REGULATION OF MINERAL NITROGEN UPTAKE IN PLANTS

the biosphere plants are exposed to different forms of N, which compri se mineral and organic N forms in soils as well as gaseous NH3, NOx, a nd molecular N2 in the atmosphere. The form of N uptake is mainly dete rmined by its abundance and accessibility, which make NO3(-) and NH4() the most important N forms for plant nutrition under agricultural co nditions. With minor importance, the form of N uptake is also subject to plant preferences, by which plants maintain their cation/anion bala nce during uptake. However, some species seem to have an obligatory pr eference which even prevents their growth on certain other N sources. In general, uptake of a certain N form closely matches the growth-rela ted demand of the plant, at least when N transport to the root surface is not limiting. In addition, many plants accumulate large pools of N during vegetative growth which are remobilized in the generative stag e. As a consequence, systems responsible for N transport need to be ti ghtly regulated in their expression and activity upon sensing N availa bility and plant demand. Employing the tools of molecular genetics, th e first plant genes encoding transporters for inorganic N have recentl y been isolated and characterized. These data can now complete the wea lth of physiological and nutritional studies on N uptake. The present article will focus on the uptake of NO3(-) and NH4(+) into root cells and tries to link data derived from physiological, genetic and molecul ar studies.