Structure, function and regulation of ammonium transporters in plants

Ammonium is an important source of nitrogen for plants. It is taken up by p lant cells via ammonium transporters in the plasma membrane and distributed to intracellular compartments such as chloroplasts, mitochondria and vacuo les probably via different transporters in each case. Ammonium is generally not used for long-distance transport of nitrogen within the plant. Instead , most of the ammonium transported into plant cells is assimilated locally via glutamine synthetases in the cytoplasm and plastids. Ammonium is also p roduced by plant cells during normal metabolism, and ammonium transporters enable it to be moved from intracellular sites of production to sites of co nsumption. Ammonium can be generated de novo from molecular nitrogen (N-2) by nitrogen-fixing bacteria in some plant cells, such as rhizobia in legume root nodule cells, and at least one ammonium transporter is implicated in the transfer of ammonium from the bacteria to the plant cytoplasm. Plant ph ysiologists have described many of these ammonium transport processes over the last few decades. However, the genes and proteins that underlie these p rocesses have been isolated and studied only recently. In this review, we c onsider in detail the molecular structure, function and regulation of plant ammonium transporters. We also attempt to reconcile recent discoveries at the molecular level with our knowledge of ammonium transport at the whole p lant level. (C) 2000 Elsevier Science B.V. All rights reserved.