RESUMO
Glutamate plays a central role in nitrogen flow and serves as a nitrogen donor for the production of amino acids. In plants, some amino acids work as buffers: during photorespiration, ammonium derived from the conversion of glycine to serine is promptly reassimilated into glutamate by the glutamine synthetase (GS-2)/ferredoxin-dependent glutamate synthase (Fd-GOGAT) cycle. The glutamate concentration is relatively stable compared with those of other amino acids under environmental changes. The few studies dealing with glutamate homeostasis have but all used knockouts or mutants of these enzymes. Here, we generated Fd-GOGAT (GLU1)-overexpressing Arabidopsis plants to analyze changes in the amino acid pool caused by glutamate overproduction under different ammonium conditions controlled by CO(2) concentration, light intensity and nitrate concentration. Under photorespiratory conditions with sufficient ammonium supply, aspartate increased and glutamine and glycine decreased, but glutamate barely changed. Under non-photorespiratory conditions, however, glutamate and most other amino acids increased. These results suggest that the synthesized glutamate is promptly converted into other amino acids, especially aspartate. In addition, ammonium supply by photorespiration does not limit glutamate biosynthesis, but glutamine and glycine are important. This study will contribute to the understanding of glutamate homeostasis in plants.
Assuntos
Aminoácido Oxirredutases/genética , Proteínas de Arabidopsis/genética , Arabidopsis/metabolismo , Expressão Gênica , Ácido Glutâmico/metabolismo , Homeostase , Aminoácido Oxirredutases/biossíntese , Aminoácido Oxirredutases/metabolismo , Aminoácidos/metabolismo , Arabidopsis/genética , Arabidopsis/efeitos da radiação , Proteínas de Arabidopsis/biossíntese , Proteínas de Arabidopsis/metabolismo , Carbono/metabolismo , Dióxido de Carbono/administração & dosagem , Respiração Celular/genética , Homeostase/efeitos da radiação , Luz , Nitratos/administração & dosagem , Nitrogênio/metabolismo , Componentes Aéreos da Planta/genética , Componentes Aéreos da Planta/metabolismo , Componentes Aéreos da Planta/efeitos da radiação , Folhas de Planta/genética , Folhas de Planta/metabolismo , Folhas de Planta/efeitos da radiação , Plantas Geneticamente Modificadas , RNA Mensageiro/metabolismo , Reação em Cadeia da Polimerase Via Transcriptase Reversa , Plântula/genética , Plântula/metabolismo , Plântula/efeitos da radiaçãoRESUMO
The metabolism of vegetative organs in plants changes during the development of the reproductive organs. The regulation of this metabolism is important in the control of crop productivity. However, the complexity of the regulatory systems makes it difficult to elucidate their mechanisms. To examine these mechanisms, we constructed model experiments using Arabidopsis to analyze metabolic and gene expression changes during leaf-stage progression and after removal of the reproductive organs. Leaf gene expression levels and content of major amino acids, both of which decreased during leaf-stage progression, increased after removal of the reproductive organs. In particular, the levels of expression of cytokinin biosynthesis genes and cytokinin-responsive genes and the cytokinin content increased after removal of the reproductive organs. Analysis of plants with knockout of a cytokinin-biosynthetic gene (AtIPT3) and a cytokinin receptor gene (AHK3) indicated that glutamate dehydrogenase genes (GDH3) were regulated by cytokinin signaling. These data suggest that cytokinins regulate communication between reproductive and vegetative organs, and that GDH3 is one target of the cytokinin-mediated regulation of nitrogen metabolism.
