Providing carbon skeletons to sustain amide synthesis in roots underlines the suitability of Brachypodium distachyon for the study of ammonium stress in cereals

Plants mainly acquire N from the soil in the form of nitrate (NO3-) or ammonium (NH4+). Ammonium-based nutrition is gaining interest because it helps to avoid the environmental pollution associated with nitrate fertilization. However, in general, plants prefer NO3- and indeed, when growing only with NH4+ they can encounter so-called ammonium stress. Since Brachypodium distachyon is a useful model species for the study of monocot physiology and genetics, we chose it to characterize performance under ammonium nutrition. Brachypodium distachyon Bd21 plants were grown hydroponically in 1 or 2.5 mM NO3- or NH4+. Nitrogen and carbon metabolism associated with NH4+ assimilation was evaluated in terms of tissue contents of NO3-, NH4+, K, Mg, Ca, amino acids and organic acids together with tricarboxylic acid (TCA) cycle and NH4+-assimilating enzyme activities and RNA transcript levels. The roots behaved as a physiological barrier preventing NH4+ translocation to aerial parts, as indicated by a sizeable accumulation of NH4+, Asn and Gln in the roots. A continuing high NH4+ assimilation rate was made possible by a tuning of the TCA cycle and its associated anaplerotic pathways to match 2-oxoglutarate and oxaloacetate demand for Gln and Asn synthesis. These results show B. distachyon to be a highly suitable tool for the study of the physiological, molecular and genetic basis of ammonium nutrition in cereals.