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 (NO ) or ammonium (NH ). Ammonium-based nutrition is gaining interest because it helps to avoid the environmental pollution associated with nitrate fertilization. However, in general, plants prefer NO and indeed, when growing only with NH they can encounter so-called ammonium stress. Since is a useful model species for the study of monocot physiology and genetics, we chose it to characterize performance under ammonium nutrition. Bd21 plants were grown hydroponically in 1 or 2.5 mM NO or NH . Nitrogen and carbon metabolism associated with NH assimilation was evaluated in terms of tissue contents of NO , NH , K, Mg, Ca, amino acids and organic acids together with tricarboxylic acid (TCA) cycle and NH -assimilating enzyme activities and RNA transcript levels. The roots behaved as a physiological barrier preventing NH translocation to aerial parts, as indicated by a sizeable accumulation of NH , Asn and Gln in the roots. A continuing high NH 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 to be a highly suitable tool for the study of the physiological, molecular and genetic basis of ammonium nutrition in cereals.