Abstract
It has been revealed that high NaCl stress (>60 mmol L−1) induced phenolics accumulation in barley seedlings, with γ-aminobutyric acid (GABA) playing a key role. Interestingly, low NaCl stimulus (20 mmol L−1) enhancing phenolics synthesis and growth of barley seedlings was also reported recently. Hence, exogenous GABA and its bio-synthesis inhibitor 3-mercaptopropionic acid (3-MP) were applied to reveal the mechanism of GABA regulating phenolics metabolism in barley seedlings treated with 20 mmol L−1 NaCl. The contents of total phenolics and flavonoids significantly increased by 11.64% and 14.52% under NaCl, respectively. The addition of GABA further increased phenolics and flavonoids contents, especially for gallic acid, protocatechuic acid, caffeic acid, and quercetin, compared with NaCl treatment. Simultaneously, GABA increased the activities and mRNA levels of phenylalanine ammonia lyase (PAL), cinnamic acid 4-hydroxylase (C4H), and 4-coumalyl CoA ligase (4CL). The addition of 3-MP suppressed the above effects, except for increasing the protein levels of PAL, C4H, and 4CL. Low concentration of NaCl not only promoted growth, but also stimulated endogenous GABA metabolism to affect key enzymes activities and mRNA levels for phenolics synthesis in barley seedlings.
Highlights
Barley grains have the characteristics of high vitamins, high dietary fiber, and high protein contents, which could be stimulated by the accumulation of γ-aminobutyric acid (GABA), phenolics, and other nutrients by germination [1]
It can be seen that the GABA content of barley seedlings increased to its maximum on the 2nd day, which was 1.25 times that of the control
The results show that there was a downward trend in the glutamate dehydrogenase (GAD) activity of barley seedlings with the increase of germination days (Figure 1b)
Summary
Barley grains have the characteristics of high vitamins, high dietary fiber, and high protein contents, which could be stimulated by the accumulation of γ-aminobutyric acid (GABA), phenolics, and other nutrients by germination [1]. GABA is a four-carbon non-protein amino acid found in prokaryotes and eukaryotes with the functions of lowering blood pressure, sedation, improving kidney and liver function, and promoting alcohol metabolism in the human body. It is mainly synthesized from glutamate catalyzed by glutamate dehydrogenase (GAD) in the cytoplasm of monocotyledonous plants [2], transported to mitochondria and degraded into succinic semialdehyde (SSA) by GABA aminotransferase (GABA-T). GABA is necessary for carbon and nitrogen metabolism in plant growth and development under abiotic stress, including salt, hypoxia, and so on [4]
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