Symbiotic nitrogen fixation (SNF) has a high energetic cost for legume plants; legumes thus reduce SNF when soil N is available. The present study aimed to increase our understanding regarding the impacts of the two principal forms of available N in soils (ammonium and nitrate) on SNF. We continuously measured the SNF of Medicago truncatula under controlled conditions. This permitted nodule sampling for comparative transcriptome profiling at points connected to the nodules' reaction following ammonium or nitrate applications. The N component of both ions systemically induced a rhythmic pattern of SNF, while the activity in control plants remained constant. This rhythmic activity reduced the per-day SNF. The nitrate ion had additional local effects; the more pronounced were a strong downregulation of leghaemoglobin, nodule cysteine-rich (NCR) peptides and nodule-enhanced nicotianamine synthase (neNAS). The neNAS has proven to be of importance for nodule functioning. Although other physiological impacts of nitrate on nodules were observed (e.g. nitrosylation of leghaemoglobin), the main effect was a rapid ion-specific and organ-specific change in gene expression levels. Contrastingly, during the first hours after ammonium applications, the transcriptome remained virtually unaffected. Therefore, nitrate-induced genes could be key for increasing the nitrate tolerance of SNF.

中文翻译：

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铵作用全身，而硝酸盐对紫花苜蓿结节产生额外的局部作用

共生固氮（SNF）对豆科植物来说具有很高的能量成本；因此，当土壤氮可用时，豆类会降低 SNF。本研究旨在加深我们对土壤中两种主要有效氮形式（铵和硝酸盐）对 SNF 影响的理解。我们在受控条件下连续测量了苜蓿的 SNF。这允许在与铵或硝酸盐应用后的结节反应相关的点处进行比较转录组分析的结节采样。两种离子的 N 组分系统地诱导了 SNF 的节律模式，而对照植物中的活性保持不变。这种有节奏的活动减少了每天的 SNF。硝酸根离子有额外的局部作用；更明显的是豆血红蛋白的强烈下调，结节富含半胱氨酸 (NCR) 肽和结节增强型烟胺合成酶 (neNAS)。neNAS 已被证明对结节功能很重要。尽管观察到硝酸盐对结节的其他生理影响（例如豆血红蛋白的亚硝基化），但主要影响是基因表达水平的快速离子特异性和器官特异性变化。相比之下，在施用铵盐后的最初几个小时内，转录组几乎不受影响。因此，硝酸盐诱导基因可能是增加 SNF 硝酸盐耐受性的关键。主要影响是基因表达水平的快速离子特异性和器官特异性变化。相比之下，在施用铵盐后的最初几个小时内，转录组几乎不受影响。因此，硝酸盐诱导基因可能是增加 SNF 硝酸盐耐受性的关键。主要影响是基因表达水平的快速离子特异性和器官特异性变化。相比之下，在施用铵盐后的最初几个小时内，转录组几乎不受影响。因此，硝酸盐诱导基因可能是增加 SNF 硝酸盐耐受性的关键。