Chapter 18 - Nitrate Assimilation in Plants

Abstract

This chapter focuses on the physiological and molecular aspects of NO3– uptake and reduction. Nitrate taken up by the roots is either reduced or stored in the vacuoles or is translocated to the shoot for subsequent reduction and vacuolar storage; it is also used for osmoregulation. Nitrate acts as both a nutrient and a signal for the initiation of various processes. It triggers the induction of NO3– assimilating enzymes but also shifts carbohydrate metabolism from starch synthesis to increased sucrose synthesis. In maize leaves, a response regulator homolog that appears involved in the N-signaling pathway is induced by added cytokinin. Few genes, which code for either a sensor protein or transcription factors, have been detected so far. Further research is required to identify the individual steps of the nitrate signaling chain. Determining the location of the PII analogue and the expression of the corresponding gene appears essential in this context. NO3– uptake capacity is not equally distributed along the root axis and is not identical in the roots of different age or ontogeny. The commonly observed alkalization of the medium during NO3– uptake is explained by a 2:1 rather than a 1:1 stoichiometry of H+ NO3– during cotransport. Lysine residues have been found to play a crucial role in the activity of the tonoplast electro-neutral NO3– /H+ antiporter.