Changes in the C/N balance caused by increasing external ammonium concentrations are driven by carbon and energy availabilities during ammonium nutrition in pea plants: the key roles of asparagine synthetase and anaplerotic enzymes

An understanding of the mechanisms underlying ammonium ( NH 4 + ) toxicity in plants requires prior knowledge of the metabolic uses for nitrogen (N) and carbon (C). We have recently shown that pea plants grown at high NH 4 + concentrations suffer an energy deficiency associated with a disruption of ionic homeostasis. Furthermore, these plants are unable to adequately regulate internal NH 4 + levels and the cell‐charge balance associated with cation uptake. Herein we show a role for an extra‐C application in the regulation of C–N metabolism in NH 4 + ‐fed plants. Thus, pea plants ( Pisum sativum ) were grown at a range of NH 4 + concentrations as sole N source, and two light intensities were applied to vary the C supply to the plants. Control plants grown at high NH 4 + concentration triggered a toxicity response with the characteristic pattern of C‐starvation conditions. This toxicity response resulted in the redistribution of N from amino acids, mostly asparagine, and lower C/N ratios. The C/N imbalance at high NH 4 + concentration under control conditions induced a strong activation of root C metabolism and the upregulation of anaplerotic enzymes to provide C intermediates for the tricarboxylic acid cycle. A high light intensity partially reverted these C‐starvation symptoms by providing higher C availability to the plants. The extra‐C contributed to a lower C4 / C5 amino acid ratio while maintaining the relative contents of some minor amino acids involved in key pathways regulating the C/N status of the plants unchanged. C availability can therefore be considered to be a determinant factor in the tolerance/sensitivity mechanisms to NH 4 + nutrition in plants.