Universität zu Köln

Mekonnen, Dereje Worku (2013). Phenotypic and chemotypic characterization of GABA-shunt mutants in Arabidopsis thaliana. PhD thesis, Universität zu Köln.

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Abstract

Gamma-Aminobutyric acid (GABA) is a four carbon non protein amino acid, and the pathway that involves its production and degradation is called the GABA shunt. The GABA shunt is a short enzymatic pathway that involves three enzymes: glutamate decarboxylase (GAD), GABA transaminase (GABA-T) and succinic semi aldehyde dehydrogenase (SSADH). GABA shunt is conserved almost in all organisms studied so far. The pathway starts in the cytosol and finishes in mitochondria in higher organisms like plants and mammals, whereas, in yeast it is exclusively cytosolic. The presence of GABA in plants has been known since over half a century ago. The induction of its accumulation under biotic and abiotic stresses has been well documented. However, the specificity of the response and the role of GABA and the GABA shunt under stress conditions have been elusive. Several Arabidopsis GABA shunt mutants have been studied and were shown to have defects in vegetative growth, reproductive growth and response to stress. To describe some, gad1 mutant was shown to have a reduced root growth. The pop2-1 mutant, defective in the activity of GABA-T, accumulated a very high level of GABA in the flower and that led to a pollen tube elongation defect. Arabidopsis ssadh mutants exhibited a severe growth phenotype, accumulated high -hydroxybutyric acid (GHB) and H2O2, and were sensitive to high-light conditions. In this work, several mutant lines have been generated and investigated to answer a range of questions; these include study on the specificity of the GABA response during abiotic stress, identifying the reasons for severe ssadh growth phenotype and suppression of the ssadh phenotype. Arabidopsis gad1/2 mutant is depleted in GABA and is unable to induce GABA synthesis under drought, salt and osmotic stress. Hence, the mutant plants were oversensitive to these stresses, indicating that GABA accumulation during abiotic stresses was a specific response. In an attempt to identify the cause of ssadh phenotype, exogenous GHB was fed to wild type and ghbdh1/2 mutants. The GHB application, however, did not elicit the severe ssadh-like phenotype. But, treatment of wild type and ghbdh1/2 mutant with SSA re-produced the ssadh phenotype, suggesting that SSA, but not GHB, was the cause of the severe ssadh phenotype. To suppress the ssadh phenotype, the potential accumulation of SSA was blocked by simultaneous mutation of Gad1/2. This, indeed, partially suppressed the ssadh phenotype. Suppression of ssadh phenotype mediated by ethyl methanesulfonate (EMS) induced mutation was also successful. Based on the range of phenotypes obtained in GABA shunt mutants, further studies on the role of the GABA shunt in plants is important.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Mekonnen, Dereje Worku dmekonne@smail.uni-koeln.de UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-53054
Journal or Publication Title:	The GABA-deficient gad1/2 mutant reveals the importance of GABA in stomatal movement and drought tolerance of Arabidopsis
Date:	15 August 2013
Publisher:	Plant Physiology
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Biology > Botanical Institute
Subjects:	Life sciences
Uncontrolled Keywords:	Keywords Language no UNSPECIFIED
Date of oral exam:	4 December 2012
Referee:	Name Academic Title Fluegge, UIf-Ingo Prof. Dr. Maurino, Veronica PD. Dr.
Funders:	NRW International Graduate School IGS DHD program
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/5305