Almost all plants have to deal with climatic factors and to develop strategies to adapt their metabolism, which may have been adversely affected, in order to acclimate and survive under these unfavourable growth conditions. Mechanisms of plant stress tolerance involve some changes in gene expression, protein modification and metabolic pathways. More particularly, plant amino acid (AA) composition is modified by environmental conditions and characterized by an elevated accumulation of specific AAs involved in plant stress tolerance. These free AAs are synthesized by various distinct metabolic networks and accumulate differentially in plant species exposed to various stresses. In response to these environmental constraints, AA metabolism plays an important regulatory role, not only because AAs are constituents of proteins but also because free AAs are potential regulatory and signalling molecules, and precursors for energy-associated metabolites, as well numerous secondary metabolites, that have several functions in plant growth and adaptive responses to various stresses. The AA synthesis pathways are complex and undergo tight regulation, particularly in respect of the metabolism of three specific AA family pathways when plants are exposed to abiotic stresses. First, the glutamate family pathway is strongly activated under stress, leading to the accumulation of proline and γ-aminobutyric acid (GABA), two stress-related molecules involved in metabolic responses to stress. Secondly, stimulation of the metabolism of the pyruvate family pathway mainly allows the production and accumulation of alanine, which is the most abundant amino acid under oxygen deficiency; it also leads to the accumulation of branched-chain amino acids, such as leucine and valine, which could play the role of compatible solutes and provide an alternative source of respiratory substrates. Thirdly, regulation of biosynthetic and catabolic fluxes through the aspartate family pathway appears to be essential for generating energy, mainly via lysine catabolism, as stress causes energy deprivation.

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Nitrate is not only an essential nutrient for plants, but also a signal involved in plant development. We have previously shown in the model legume Medicago truncatula, that the nitrate signal, which restricts primary root growth, is mediated by MtNPF6.8, a nitrate transporter. Nitrate signal also induces changes in reactive oxygen species accumulation in the root tip due to changes in cell wall peroxidase (PODs) activity. Thus, it was interesting to determine the importance of the role of MtNPF6.8 in the regulation of the root growth by nitrate and identify the POD isoforms responsible for the changes in POD activity. For this purpose, we compared in M. truncatula a npf6.8 mutant and nitrate insensitive line deficient in MtNPF6.8 and the corresponding wild and sensitive genotype for their transcriptomic and proteomic responses to nitrate. Interestingly, only 13 transcripts and no protein were differently accumulated in the primary root tip of the npf6.8-3 mutant line in response to nitrate. The sensitivity of the primary root tip to nitrate appeared therefore to be strongly linked to the integrity of MtNPF6.8 which acts as a master mediator of the nitrate signal involved in the control of the root system architecture. In parallel, 7,259 and 493 genes responded, respectively, at the level of transcripts or proteins in the wild type, 196 genes being identified by both their transcript and protein. By focusing on these 196 genes, a concordance of expression was observed for most of them with 143 genes being up-regulated and 51 being down-regulated at the two gene expression levels. Their ontology analysis uncovered a high enrichment in POD genes, allowing the identification of POD candidates involved in the changes in POD activity previously observed in response to nitrate.

In this paper we discuss thc ways in which our understanding of the nature o f the molecular controls o f nitrogen assimilation has hccn increased by the use of non-leguminous and lcguminous plants with genetically-altered capacities for ammonia assimilation. Using tobacco or Lorus as model plants. Glutamine synthetase (GS) and glut;imate synthasc (GOGAT) activities have hecn altered by stimulating or inhibiting in an organ-or tissue-specific manner the expression of the corresponding genes. In a few selected examples, the physiological impact o f these genetic manipulations h;is been studied on plants grown under diffcrent nitrogen regimcs. The use o f such genclicallymodified plants will allow us to bcttcr understand the molecular control ofthis metabolic pathway. It is also potentially ot'great importance in agriculture if such internal and stable modilications are benelicial in terms of nitrogen use enicicncy. thus avoiding an excessive uliliration of fertilizers or herbicides (GS inhibitors). Our current knowledge and prospects for future development are explored.