The Role of Reactive Oxygen Species Under Ammonium Nutrition

Abstract

Nitrogen (N) is one of the most important macroelements for living organisms, serving as a central precursor for nucleic acids, amino acids, and other organic molecules. Because N is a ubiquitous intermediate of plant metabolic pathways, the element is undoubtedly crucial to optimal plant growth and development. Plants are capable of utilizing different inorganic N sources, mainly nitrate (NO 3 À) and ammonium (NH 4 +) (Barker and Mills 1980; Lewis 1992). NO 3 À and NH 4 + are taken up from the soil by roots; both N forms are actively absorbed via several low-and high-affinity transporters (Glass et al. 2002). Nitrate is cotransported with H + and is then directly reduced or stored in vacuoles after importation; however, in many plants, such as Arabidopsis, most NO 3 À is translocated to the shoots (Masclaux-Daubresse et al. 2010). NO 3 À is reduced to NO 2 À by the cytosolic nitrate reductase (NR), which uses NAD(P)H. NO 2 À is then further reduced by the plastidic nitrite reductase (NiR), which consumes 6 electrons from reduced ferredoxin (Fd). The resulting product (NH 4 +) is incorporated into amino acid structures in the glutamine synthetase-glutamine:2-oxoglutarate (2-OG) amino-transferase (GS-GOGAT) cycle (Noctor and Foyer 1998; Masclaux-Daubresse et al. 2010). Ammonium ions can be imported into the plant cell through plasmalemma transporters, but free diffusion of NH 3 is also possible (Loqué et al. 2007; Ludewig et al. 2007). Primary NH 4 + assimilation occurs mainly in root plastids, and the major responsible GS has a very high affinity for NH 4 + (Miflin and Habash 2002). The resulting amide product, glutamine (Gln), is transported via xylem sap to the