Nitrate Reduction and signalling

Abstract

The ability to use nitrate as sole nitrogen source to sustain growth is a property shared by some bacteria and fungi and by most algae and plants. The biochemical pathway responsible for nitrate assimilation seems to be the same in both prokaryotes and eukaryotes. Soil nitrate is the preferred inorganic nitrogen source for many wild or cultivated plants. It seems, indeed, that most plants, unlike bacteria or fungi, which preferentially use ammonium as a nitrogen source, show better growth when nitrate is present. The generalisation of the use of chemical fertilizers has allowed a tremendous increase in crop yield during the past 50 years. However, there is now a growing concern about the effect of nitrate, on both the environment and human health (Chap. 8). Indeed, nitrate can accumulate in high concentrations in the leaves of edible plants or in drinking water. Once taken up from the soil by an active process (see Chap. 1.1), nitrate is either stored in the plant root system or translocated to aerial parts via the xylem. High concentrations of nitrate can be found in vacuoles and it seems that nitrate, beside its role as a nutrient, participates in the maintenance of the plant osmoticum. The first committed step of the nitrate assimilation pathway is the reduction of nitrate to nitrite, catalysed by assimilatory nitrate reductase (NR, Fig. 1). In some bacteria, dissimilatory nitrate reduction, in which nitrate replaces oxygen as a terminal electron acceptor for respiration, is also found, but the utilisation of nitrate for respiration in anaerobic plant cells is still debated. The nitrite formed by NR activity is translocated to the chloroplast, where it is further reduced to ammonium by nitrite reductase (NiR). Ammonium is subsequently incorporated into the amino acid pool through the action of glutamine synthetase (GS) and glutamate synthase (GOGAT) (see Chap. 2.2 and Fig. 1).