Abstract:
Rapid and large accumulation of GABA (y-aminobutyric acid) in response to a number of
plant stresses has been well documented. But the role(s) of GABA in plants is not well defined. In
recent years, the possibility of GABA involvement in regulating plant growth and development has
been raised. In the present study, this possibility was examined.
First, to rapidly and accurately determine GABA levels in plant tissues, a spectrometric
method for GABA determination was developed based on a commercially available enzyme Gabase.
Seventy mM LaCb almost completely removed water-soluble pigments from plant tissues which
greatly interfere with the absorbance reading at 340nm. Inactivation of GAD (glutamate
decarboxylase) by immediately adding methanol to a frozen plant tissue powder was suggested to
prevent GABA production during extraction. The recovery of GABA with this method was
approximately 100%.
Second, the relationship between GABA levels and hypocotyl elongation in soybean
seedlings was analyzed using different approaches to regulate in vivo GABA levels and the
elongation of hypocotyls. The following major observations were made.
(1) Mechanical stimulation by stroking elevated GABA levels and concurrently induced a rapid and
significant reduction in hypocotyl elongation.
(2) External GABA was demonstrated to penetrate into the hypocotyls using '*C-GABA.
Application of external GABA elevated in vivo GABA levels, but failed to inhibit hypocotyl
elongation.
(3) LaCla and blue light irradiation caused an inhibition in the elongation of dark-grown hypocotyls,
whereas GABA levels were not significantly affected. (4) Ca^was suggested to be involved in the signal transduction pathway leading from mechanical
stimulation to GABA production, as indicated by the ability of La'* to inhibit GABA production
in stimulated hypocotyls.
(5) Bicuculline, saclofen and baclofen (agonists and antagonists of GABA receptors in animals) had
no effect on hypocotyl elongation. It might indicate that GABA-binding components which are
structurally similar to animal GABA receptors and functionally capable of regulating plant
growth may not exist in plants.
Therefore, the conclusion was drawn that GABA alone is not sufficient to inhibit hypocotyl
elongation.
Third, chloride influx in isolated Asparagus cells was enhanced by lOmM GABA during a 3
hour incubation, but the effect was not specific for GABA. Chloride efflux was not influenced by
GABA. Both influx and efflux of chloride were significantly inhibited by NPPB, a chloride channel
blocker. These results suggest that GABA does not influence the activity of plant chloride channels.
