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An adequate carbon supply is fundamental for plants to thrive under ammonium stress. In this work, we studied the mechanisms involved in tomato (Solanum lycopersicum L.) response to ammonium toxicity when grown under ambient or elevated CO2 conditions (400 or 800 p.p.m. CO2). Tomato roots were observed to be the primary organ dealing with ammonium nutrition. We therefore analyzed nitrogen (N) and carbon (C) metabolism in the roots, integrating the physiological response with transcriptomic regulation. Elevated levels of CO2 preferentially stimulated root growth despite the high ammonium content. The induction of anaplerotic enzymes from the tricarboxylic acid (TCA) cycle led to enhanced amino acid synthesis under ammonium nutrition. Furthermore, the root transcriptional response to ammonium toxicity was improved by CO2-enriched conditions, leading to higher expression of stress-related genes, as well as enhanced modulation of genes related to signaling, transcription, transport and hormone metabolism. Tomato roots exposed to ammonium stress also showed a defense-like transcriptional response according to the modulation of genes related to detoxification and secondary metabolism, involving principally terpenoid and phenolic compounds. These results indicate that increasing C supply allowed the co-ordinated regulation of root defense mechanisms when dealing with ammonium toxicity.

Proper carbon (C) supply is essential for nitrogen (N) assimilation especially when plants are grown under ammonium (NH 4 + ) nutrition. However, how C and N metabolic fluxes adapt to achieve so remains uncertain. In this work, roots of wheat ( Triticum aestivum L.) plants grown under exclusive NH 4 + or nitrate (NO 3 − ) supply were incubated with isotope-labelled substrates ( 15 NH 4 + , 15 NO 3 − , or [ 13 C]Pyruvate) to follow the incorporation of 15 N or 13 C into amino acids and organic acids. Roots of plants adapted to ammonium nutrition presented higher capacity to incorporate both 15 NH 4 + and 15 NO 3 − into amino acids, thanks to the previous induction of the NH 4 + assimilative machinery. The 15 N label was firstly incorporated into [ 15 N]Gln vía glutamine synthetase; ultimately leading to [ 15 N]Asn accumulation as an optimal NH 4 + storage. The provision of [ 13 C]Pyruvate led to [ 13 C]Citrate and [ 13 C]Malate accumulation and to rapid [ 13 C]2-OG consumption for amino acid synthesis and highlighted the importance of the anaplerotic routes associated to tricarboxylic acid (TCA) cycle. Taken together, our results indicate that root adaptation to ammonium nutrition allowed efficient assimilation of N thanks to the promotion of TCA cycle open flux modes in order to sustain C skeleton availability for effective NH 4 + detoxification into amino acids.

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