This review first summarizes the numerous studies that have described the interaction between the nitrogen supply and the response of photosynthesis, metabolism and growth to elevated [CO 2 ]. The initial stimulation of photosynthesis in elevated [CO 2 ] is often followed by a decline of photosynthesis, that is typically accompanied by a decrease of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), an accumulation of carbohydrate especially starch, and a decrease of the nitrogen concentration in the plant. These changes are particularly marked when the nitrogen supply is low, whereas when the nitrogen supply is adequate there is no acclimation of photosynthesis, no major decrease in the internal concentration of nitrogen or the levels of nitrogen metabolites, and growth is stimulated markedly. Second, emerging evidence is discussed that signals derived from nitrate and nitrogen metabolites such as glutamine act to regulate the expression of genes involved in nitrate and ammonium uptake and assimilation, organic acid synthesis and starch accumulation, to modulate the sugar-mediated repression of the expression of genes involved in photosynthesis, and to modulate whole plant events including shoot-root allocation, root architecture and flowering. Third, increased rates of growth in elevated [CO 2 ] will require higher rates of inorganic nitrogen uptake and assimilation. Recent evidence is discussed that an increased supply of sugars can increase the rates of nitrate and ammonium uptake and assimilation, the synthesis of organic acid acceptors, and the synthesis of amino acids.

Nitrate is both an important nutrient and a signalling molecule for plants. Although several components of the nitrate signalling pathway have been identified, their hierarchical organization remains unclear. Here we show that the localization of NLP7, a member of the RWP-RK transcription factor family, is regulated by nitrate via a nuclear retention mechanism. Genome-wide analyses revealed that NLP7 binds and modulates a majority of known nitrate signalling and assimilation genes. Our findings indicate that plants, like fungi and mammals, rely on similar nuclear retention mechanisms to instantaneously respond to the availability of key nutrients.

Plants have evolved a variety of mechanisms to adapt to N starvation. NITRATE TRANSPORTER2.4 (NRT2.4) is one of seven NRT2 family genes in Arabidopsis thaliana, and NRT2.4 expression is induced under N starvation. Green fluorescent protein and b-glucuronidase reporter analyses revealed that NRT2.4 is a plasma membrane transporter expressed in the epidermis of lateral roots and in or close to the shoot phloem. The spatiotemporal expression pattern of NRT2.4 in roots is complementary with that of the major high-affinity nitrate transporter NTR2.1. Functional analysis in Xenopus laevis oocytes and in planta showed that NRT2.4 is a nitrate transporter functioning in the high-affinity range. In N-starved nrt2.4 mutants, nitrate uptake under low external supply and nitrate content in shoot phloem exudates was decreased. In the absence of NRT2.1 and NRT2.2, loss of function of NRT2.4 (triple mutants) has an impact on biomass production under low nitrate supply. Together, our results demonstrate that NRT2.4 is a nitrate transporter that has a role in both roots and shoots under N starvation. INTRODUCTIONNitrate (NO 3 -) uptake from the soil and distribution through the plant can profoundly affect plant growth and productivity. Nitrogen (N) limitation decreases crop yield worldwide. To meet expanding food demands, the global use of N fertilizer in agricultural production is projected to increase threefold to reach 249 million tons annually by the year 2050 (Tilman et al., 2001). However, the recovery of N fertilizer by crops is low, with in some cases only 30 to 50% of the applied N being taken up by the crop (Peoples et al., 1995;Sylvester-Bradley and Kindred, 2009). The remainder is partly used by subsequent crops but can also be lost from the agro-ecosystem, and fertilizer runoff into aquatic systems leads to environmentally harmful eutrophication (Tilman, 1998). Therefore, improving N uptake efficiency is important to reduce the costs of crop production and pollution damage. Beside N uptake, N remobilization is another key step to improve N use efficiency in crops (Mickelson et al., 2003;Masclaux-Daubresse et al., 2008).Plants have evolved versatile mechanisms to cope with N limitation and N starvation, and besides major adaptive changes of the root system architecture (Drew and Saker, 1975), root NO 3 -uptake characteristics are regulated in response to N availability (Clarkson et al., 1986;Lejay et al., 1999;Glass, 2003). Physiological studies have led to the conclusion that at least three NO 3 -uptake systems are responsible for the influx of NO 3 -into roots (reviewed in Crawford and Glass, 1998;Daniel-Vedele et al., 1998;Forde, 2000). Two high-affinity transport systems (HATS) operate to take up NO 3 -at low concentrations in the external medium, and both display saturable kinetics as a function of the external NO 3 -concentration, with saturation in the range of 0.2 to 0.5 mM. The first one, constitutive HATS, is active in plants that have not been supplied with NO 3 -, whereas the second HATS is induced by NO ...

Nitrogen (N) is an essential macronutrient for plants. N levels in soil vary widely, and plants have developed strategies to cope with N deficiency. However, the regulation of these adaptive responses and the coordinating signals that underlie them are still poorly understood. The aim of this study was to characterize N starvation in adult Arabidopsis (Arabidopsis thaliana) plants in a spatiotemporal manner by an integrative, multilevel global approach analyzing growth, metabolites, enzyme activities, and transcript levels. We determined that the remobilization of N and carbon compounds to the growing roots occurred long before the internal N stores became depleted. A global metabolite analysis by gas chromatography-mass spectrometry revealed organ-specific differences in the metabolic adaptation to complete N starvation, for example, for several tricarboxylic acid cycle intermediates, but also for carbohydrates, secondary products, and phosphate. The activities of central N metabolism enzymes and the capacity for nitrate uptake adapted to N starvation by favoring N remobilization and by increasing the high-affinity nitrate uptake capacity after long-term starvation. Changes in the transcriptome confirmed earlier studies and added a new dimension by revealing specific spatiotemporal patterns and several unknown N starvation-regulated genes, including new predicted small RNA genes. No global correlation between metabolites, enzyme activities, and transcripts was evident. However, this multilevel spatiotemporal global study revealed numerous new patterns of adaptation mechanisms to N starvation. In the context of a sustainable agriculture, this work will give new insight for the production of crops with increased N use efficiency.

The plant specific RWP-RK family of transcription factors, initially identified in legumes and Chlamydomonas, are found in all vascular plants, green algae, and slime molds. These proteins possess a characteristic RWP-RK motif, which mediates DNA binding. Based on phylogenetic and domain analyses, we classified the RWP-RK proteins of six different species in two subfamilies: the NIN-like proteins (NLPs), which carry an additional PB1 domain at their C-terminus, and the RWP-RK domain proteins (RKDs), which are divided into three subgroups. Although, the functional analysis of this family is still in its infancy, several RWP-RK proteins have a key role in regulating responses to nitrogen availability. The nodulation-specific NIN proteins are involved in nodule organogenesis and rhizobial infection under nitrogen starvation conditions. Arabidopsis NLP7 in particular is a major player in the primary nitrate response. Several RKDs act as transcription factors involved in egg cell specification and differentiation or gametogenesis in algae, the latter modulated by nitrogen availability. Further studies are required to extend the general picture of the functional role of these exciting transcription factors.

Here, we report that SUGARS WILL EVENTUALLY BE EXPORTED TRANSPORTER (SWEET16) from Arabidopsis (Arabidopsis thaliana) is a vacuole-located carrier, transporting glucose (Glc), fructose (Fru), and sucrose (Suc) after heterologous expression in Xenopus laevis oocytes. The SWEET16 gene, similar to the homologs gene SWEET17, is mainly expressed in vascular parenchyma cells. Application of Glc, Fru, or Suc, as well as cold, osmotic stress, or low nitrogen, provoke the down-regulation of SWEET16 messenger RNA accumulation. SWEET16 overexpressors (35S Pro :SWEET16) showed a number of peculiarities related to differences in sugar accumulation, such as less Glc, Fru, and Suc at the end of the night. Under cold stress, 35S Pro :SWEET16 plants are unable to accumulate Fru, while under nitrogen starvation, both Glc and Fru, but not Suc, were less abundant. These changes of individual sugars indicate that the consequences of an increased SWEET16 activity are dependent upon the type of external stimulus. Remarkably, 35S Pro :SWEET16 lines showed improved germination and increased freezing tolerance. The latter observation, in combination with the modified sugar levels, points to a superior function of Glc and Suc for frost tolerance. 35S Pro :SWEET16 plants exhibited increased growth efficiency when cultivated on soil and showed improved nitrogen use efficiency when nitrate was sufficiently available, while under conditions of limiting nitrogen, wild-type biomasses were higher than those of 35S Pro :SWEET16 plants. Our results identify SWEET16 as a vacuolar sugar facilitator, demonstrate the substantial impact of SWEET16 overexpression on various critical plant traits, and imply that SWEET16 activity must be tightly regulated to allow optimal Arabidopsis development under nonfavorable conditions.

The inhibition of photosynthesis after supplying glucose to detached leaves of spinach (Spinacia oleracea L.) was used as a model system to search for mechanisms which potentially contribute to the "sink" regulation of photosynthesis. Detached leaves were supplied with 50 mM glucose or water for 7 d through the transpiration stream, holding the leaves in low irradiance (16 μmol photons · m(-2) · s(-1)) and a cycle of 9 h light/15 h darkness to prevent any endogenous accumulation of carbohydrate. Leaves supplied with water only showed marginal changes of photosynthesis, respiration, enzyme levels or metabolites. When leaves were supplied with 50 mM glucose, photosynthesis was gradually inhibited over several days. The inhibition was most marked when photosynthesis was measured in saturating irradiance and ambient CO2, less marked in saturating irradiance and saturating CO2, and least marked in limiting irradiance. There was a gradual loss of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) protein, fructose-1,6-bisphosphatase, NADP-glyceraldehyde-3-phosphate dehydrogenase and chlorophyll. The inhibition of photosynthesis was accompanied by a large decrease of glycerate-3-phosphate, an increase of triose-phosphates and fructose-1,6-bisphospate, and a small decrease of ribulose-1,5-bisphosphate. The stromal NADPH/NADP ratio increased (as indicated by increased activation of NADP-malate dehydrogenase), and the ATP/ADP ratio increased. Chlorophyll-fluorescence analysis indicated that thylakoid energisation was increased, and that the acceptor side of photosystem II was more reduced. Similar results were obtained when glucose was supplied by floating leaf discs in low irradiance on glucose solution, and when detached spinach leaves were held in high light to produce an endogenous accumulation of carbohydrate. Feeding glucose also led to an increased rate of respiration. This was not accompanied by any changes of pyruvate kinase, phosphofructokinase, or pyrophosphate: fructose-6-phosphate phosphotransferase activity. There was a decrease of phosphoenolpyruvate, glycerate-3-phosphate and glycerate-2-phosphate, an increase of pyruvate and triose-phosphates, and an increased ATP/ADP ratio. These results show (i) that accumulation of carbohydrate can inhibit photosynthesis via a long-term mechanism involving a decrease of Rubisco and other Calvin-cycle enzymes and (ii) that respiration is stimulated due to an unknown mechanism, which increases the utilisation of phosphoenolpyruvate.

expression of NR. Activity of all four enzymes decreased in nitrate-limited wild-type plants. (iv)In the light, malate accumulated, citrate decreased, and about 30% of the assimilated nitrate accumulated temporarily as glutamine, ammonium, glycine and serine. These changes were reversed during the night. (v) It is proposed that the diurnal changes of expression facilitate preferential synthesis of malate to act as a counter-anion for pH regulation during the first part of the light period when NR activity is high, and preferential synthesis of 2-oxoglutarate to act as a nitrogen acceptor later in the day when large amounts of nitrogen have accumulated in ammonium, glutamine and other amino acids including glycine in the photorespiration pathway, and NR activity has been decreased. Key INTRODUCTIONPhotosynthetic metabolism in source leaves is subject to dramatic diurnal changes (Geiger & Servaites 1994). The most obvious is the alternation between photosynthesis in the light and respiration in the dark. Superimposed is a cycle of starch accumulation and mobilization which allows part of the assimilated carbon to be temporarily stored in the leaf to support sucrose export during the night (Stitt, Huber & Kerr 1987;Heineke et al. 1994;Geiger & Servaites 1994). The balance between carbon export and accumulation even shifts during the light period, with the early part of the light period being characterized by high sucrose phosphate synthase (SPS, EC 2.4.1.14) activity (Stitt et al. 1987;Stitt 1996) and the later part by lower SPS activity and higher rates of starch synthesis (Stitt et al. 1987;Stitt 1996). Analogous diurnal changes occur in nitrogen metabolism. Nitrate reductase (NR) activity is highest in the first part of the light period, declines in the later part of the light period, and is low or negligible at night (Galangau et al.