该研究探讨了小麦(Triticum aestivum)根系NH₄⁺毒性以及NO₃^-缓解其毒性机理。与7.5 mmol·L^-1 NO₃^-处理(CK)相比, 7.5 mmol·L^-1 NH₄⁺处理(SA)抑制小麦根系生长, 添加1 mmol·L^-1 NO₃^- (AN)后缓解了对根系生长的抑制。转录组分析表明, 与CK相比, SA处理下, 编码糖酵解途径酶、发酵关键酶、呼吸爆发性氧化酶同源物(Rbohs)、交替氧化酶(AOX)和双加氧酶相关基因显著上调表达; 编码TCA循环酶、ATP合酶和水通道蛋白(AQPs)相关基因显著下调表达。与SA相比, AN处理下, 糖酵解、发酵途径、Rbohs、AOX和双加氧酶相关基因的表达下调, 编码TCA循环酶、ATP合酶和AQPs的基因表达上调。蛋白质组分析表明, SA处理下, 糖酵解与发酵相关酶以及AOX相关基因表达上调, 而AQPs相关基因表达下调。AN处理下, 糖酵解与发酵相关酶以及AOX相关基因表达下调, AQPs相关基因表达上调。综上所述, 单独NH₄⁺处理促进糖酵解和发酵途径, 抑制TCA循环, 能量生成减少, 最终抑制小麦根系生长, 这可能与NH₄⁺处理引起根系缺O₂胁迫有关。添加NO₃^-后抑制了糖酵解和发酵途径, 促进TCA循环和能量产生, 显著缓解了根系缺O₂胁迫以及NH₄⁺对根系生长的抑制。

INTRODUCTION Nitrogen (N) is one of the most essential nutrients for the plant growth and development, and NH₄⁺-N is the main form of N source. Appropriate amount of NH₄⁺ promotes plant growth and increase crop yield. However, when used as the only N source, NH₄⁺ suppresses the growth and production of crops. Wheat (Triticum aestivum) is the third largest cereal crop in China, and its production has a profound impact on the food production. As the indispensable raw material for foods, wheat is crucial to daily life, so studying NH₄⁺ toxicity and mitigation mechanisms are of great significance for wheat production.

RATIONALE NH₄⁺toxicity is a ubiquitous issue in plants. The mechanisms of how NH₄⁺ causes phytotoxicity are not fully understood. To explore the mechanisms of NO₃^--dependent alleviation of NH₄⁺toxicity in the roots of wheat, both transcriptomic and proteomic approaches were used to investigate the differential expressions of genes and proteins under different nitrogen treatments.

RESULTS Compared with 7.5 mmol·L^-1NO₃^- (control), 7.5 mmol·L^-1NH₄⁺treatment inhibited the root growth of wheat seedlings. Transcriptome analysis showed that sole NH₄⁺ treatment upregulated the expressions of genes encoding glycolysis- and fermentation-related enzymes, including pyruvate decarboxylase, alcohol dehydrogenase and lactate dehydrogenase, while downregulated the expressions of genes encoding TCA cycle enzymes and the ATP synthases in the roots compared with control. Expressions of genes encoding the respiratory burst oxidase homologs (Rbohs), alternative oxidase (AOX) and dioxygenases were significantly upregulated, and expression of the PIP-type aquaporin genes were downregulated. The addition of 1 mmol·L^-1NO₃^- to the solution containing 7.5 mmol·L^-1NH₄⁺downregulated the expression of genes encoding glycolysis enzymes, fermentation enzymes, Rbohs, AOX and dioxygenases, and increased the expression of genes encoding the TCA cycle enzymes, the ATP synthases and PIP-type aquaporins. Proteome analysis showed that expressions of glycolysis enzymes, fermentation enzymes and AOX were upregulated, while PIP-type aquaporins were downregulated under sole NH₄⁺conditions compared with the control. The addition of NO₃^- downregulated the expressions of glycolysis enzymes, fermentation enzymes, Rbohs and AOX and upregulated expressions of PIP-type aquaporins.

CONCLUSION In conclusion, sole NH₄⁺ treatment promotes glycolytic and fermentation pathways, inhibits the TCA cycle and energy generation, and ultimately inhibits root growth of wheat seedlings. The inhibition of root growth may be due to the sole NH₄⁺-induced hypoxic stress in the roots. The addition of NO₃^- inhibits the glycolysis and fermentation pathways, promotes the TCA cycle and energy production, significantly alleviates the hypoxia stress and thereby attenuates the inhibitory effect of NH₄⁺ on root growth.

Changes in transcription level of DEGs involved in fermentation pathway and TCA cycle under different N treatments in the roots of wheat seedlings. Sole NH₄⁺ treatment may induce the fermentation pathway, inhibit the capacity of TCA cycle, and ultimately inhibit the root growth in wheat. The addition of NO₃^- remarkably alleviates the sole NH₄⁺-induced inhibitory effects on the root growth.