y-Aminonbutyric acid (GABA) is a ubiquitous non-protein amino
acid found ill many plants and organisms. GABA accumulation in
plants has previously been reported as result of
stresses such as water deprivation, high salinity and
temperature extremes. It is thought that GABA accumulates as a
compatible solute in the cytoplasm where it becomes a major
constituent of the free amino acid pool. GABA is synthesised
from the decarboxylation of glutamate by glutamate
decarboxylase (GDC). In some plants, GDC is activated by the lowering
of the cytoplasmic pH and the presence of calmodulin and Ca²+
A calmodulin-induced activation of may be due to
the physiological factors and environmental stimuli acting
in concert leading to the synthesis and accumulation of GABA.
The GABA content of Arabidopsis thaliana var. Columbia (L)
Heynh leaves was found to increase by over 130% due to water deprivation.
NaCl concentrations of up to 100 mM seemed to cause GABA accumulation due to a decrease in osmotic potential. Concentrations of NaCl above 100 mM probably caused GABA accumulation due to combined hyperosmosis and salt
toxicity effects. The high levels of GABA in the leaves were
maintained throughout a 24 h stress-application period,
consistent with its role as compatible solute.
The accumulation of GABA followed by its decline in the dark
could be attributed to its rapid metabolism because of an
active GABA shunt. This is in contrast to the absence of major
variations in the amount of GABA in the light confirming its
decreased role as a channel for the glutamate carbon and
nitrogen under such conditions. A substantial increase in the
GABA content was followed by a dramatic decrease in the last
12 h of incubation. This profile of GABA could support its
proposed role as a temporary sink for nitrogen and carbon from
glutamate during environmental stress.
Glutamate decarboxylase appears to be encoded by a single gene
in the genome of Arabidopsis. Sequence analysis reveals that
the protein possesses what could be a carboxy-terminal,
calmodulin- binding domain, which is consistent with other
glutamate decarboxylases. The 30-amino acid peptide contains a
TrpLysLys motif found in some calmodulin targets. The
secondary structure predictions of this peptide suggest a
potential to form an a- helix which is also consistent with
proteins known calmodulin- binding domains. / Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 1997.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/10304 |
| Date | January 1997 |
| Creators | Molatudi, Mohohlo W. |
| Contributors | Van Staden, Johannes., Cress, William A. |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
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