Higher Ammonium Transamination Capacity Can Alleviate Glutamate Inhibition on Winter Wheat (Triticum aestivum L.) Root Growth under High Ammonium Stress.

Feng Wang,Tingbo Dai,Abid Muhammad,Dong Jiang,Zhongwei Tian,Yang Liu,Jingwen Gao,Weixing Cao,Yixuan Zhang,Aimin Zhang

doi:10.1371/journal.pone.0160997

Abstract

Most of the studies about NH4+ stress mechanism simply address the effects of free NH4+, failing to recognize the changed nitrogen assimilation products. The objective of this study was to elucidate the effects of glutamate on root growth under high ammonium (NH4+) conditions in winter wheat (Triticum aestivum L.). Hydroponic experiments were conducted using two wheat cultivars, AK58 (NH4+-sensitive) and Xumai25 (NH4+-tolerant) with either 5 mM NH4+ nitrogen (AN) as stress treatment or 5 mM nitrate (NO3-) nitrogen as control. To evaluate the effects of NH4+-assimilation products on plant growth, 1 μM L-methionine sulfoximine (MSO) (an inhibitor of glutamine synthetase (GS)) and 1 mM glutamates (a primary N assimilation product) were added to the solutions, respectively. The AN significantly reduced plant biomass, total root length, surface area and root volume in both cultivars, but less effect was observed in Xumai25. The inhibition effects were alleviated by the application of MSO but strengthened by the application of glutamate. The AN increased the activities of GS, glutamate dehydrogenase (GDH) in both cultivars, resulting in higher glutamate contents. However, its contents were decreased by the application of MSO. Compared to AK58, Xumai25 showed lower glutamate contents due to its higher activities of glutamic-oxaloacetic transaminase (GOT) and glutamic-pyruvic transaminase (GPT). With the indole-3-acetic acid (IAA) contents decreasing in roots, the ratio of shoot to root in IAA was increased, and further increased by the application of glutamate, and reduced by the application of MSO, but the ratio was lower in Xumai25. Meanwhile, the total soluble sugar contents and its root to shoot ratio also showed similar trends. These results indicate that the NH4+-tolerant cultivar has a greater transamination ability to prevent glutamate over-accumulation to maintain higher IAA transport ability, and consequently promoted soluble sugar transport to roots, further maintaining root growth.

Highlights

Ammonium (NH4+) and nitrate (NO3-) are the two major nitrogen (N) forms uptaken by higher plants
The plant dry biomass significantly increased in the presence of 1 μM methionine sulfoximine (MSO) than in its absence under NH4+ conditions, but MSO had little effect on the growth of plant grown under NO3- conditions
The plant dry biomass of two cultivars was markedly lower after adding glutamate, and under NO3- condition, the decreases were 17.9% in AK58 and 15.8% in Xumai25, while the dry matter inhibition effect

Summary

Introduction

Ammonium (NH4+) and nitrate (NO3-) are the two major nitrogen (N) forms uptaken by higher plants. In China, a large amount of N fertilizer is applied as the base fertilizer, which along with the atmospheric NH4+ deposition and slow-release N fertilizer leads to short term high NH4+ concentrations in the soil, which can exceed up to 20 mM [7], much higher than the optimum NH4+ concentrations (0.1 to 0.5 mM) for the terrestrial crops [8,9]. These contexts of NH4+ in the soil result in high NH4+ stress for the crops. It is necessary to explore the regulatory mechanisms of NH4+ stress on the plant growth to increase the crop growth and yields

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Conclusion