In seeds, glutamate decarboxylase (GAD) operates at the metabolic nexus between carbon and nitrogen metabolism by catalyzing the unidirectional decarboxylation of glutamate to form g-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 carboxyl-terminal regulatory Ca 2+ -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.Efficient assimilation of carbon (C) and nitrogen (N) is essential for optimal plant growth, productivity, and yield (Stitt, 1999), particularly in seeds in which the content of essential amino acids is low. Hence, a comprehensive understanding of seed development and metabolism is central to the enhancement of crop yield and quality. Seed development can be divided into three phases: cell division, maturation (accumulation of food reserves), and desiccation (Weber et al., 2005). The shift from one phase to the other is affected by sugars (Suc, hexoses, trehalose) and abscisic acid. In the early stages of seed development, maternal regulation is maintained via assimilate unloading and supply of nutrients, thereby sustaining cell division. During maturation, seed metabolism changes and storage reserves accumulate in expanding cells. At this time, photosynthetic activity is initiated in the seed, which is believed to improve oxygen supply and the energy state in the seed, thus counteracting increasingly hypoxic conditions (Borisjuk and Rolletschek, 2009). The maturation stage is followed by a phase termed "maturation drying," in which the metabolism of Arabidopsis (Arabidopsis thaliana) seeds shifts from a gen...

No abstract