in rice seedlings, reduced NH toxicity symptoms and promoted plant growth. GABA addition also reduced rhizosphere acidification and alleviated the inhibition of Ca, Mg, Fe and Zn absorption caused by excessive NH . Furthermore, we found that the activity of glutamine synthetase/NADH-glutamate synthase (GS; EC 6.3.1.2/NADH-GOGAT; EC1.4.1.14) in root increased gradually as the NH concentration increased. However, when the concentration of NH is more than 3 mM, GABA treatment inhibited NH -induced increases in GS/NADH-GOGAT activity. The inhibition of ammonium assimilation may restore the elongation of seminal rice roots repressed by high NH . These results suggest that mitigation of ammonium accumulation and assimilation is essential for GABA-dependent alleviation of ammonium toxicity in rice seedlings.