ABSTRACT
Nitrogen assimilation is a multistep process that involves a set of enzymatic reactions located in different subcellular compartments and plant organs (Fig. 1). Although most plants are able to absorb and incorporate both inorganic and organic nitrogen compounds, the inorganic forms present in the soil represent the main nitrogen source in most habitats. A general scheme of the inorganic nitrogen assimilation requires the distinction of the sources implicated in the supply of inorganic nitrogen. In addition to the primary sources, in which nitrogen is incorporated into plants as nitrate or ammonium from water or soil or as dinitrogen in symbiotic plants with nitrogen-fixing bacteria, a number of metabolic processes such as photorespiration, the biosynthesis of phenylpropanoids, or the catabolism of amino acids supply ammonium ion as a secondary source. These secondary nitrogen sources are of great relevance in some tissues and phases of the plant life cycle. Thus, after germination, most nitrogen is mobilized from storage proteins in the embryo and cotyledons to support the early plant development. This nitrogen mobilization involves protein degradation and amino acid deamination, releasing ammonium ion that will be used in plants as a unique nitrogen source
FIGURE 1 General scheme of nitrogen assimilation in higher plants. Only nitrate assimilation is considered for simplicity. Surplus of nitrate in root cells can be stored in the vacuole or translocated to the shoot via xylem. Nitrate reduction to ammonium ion through nitrate and nitrite reductase can operate in both root and photosynthetic cells. Photorespiration is a main source of ammonium ion in photosynthetic cells of C3 plants. Aas, amino acids.
