Regulatory Levels for the Transport of Ammonium in Plant Roots

Ammonium is an attractive nitrogen form for root uptake due to its permanent availability and the reduced state of the nitrogen. On the other hand, ammonium fluxes are difficult to control because ammonium represents an equilibrium between NH4+ and NH3, which are two N forms with different membrane permeabilities. There is increasing evidence that AMT-type ammonium transporters represent the major entry pathways for root uptake of NH4+. Since excess uptake of ammonium might cause toxicity and since ammonium is also released from catabolic processes within the cell, ammonium uptake across the root plasma membrane has to be tightly regulated. To take over a function in cellular ammonium homeostasis, various AMT transporters are synthesized that differ in their biochemical properties, their localization, and in their regulation at the transcriptional level. At the same time, AMT-driven transport is subject to control by the nitrogen status of a local root portion as well as of the whole plant. In this review, the focus is on the different levels at which AMT-dependent ammonium uptake is regulated and the gaps in current knowledge are highlighted.