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.

The physiological and biochemical role of the γ-aminobutyric acid (GABA) shunt pathway in green pea seedlings (Pisum sativum L.) was studied in response to soil water holding capacity levels: 80%, 60%, 40%, 20%, and 10% grown under continuous light at 25 °C for 7 days and 14 days, separately. Characterization of seeds germination pattern, seedlings growth (plant height, fresh and dry weight, and chlorophyll contents), GABA shunt metabolite (GABA, glutamate, and alanine) levels, total protein and carbohydrate levels, and oxidative damage (MDA level) were examined. Data showed a significant effect of drought stress on seed germination, plant growth, GABA shunt metabolites level, total protein and carbohydrate contents, and MDA level. A significant decline in seed germination percentage was recorded at a 20% drought level, which indicated that 20% of soil water holding capacity is the threshold value of water availability for normal germination after 14 days. Seedling fresh weight, dry weight, and plant height were significantly reduced with a positive correlation as water availability was decreased. There was a significant decrease with a positive correlation in Chl a and Chl b contents in response to 7 days and 14 days of drought. GABA shunt metabolites were significantly increased with a negative correlation as water availability decreased. Pea seedlings showed a significant increase in protein content as drought stress was increased. Total carbohydrate levels increased significantly when the amount of water availability decreased. MDA content increased slightly but significantly after 7 days and sharply after 14 days under all water stress levels. The maximum increase in MDA content was observed at 20% and 10% water levels. Overall, the significant increases in GABA, protein and carbohydrate contents were to cope with the physiological impact of drought stress on Pisum sativum L. seedlings by maintaining cellular osmotic adjustment, protecting plants from oxidative stress, balancing carbon and nitrogen (C:N) metabolism, and maintaining cell metabolic homeostasis and cell turgor. The results presented in this study indicated that severe (less than 40% water content of the holding capacity) and long-term drought stress should be avoided during the germination stage to ensure proper seedling growth and metabolism in Pisum sativum L.

In the present study, three Arabidopsis thaliana pop2 mutant lines with different T-DNA insertions in a gene coding γ-aminobutyric acid transaminase (GABA-TA) were screened for seed germination percentage, stress-induced oxidative damage, and GABA content and metabolism under various abiotic stresses including high temperature (42 ºC), low temperature (4 ºC), salinity (NaCl), and osmotic stress (mannitol). All mutant lines showed a decreased germination under all the stress treatments with a significant reduction in the pop2-1 and pop2-3 mutant lines. Content of GABA and MDA increased significantly in all pop2 mutants and wild type (WT) seedlings in response to all the treatments. However, content of GABA and MDA was lower in all pop2 mutants comparing to the WT under the same treatments. GABA increased already after 30 min and increased significantly after 2 h at 42 ºC especially in the pop2-3 and WT seedlings. In response to the cold treatment, GABA content increased up to 4-fold compared to the control in all pop2 mutants and WT seedlings. In response to the NaCl treatment, GABA accumulated slightly in the WT and all pop2 mutants. On the contrary, GABA content increased significantly in the pop2, pop2-1, and pop2-3 mutants and WT under all mannitol treatments.