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PLANT PHYSIOLOGY , Vol 106, Issue 2 513-520, Copyright © 1994 by American Society of Plant Biologists
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ENVIRONMENTAL AND STRESS PHYSIOLOGY |
Ammonium Assimilation and the Role of [gamma]-Aminobutyric Acid in pH Homeostasis in Carrot Cell Suspensions
A. D. Carroll, G. G. Fox, S. Laurie, R. Phillips, R. G. Ratcliffe and G. R. Stewart
Department of Biology, Darwin Building, University College London, Gower Street, London WC1E 6BT, United Kingdom (A.D.C., S.L., R.P.)
In vivo 15N NMR spectroscopy was used to monitor the assimilation of
ammonium by cell-suspension cultures of carrot (Daucus carota L. cv
Chantenay). The cell suspensions were supplied with oxygen in the form of
either pure oxygen ("oxygenated cells") or air ("aerated cells"). In
contrast to oxygenated cells, in which ammonium assimilation had no effect
on cytoplasmic pH, ammonium assimilation by aerated cells caused a decrease
in cytoplasmic pH of almost 0.2 pH unit. This led to a change in nitrogen
metabolism resulting in the accumulation of [gamma]-aminobutyric acid. The
metabolic effect of the reduced oxygen supply under aerated conditions
could be mimicked by artificially decreasing the cytoplasmic pH of
oxygenated cells and was abolished by increasing the cytoplasmic pH of
aerated cells. The activity of glutamate decarboxylase increased as the
cytoplasmic pH declined and decreased as the pH recovered. These findings
are consistent with a role for the decarboxylation of glutamate, a
proton-consuming reaction, in the short-term regulation of cytoplasmic pH,
and they demonstrate that cytoplasmic pH influences the pathways of
intermediary nitrogen metabolism.
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