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Plant and Cell Physiology Advance Access originally published online on December 11, 2007
Plant and Cell Physiology 2008 49(1):92-102; doi:10.1093/pcp/pcm171
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© The Author 2007. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For permissions, please email: journals.permissions@oxfordjournals.org

Contribution of the GABA shunt to hypoxia-induced alanine accumulation in roots of Arabidopsis thaliana

Yo Miyashita* and Allen G. Good

Department of Biological Sciences, University of Alberta, Edmonton, AB, T6G 2E9, Canada

*Corresponding author: E-mail, yo{at}ualberta.ca; Fax, +1 780-492-9234.


  Abstract

When subjected to low oxygen stress, plants accumulate alanine and {gamma}-aminobutyric acid (GABA). To investigate the function of GABA metabolism under hypoxia and its contribution to alanine accumulation, we studied the genes that encode the two key enzymes of the GABA shunt, glutamate decarboxylase (GAD) and GABA transaminase (GABA-T). Among the five homologous GAD genes found in Arabidopsis thaliana, GAD1 expression was predominantly found in roots, while GAD2 expression was evident in all organs. Expression of the other three GAD genes was generally weak. In response to hypoxia, transcriptional induction was observed for GAD4 only. For GABA-T1, its expression was detected in all organs, but there was no significant transcriptional change under hypoxic conditions. Moreover, we have isolated and characterized Arabidopsis mutants defective in GAD1 and GABA-T1. In gad1 mutants, GAD activity was significantly reduced in roots but was not affected in shoots. In the gaba-t1 mutant, GABA-T activity was decreased to negligible levels in both shoots and roots. These mutants were phenotypically normal under normal growth conditions except for the reduced seed production of the pop2 mutants as described previously. However, metabolite analysis revealed significant changes in GABA content in gad1 and gaba-t1 mutants. The levels of alanine under hypoxic conditions were also affected in the roots of gad1 and gaba-t1 mutants. The partial inhibition of the hypoxia-induced alanine accumulation in roots of these mutants suggests that the GABA shunt is, in part, responsible for the alanine accumulation under hypoxia.

Keywords: Alanine — Amino acid metabolism — Anaerobic stress — GABA — Hypoxia — Low oxygen stress

Abbreviations: ADH, alcohol dehydrogenase; AlaAT, alanine aminotransferase; AlaDH, alanine dehydrogenase; DTT, dithiothreitol; ELISA, enzyme-linked immunosorbent assay; GABA, {gamma}-aminobutyric acid; GABA-T, GABA transaminase; GAD, glutamate decarboxylase; GDH, glutamate dehydrogenase; GHB, {gamma}-hydroxybutyrate; PLP, pyridoxal-5-phosphate; PMSF, phenylmethylsulfonyl fluoride; ROI, reactive oxygen intermediate; SSA, succinic semialdehyde; TCA cycle, tricarboxylic acid cycle; RT–PCR, reverse transcriptase–PCR.

(Received October 16, 2007; Accepted December 4, 2007)
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