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Abstract

In seeds, glutamate decarboxylase (GAD) operates at the metabolic nexus between carbon and nitrogen metabolism by catalysing the unidirectional decarboxylation of Glu to form gamma-aminobutyric acid (GABA). To elucidate the regulatory role of GAD in seed development, we generated Arabidopsis thaliana transgenic plants expressing a truncated GAD from Petunia hybrida missing the C-terminal regulatory Ca2+-calmodulin (CaM) 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, though not Glu or Pro. Dry transgenic seeds had higher protein content than the wild-type seeds, but lower amounts of the intermediates of glycolysis, glycerol and malate. The total fatty acid (TFA) content of the transgenic seeds was 50% lower than in that of the wild-type, while acyl CoAs 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 upregulated at the transcript level included the tricarboxylic acid cycle, fatty acid elongation, the shikimate pathway and Trp metabolism, N-C remobilization and programmed cell death. Genes involved in the regulation of germination were similarly upregulated. Taken together these results indicate that the GAD-mediated conversion of Glu to GABA during seed development plays an important role in balancing carbon and nitrogen metabolism and in storage reserve accumulation.