Roles of GABA synthesis
Biochemical pH-stat
Because GAD activity consumes H1 (Fig. 1), it has been proposed
that stress-induced GABA synthesis can contribute to pH regulation6.
Early in vivo NMR spectroscopy data demonstrated that
the imposition of anoxia on corn root tips and the corresponding
reduction in cytosolic pH involves the transient production of lactate,
and a lag in the synthesis of ethanol39. However, it was
argued that most of the GABA accumulation occurs after the predominant
acid-generating reactions have ceased. Two independent
investigations have provided direct evidence for GABA
accumulation in response to cytosolic acidification. A fluorescent
pH probe and an enzymatic assay for GABA were employed to
measure cytosolic pH changes and GABA accumulation in photosynthetic
asparagus cells exposed to permeant weak acids26.
Cytosolic pH decreases by 0.6 with a half-time of 2 s, and GABA
levels increase by 200–300% within 15 s. It is calculated that after
45 s of weak acid treatment, H1-consuming GABA production
accounts for ~50% of the imposed acid load. In vivo 31P and 15NNMR
spectroscopy was employed to monitor cytosolic pH and
GABA levels in aerated, cultured carrot cells25. The initiation of
ammonium assimilation causes a decline in cytosolic pH by 0.2
units, followed by an accumulation of GABA. GAD activity
increases with reduced pH, and declines as the pH recovers. Acidstimulated
GABA synthesis does not involve Ca21 flux because of
acidification31. Thus, GABA accumulation can ameliorate cytosolic
acidification.