From the soil to the seeds: the long journey of nitrate in plants.

Under temperate climates and in cultivated soils, nitrate is the most important source of nitrogen (N) available for crops and, before its reduction and assimilation into amino acids, must enter the root cells and then move in the whole plant. The aim of this review is to provide an overall picture of the numerous membrane proteins that achieve these processes by being localized in different compartments and in different tissues. Nitrate transporters (NRT) from the NRT1 and NRT2 families ensure the capacity of root cells to take up nitrate, through high- and low-affinity systems (HATS and LATS) depending on nitrate concentrations in the soil solution. Other members of the NRT1 family are involved subsequently in loading and unloading of nitrate to and from the xylem vessels, allowing its distribution to aerial organs or its remobilization from old leaves. Once in the cell, nitrate can be stored in the vacuole by passing through the tonoplast, a step that involves chloride channels (CLC) or a NRT2 member. Finally, with the exception of one NRT1 member, the transport of nitrite towards the chloroplast is still largely unknown. All these fluxes are controlled by key factors, the 'major tour operators' like the internal nutritional status of the plant but also by external abiotic factors.