Targeted Enhancement of Glutamate-to-γ-Aminobutyrate Conversion in Arabidopsis Seeds Affects Carbon-Nitrogen Balance and Storage Reserves in a Development-Dependent Manner

• Published in: Plant physiology (Bethesda) 2011-11, Vol.157 (3), p.1026-1042
• Main Authors: Fait, Aaron, Nesi, Adriano Nunes, Angelovici, Ruthie, Lehmann, Martin, Pham, Phuong Anh, Song, Luhua, Haslam, Richard P., Napier, Johnathan A., Galili, Gad, Fernie, Alisdair R.
• Format: Article
• Language: English
• Subjects: Amino acid metabolism; Amino acids; Arabidopsis - enzymology; Arabidopsis - genetics; Arabidopsis - growth & development; Arabidopsis - metabolism; BIOCHEMICAL PROCESSES AND MACROMOLECULAR STRUCTURES; Biological and medical sciences; Calmodulin - metabolism; Carbon - metabolism; Carbon Isotopes; Citric Acid Cycle; Desiccation; Electron Transport; Enzymes; Fatty acids; Fatty Acids - metabolism; Fundamental and applied biological sciences. Psychology; gamma-Aminobutyric Acid - metabolism; Gene Expression Regulation, Plant; Genes, Plant - genetics; Germination; Glutamate Decarboxylase - metabolism; Glutamic Acid - metabolism; Isotope Labeling; Lipid metabolism; Metabolism; Mitochondria - metabolism; Mutant Proteins - metabolism; Nitrogen - metabolism; Petunia - enzymology; Plant physiology and development; Plants; Plants, Genetically Modified; Protein Binding; Protein metabolism; Protein Structure, Tertiary; Seeds; Seeds - growth & development; Seeds - metabolism; Transgenic plants; Up-Regulation
• ISSN: 0032-0889; 1532-2548; 1532-2548
• DOI: 10.1104/pp.111.179986

In seeds, glutamate decarboxylase (GAD) operates at the metabolic nexus between carbon and nitrogen metabolism by catalyzing the unidirectional decarboxylation of glutamate to form y-aminobutyric acid (GABA). To elucidate the regulatory role of GAD in seed development, we generated Arabidopsis (Arabidopsis thaliana) transgenic plants expressing a truncated GAD from Petunia hybrida missing the carboxy1-terminal regulatory Ca²⁺ -calmodulin-binding domain under the transcriptional regulation of the seed maturation-specific phaseolin promoter. Dry seeds of the transgenic plants accumulated considerable amounts of GABA, and during desiccation the content of several amino acids increased, although not glutamate or proline. Dry transgenic seeds had higher protein content than wild-type seeds but lower amounts of the intermediates of glycolysis, glycerol and malate. The total fatty acid content of the transgenic seeds was 50% lower than in the wild type, while acyl-coenzyme A accumulated in the transgenic seeds. Labeling experiments revealed altered levels of respiration in the transgenic seeds, and fractionation studies indicated reduced incorporation of label in the sugar and lipid fractions extracted from transgenic seeds. Comparative transcript profiling of the dry seeds supported the metabolic data. Cellular processes up-regulated at the transcript level included the tricarboxylic acid cycle, fatty acid elongation, the shikimate pathway, tryptophan metabolism, nitrogen-carbon remobilization, and programmed cell death. Genes involved in the regulation of germination were similarly up-regulated. Taken together, these results indicate that the GAD-mediated conversion of glutamate to GABA during seed development plays an important role in balancing carbon and nitrogen metabolism and in storage reserve accumulation.