Phenotypic effects from the expression of a deregulated AtGAD1 transgene and GABA pathway suppression mutants in maize

Glutamate decarboxylase-1 is essential for efficient acclimation of Arabidopsis thaliana to nutritional phosphorus deprivation

Glutamate decarboxylase (GAD) is a Ca2+ -calmodulin-activated, cytosolic enzyme that produces γ-aminobutyrate (GABA) as the committed step of the GABA shunt. This pathway bypasses the 2-oxoglutarate to succinate reactions of the tricarboxylic acid (TCA) cycle. GABA also accumulates during many plant stresses. We tested the hypothesis that AtGAD1 (At5G17330) facilitates Arabidopsis acclimation to Pi deprivation. Quantitative RT-PCR and immunoblotting revealed that AtGAD1 transcript and protein expression is primarily root-specific, but inducible at lower levels in shoots of Pi-deprived (-Pi) plants. Pi deprivation reduced levels of the 2-oxoglutarate dehydrogenase (2-OGDH) cofactor thiamine diphosphate (ThDP) in shoots and roots by > 50%. Growth of -Pi atgad1 T-DNA mutants was significantly attenuated relative to wild-type plants. This was accompanied by: (i) an > 60% increase in shoot and root GABA levels of -Pi wild-type, but not atgad1 plants, and (ii) markedly elevated anthocyanin and reduced free and total Pi levels in leaves of -Pi atgad1 plants. Treatment with 10 mM GABA reversed the deleterious development of -Pi atgad1 plants. Our results indicate that AtGAD1 mediates GABA shunt upregulation during Pi deprivation. This bypass is hypothesized to circumvent ThDP-limited 2-OGDH activity to facilitate TCA cycle flux and respiration by -Pi Arabidopsis.

Characterization of GABA-Transaminase Gene from Mulberry (Morus multicaulis) and Its Role in Salt Stress Tolerance

Gamma-aminobutyric acid (GABA) has been reported to accumulate in plants when subjected to salt stress, and GABA-transaminase (GABA-T) is the main GABA-degrading enzyme in the GABA shunt pathway. So far, the salt tolerance mechanism of the GABA-T gene behind the GABA metabolism remains unclear. In this study, the cDNA (designated MuGABA-T) of GABA-T gene was cloned from mulberry, and our data showed that MuGABA-T protein shares some conserved characteristics with its homologs from several plant species. MuGABA-T gene was constitutively expressed at different levels in mulberry tissues, and was induced substantially by NaCl, ABA and SA. In addition, our results demonstrated that exogenous application of GABA significantly reduced the salt damage index and increased plant resistance to NaCl stress. We further performed a functional analysis of MuGABA-T gene and demonstrated that the content of GABA was reduced in the transgenic MuGABA-T Arabidopsis plants, which accumulated more ROS and exhibited more sensitivity to salt stress than wild-type plants. However, exogenous application of GABA significantly increased the activities of antioxidant enzymes and alleviated the active oxygen-related injury of the transgenic plants under NaCl stress. Moreover, the MuGABA-T gene was overexpressed in the mulberry hairy roots, and similar results were obtained for sensitivity to salt stress in the transgenic mulberry plants. Our results suggest that the MuGABA-T gene plays a pivotal role in GABA catabolism and is responsible for a decrease in salt tolerance, and it may be involved in the ROS pathway in the response to salt stress. Taken together, the information provided here is helpful for further analysis of the function of GABA-T genes, and may promote mulberry resistance breeding in the future.