The GABA shunt pathway in germinating seeds of wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.) under salt stress

Abstract

Soil salinity is one of the major abiotic stresses affecting seed germination, crop growth and productivity. In this study, seeds of three wheat (Triticum aestiveum L.) and three barley (Hordeum vulgare L.) cultivars were treated with different concentrations of NaCl to investigate the effect of salt on seed germination physiology and metabolism through the characterization of seed germination pattern, gamma-aminobutyric acid (GABA) shunt metabolite accumulation [GABA, glutamate (Glu) and alanine (Ala)] and glutamate decarboxylase (GAD) expression using RT-PCR. A trend of decreasing germination percentage with increasing NaCl concentrations was observed. Under all salt stress treatments, data showed significant increase with positive correlation (r = 0.50–0.99) between abundance of GABA shunt metabolites and salt concentration in all wheat and barley cultivars for 5 days. Increased GABA content was associated with a small but significant increase in Ala and Glu content in all cultivars. In all NaCl treatments, the transcription of GAD in terms of RNA abundance showed a significant increase in all cultivars with positive correlation (r = 0.50–0.98). Data showed significant association between GAD RNA transcription and the response of germinating seeds to salt stress in terms of GABA shunt metabolite accumulation. The elevated expression of GAD under salinity suggests the need for elevated activity of the GAD-mediated conversion of Glu to GABA during seed germination, which provides alternative metabolic routes to the respiratory machinery, balancing carbon and nitrogen metabolism and osmolyte synthesis in germinating seeds of wheat and barley under salt stress.

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

“…It was proposed that when carbon shortage becomes a limiting factor for plant growth and development, GABA can be used as a carbon source to support the TCA cycle under salt stress [ 23 , 51 , 52 ]. Since GABA-T catalyzes GABA conversion into succinic semialdehyde in the GABA shunt, its overexpression will accelerate the metabolism of GABA and provide more carbon sources for the TCA cycle, which may not have an adverse effect on the TCA cycle in the transgenic plants [ 26 , 53 , 54 ]. In addition, it was suggested that GABA is also involved in cytosolic pH and osmotic adjustment mechanisms defending against various stresses [ 55 , 56 , 57 ].…”

Can Alternative Metabolic Pathways and Shunts Overcome Salinity Induced Inhibition of Central Carbon Metabolism in Crops?

“…The GABA shunt produces glutamate that enters from cytosol to the mitochondria for further catabolic activity and its biochemical interactions with other cytosolic respiratory pathways such as glycolysis and the oxidative pentose pathway remain to be resolved. The GABA shunt is a conserved pathway in eukaryotes ( Bouche and Fromm, 2004 ) and the important role it plays in respiratory metabolism during salinity exposure has been shown in barley and wheat during a range of developmental stages including seed germination, seedling growth, and in mature plants ( Al-Quraan et al, 2019 ; Che-Othman et al, 2020 ). This functional bypass of 2-oxoglutarte dehydrogenase to supply succinate to the TCA cycle may be one of many metabolic bypasses and shunts that are induced during salinity exposure to overcome salinity induced enzyme inhibition.…”

“…Salt exposure also increases two other GABA shunt metabolites, glutamate and alanine, and the activity GABA metabolism enzymes (Renault et al, 2010;Al-Quraan et al, 2019;Che-Othman et al, 2020). Because GABA production is closely connected to the abundance of glutamate, GABA synthesis plays an important role in determining glutamate abundance with GABA acting as a nitrogen storage metabolite.…”

The versatile GABA in plants

“…GABA is involved in seed gemination and primary and adventitious root growth. GABA levels increase in germination of soybean, 41 oats, 42 barnyard millet, 43 adlay, 44 rice, 45 wheat, 46 , 47 barley, 48 and Chinese wild rice 49 seeds. GABA activates α-amylase gene expression and promotes seed starch degradation in a dose-dependent pattern in seed germination.…”