A molecular study of y-Aminobutyric acid synthesis in Arabidopsis thaliana under abiotic stress.

Abstract

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.