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

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.