Effects of calmodulin overexpression on gamma-aminobutyric acid (GABA) levels and glutamate decarboxylase activity in rice seedlings

The transgenic 'Khao Dawk Mali 105' (KDML105) rice (Oryza sativa L.) lines overexpressing OsCam1-1 calmodulin gene are more tolerant to salt stress and drought than the wild type (WT) plants. Here, we examined the production of gamma-aminobutyric acid (GABA) and glutamate decarboxylase (GAD) during germination in such plants. During the first 24 h of soaking in water under normal conditions, the GABA content increased to a higher degree in all transgenic rice lines than in the WT. During seed germination (from day 2 to day 4) under both normal and salt stress conditions, the GABA content in all rice lines rapidly increased. GABA content under salt stress conditions then continued to increase and reached higher levels in all transgenic overexpression lines than in WT after 6-8 days of germination. Two of the three transgenic overexpression lines also exhibited statistically significantly higher GAD activity while the other line had a trend of having higher activity compared to the WT. These results suggest that transgenic rice plants overexpressing OsCam1-1 had enhanced GAD activity facilitated by calmodulin overexpression, resulting in higher GABA content.