Plants have developed adaptive responses allowing them to cope with nitrogen (N) fluctuation in the soil and maintain growth despite changes in external N availability. Nitrate is the most important N form in temperate soils. Nitrate uptake by roots and its transport at the whole-plant level involves a large panoply of transporters and impacts plant performance. Four families of nitrate-transporting proteins have been identified so far: nitrate transporter 1/peptide transporter family (NPF), nitrate transporter 2 family (NRT2), the chloride channel family (CLC), and slow anion channel-associated homologues (SLAC/SLAH). Nitrate transporters are also involved in the sensing of nitrate. It is now well established that plants are able to sense external nitrate availability, and hence that nitrate also acts as a signal molecule that regulates many aspects of plant intake, metabolism, and gene expression. This review will focus on a global picture of the nitrate transporters so far identified and the recent advances in the molecular knowledge of the so-called primary nitrate response, the rapid regulation of gene expression in response to nitrate. The recent discovery of the NIN-like proteins as master regulators for nitrate signalling has led to a new understanding of the regulation cascade.

C and N metabolism are co-regulated in higher plants. Energy and C skeletons required for N assimilation are provided either directly or indirectly (via Suc) by photosynthesis. A high rate of CO 2 assimilation favors a high rate of N assimilation and vice versa (Ferrario et al., 1995). Molecular and metabolic controls are implicated in the C to N interaction, involving reciprocal regulation between the pathways of C and N assimilation (Champigny and Foyer,

Glutamate (Glu) dehydrogenase (GDH) catalyses the reversible amination of 2-oxoglutarate for the synthesis of Glu using ammonium as a substrate. This enzyme preferentially occurs in the mitochondria of companion cells of a number of plant species grown on nitrate as the sole nitrogen source. For a better understanding of the controversial role of GDH either in ammonium assimilation or in the supply of 2-oxoglutarate (F. Dubois, T. Tercé-Laforgue, M.B. Gonzalez-Moro, M.B. Estavillo, R. Sangwan, A. Gallais, B. Hirel [2003] Plant Physiol Biochem 41: 565-576), we studied the localization of GDH in untransformed tobacco (Nicotiana tabacum) plants grown either on low nitrate or on ammonium and in ferredoxin-dependent Glu synthase antisense plants. Production of GDH and its activity were strongly induced when plants were grown on ammonium as the sole nitrogen source. The induction mainly occurred in highly vascularized organs such as stems and midribs and was likely to be due to accumulation of phloem-translocated ammonium in the sap. GDH induction occurred when ammonia was applied externally to untransformed control plants or resulted from photorespiratory activity in transgenic plants down-regulated for ferredoxin-dependent Glu synthase. GDH was increased in the mitochondria and appeared in the cytosol of companion cells. Taken together, our results suggest that the enzyme plays a dual role in companion cells, either in the mitochondria when mineral nitrogen availability is low or in the cytosol when ammonium concentration increases above a certain threshold.Ammonium is the ultimate form of inorganic nitrogen available to the plant. It can originate from a wide variety of metabolic processes such as nitrate reduction, photorespiration, phenylpropanoid metabolism, utilization of nitrogen transport compounds, amino acid catabolism, symbiotic nitrogen fixation (Hirel and Lea, 2001), and insect digestion in carnivorous plants (Schulze et al., 1997). It is then incorporated into an organic molecule, 2-oxoglutarate, by the combined action of the enzymes Gln synthetase (GS) and Glu synthase (Gln-2-oxoglutarate aminotransferase; GOGAT) to allow the synthesis of Gln and Glu. Both amino acids are further used as amino group donors for the synthesis of virtually all the nitrogencontaining molecules including the other amino acids needed for protein synthesis and nucleotides used as basic molecules for RNA and DNA synthesis (Hirel and Lea, 2001). The GS/GOGAT cycle is the major mechanism of ammonium assimilation in higher plants regardless of the various sources of ammonium listed above. However, it has often been argued that other enzymes have the capacity to assimilate ammonium, leading to the hypothesis that alternative pathways might operate under particular physiological conditions when the GS/GOGAT pathway may not be able to fulfill its function (Harrison et al., 2003).One of these alternative pathways is the reaction catalyzed by the mitochondrial NAD(H)-dependent Glu dehydrogenase (GDH; EC 1.4.1.2), which possesses the ...

The PII signal transducing protein is involved in carbon/nitrogen (C/N) sensing in bacteria and cyanobacteria. In higher plants the function of the PII homolog GLB1 is not known. GLB1 transcripts were found in all plant organs tested, while in Arabidopsis leaves GLB1 expression and PII protein levels were not significantly affected by either the day/night cycle or N-nutrition. Its putative regulatory role in plants has been studied by analysing Arabidopsis thaliana T-DNA insertion lines in the GLB1 gene. These PII mutants showed an 80% (PIIV1 mutant) and 100% (PIIS2 mutant) reduced AtGLB1 transcript level and no detectable PII protein. They did not display an altered growth or developmental phenotype when grown under non-limiting conditions suggesting that the PII protein does not play a crucial role in plants. However, in vitro grown PII mutants did show a higher sensitivity to nitrite (NO (2) (-) ) compared to the wild-type plants. This observation is reminiscent of the role of PII in the regulation of NO (2) (-) metabolism in cyanobacteria. Furthermore, when grown hydroponically, the PII mutants displayed a slight increase in carbohydrate (starch and sugars) levels in response to N starvation and a slight decrease in the levels of ammonium (NH (4) (+) ) and amino acids (mainly Gln) in response to NH (4) (+) resupply. Although the phenotypic changes are rather small in the mutant lines, these data support the hypothesis of a subtle involvement of the PII protein in the regulation of some steps of primary C and N metabolism.