Futile transmembrane NH ₄ ⁺ cycling: A cellular hypothesis to explain ammonium toxicity in plants

Ammonium Toxicity in Bacteria

“…Ammonium is in contrast to the situation in animal cells (e. g. see [22]) and plants (e.g., [4,5]) not toxic for the studied model bacteria C. glutamicum, E. coli, and B. subtilis, even in molar concentrations. Because most bacteria prefer ammonium as nitrogen source and some species even produce ammonium, for example, rhizobia and cyanobacteria by N 2 -fixation and proteolytic clostridia by amino acid fermentation, we assume that ammonium resistance is a general phenomenon in bacteria.…”

“…Also in this case, a specific ammonium effect was not observed ( Table 2). As in E. coli, addition of 750 mM and 1000 mM ammonium (375 mM and 500 mM (NH 4 ) 2 SO 4 ) impaired growth; however, similar effects were observed when (NH 4 ) 2 SO 4 was exchanged against (Na) 2 SO 4 .…”

“…As one reason for this strict regulation, prevention of a putative energy-wasting futile cycling of ammonium was discussed. In such a situation, NH 4 + would be transported into the cell by an energy-dependent uptake mechanism and would subsequently diffuse across the cytoplasmic membrane back into the medium in the form of NH 3 . For C. glutamicum, such an effect of ammonium cycling could not be shown.…”

“…Ammonium (in this communication, the term ''ammonium'' is used in a general sense, including NH 4 + and NH 3 ; distinct ammonium species are indicated by chemical formula) is the preferred nitrogen source for most bacteria and fungi, and also plants acquire ammonium as a nitrogen source from the soil [35]. When present in high concentrations, diffusion of NH 3 , which is in equilibrium with NH 4 + , across the cytoplasmic membrane into the cell is sufficient to promote growth.…”

“…When present in high concentrations, diffusion of NH 3 , which is in equilibrium with NH 4 + , across the cytoplasmic membrane into the cell is sufficient to promote growth. In a situation of ammonium limitation, diffusion becomes negligible and special carriers are synthesized in bacteria, fungi, and plants (for a recent review, see [36]).…”