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PLANT PHYSIOLOGY , Vol 104, Issue 3 865-871, Copyright © 1994 by American Society of Plant Biologists
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ENVIRONMENTAL AND STRESS PHYSIOLOGY |
The Synthesis of [gamma]-Aminobutyric Acid in Response to Treatments Reducing Cytosolic pH
L. A. Crawford, A. W. Bown, K. E. Breitkreuz and F. C. Guinel
Department of Biological Sciences, Brock University, St. Catharines, Ontario, Canada L2S 3A1
[gamma]-Aminobutyric acid (GABA) synthesis (L-glutamic acid + H+ -> GABA
+ CO2) is rapidly stimulated by a variety of stress conditions including
hypoxia. Recent literature suggests that GABA production and concomitant H+
consumption ameliorates the cytosolic acidification associated with hypoxia
or other stresses. This proposal was investigated using isolated asparagus
(Asparagus sprengeri Regel) mesophyll cells. Cell acidification was
promoted using hypoxia, H+/L-glutamic acid symport, and addition of
butyrate or other permeant weak acids. Sixty minutes of all three
treatments stimulated the levels of both intracellular and extracellular
GABA by values ranging from 100 to 1800%. At an external pH of 5.0,
addition of 5 mM butyrate stimulated an increase in overall GABA level from
3.86 (0.56 [plus or minus] SE) to 20.4 (2.16 [plus or minus] SE) nmol of
GABA/106 cell. Butyrate stimulated GABA levels by 200 to 300% within 15 s,
and extracellular GABA was observed after 10 min. The acid load due to
butyrate addition was assayed by measuring [14C]butyrate uptake. After 45 s
of butyrate treatment, H+-consuming GABA production accounted for 45% of
the imposed acid load. The cytosolic location of a fluorescent pH probe was
confirmed using fluorescent microscopy. Spectrofluorimetry indicated that
butyrate addition reduced cytosolic pH by 0.60 units with a half-time of
approximately 2 s. The proposal that GABA synthesis ameliorates cytosolic
acidification is supported by the data. The possible roles of H+ and Ca2+
in stimulating GABA synthesis are discussed.
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