Fluctuations of γ-aminobutyrate, γ-hydroxybutyrate, and related amino acids in Arabidopsis leaves as a function of the light-dark cycle, leaf age, and N stress

To gain further insight into the metabolic role of gamma-aminobutyrate (GABA), we determined the pool sizes of GABA and its catabolic products, alanine and gamma-hydroxybutyrate (GHB), as well as key amino acids (Glu, Gln, Asp, Asn, Pro, Gly, Ser), in Arabidopsis leaves as a function of the light-dark cycle, leaf age (old versus young), and N stress (continuous versus interrupted N supply). Regardless of time of day and leaf age, there was a close relationship among Glu, GABA, and GHB when N was supplied continuously, indicating that GABA and GHB were probably derived exclusively from Glu and GABA, respectively. Ala was also closely linked to GABA in young leaves, but not in old leaves, a result consistent with the existence of multiple sources of Ala. The nature of the responses of GABA and GHB to an interrupted N supply depended on leaf age, and differed from responses exhibited by Glu, Gln, and Asn. Overall fluctuations in primary amino acids under both continuous and interrupted N supply, as well as those associated with photorespiration, aging, and stress, suggest that the old and young leaves chosen for study here function in Arabidopsis as source and sink leaves, respectively.