Exploring ammonium tolerance in a large panel of Arabidopsis thaliana natural accessions

Summary

Ammonium nutrition is toxic to many plants. Arabidopsis displays high intraspecific variability in ammonium tolerance (shoot biomass), and ammonium accumulation seems to be an important player in this variability.

Abstract

Plants are dependent on exogenous nitrogen (N) supply. Ammonium (NH ₄ ⁺), together with nitrate (NO ₃ ^–), is one of the main nitrogenous compounds available in the soil. Paradoxically, although NH ₄ ⁺ assimilation requires less energy than that of NO ₃ ^–, many plants display toxicity symptoms when grown with NH ₄ ⁺ as the sole N source. However, in addition to species-specific ammonium toxicity, intraspecific variability has also been shown. Thus, the aim of this work was to study the intraspecific ammonium tolerance in a large panel of Arabidopsis thaliana natural accessions. Plants were grown with either 1mM NO ₃ ^– or NH ₄ ⁺ as the N source, and several parameters related to ammonium tolerance and assimilation were determined. Overall, high variability was observed in A. thaliana shoot growth under both forms of N nutrition. From the parameters determined, tissue ammonium content was the one with the highest impact on shoot biomass, and interestingly this was also the case when N was supplied as NO ₃ ^–. Enzymes of nitrogen assimilation did not have an impact on A. thaliana biomass variation, but the N source affected their activity. Glutamate dehydrogenase (GDH) aminating activity was, in general, higher in NH ₄ ⁺-fed plants. In contrast, GDH deaminating activity was higher in NO ₃ ^–-fed plants, suggesting a differential role for this enzyme as a function of the N form supplied. Overall, NH ₄ ⁺ accumulation seems to be an important player in Arabidopsis natural variability in ammonium tolerance rather than the cell NH ₄ ⁺ assimilation capacity.