One of the most important roles of the roots is to acquire mineral resources from the soil. Among the various nutrients required by the plants, nitrogen (N) is the one needed in greatest amounts because it typically accounts for 1%–3% of the dry biomass (Marschner 1995). Most plants take up nitrogen in the form of nitrate (NO3-) and/or ammonium (NH4+) present in the soil solution. Indeed, only a small proportion of all plant species (notably the legumes) are able to set up symbioses that allow them to indirectly use the unlimited source of atmospheric N2. For the vast majority of species root N acquisition is strongly constrained by the dramatic uctuations that occur, in both time and space, for NO3-and NH4+ availability in the soils (Miller et al. 2007). Modern agriculture has solved this problem by a massive use of N fertilizers that in turn led to a major disturbance of the overall nitrogen cycle between the atmosphere, hydrosphere, and biosphere (Galloway et al. 2003). Both these harmful environmental consequences and the high economic and energetic costs of excessive inputs of N fertilizers are now concerning enough to raise the improvement of N use eciency by crops as a major goal in agronomy and plant sciences (Good et al. 2004).