Metabolic alterations in free amino acid contents of rice (Oryza sativa l.) seeds during high-humidity storage

Numerous physicochemical and physiological changes occur in seed cells during their aging process. To elucidate the functional biology related to aging, the investigation of cellular metabolites is an important strategy. Changes of free amino acid contents during seed deterioration were measured using capillary electrophoresis-mass spectrometry. Under high-humidity conditions, contents of aspartic acid, glutamine and glutamic acid in seed decreased. In contrast, contents of Gamma Amino Butyric Acid (GABA), alanine, threonine, glycine, histidine and lysine increased. The largest increase was observed in GABA. Additionally, the gene of glutamic acid decarboxylase, which synthesizes GABA from glutamic acid, was expressed stably in seeds that had been stored for several months under high-humidity conditions. The expression levels of GABA transaminase, which synthesizes succinic semialdehyde and alanine from GABA, decreased in seeds stored under identical conditions. These results suggested that the GABA metabolism including gene expression was probably influenced by high-humidity conditions during seed storage. Therefore, the metabolism of free amino acid, especially GABA, must be focused on to understand molecular mechanism of seed deterioration