Study on modulation of root glutamine synthetase isoforms and plant amino acid balance in response to nitrogen nutrition in maize

Glutamine synthetase (EC:6.3.1.2) catalyzes the first step in mineral nitrogen (N) assimilation. In maize (Zea mays L.) five cytosolic (GS1) isoforms and one plastidic (GS2) enzymes are known. This study was focused on plants exposed by hydroponic systems to different inorganic N source: NO3-, NH4+or both. The proteomic characterization of root profiles put in evidence that, while the GS2 was exclusively involved in NO3- assimilation, the GS1 isoforms were differently and specifically regulated in response to both nutrients. At the same time, the LC-ESI-MS analysis of amino acid speciation in root, xylem and leaves highlighted as the assimilatory pathways were deeply influenced by N sources. In particular, while NO3- assimilation was almost equally distributed in the two apparatus, the NH4+ organication was predominantly located in the roots. This compartmentalization seemed to be allowed by a modulation of whole plant metabolism. In fact, the increased accumulation of asparagine in root was related to an increase in xylem translocation of glutamine, which in leaves appeared recycled in alanine and glutamate to cope with the higher request of carbon skeletons.