RESUMO
BACKGROUND: Glutamate plays a central position in the synthesis of a variety of organic molecules in plants and is synthesised from nitrate through a series of enzymatic reactions. Glutamate synthases catalyse the last step in this pathway and two types are present in plants: NADH- or ferredoxin-dependent. Here we report a genome wide microarray analysis of the transcriptional reprogramming that occurs in leaves and roots of the A. thaliana mutant glu1-2 knocked-down in the expression of Fd-GOGAT1 (GLU1; At5g04140), one of the two genes of A. thaliana encoding ferredoxin-dependent glutamate synthase. RESULTS: Transcriptional profiling of glu1-2 revealed extensive changes with the expression of more than 5500 genes significantly affected in leaves and nearly 700 in roots. Both genes involved in glutamate biosynthesis and transformation are affected, leading to changes in amino acid compositions as revealed by NMR metabolome analysis. An elevated glutamine level in the glu1-2 mutant was the most prominent of these changes. An unbiased analysis of the gene expression datasets allowed us to identify the pathways that constitute the secondary response of an FdGOGAT1/GLU1 knock-down. Among the most significantly affected pathways, photosynthesis, photorespiratory cycle and chlorophyll biosynthesis show an overall downregulation in glu1-2 leaves. This is in accordance with their slight chlorotic phenotype. Another characteristic of the glu1-2 transcriptional profile is the activation of multiple stress responses, mimicking cold, heat, drought and oxidative stress. The change in expression of genes involved in flavonoid biosynthesis is also revealed. The expression of a substantial number of genes encoding stress-related transcription factors, cytochrome P450 monooxygenases, glutathione S-transferases and UDP-glycosyltransferases is affected in the glu1-2 mutant. This may indicate an induction of the detoxification of secondary metabolites in the mutant. CONCLUSIONS: Analysis of the glu1-2 transcriptome reveals extensive changes in gene expression profiles revealing the importance of Fd-GOGAT1, and indirectly the central role of glutamate, in plant development. Besides the effect on genes involved in glutamate synthesis and transformation, the glu1-2 mutant transcriptome was characterised by an extensive secondary response including the downregulation of photosynthesis-related pathways and the induction of genes and pathways involved in the plant response to a multitude of stresses.
