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

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.

Fig 1

Free NH₄⁺ contents in shoots (A) and roots (B) of wheat plants grown under NO₃^- or NH₄⁺ conditions with applying MSO or glutamate in AK58 (Left) and Xumai25 (Right). Nitrate refers to nitrate-fed plants; Nitrate-MSO refers to nitrate-fed with the application of MSO; Nitrate-Glu refers to nitrate-fed with the application of glutamate; Ammonium refers to ammonium-fed plants; Ammonium-MSO refers to ammonium-fed with the application of MSO; Ammonium-Glu refers to ammonium-fed with the application of glutamate. Lowercase letters refer to significant difference between treatments (P<0.05). Whiskers on the top of the bars indicate standard error (n = 6).

Fig 2

Glutamate concentrations in shoots (A) and roots (B) of wheat plants grown under NO₃^- or NH₄⁺ conditions with applying MSO or glutamate in AK58 (Left) and Xumai25 (Right). Nitrate refers to nitrate-fed plants; Nitrate-MSO refers to nitrate-fed with the application of MSO; Nitrate-Glu refers to nitrate-fed with the application of glutamate; Ammonium refers to ammonium-fed plants; Ammonium-MSO refers to ammonium-fed with the application of MSO; Ammonium-Glu refers to ammonium-fed with the application of glutamate. Lowercase letters refer to significant difference between treatments (P<0.05). Whiskers on the top of the bars indicate standard error (n = 6).

Fig 3

The activity of glutamine synthetase (GS) (A, B, respectively), glutamate dehydrogenase aminating enzyme (NADH-GDH) (C, D, respectively) and glutamate dehydrogenase deaminating enzyme (NAD⁺-GDH) (E, F, respectively) in shoots or roots of wheat plants grown under NO₃^- or NH₄⁺ conditions with applying MSO or glutamate in AK58 (Left) and Xumai25 (Right). Nitrate refers to nitrate-fed plants; Nitrate-MSO refers to nitrate-fed with the application of MSO; Nitrate-Glu refers to nitrate-fed with the application of glutamate; Ammonium refers to ammonium-fed plants; Ammonium-MSO refers to ammonium-fed with the application of MSO; Ammonium-Glu refers to ammonium-fed with the application of glutamate. Lowercase letters refer to significant difference between treatments (P< 0.05). Whiskers on the top of the bars indicate standard error (n = 6).

Fig 4

The activity of glutamic-oxaloacetic transaminase (GOT) (A, B, respectively) and glutamic-pyruvic transaminase (GPT) (C, D, respectively) in shoots or roots of wheat plants grown under NO₃^- or NH₄⁺ conditions with applying MSO or glutamate in AK58 (Left) and Xumai25 (Right). Nitrate refers to nitrate-fed plants; Nitrate-MSO refers to nitrate-fed with the application of MSO; Nitrate-Glu refers to nitrate-fed with the application of glutamate; Ammonium refers to ammonium-fed plants; Ammonium-MSO refers to ammonium-fed with the application of MSO; Ammonium-Glu refers to ammonium-fed with the application of glutamate. Lowercase letters refer to significant difference between treatments (P< 0.05). Whiskers on the top of the bars indicate standard error (n = 6).

Fig 5

Indole-3-acetic acid (IAA) concentrations in shoots (A) and roots(B) of wheat plants grown under NO₃^- or NH₄⁺ conditions with applying MSO or glutamate in AK58 (Left) and Xumai25 (Right). Nitrate refers to nitrate-fed plants; Nitrate-MSO refers to nitrate-fed with the application of MSO; Nitrate-Glu refers to nitrate-fed with the application of glutamate; Ammonium refers to ammonium-fed plants; Ammonium-MSO refers to ammonium-fed with the application of MSO; Ammonium-Glu refers to ammonium-fed with the application of glutamate. Lowercase letters refer to significant difference between treatments (P< 0.05). Whiskers on the top of the bars indicate standard error (n = 6).

Fig 6

Ratio of shoot to root in IAA of wheat plants grown under NO₃^- or NH₄⁺ conditions with applying MSO or glutamate in AK58 (Left) and Xumai25 (Right). IAA refers to Indole-3-acetic acid. Nitrate refers to nitrate-fed plants; Nitrate-MSO refers to nitrate-fed with the application of MSO; Nitrate-Glu refers to nitrate-fed with the application of glutamate; Ammonium refers to ammonium-fed plants; Ammonium-MSO refers to ammonium-fed with the application of MSO; Ammonium-Glu refers to ammonium-fed with the application of glutamate. Lowercase letters refer to significant difference between treatments (P< 0.05). Whiskers on the top of the bars indicate standard error (n = 6).