Disruption of a putative "urea cycle" and its connection to polyamine metabolismn in plants under ammonium nutrition

Environmental stresses are the major cause of crop loss worldwide. The augmented use of fertilizers by industrial farming results in an over accumulation of nutrients in the soil, which originates lixiviation and eutrophication. Ammonium nutrition represents a valid alternative which binds tighter to the soil particles and then reduces the lixiviation. However, ammonium as sole source of nitrogen induces higher intracellular ammonium content leading to physiological and morphological disorders, impairing the optimal growth and development of the plants. In the present study, the disruption of the ‘urea cycle’ and polyamine (PA) metabolism by high contents of ammonium is proposed as a key mechanism underlying the tolerance response of Medicago truncatula to ammonium stress. In order to elucidate the involvement of M.truncatula amine oxidases in ammonium tolerance response, the effect of different nitrogen sources and doses on these enzymes were studied at phylogenetic, transcriptional and enzymatic level. In addition, hydrogen peroxide content was analysed to clarify whether it could be involved in ammonium tolerance of M.truncatula plants. In general terms, transcript levels of genes encoding diamine oxidases decreased in ammonium-fed seedlings in comparison to nitrate-fed plants, as well as the enzymatic activity did. In contrast, the expression of polyamine oxidases encoding genes did not show acute significant differences between treatments. However, the enzymatic activity of polyamine oxidases increased in shoot tissue of high ammonium-fed seedlings, and showed an increase tend in roots. Finally, the content of hydrogen peroxide was higher in M.truncatula plants grown under high nitrogen nutrition, especially in root tissue of high ammonium-fed seedlings. These results suggest the importance of the ‘urea cycle’ and PA metabolism in the tolerance against the abiotic stress induced by ammonium nutrition, as source of GABA and hydrogen peroxide, which are involved in abiotic stress responses. Finally, the accumulation of polyamine-derived hydrogen peroxide is suggested to be related to lignin deposition in plant cell walls of M. truncatula plants grown under high nitrogen nutrition. Anyway, future research should be performed to fully elucidate the mechanisms underlying plant ammonium tolerance responses.