Ammonium acts systemically while nitrate exerts an additional local effect on Medicago truncatula nodules

Area of Science:

• Plant biology
• Molecular biology
• Biochemistry

Background:

• Symbiotic nitrogen fixation (SNF) is energetically costly for legumes.
• Legumes decrease SNF when soil nitrogen (N) is abundant.

Purpose of the Study:

• To investigate the distinct impacts of ammonium and nitrate on SNF in Medicago truncatula.
• To understand the molecular mechanisms underlying nitrogen's regulation of SNF.

Main Methods:

• Continuous measurement of SNF under controlled conditions.
• Transcriptome profiling of nodules following ammonium or nitrate application.
• Comparative analysis of gene expression changes.

Main Results:

• Both ammonium and nitrate induced a rhythmic pattern of SNF, reducing daily fixation rates.
• Nitrate specifically caused local effects, including downregulation of leghaemoglobin, nodule cysteine-rich (NCR) peptides, and neNAS.
• Ammonium application had minimal impact on nodule transcriptome in the initial hours.

Conclusions:

• Nitrate significantly alters gene expression in nodules, impacting key functional components like leghaemoglobin and neNAS.
• The observed ion-specific and organ-specific gene expression changes highlight nitrate's complex regulation of SNF.
• Understanding nitrate-induced genes is crucial for enhancing nitrate tolerance in legume symbiosis.