New insights towards the function of glutamate dehydrogenase revealed during source-sink transition of tobacco (Nicotiana tabacum) plants grown under different nitrogen regimes

Glutamate dehydrogenase in "Liverworld"-A study in selected species to explore a key enzyme of plant primary metabolism in Marchantiophyta

In plants, glutamate dehydrogenase (GDH) is an ubiquitous enzyme that catalyzes the reversible amination of 2-oxoglutarate in glutamate. It contributes to both the amino acid homeostasis and the management of intracellular ammonium, and it is regarded as a key player at the junction of carbon and nitrogen assimilation pathways. To date, information about the GDH of terrestrial plants refers to a very few species only. We focused on selected species belonging to the division Marchantiophyta, providing the first panoramic overview of biochemical and functional features of GDH in liverworts. Native electrophoretic analyses showed an isoenzymatic profile less complex than what was reported for Arabidposis thaliana and other angiosperms: the presence of a single isoform corresponding to an α-homohexamer, differently prone to thermal inactivation on a species- and organ-basis, was found. Sequence analysis conducted on amino acid sequences confirmed a high similarity of GDH in modern liverworts with the GDH2 protein of A. thaliana, strengthening the hypothesis that the duplication event that gave origin to GDH1-homolog gene from GDH2 occurred after the evolutionary bifurcation that separated bryophytes and tracheophytes. Experiments conducted on Marchantia polymorpha and Calypogeia fissa grown in vitro and compared to A. thaliana demonstrated through in gel activity detection and monodimensional Western Blot that the aminating activity of GDH resulted in strongly enhanced responses to ammonium excess in liverworts as well, even if at a different extent compared to Arabidopsis and other vascular species. The comparative analysis by bi-dimensional Western Blot suggested that the regulation of the enzyme could be, at least partially, untied from the protein post-translational pattern. Finally, immuno-electron microscopy revealed that the GDH enzyme localizes at the subcellular level in both mitochondria and chloroplasts of parenchyma and is specifically associated to the endomembrane system in liverworts.

Nitrogen metabolic rate and differential ammonia volatilization regulate resistance against opportunistic fungus Alternaria alternata in tobacco

Nutritional correlations between plants and pathogens can crucially affect disease severity. As an essential macronutrient, the availability of nitrogen (N) and the types of N content play a fundamental part not only in energy metabolism and protein synthesis but also in pathogenesis. However, a direct connection has not yet been established between differences in the level of resistance and N metabolism. Pertinently, former studies hold ammonia (NH₃) accountable for the development of diseases in tobacco (Nicotiana tabacum L.) and in some post-harvest fruits. With a purpose of pinpointing the function of NH₃ volatilization on Alternaria alternata (Fries) Keissl pathogenesis and its correlation with both N metabolism and resistance differences to Alternaria alternata infection in tobacco, leaf tissue of two tobacco cultivars with susceptibility (Changbohuang; CBH), or resistance (Jingyehuang; JYH) were analyzed apropos of ammonia compensation point, apoplastic NH₄ ⁺ concentration, pH value as well as activities of key enzymes and N status. At the leaf age of 40 to 60 d, the susceptible cultivar had a significantly higher foliar apoplastic ammonium (NH₄ ⁺) concentration, pH value and NH₃ volatilization potential compared to the resistant one accompanied by a significant reduction in glutamine synthetase (GS), which in particular was a primary factor causing the NH₃ volatilization. The NH₄ ⁺ concentration in CBH was 1.44 times higher than that in JYH, and CBH had NH₃ compensation points that were 7.09, 6.15 and 4.35-fold higher than those of JYH at 40, 50 and 60 d, respectively. Moreover, the glutamate dehydrogenase (GDH) activity had an upward tendency related to an increased NH₄ ⁺ accumulation in both leaf tissues and apoplast but not with the NH₃ compensation point. Collectively, our results strongly suggest that the accumulation of NH₃ volatilization, rather than NH₄ ⁺ and total N, was the primary factor inducing the Alternaria alternata infection in tobacco. Meanwhile, the susceptible cultivar was characterized by a higher N re-transfer ability of NH₃ volatilization, in contrast to the disease-resistant cultivar, and had a stronger capability of N assimilation and reutilization. This study provides a deeper understanding of the pathogenicity mechanism induced by Alternaria alternata, which is useful for breeding Alternaria alternata-resistant varieties of tobacco, at the same time, our research is also conducive to control tobacco brown spot caused by Alternaria alternata in the field.

Integrated physiological, proteome and gene expression analyses provide new insights into nitrogen remobilization in citrus trees

Nitrogen (N) remobilization is an important physiological process that supports the growth and development of trees. However, in evergreen broad-leaved tree species, such as citrus, the mechanisms of N remobilization are not completely understood. Therefore, we quantified the potential of N remobilization from senescing leaves of spring shoots to mature leaves of autumn shoots of citrus trees under different soil N availabilities and further explored the underlying N metabolism characteristics by physiological, proteome and gene expression analyses. Citrus exposed to low N had an approximately 38% N remobilization efficiency (NRE), whereas citrus exposed to high N had an NRE efficiency of only 4.8%. Integrated physiological, proteomic and gene expression analyses showed that photosynthesis, N and carbohydrate metabolism interact with N remobilization. The improvement of N metabolism and photosynthesis, the accumulation of proline and arginine, and delayed degradation of storage protein in senescing leaves are the result of sufficient N supply and low N remobilization. Proteome further showed that energy generation proteins and glutamate synthase were hub proteins affecting N remobilization. In addition, N requirement of mature leaves is likely met by soil supply at high N nutrition, thereby resulting in low N remobilization. These results provide insight into N remobilization mechanisms of citrus that are of significance for N fertilizer management in orchards.

Effects of partial replacement of red by green light in the growth spectrum on photomorphogenesis and photosynthesis in tomato plants

The artificial light used in growth chambers is usually devoid of green (G) light, which is considered to be less photosynthetically efficient than blue (B) or red (R) light. To verify the role of G light supplementation in the spectrum, we modified the RB spectrum by progressively replacing R light with an equal amount of G light. The tomato plants were cultivated under 100 µmol m^-2 s^-1 of five different combinations of R (35-75%) and G light (0-40%) in the presence of a fixed proportion of B light (25%) provided by light-emitting diodes (LEDs). Substituting G light for R altered the plant's morphology and partitioning of biomass. We observed a decrease in the dry biomass of leaves, which was associated with increased biomass accumulation and the length of the roots. Moreover, plants previously grown under the RGB spectrum more efficiently utilized the B light that was applied to assess the effective quantum yield of photosystem II, as well as the G light when estimated with CO₂ fixation using RB + G light-response curves. At the same time, the inclusion of G light in the growth spectrum reduced stomatal conductance (g_s), transpiration (E) and altered stomatal traits, thus improving water-use efficiency. Besides this, the increasing contribution of G light in place of R light in the growth spectrum resulted in the progressive accumulation of phytochrome interacting factor 5, along with a lowered level of chalcone synthase and anthocyanins. However, the plants grown at 40% G light exhibited a decreased net photosynthetic rate (P_n), and consequently, a reduced dry biomass accumulation, accompanied by morphological and molecular traits related to shade-avoidance syndrome.

Dissecting the metabolic reprogramming of maize root under nitrogen-deficient stress conditions

The growth and development of maize (Zea mays L.) largely depends on its nutrient uptake through the root. Hence, studying its growth, response, and associated metabolic reprogramming to stress conditions is becoming an important research direction. A genome-scale metabolic model (GSM) for the maize root was developed to study its metabolic reprogramming under nitrogen stress conditions. The model was reconstructed based on the available information from KEGG, UniProt, and MaizeCyc. Transcriptomics data derived from the roots of hydroponically grown maize plants were used to incorporate regulatory constraints in the model and simulate nitrogen-non-limiting (N+) and nitrogen-deficient (N-) condition. Model-predicted flux-sum variability analysis achieved 70% accuracy compared with the experimental change of metabolite levels. In addition to predicting important metabolic reprogramming in central carbon, fatty acid, amino acid, and other secondary metabolism, maize root GSM predicted several metabolites (l-methionine, l-asparagine, l-lysine, cholesterol, and l-pipecolate) playing a regulatory role in the root biomass growth. Furthermore, this study revealed eight phosphatidylcholine and phosphatidylglycerol metabolites which, even though not coupled with biomass production, played a key role in the increased biomass production under N-deficient conditions. Overall, the omics-integrated GSM provides a promising tool to facilitate stress condition analysis for maize root and engineer better stress-tolerant maize genotypes.

Light Quality-Dependent Regulation of Non-Photochemical Quenching in Tomato Plants

Simple Summary

Photosynthetic organisms, such as land plants, evolved to utilize available light and to use its energy to assimilate carbon dioxide and produce carbohydrates. However, the light intensity often exceeds the ability of plants to successfully utilize absorbed energy, thus inducing stress, manifested by an increased radical concentration inside plant cells and disruption of the inner structures, and consequently decreased plant yield. Plants solve this problem by using a mechanism termed non-photochemical quenching, by which they can dissipate the energy not used in photosynthesis. Modern agriculture, however, also involves indoor plant farming. For indoor plant farming LED-based lighting systems, with non-saturating light intensities, are suitable based on their restricted energy consumption. However, the composition of applied light should first be optimized to maximize its utilization. Our study examined the influence of monochromatic LEDs (red, green, and blue) on the photoprotective and photosynthetic properties of tomato plants. We indicate that monochromatic green light could be considered an important component of lighting systems to alleviate energy dissipation, while blue light enhances photosynthetic efficiency. Our study not only proves the crucial importance of spectrum optimization but also provides evidence that different light wavelengths modify photosynthetic and photoprotective properties.

Abstract

Photosynthetic pigments of plants capture light as a source of energy for photosynthesis. However, the amount of energy absorbed often exceeds its utilization, thus causing damage to the photosynthetic apparatus. Plants possess several mechanisms to minimize such risks, including non-photochemical quenching (NPQ), which allows them to dissipate excess excitation energy in the form of harmless heat. However, under non-stressful conditions in indoor farming, it would be favorable to restrict the NPQ activity and increase plant photosynthetic performance by optimizing the light spectrum. Towards this goal, we investigated the dynamics of NPQ, photosynthetic properties, and antioxidant activity in the leaves of tomato plants grown under different light qualities: monochromatic red (R), green (G), or blue (B) light (L) at 80 µmol m⁻² s⁻¹ and R:G:B = 1:1:1 (referred to as the white light (WL)) at 120 µmol m⁻² s⁻¹. The results confirm that monochromatic BL increased the quantum efficiency of PSII and photosynthetic pigments accumulation. The RL and BL treatments enhanced the NPQ amplitude and showed negative effects on antioxidant enzyme activity. In contrast, plants grown solely under GL or WL presented a lower amplitude of NPQ due to the reduced accumulation of NPQ-related proteins, photosystem II (PSII) subunit S (PsbS), PROTON GRADIENT REGULATION-LIKE1 (PGRL1), cytochrome b₆f subunit f (cytf) and violaxanthin de-epoxidase (VDE). Additionally, we noticed that plants grown under GL or RL presented an increased rate of lipid peroxidation. Overall, our results indicate the potential role of GL in lowering the NPQ amplitude, while the role of BL in the RGB spectrum is to ensure photosynthetic performance and photoprotective properties.

Ammonium supply induces differential metabolic adaptive responses in tomato according to leaf phenological stage

Nitrate (NO3-) and ammonium (NH4+) are the main inorganic nitrogen sources available to plants. However, exclusive ammonium nutrition may lead to stress characterized by growth inhibition, generally associated with a profound metabolic reprogramming. In this work, we investigated how metabolism adapts according to leaf position in the vertical axis of tomato (Solanum lycopersicum cv. M82) plants grown with NH4+, NO3-, or NH4NO3 supply. We dissected leaf biomass composition and metabolism through an integrative analysis of metabolites, ions, and enzyme activities. Under ammonium nutrition, carbon and nitrogen metabolism were more perturbed in mature leaves than in young ones, overall suggesting a trade-off between NH4+ accumulation and assimilation to preserve young leaves from ammonium stress. Moreover, NH4+-fed plants exhibited changes in carbon partitioning, accumulating sugars and starch at the expense of organic acids, compared with plants supplied with NO3-. We explain such reallocation by the action of the biochemical pH-stat as a mechanism to compensate the differential proton production that depends on the nitrogen source provided. This work also underlines that the regulation of leaf primary metabolism is dependent on both leaf phenological stage and the nitrogen source provided.

Effect of exogenously-applied abscisic acid, putrescine and hydrogen peroxide on drought tolerance of barley

The objective of this study was to identify the effect of abscisic acid (ABA), putrescine (Put) and hydrogen peroxide (H2O2) foliar pre-treatment on drought tolerance of barley. Despite water limitation, ABA-sprayed plants preserved increased water content, photosynthetic efficiency of PSII (ΦPSII) and CO2 assimilation rate (Pn) compared to untreated stressed plants. The ABA-treated plants presented also the lowest rate of lipid peroxidation (MDA), lowered the rate of PSII primary acceptor reduction (1 – qP) and increased the yield of regulated energy dissipation (NPQ) with higher accumulation of PGRL1 (PROTON GRADIENT REGULATION LIKE1) protein. These plants preserved a similar level of photochemical efficiency and the rate of electron transport of PSII (ETRII) to the well-watered samples. The significantly less pronounced response was observed in Put-sprayed samples under drought. Additionally, the combined effects of drought and H2O2 application increased the 1 – qP and quantum yield of non-regulated energy dissipation in PSII (ΦNO) and reduced the accumulation of Rubisco activase (RCA). In conclusion, ABA foliar application allowed to balance water retention and preserve antioxidant capacity resulting in efficient photosynthesis and the restricted risk of oxidative damage under drought. Neither hydrogen peroxide nor putrescine has been able to ameliorate drought stress as effectively as ABA.

Glutamate dehydrogenase is essential in the acclimation of Virgilia divaricata, a legume indigenous to the nutrient-poor Mediterranean-type ecosystems of the Cape Fynbos

Glutamate dehydrogenase (NAD(H)- GDH, EC 1.4.1.2) is an important enzyme in nitrogen (N) metabolism. It serves as a link between C and N metabolism, in its role of assimilating ammonia into glutamine or deaminating glutamate into 2-oxoglutarate and ammonia. GDH may also have a key in the N assimilation of legumes growing in P-poor soils. Virgilia divaricata is such a legume, growing in the nutrient limited soils of the mediterranean-type Cape fynbos ecosystem. In order to understand the role of GDH in the nitrogen nutrition of V. divaricata, the aim of this study was to identify the GDH gene transcripts, their relative expressions and enzyme activity in P-stressed roots and nodules during N metabolism. During P deficiency there was a reduction in total plant biomass as well as total plant P concentration. The analysis of the GDH cDNA sequences in V. divaricata revealed the presence of GHD1 and GHD2 subunits, these corresponding to the GDH1, GDH-B and GDH3 genes of legumes and non-legume plants. The relative expression of GDH1 and GDH2 genes in the roots and nodules, indicates that two the subunits were differently regulated depending on the organ type, rather than P supply. Although both transcripts appeared to be ubiquitously expressed in the roots and nodules, the GDH2 transcript evidently predominated over those of GDH1. Furthermore, the higher expression of both GDH transcripts in the roots than nodules, suggests that roots are more reliant on on GDH in P-poor soils, than nodules. With regards to GHD activity, both aminating and deaminating GDH activities were differently affected by P deficiency in roots and nodules. This may function to assimilate N and regulate internal C and N in the roots and nodules. The variation in GDH1 and GDH2 transcript expression and GDH enzyme activities, indicate that the enzyme may be regulated by post-translational modification, instead of by gene expression during P deficiency in V. divaricata.

Variability of leaf photosynthetic characteristics in rice and its relationship with resistance to water stress under different nitrogen nutrition regimes

The negative effects of water stress on rice can be alleviated by NH₄ ⁺ nutrition. However, the effects of mixed nitrogen (N) nutrition (NO₃ ^-  + NH₄ ⁺ ) on resistance to water stress are still not well known. To investigate the response of rice growth to water stress and its relationship with photosynthetic characteristics, a hydroponic experiment supplying different N forms was conducted. Compared with NO₃ ^- nutrition, mixed-N and NH₄ ⁺ nutrition greatly alleviated the reduction of leaf area, chlorophyll content, and photosynthesis under water stress, whilst subsequently maintaining higher biomass. In contrast, water stress inhibited the root-shoot ratios in NH₄ ⁺ - and mixed-N-supplied plants, indicating reduced root growth and higher photosynthate availability to shoots. The following key observations were made: (1) a similar stomatal limitation and low proportion of activated Rubisco were observed among the three different N nutrition regimes; (2) increased mesophyll conductance in NH₄ ⁺ - and mixed-N-supplied plants simultaneously stimulated leaf photosynthesis and improved the water use efficiency and (3), the maximum carboxylation rate and actual photochemical efficiency of photosystem II in NH₄ ⁺ - and mixed-N-supplied plants were significantly higher than that in NO₃ ^- -supplied plants, thus resulting in higher photochemical efficiency under water stress. In conclusion, mixed-N and NH₄ ⁺ nutrition may be used to develop strategies for improved water stress resistance and stimulated biomass production under conditions of osmotic stress and possibly drought.

Foxtail Millet [Setaria italica (L.) Beauv.] Grown under Low Nitrogen Shows a Smaller Root System, Enhanced Biomass Accumulation, and Nitrate Transporter Expression

Foxtail millet (FM) [Setaria italica (L.) Beauv.] is a grain and forage crop well adapted to nutrient-poor soils. To date little is known how FM adapts to low nitrogen (LN) at the morphological, physiological, and molecular levels. Using the FM variety Yugu1, we found that LN led to lower chlorophyll contents and N concentrations, and higher root/shoot and C/N ratios and N utilization efficiencies under hydroponic culture. Importantly, enhanced biomass accumulation in the root under LN was in contrast to a smaller root system, as indicated by significant decreases in total root length; crown root number and length; and lateral root number, length, and density. Enhanced carbon allocation toward the root was rather for significant increases in average diameter of the LN root, potentially favorable for wider xylem vessels or other anatomical alterations facilitating nutrient transport. Lower levels of IAA and CKs were consistent with a smaller root system and higher levels of GA may promote root thickening under LN. Further, up-regulation of SiNRT1.1, SiNRT2.1, and SiNAR2.1 expression and nitrate influx in the root and that of SiNRT1.11 and SiNRT1.12 expression in the shoot probably favored nitrate uptake and remobilization as a whole. Lastly, more soluble proteins accumulated in the N-deficient root likely as a result of increases of N utilization efficiencies. Such "excessive" protein-N was possibly available for shoot delivery. Thus, FM may preferentially transport carbon toward the root facilitating root thickening/nutrient transport and allocate N toward the shoot maximizing photosynthesis/carbon fixation as a primary adaptive strategy to N limitation.

Elevated CO2 Induces Root Defensive Mechanisms in Tomato Plants When Dealing with Ammonium Toxicity

An adequate carbon supply is fundamental for plants to thrive under ammonium stress. In this work, we studied the mechanisms involved in tomato (Solanum lycopersicum L.) response to ammonium toxicity when grown under ambient or elevated CO2 conditions (400 or 800 p.p.m. CO2). Tomato roots were observed to be the primary organ dealing with ammonium nutrition. We therefore analyzed nitrogen (N) and carbon (C) metabolism in the roots, integrating the physiological response with transcriptomic regulation. Elevated levels of CO2 preferentially stimulated root growth despite the high ammonium content. The induction of anaplerotic enzymes from the tricarboxylic acid (TCA) cycle led to enhanced amino acid synthesis under ammonium nutrition. Furthermore, the root transcriptional response to ammonium toxicity was improved by CO2-enriched conditions, leading to higher expression of stress-related genes, as well as enhanced modulation of genes related to signaling, transcription, transport and hormone metabolism. Tomato roots exposed to ammonium stress also showed a defense-like transcriptional response according to the modulation of genes related to detoxification and secondary metabolism, involving principally terpenoid and phenolic compounds. These results indicate that increasing C supply allowed the co-ordinated regulation of root defense mechanisms when dealing with ammonium toxicity.

Metabolite Profiling of Wheat Seedlings Induced by Chitosan: Revelation of the Enhanced Carbon and Nitrogen Metabolism

Chitosan plays an important role in regulating growth and eliciting defense in many plant species. However, the exact metabolic response of plants to chitosan is still not clear. The present study performed an integrative analysis of metabolite profiles in chitosan-treated wheat seedlings and further investigated the response of enzyme activities and transcript expression related to the primary carbon (C) and nitrogen (N) metabolism. Metabolite profiling revealed that chitosan could induce significant difference of organic acids, sugars and amino acids in leaves of wheat seedlings. A higher accumulation of sucrose content was observed after chitosan treatment, accompanied by an increase in sucrose phosphate synthase (SPS) and fructose 1, 6-2 phosphatase (FBPase) activities as well as an up-regulation of relative expression level. Several metabolites associated with tricarboxylic acid (TCA) cycle, including oxaloacetate and malate, were also improved along with an elevation of phosphoenolpyruvate carboxylase (PEPC) and pyruvate dehydrogenase (PDH) activities. On the other hand, chitosan could also enhance the N reduction and N assimilation. Glutamate, aspartate and some other amino acids were higher in chitosan-treated plants, accompanied by the activation of key enzymes of N reduction and glutamine synthetase/glutamate synthase (GS/GOGAT) cycle. Together, these results suggested a pleiotropic modulation of carbon and nitrogen metabolism in wheat seedlings induced by chitosan and provided a significant insight into the metabolic mechanism of plants in response to chitosan for the first time, and it would give a basic guidance for the future application of chitosan in agriculture.

Perigone Lobe Transcriptome Analysis Provides Insights into Rafflesia cantleyi Flower Development

Rafflesia is a biologically enigmatic species that is very rare in occurrence and possesses an extraordinary morphology. This parasitic plant produces a gigantic flower up to one metre in diameter with no leaves, stem or roots. However, little is known about the floral biology of this species especially at the molecular level. In an effort to address this issue, we have generated and characterised the transcriptome of the Rafflesia cantleyi flower, and performed a comparison with the transcriptome of its floral bud to predict genes that are expressed and regulated during flower development. Approximately 40 million sequencing reads were generated and assembled de novo into 18,053 transcripts with an average length of 641 bp. Of these, more than 79% of the transcripts had significant matches to annotated sequences in the public protein database. A total of 11,756 and 7,891 transcripts were assigned to Gene Ontology categories and clusters of orthologous groups respectively. In addition, 6,019 transcripts could be mapped to 129 pathways in Kyoto Encyclopaedia of Genes and Genomes Pathway database. Digital abundance analysis identified 52 transcripts with very high expression in the flower transcriptome of R. cantleyi. Subsequently, analysis of differential expression between developing flower and the floral bud revealed a set of 105 transcripts with potential role in flower development. Our work presents a deep transcriptome resource analysis for the developing flower of R. cantleyi. Genes potentially involved in the growth and development of the R. cantleyi flower were identified and provide insights into biological processes that occur during flower development.

Characterization of natural leaf senescence in tobacco (Nicotiana tabacum) plants grown in vitro

Leaf senescence is a highly regulated final phase of leaf development preceding massive cell death. It results in the coordinated degradation of macromolecules and the subsequent nutrient relocation to other plant parts. Very little is still known about early stages of leaf senescence during normal leaf ontogeny that is not triggered by stress factors. This paper comprises an integrated study of natural leaf senescence in tobacco plants grown in vitro, using molecular, structural, and physiological information. We determined the time sequence of ultrastructural changes in mesophyll cells during leaf senescence, showing that the degradation of chloroplast ultrastructure fully correlated with changes in chlorophyll content. The earliest degenerative changes in chloroplast ultrastructure coinciding with early chromatin condensation were observed already in mature green leaves. A continuum of degradative changes in chloroplast ultrastructure, chromatin condensation and aggregation, along with progressive decrease in cytoplasm organization and electron density were observed in the course of mesophyll cells ageing. Although the total amounts of endogenous cytokinins gradually increased during leaf ontogenesis, the proportion of bioactive cytokinin forms, as well as their phosphate precursors, in total cytokinin content rapidly declined with ageing. Endogenous indole-3-acetic acid (IAA) levels were strongly reduced in senescent leaves, and a decreasing tendency was also observed for abscisic acid (ABA) levels. Senescence-associated tobacco cysteine proteases (CP, E.C. 3.4.22) CP1 and CP23 genes were induced in the initial phase of senescence. Genes encoding glutamate dehydrogenase (GDH, E.C. 1.4.1.2) and one isoform of cytosolic glutamine synthetase (GS1, E.C. 6.3.1.2) were induced in the late stage of senescence, while chloroplastic GS (GS2) gene showed a continuous decrease with leaf ageing.

Variable P supply affects N metabolism in a legume tree, Virgilia divaricata, from nutrient-poor Mediterranean-type ecosystems

Virgilia divaricata Adamson is a forest margin legume that is known to invade the N- and P-poor soils of the mature fynbos, implying that it tolerates variable soil N and P levels. It is not known how the legume uses inorganic N from soil and atmospheric sources under variable P supply. Little is known about how P deficiency affects the root nodule metabolic functioning of V. divaricata and the associated energy costs of N assimilation. This study aimed to determine whether P deficiency affects the metabolic status of roots and nodules, and the impact on the routes of N assimilation in V. divaricata.V. divaricata had reduced biomass, plant P concentration and biological nitrogen fixation during P deficiency. Based on adenylate data, P-stressed nodules maintained their P status better than P-stressed roots. V. divaricata was able to alter C and N metabolism differently in roots and nodules under P stress. This was achieved via internal P cycling by possible replacement of membrane phospholipids with sulfolipids and galactolipids, and increased reliance on the pyrophosphate (PPi)-dependent metabolism of sucrose via UDP-glucose (UDPG) and to fructose-6-phosphate (Fru-6-P). P-stressed roots mostly exported ureides as organic N and recycled amino acids via deaminating glutamate dehydrogenase. In contrast, P-stressed nodules largely exported amino acids. Compared with roots, nodules showed more P conservation during low P supply. The roots and nodules of V. divaricata metabolised N differently during P stress, meaning that these organs may contribute differently to the success of this plant in soils from forest to fynbos.

CO2 enrichment modulates ammonium nutrition in tomato adjusting carbon and nitrogen metabolism to stomatal conductance

Ammonium (NH4(+)) toxicity typically occurs in plants exposed to high environmental NH4(+) concentration. NH4(+) assimilating capacity may act as a biochemical mechanism avoiding its toxic accumulation but requires a fine tuning between nitrogen assimilating enzymes and carbon anaplerotic routes. In this work, we hypothesized that extra C supply, exposing tomato plants cv. Agora Hybrid F1 to elevated atmospheric CO2, could improve photosynthetic process and thus ameliorate NH4(+) assimilation and tolerance. Plants were grown under nitrate (NO3(-)) or NH4(+) as N source (5-15mM), under two atmospheric CO2 levels, 400 and 800ppm. Growth and gas exchange parameters, (15)N isotopic signature, C and N metabolites and enzymatic activities were determined. Plants under 7.5mM N equally grew independently of the N source, while higher ammonium supply resulted toxic for growth. However, specific stomatal closure occurred in 7.5mM NH4(+)-fed plants under elevated CO2 improving water use efficiency (WUE) but compromising plant N status. Elevated CO2 annulled the induction of TCA anaplerotic enzymes observed at non-toxic NH4(+) nutrition under ambient CO2. Finally, CO2 enrichment benefited tomato growth under both nutritions, and although it did not alleviate tomato NH4(+) tolerance it did differentially regulate plant metabolism in N-source and -dose dependent manner.

Resolving the Role of Plant NAD-Glutamate Dehydrogenase: III. Overexpressing Individually or Simultaneously the Two Enzyme Subunits Under Salt Stress Induces Changes in the Leaf Metabolic Profile and Increases Plant Biomass Production

NAD-dependent glutamate dehydrogenase (NAD-GDH) of higher plants has a central position at the interface between carbon and nitrogen metabolism due to its ability to carry out the deamination of glutamate. In order to obtain a better understanding of the physiological function of NAD-GDH under salt stress conditions, transgenic tobacco (Nicotiana tabacum L.) plants that overexpress two genes from Nicotiana plumbaginifolia individually (GDHA and GDHB) or simultaneously (GDHA/B) were grown in the presence of 50 mM NaCl. In the different GDH overexpressors, the NaCl treatment induced an additional increase in GDH enzyme activity, indicating that a post-transcriptional mechanism regulates the final enzyme activity under salt stress conditions. A greater shoot and root biomass production was observed in the three types of GDH overexpressors following growth in 50 mM NaCl, when compared with the untransformed plants subjected to the same salinity stress. Changes in metabolites representative of the plant carbon and nitrogen status were also observed. They were mainly characterized by an increased amount of starch present in the leaves of the GDH overexpressors as compared with the wild type when plants were grown in 50 mM NaCl. Metabolomic analysis revealed that overexpressing the two genes GDHA and GDHB, individually or simultaneously, induced a differential accumulation of several carbon- and nitrogen-containing molecules involved in a variety of metabolic, developmental and stress-responsive processes. An accumulation of digalactosylglycerol, erythronate and porphyrin was found in the GDHA, GDHB and GDHA/B overexpressors, suggesting that these molecules could contribute to the improved performance of the transgenic plants under salinity stress conditions.

ZD958 is a low-nitrogen-efficient maize hybrid at the seedling stage among five maize and two teosinte lines

ZD958 was the most low-N-efficient line among five maize and two teosinte lines. Zea parviglumis and Zea diploperennis were insensitive to N limitation. Maize and teosinte genetically and evolutionarily diverged in gene regulation. GDH2, ASN2, and T4 were consistently down-regulated across seven lines. Maternal asymmetric inheritance and heterosis vigor made ZD958 low-N-efficient. Nitrogen (N) deficiency remains a serious limiting factor for maize production in many developing countries. It is particularly important to better understand how hybrid maize responds to N limitation. ZD958, a dominant high-yield hybrid in North China, was comparatively analyzed with four other maize and two teosinte lines at physiological and transcriptional levels. ZD958 was the most low-N-efficient line among five maize and two teosinte lines due to its largest biomass accumulation at a lowest N concentration under N limitation; while Zea parviglumis and Zea diploperennis had large root systems and were insensitive to N limitation. In anti-parallel with down-regulation of N metabolic genes in the ZD958 root, carbon allocation towards the root was enhanced for the significant increase in the root length. Variations in expression patterns of ten genes mediating N uptake, transport, and metabolism indicated large genetic and evolutionary divergence among seven lines under N limitation. Notably, GDH2, ASN2, and VAAT5 were consistently down-regulated under N limitation across these maize and teosinte lines, suggesting essential evolutionary conservation of gene regulation in response to N limitation and providing molecular markers for N nutritional diagnosis. Asymmetric inheritance, mostly from its maternal donor Z58, and heterosis vigor made ZD958 low-N-efficient at the seedling stage. The superior traits in crown roots in ZD958 may be derived from its paternal donor Chang7-2. Thus, Z58, Chang7-2, and two wild maize lines (Z. parviglumis and Z. diploperennis) provide valuable germplasms for N-efficient and large-root maize breeding.

Over-expression of a glutamate dehydrogenase gene, MgGDH, from Magnaporthe grisea confers tolerance to dehydration stress in transgenic rice

Heterologous expression of a fungal NADP(H)-GDH gene ( MgGDH ) from Magnaporthe grisea can improve dehydration stress tolerance in rice by preventing toxic accumulation of ammonium. Glutamate dehydrogenase (GDH; EC 1.4.1.2 and EC 1.4.1.4) may act as a stress-responsive enzyme in detoxification of high intracellular ammonia and production of glutamate for proline synthesis under stress conditions. In present study, a fungal NADP(H)-GDH gene (MgGDH) from Magnaporthe grisea was over-expressed in rice (Oryza sativa L. cv. 'kitaake'), and the transgenic plants showed the improvement of tolerance to dehydration stress. The kinetic analysis showed that His-TF-MgGDH preferentially utilizes ammonium to produce L-glutamate. Moreover, the affinity of His-TF-MgGDH for ammonium was dramatically higher than that of His-TF-OsGDH for ammonium. Over-expressing MgGDH transgenic rice plants showed lower water-loss rate and higher completely close stomata than the wild-type plants under dehydration stress conditions. In transgenic plants, the NADP(H)-GDH activities were markedly higher than those in wild-type plants and the amination activity was significantly higher than the deamination activity. Compared with wild-type plants, the transgenic plants accumulated much less NH4 (+) but higher amounts of glutamate, proline and soluble sugar under dehydration stress conditions. These results indicate that heterologous expression of MgGDH can prevent toxic accumulation of ammonium and in return improve dehydration stress tolerance in rice.

Assessing the metabolic impact of nitrogen availability using a compartmentalized maize leaf genome-scale model

Maize (Zea mays) is an important C4 plant due to its widespread use as a cereal and energy crop. A second-generation genome-scale metabolic model for the maize leaf was created to capture C4 carbon fixation and investigate nitrogen (N) assimilation by modeling the interactions between the bundle sheath and mesophyll cells. The model contains gene-protein-reaction relationships, elemental and charge-balanced reactions, and incorporates experimental evidence pertaining to the biomass composition, compartmentalization, and flux constraints. Condition-specific biomass descriptions were introduced that account for amino acids, fatty acids, soluble sugars, proteins, chlorophyll, lignocellulose, and nucleic acids as experimentally measured biomass constituents. Compartmentalization of the model is based on proteomic/transcriptomic data and literature evidence. With the incorporation of information from the MetaCrop and MaizeCyc databases, this updated model spans 5,824 genes, 8,525 reactions, and 9,153 metabolites, an increase of approximately 4 times the size of the earlier iRS1563 model. Transcriptomic and proteomic data have also been used to introduce regulatory constraints in the model to simulate an N-limited condition and mutants deficient in glutamine synthetase, gln1-3 and gln1-4. Model-predicted results achieved 90% accuracy when comparing the wild type grown under an N-complete condition with the wild type grown under an N-deficient condition.

Resolving the role of plant glutamate dehydrogenase: II. Physiological characterization of plants overexpressing the two enzyme subunits individually or simultaneously

Glutamate dehydrogenase (GDH; EC 1.4.1.2) is able to carry out the deamination of glutamate in higher plants. In order to obtain a better understanding of the physiological function of GDH in leaves, transgenic tobacco (Nicotiana tabacum L.) plants were constructed that overexpress two genes from Nicotiana plumbaginifolia (GDHA and GDHB under the control of the Cauliflower mosiac virus 35S promoter), which encode the α- and β-subunits of GDH individually or simultaneously. In the transgenic plants, the GDH protein accumulated in the mitochondria of mesophyll cells and in the mitochondria of the phloem companion cells (CCs), where the native enzyme is normally expressed. Such a shift in the cellular location of the GDH enzyme induced major changes in carbon and nitrogen metabolite accumulation and a reduction in growth. These changes were mainly characterized by a decrease in the amount of sucrose, starch and glutamine in the leaves, which was accompanied by an increase in the amount of nitrate and Chl. In addition, there was an increase in the content of asparagine and a decrease in proline. Such changes may explain the lower plant biomass determined in the GDH-overexpressing lines. Overexpressing the two genes GDHA and GDHB individually or simultaneously induced a differential accumulation of glutamate and glutamine and a modification of the glutamate to glutamine ratio. The impact of the metabolic changes occurring in the different types of GDH-overexpressing plants is discussed in relation to the possible physiological function of each subunit when present in the form of homohexamers or heterohexamers.

High irradiance improves ammonium tolerance in wheat plants by increasing N assimilation

Ammonium is a paradoxical nutrient ion. Despite being a common intermediate in plant metabolism whose oxidation state eliminates the need for its reduction in the plant cell, as occurs with nitrate, it can also result in toxicity symptoms. Several authors have reported that carbon enrichment in the root zone enhances the synthesis of carbon skeletons and, accordingly, increases the capacity for ammonium assimilation. In this work, we examined the hypothesis that increasing the photosynthetic photon flux density is a way to increase plant ammonium tolerance. Wheat plants were grown in a hydroponic system with two different N sources (10mM nitrate or 10mM ammonium) and with two different light intensity conditions (300 μmol photon m(-2)s(-1) and 700 μmol photon m(-2)s(-1)). The results show that, with respect to biomass yield, photosynthetic rate, shoot:root ratio and the root N isotopic signature, wheat behaves as a sensitive species to ammonium nutrition at the low light intensity, while at the high intensity, its tolerance is improved. This improvement is a consequence of a higher ammonium assimilation rate, as reflected by the higher amounts of amino acids and protein accumulated mainly in the roots, which was supported by higher tricarboxylic acid cycle activity. Glutamate dehydrogenase was a key root enzyme involved in the tolerance to ammonium, while glutamine synthetase activity was low and might not be enough for its assimilation.

Cessation of photosynthesis in Lotus japonicus leaves leads to reprogramming of nodule metabolism

Symbiotic nitrogen fixation (SNF) involves global changes in gene expression and metabolite accumulation in both rhizobia and the host plant. In order to study the metabolic changes mediated by leaf-root interaction, photosynthesis was limited in leaves by exposure of plants to darkness, and subsequently gene expression was profiled by real-time reverse transcription-PCR (RT-PCR) and metabolite levels by gas chromatography-mass spectrometry in the nodules of the model legume Lotus japonicus. Photosynthetic carbon deficiency caused by prolonged darkness affected many metabolic processes in L. japonicus nodules. Most of the metabolic genes analysed were down-regulated during the extended dark period. In addition to that, the levels of most metabolites decreased or remained unaltered, although accumulation of amino acids was observed. Reduced glycolysis and carbon fixation resulted in lower organic acid levels, especially of malate, the primary source of carbon for bacteroid metabolism and SNF. The high amino acid concentrations together with a reduction in total protein concentration indicate possible protein degradation in nodules under these conditions. Interestingly, comparisons between amino acid and protein content in various organs indicated systemic changes in response to prolonged darkness between nodulated and non-nodulated plants, rendering the nodule a source organ for both C and N under these conditions.

Exotic herbivores on a shared native host: tissue quality after individual, simultaneous, and sequential attack

Plants in nature are often attacked by multiple enemies whose effect on the plant cannot always be predicted based on the outcome of individual attacks. We investigated how two invasive herbivores, the hemlock woolly adelgid (Adelges tsugae) (HWA) and the elongate hemlock scale (Fiorinia externa) (EHS), alter host plant quality (measured as amino acid concentration and composition) when feeding individually or jointly on eastern hemlock (Tsuga canadensis), an important long-lived forest tree that is in severe decline. The joint herbivore treatments included both simultaneous and sequential infestations by the two herbivores. We expected resource depletion over time, particularly in response to feeding by HWA. In contrast, HWA dramatically increased the concentration and altered the composition of individual free amino acids. Compared to control trees, HWA increased total amino acid concentration by 330% after 1 year of infestation. Conversely, EHS had a negligible effect when feeding individually. Interestingly, there was a marginally significant HWA × EHS interaction that suggests the potential for EHS presence to reduce the impact of HWA on foliage quality when the two species co-occur. We suggest indirect effects of water stress as a possible physiological mechanism for our results. Understanding how species interactions change the physiology of a shared host is crucial to making more accurate predictions about host mortality and subsequent changes in affected communities and ecosystems, and to help design appropriate management plans.

Ammonium formation and assimilation in P(SARK)∷IPT tobacco transgenic plants under low N

Wild Type (WT) and transgenic tobacco plants expressing isopentenyltransferase (IPT), a gene encoding the enzyme regulating the rate-limiting step in cytokinins (CKs) synthesis, were grown under limited nitrogen (N) conditions. We analyzed nitrogen forms, nitrogen metabolism related-enzymes, amino acids and photorespiration related-enzymes in WT and P(SARK)∷IPT tobacco plants. Our results indicate that the WT plants subjected to N deficiency displayed reduced nitrate (NO₃⁻) assimilation. However, an increase in the production of ammonium (NH₄⁺), by the degradation of proteins and photorespiration led to an increase in the glutamine synthetase/glutamate synthase (GS/GOGAT) cycle in WT plants. In these plants, the amounts of amino acids decreased with N deficiency, although the relative amounts of glutamate and glutamine increased with N deficiency. Although the transgenic plants expressing P(SARK)∷IPT and growing under suboptimal N conditions displayed a significant decline in the N forms in the leaf, they maintained the GS/GOGAT cycle at control levels. Our results suggest that, under N deficiency, CKs prevented the generation and assimilation of NH₄⁺ by increasing such processes as photorespiration, protein degradation, the GS/GOGAT cycle, and the formation of glutamine.

Natural variation of Arabidopsis response to nitrogen availability

Our understanding of plant growth in response to nitrogen (N) supply is mainly based on studies of mutants and transformants. This study explored the natural variability of Arabidopsis thaliana first to find out its global response to N availability and secondly to characterize the plasticity for growth and N metabolism among 23 genetically distant accessions under normal (N+), limited (N-), and starved (N0) N supplies. Plant growth was estimated by eight morphological traits characterizing shoot and root growth and 10 metabolic parameters that represented N and carbon metabolism. Most of the studied traits showed a large variation linked to genotype and nutrition. Furthermore, Arabidopsis growth was coordinated by master traits such as the shoot to root ratio of nitrate content in N+, root fresh matter and root amino acids in N-, and shoot fresh matter together with root thickness in N0. The 23 accessions could be gathered into four different groups, according to their growth in N+, N-, and N0. Phenotypic profiling characterized four different adaptative responses to N- and N0. Class 1 tolerated N limitation with the smallest decrease in shoot and root biomass compared with N+, while class 2 presented the highest resistance to N starvation by preferential increased root growth, huge starch accumulation, and high shoot nitrate content. In contrast, class 3 plants could tolerate neither N limitation nor N starvation. Small plants of class 4 were different, with shoot biomass barely affected in N- and root biomass unaffected in N0.

Nitrogen metabolism in leaves of a tank epiphytic bromeliad: characterization of a spatial and functional division

The leaf is considered the most important vegetative organ of tank epiphytic bromeliads due to its ability to absorb and assimilate nutrients. However, little is known about the physiological characteristics of nutrient uptake and assimilation. In order to better understand the mechanisms utilized by some tank epiphytic bromeliads to optimize the nitrogen acquisition and assimilation, a study was proposed to verify the existence of a differential capacity to assimilate nitrogen in different leaf portions. The experiments were conducted using young plants of Vriesea gigantea. A nutrient solution containing NO₃⁻/NH₄⁺ or urea as the sole nitrogen source was supplied to the tank of these plants and the activities of urease, nitrate reductase (NR), glutamine synthetase (GS) and glutamate dehydrogenase (NADH-GDH) were quantified in apical and basal leaf portions after 1, 3, 6, 9, 12, 24 and 48 h. The endogenous ammonium and urea contents were also analyzed. Independent of the nitrogen sources utilized, NR and urease activities were higher in the basal portions of leaves in all the period analyzed. On the contrary, GS and GDH activities were higher in apical part. It was also observed that the endogenous ammonium and urea had the highest contents detected in the basal region. These results suggest that the basal portion was preferentially involved in nitrate reduction and urea hydrolysis, while the apical region could be the main area responsible for ammonium assimilation through the action of GS and GDH activities. Moreover, it was possible to infer that ammonium may be transported from the base, to the apex of the leaves. In conclusion, it was suggested that a spatial and functional division in nitrogen absorption and NH₄⁺ assimilation between basal and apical leaf areas exists, ensuring that the majority of nitrogen available inside the tank is quickly used by bromeliad's leaves.

Effects of nitrogen form on growth, CO₂ assimilation, chlorophyll fluorescence, and photosynthetic electron allocation in cucumber and rice plants

Cucumber and rice plants with varying ammonium (NH(4)(+)) sensitivities were used to examine the effects of different nitrogen (N) sources on gas exchange, chlorophyll (Chl) fluorescence quenching, and photosynthetic electron allocation. Compared to nitrate (NO(3)(-))-grown plants, cucumber plants grown under NH(4)(+)-nutrition showed decreased plant growth, net photosynthetic rate, stomatal conductance, intercellular carbon dioxide (CO(2)) level, transpiration rate, maximum photochemical efficiency of photosystem II, and O(2)-independent alternative electron flux, and increased O(2)-dependent alternative electron flux. However, the N source had little effect on gas exchange, Chl a fluorescence parameters, and photosynthetic electron allocation in rice plants, except that NH(4)(+)-grown plants had a higher O(2)-independent alternative electron flux than NO(3)(-)-grown plants. NO(3)(-) reduction activity was rarely detected in leaves of NH(4)(+)-grown cucumber plants, but was high in NH(4)(+)-grown rice plants. These results demonstrate that significant amounts of photosynthetic electron transport were coupled to NO(3)(-) assimilation, an effect more significant in NO(3)(-)-grown plants than in NH(4)(+)-grown plants. Meanwhile, NH(4)(+)-tolerant plants exhibited a higher demand for the reduced form of nicotinamide adenine dinucleotide phosphate (NADPH) for NO(3)(-) reduction, regardless of the N form supplied, while NH(4)(+)-sensitive plants had a high water-water cycle activity when NH(4)(+) was supplied as the sole N source.

Induction of leaf senescence by low nitrogen nutrition in sunflower (Helianthus annuus) plants

Different parameters which vary during the leaf development in sunflower plants grown with nitrate (2 or 20 mM) for a 42-day period have been determined. The plants grown with 20 mM nitrate (N+) showed greater leaf area and specific leaf mass than the plants grown with 2 mM nitrate (N-). The total chlorophyll content decreased with leaf senescence, like the photosynthetic rate. This decline of photosynthetic activity was greater in plants grown with low nitrogen level (N-), showing more pronounced senescence symptoms than with high nitrogen (N+). In both treatments, soluble sugars increased with aging, while starch content decreased. A significant increase of hexose to sucrose ratio was observed at the beginning of senescence, and this raise was higher in N- plants than in N+ plants. These results show that sugar senescence regulation is dependent on nitrogen, supporting the hypothesis that leaf senescence is regulated by the C/N balance. In N+ and N- plants, ammonium and free amino acid concentrations were high in young leaves and decreased progressively in the senescent leaves. In both treatments, asparagine, and in a lower extent glutamine, increased after senescence start. The drop in the (Glu+Asp)/(Gln+Asn) ratio associated with the leaf development level suggests a greater nitrogen mobilization. Besides, the decline in this ratio occurred earlier and more rapidly in N- plants than in N+ plants, suggesting that the N- remobilization rate correlates with leaf senescence severity. In both N+ and N- plants, an important oxidative stress was generated in vivo during sunflower leaf senescence, as revealed by lipid peroxidation and hydrogen peroxide accumulation. In senescent leaves, the increase in hydrogen peroxide levels occurred in parallel with a decline in the activity of antioxidant enzymes. In N+ plants, the activities of catalase and ascorbate peroxidase (APX) increased to reach their highest values at 28 days, and later decreased during senescence, whereas in N- plants these activities started to decrease earlier, APX after 16 days and catalase after 22 days, suggesting that senescence is accelerated in N-leaves. It is probable that systemic signals, such as a deficit in amino acids or other metabolites associated with the nitrogen metabolism produced in plants grown with low nitrogen, lead to an early senescence and a higher oxidation state of the cells of these plant leaves.

Resolving the role of plant glutamate dehydrogenase. I. In vivo real time nuclear magnetic resonance spectroscopy experiments

In higher plants the glutamate dehydrogenase (GDH) enzyme catalyzes the reversible amination of 2-oxoglutarate to form glutamate, using ammonium as a substrate. For a better understanding of the physiological function of GDH either in ammonium assimilation or in the supply of 2-oxoglutarate, we used transgenic tobacco (Nicotiana tabacum L.) plants overexpressing the two genes encoding the enzyme. An in vivo real time (15)N-nuclear magnetic resonance (NMR) spectroscopy approach allowed the demonstration that, when the two GDH genes were overexpressed individually or simultaneously, the transgenic plant leaves did not synthesize glutamate in the presence of ammonium when glutamine synthetase (GS) was inhibited. In contrast we confirmed that the primary function of GDH is to deaminate Glu. When the two GDH unlabeled substrates ammonium and Glu were provided simultaneously with either [(15)N]Glu or (15)NH(4)(+) respectively, we found that the ammonium released from the deamination of Glu was reassimilated by the enzyme GS, suggesting the occurrence of a futile cycle recycling both ammonium and Glu. Taken together, these results strongly suggest that the GDH enzyme, in conjunction with NADH-GOGAT, contributes to the control of leaf Glu homeostasis, an amino acid that plays a central signaling and metabolic role at the interface of the carbon and nitrogen assimilatory pathways. Thus, in vivo NMR spectroscopy appears to be an attractive technique to follow the flux of metabolites in both normal and genetically modified plants.

A quantitative genetic study for elucidating the contribution of glutamine synthetase, glutamate dehydrogenase and other nitrogen-related physiological traits to the agronomic performance of common wheat

To better understand the genetic variability for nitrogen use efficiency in winter wheat is a necessity in the frame of the present economic and ecological context. The objective of this work was to investigate the role of the enzymes glutamine synthetase (GS) and glutamate dehydrogenase (GDH), and other nitrogen (N)-related physiological traits in the control of agronomic performance in wheat. A quantitative genetics approach was developed using the Arche x Récital population of doubled haploid lines grown for 3 years in the field. GS and GDH activities, ammonium, amino acid and protein contents were measured at different stages of plant development in different organs after flowering. Significant genotypic effects were observed for all measured physiological and agronomical traits. Heading date was negatively correlated with ammonium, amino acid, protein contents and GS activity in the flag leaf lamina. Grain protein content was positively correlated with both ammonium and amino acid content, and to a lesser extent with soluble protein content and GS activity. A total of 148 quantitative trait loci (QTLs) were detected, 104 QTLs for physiological traits and 44 QTLs for agronomic traits. Twenty-six QTLs were detected for GDH activity spread over 13 chromosomes and 25 QTLs for GS activity spread over 12 chromosomes. We found only a co-localization between a QTL for GS activity and GSe, a structural gene encoding cytosolic GS on chromosome 4B. A coincidence between a QTL for GDH activity and a gene encoding GDH was also found on chromosome 2B. QTL regions combining both physiological and agronomical QTLs were mainly identified on linkage groups 2A, 2B, 2D, 5A, 5B and 5D. This approach allowed us to propose possible functions of physiological traits to explain the variation observed for agronomic traits including yield and its components.

Regulation of glutamine synthetase 1 and amino acids transport in the phloem of young wheat plants

The possible regulation of amino acid remobilization via the phloem in wheat (Triticum aestivum L.) by the primary enzyme in nitrogen (N) assimilation and re-assimilation, glutamine synthetase (GS, E.C. 6.3.1.2) was studied using two conditions known to alter N phloem transport, N deficiency and cytokinins. The plants were grown for 15 days in controlled conditions with optimum N supply and then N was depleted from and/or 6-benzylaminopurine was added to the nutrient solution. Both treatments generated an induction of GS1, monitored at the level of gene expression, protein accumulation and enzyme activity, and a decrease in the exudation of amino acids to the phloem, obtained with EDTA technique, which correlated negatively. GS inhibition by metionine sulfoximide (MSX) produced an increase of amino acids exudation and the inhibitor successfully reversed the effect of N deficiency and cytokinin addition over phloem exudation. Our results point to an important physiological role for GS1 in the modulation of amino acids export levels in wheat plants.

Adjustment of growth and central metabolism to a mild but sustained nitrogen-limitation in Arabidopsis

We have established a simple soil-based experimental system that allows a small and sustained restriction of growth of Arabidopsis by low nitrogen (N). Plants were grown in a large volume of a peat-vermiculite mix that contained very low levels of inorganic N. As a control, inorganic N was added in solid form to the peat-vermiculite mix, or plants were grown in conventional nutrient-rich solids. The low N growth regime led to a sustained 20% decrease of the relative growth rate over a period of 2 weeks, resulting in a two- to threefold decrease in biomass in 35- to 40-day-old plants. Plants in the low N regime contained lower levels of nitrate, lower nitrate reductase activity, lower levels of malate, fumarate and other organic acids and slightly higher levels of starch, as expected from published studies of N-limited plants. However, their rosette protein content was unaltered, and total and many individual amino acid levels increased compared with N-replete plants. This metabolic phenotype reveals that Arabidopsis responds adaptively to low N by decreasing the rate of growth, while maintaining the overall protein content, and maintaining or even increasing the levels of many amino acids.

Genetic variation for N-remobilization and postsilking N-uptake in a set of maize recombinant inbred lines. 3. QTL detection and coincidences

The objective of this study was to map and characterize QTLs for traits related to nitrogen utilization efficiency (NUE), grain N yield, N-remobilization and post-silking N-uptake. Furthermore, to examine whether QTLs detected with recombinant inbred lines (RILs) crossed to a tester are common to those detected with line per se evaluation, both types of evaluations were developed from the same set of RILs. The material was studied over two years at high N-input, and one year at low N-input. We used (15)N-labelling to evaluate with accuracy the proportion of N remobilized from stover to kernels and the proportion of postsilking N-uptake allocated to kernels. With 59 traits studied in three environments, 608 QTLs were detected. Using a method of QTL clustering, 72 clusters were identified, with few QTLs being specific to one environment or to the type of plant material (lines or testcross families). However, considering each trait separately, few QTLs were common to both line per se and testcross evaluation. This shows that genetic variability is expressed differently according to the type of progeny. Studies of coincidences among QTLs within the clusters showed an antagonism between N-remobilization and N-uptake in several QTL-clusters. QTLs for N-uptake, root system architecture and leaf greenness coincided positively in eight clusters. QTLs for remobilization mainly coincided in clusters with QTLs for leaf senescence. On the whole, sign of coincidences between QTLs underlined the role of a "stay-green" phenotype in favouring N-uptake capacity, and thus grain yield and N grain yield.

Enzymatic and metabolic diagnostic of nitrogen deficiency in Arabidopsis thaliana Wassileskija accession

Adaptation to steady-state low-nutrient availability was investigated by comparing the Wassileskija (WS) accession of Arabidopsis thaliana grown on 2 or 10 mM nitrate. Low nitrogen conditions led to a limited rosette biomass and seed yield. The latter was mainly due to reduced seed number, while seed weight was less affected. However, harvest index was lower in high nitrate compared with limited nitrate conditions. Under nitrogen-limiting conditions, nitrate reductase activity was decreased while glutamine synthetase activity was increased due to a higher accumulation of the cytosolic enzyme. The level of nitrogen remobilization to the seeds was higher under low nitrogen, and the vegetative parts of the plants remaining after seed production stored very low residual nitrogen. Through promoting nitrogen remobilization and recycling pathways, nitrogen limitation modified plant and seed compositions. Rosette leaves contained more sugars and less free amino acids when grown under nitrogen-limiting conditions. Compared with high nitrogen, the levels of proline, asparagine and glutamine were decreased. The seed amino acid composition reflected that of the rosette leaves, thus suggesting that phloem loading for seed filling was poorly selective. The major finding of this report was that together with decreasing biomass and yield, nitrogen limitation triggers large modifications in vegetative products and seed quality.

NAD(H)-dependent glutamate dehydrogenase is essential for the survival of Arabidopsis thaliana during dark-induced carbon starvation

Interconversion between glutamate and 2-oxoglutarate, which can be catalysed by glutamate dehydrogenase (GDH), is a key reaction in plant carbon (C) and nitrogen (N) metabolism. However, the physiological role of plant GDH has been a controversial issue for several decades. To elucidate the function of GDH, the expression of GDH in various tissues of Arabidopsis thaliana was studied. Results suggested that the expression of two Arabidopsis GDH genes was differently regulated depending on the organ/tissue types and cellular C availability. Moreover, Arabidopsis mutants defective in GDH genes were identified and characterized. The two isolated mutants, gdh1-2 and gdh2-1, were crossed to make a double knockout mutant, gdh1-2/gdh2-1, which contained negligible levels of NAD(H)-dependent GDH activity. Phenotypic analysis on these mutants revealed an increased susceptibility of gdh1-2/gdh2-1 plants to C-deficient conditions. This conditional phenotype of the double knockout mutant supports the catabolic role of GDH and its role in fuelling the TCA cycle during C starvation. The reduced rate of glutamate catabolism in the gdh2-1 and gdh1-2/gdh2-1 plants was also evident by the growth retardation of these mutants when glutamate was supplied as the alternative N source. Furthermore, amino acid profiles during prolonged dark conditions were significantly different between WT and the gdh mutant plants. For instance, glutamate levels increased in WT plants but decreased in gdh1-2/gdh2-1 plants, and aberrant accumulation of several amino acids was detected in the gdh1-2/gdh2-1 plants. These results suggest that GDH plays a central role in amino acid breakdown under C-deficient conditions.

Differential capacity of nitrogen assimilation between apical and basal leaf portions of a tank epiphytic bromeliad

The leaf is the main organ of the vegetative body of tank epiphytic bromeliads, which may be subdivided into apical and basal parts. Little is known about the existence of morphological, anatomical or physiological differences between these two leaf portions. The objective of this study was to verify the existence of a differential capacity to assimilate nitrogen from urea between basal and apical leaf portions of Vriesea gigantea, a tank epiphytic bromeliad. Plants were cultivated in vitro in the presence of 5 mM urea for 15 days. During this period, the activities of the enzymes glutamine synthetase (GS, EC 6.3.1.2) and NADH-dependent glutamate dehydrogenase (NADH-GDH, EC 1.4.1.2) were quantified, as well as the densities of trichomes and stomata found at the leaf surface of both regions. The highest activities of GS and NADH-GDH were always detected at the top portion, and on the 3rd day of in vitro cultivation an increase in these activities was detected. An inverse correlation between number of trichomes and stomata in these two leaf portions was verified: the top region presented 30% less trichomes and twice as many stomata as the basal region. This suggests that the upper region of the leaf may be preferentially involved with the assimilation of nitrogen from urea via GS and NADH-GDH, whereas the basal region may be involved with its absorption. This is the first study to report that ammonium assimilation may take place preferentially in a specific portion of tank-forming bromeliad leaves.

Partial substitution of NO(3)(-) by NH(4)(+) fertilization increases ammonium assimilating enzyme activities and reduces the deleterious effects of salinity on the growth of barley

Productivity of cereal crops is restricted in saline soils but may be improved by nitrogen nutrition. In this study, the effect of ionic nitrogen form on growth, mineral content, protein content and ammonium assimilation enzyme activities of barley (Hordeum vulgare cv. Alexis L.) irrigated with saline water, was determined. Leaf and tiller number as well as plant fresh and dry weights declined under salinity (120 mM NaCl). In non-saline conditions, growth parameters were increased by application of NH(4)(+)/NO(3)(-) (25:75) compared to NO(3)(-) alone. Under saline conditions, application of NH(4)(+)/NO(3)(-) led to a reduction of the detrimental effects of salt on growth. Differences in growth between the two nitrogen regimes were not due to differences in photosynthesis. The NH(4)(+)/NO(3)(-) regime led to an increase in total N in control and saline treatments, but did not cause a large decrease in plant Na(+) content under salinity. Activities of GS (EC 6.3.1.2), GOGAT (EC 1.4.1.14), PEPC (EC 4.1.1.31) and AAT (EC 2.6.1.1) increased with salinity in roots, whereas there was decreased activity of the alternative ammonium assimilation enzyme GDH (EC 1.4.1.2). The most striking effect of nitrogen regime was observed on GDH whose salinity-induced decrease in activity was reduced from 34% with NO(3)(-) alone to only 14% with the mixed regime. The results suggest that the detrimental effects of salinity can be reduced by partial substitution of NO(3)(-) with NH(4)(+) and that this is due to the lower energy cost of N assimilation with NH(4)(+) as opposed to NO(3)(-) nutrition.

Effect of ammonium and nitrate nutrition on some physiological processes in higher plants - growth, photosynthesis, photorespiration, and water relations

Ammonium and nitrate as different forms of nitrogen nutrients impact differently on some physiological and biochemical processes in higher plants. Compared to nitrate, ammonium results in small root and small leaf area, which may contribute to a low carbon gain, and an inhibition on growth. On the other hand, due to (photo)energy saving, a higher CO (2) assimilation rate per leaf area was observed frequently in plants supplied with ammonium than in those supplied with nitrate. These results were dependent not only on higher Rubisco content and/or activity, but also on RuBP regeneration rate. The difference in morphology such as chloroplast volume and specific leaf weight might be the reason why the CO (2) concentration in the carboxylation site and hence the photorespiration rate differs in plants supplied with the two nitrogen forms. The effect of nitrogen form on water uptake and transportation in plants is dependent both on leaf area or shoot parameter, and on the root activity (i.e., root hydraulic conductivity, aquaporin activity).

Wheat nitrogen metabolism during grain filling: comparative role of glumes and the flag leaf

The mobilization of nitrogen (N) compounds and the roles played by glumes and the flag leaf during grain filling were studied in bread wheat (Triticum aestivum L. cv. Florida) grown under field conditions. Glumes lost twice as much of their total N content as that lost by the flag leaf between the milk and early dough stages. In the flag leaf, glumes and grains, Glu, Asp, Ser and Ala accounted for 85% of all the reductions in the free amino acid pool. Principal component analysis of free amino acid pools separated grains from the glumes and the flag leaf, suggesting grain specific regulations in the use of free amino acids in protein synthesis. In all three organs, no decrease in Gln was detected, probably due to steady glutamine synthetase (GS; EC 6.3.1.2) activities per soluble protein in both the flag leaf and glumes. Compared with the flag leaf, glumes presented relatively smaller amounts of the chloroplast GS associated isoform. This we show is due to a lower relative number of mesophyll cells in glumes as supported by the different anatomy and the cellular pattern of the GS immunolocalization. We argue that cellular distribution plays a key role in supporting metabolism to enable the various functions undertaken by glume tissue.

Abiotic stress generates ROS that signal expression of anionic glutamate dehydrogenases to form glutamate for proline synthesis in tobacco and grapevine

Glutamate dehydrogenase (GDH) may be a stress-responsive enzyme, as GDH exhibits considerable thermal stability, and de novo synthesis of the alpha-GDH subunit is induced by exogenous ammonium and senescence. NaCl treatment induces reactive oxygen species (ROS), intracellular ammonia, expression of tobacco (Nicotiana tabacum cv Xanthi) gdh-NAD;A1 encoding the alpha-subunit of GDH, increase in immunoreactive alpha-polypeptide, assembly of the anionic isoenzymes, and in vitro GDH aminating activity in tissues from hypergeous plant organs. In vivo aminating GDH activity was confirmed by gas chromatorgraphy-mass spectrometry monitoring of (15)N-Glu, (15)N-Gln, and (15)N-Pro in the presence of methionine sulfoximine and amino oxyacetic acid, inhibitors of Gln synthetase and transaminases, respectively. Along with upregulation of alpha-GDH by NaCl, isocitrate dehydrogenase genes, which provide 2-oxoglutarate, are also induced. Treatment with menadione also elicits a severalfold increase in ROS and immunoreactive alpha-polypeptide and GDH activity. This suggests that ROS participate in the signaling pathway for GDH expression and protease activation, which contribute to intracellular hyperammonia. Ammonium ions also mimic the effects of salinity in induction of gdh-NAD;A1 expression. These results, confirmed in tobacco and grape (Vitis vinifera cv Sultanina) tissues, support the hypothesis that the salinity-generated ROS signal induces alpha-GDH subunit expression, and the anionic iso-GDHs assimilate ammonia, acting as antistress enzymes in ammonia detoxification and production of Glu for Pro synthesis.

Effect of sugar-induced senescence on gene expression and implications for the regulation of senescence in Arabidopsis

There has been some debate whether leaf senescence is induced by sugar starvation or by sugar accumulation. External supply of sugars has been shown to induce symptoms of senescence such as leaf yellowing. However, it was so far not clear if sugars have a signalling function during developmental senescence. Glucose and fructose accumulate strongly during senescence in Arabidopsis thaliana (L.) Heynh. leaves. Using Affymetrix GeneChip analysis we determined the effect of sugar-induced senescence on gene expression. Growth on glucose in combination with low nitrogen supply induced leaf yellowing and changes in gene expression that are characteristic of developmental senescence. Most importantly, the senescence-specific gene SAG12, which was previously thought to be sugar-repressible, was induced over 900-fold by glucose. Induction of SAG12, which is expressed during late senescence, demonstrates that processes characteristic for late stages are sugar-inducible. Two MYB transcription factor genes, PAP1 and PAP2, were identified as senescence-associated genes that are induced by glucose. Moreover, growth on glucose induced genes for nitrogen remobilisation that are typically enhanced during developmental senescence, including the glutamine synthetase gene GLN1;4 and the nitrate transporter gene AtNRT2.5. In contrast to wild-type plants, the hexokinase-1 mutant gin2-1 did not accumulate hexoses and senescence was delayed. Induction of senescence by externally supplied glucose was partially abolished in gin2-1, indicating that delayed senescence was a consequence of decreased sugar sensitivity. Taken together, our results show that Arabidopsis leaf senescence is induced rather than repressed by sugars.

Fluctuations of γ-aminobutyrate, γ-hydroxybutyrate, and related amino acids in Arabidopsis leaves as a function of the light–dark cycle, leaf age, and N stressEditorial decisions for this paper were made by Robert Ireland, Associate Editor, Canadian Journal of Botany

To gain further insight into the metabolic role of γ-aminobutyrate (GABA), we determined the pool sizes of GABA and its catabolic products, alanine and γ-hydroxybutyrate (GHB), as well as key amino acids (Glu, Gln, Asp, Asn, Pro, Gly, Ser), in Arabidopsis leaves as a function of the light–dark cycle, leaf age (old versus young), and N stress (continuous versus interrupted N supply). Regardless of time of day and leaf age, there was a close relationship among Glu, GABA, and GHB when N was supplied continuously, indicating that GABA and GHB were probably derived exclusively from Glu and GABA, respectively. Ala was also closely linked to GABA in young leaves, but not in old leaves, a result consistent with the existence of multiple sources of Ala. The nature of the responses of GABA and GHB to an interrupted N supply depended on leaf age, and differed from responses exhibited by Glu, Gln, and Asn. Overall fluctuations in primary amino acids under both continuous and interrupted N supply, as well as those associated with photorespiration, aging, and stress, suggest that the old and young leaves chosen for study here function in Arabidopsis as source and sink leaves, respectively.

Arabidopsis cytosolic glutamine synthetase AtGLN1;1 is a potential substrate of AtCRK3 involved in leaf senescence

While considerable progress has been achieved in plant CDPK studies in the past decade, there is relatively no information about the potential substrates of CRKs. In this report, a yeast two-hybrid screen was performed with truncated form of AtCRK3 as bait to identify its interacting proteins in an effort to dissect its physiological roles. One gene encoding cytosolic glutamine synthetase AtGLN1;1 was isolated. Further analyses indicated that AtGLN1;1 could interact specifically with AtCRK3 and the kinase domain of AtCRK3 and the catalytic domain of AtGLN1;1 were responsible for such interaction, respectively. Furthermore, in vitro and in vivo co-immunoprecipitation results strongly supported that they could physically interact with each other. Phosphorylation assays revealed that AtGLN1;1 could be specifically phosphorylated by AtCRK3 in vitro. All the results demonstrate that AtGLN1;1 may be a substrate of AtCRK3. In addition, both AtGLN1;1 and AtCRK3 could be induced by natural or artificially induced leaf senescence, implying that such interaction may be involved in the regulation of nitrogen remobilization during leaf senescence.

Antisense repression reveals a crucial role of the plastidic 2-oxoglutarate/malate translocator DiT1 at the interface between carbon and nitrogen metabolism

Ammonia assimilation by the plastidic glutamine synthetase/glutamate synthase system requires 2-oxoglutarate (2-OG) as a carbon precursor. Plastids depend on 2-OG import from the cytosol. A plastidic dicarboxylate translocator 1-[2-OG/malate translocator (DiT1)] has been identified and its substrate specificity and kinetic constants have been analyzed in vitro. However, the role of DiT1 in intact plants and its significance for ammonia assimilation remained uncertain. Here, to study the role of DiT1 in intact plants, its expression was antisense-repressed in transgenic tobacco plants. This resulted in a reduced transport capacity for 2-OG across the plastid envelope membrane. In consequence, allocation of carbon precursors to amino acid synthesis was impaired, organic acids accumulated and protein content, photosynthetic capacity and sugar pools in leaves were strongly decreased. The phenotype was consistent with a role of DIT1 in both, primary ammonia assimilation and the re-assimilation of ammonia resulting from the photorespiratory carbon cycle. Unexpectedly, the in situ rate of nitrate reduction was extremely low in alpha-DiT1 leaves, although nitrate reductase (NR) expression and activity remained high. We hypothesize that this discrepancy between extractable NR activity and in situ nitrate reduction is due to substrate limitation of NR. These findings and the severe phenotype of the antisense plants point to a crucial role of DiT1 at the interface between carbon and nitrogen metabolism.

Combined agronomic and physiological aspects of nitrogen management in wheat highlight a central role for glutamine synthetase

In wheat the period of grain filling is characterized by a transition for all vegetative organs from sink to source status. To study this transition, the progression of physiological markers and enzyme activities representative of nitrogen metabolism was monitored from the vegetative stage to maturity in different leaf stages and stem sections of two wheat (Triticum aestivum) cultivars grown at high and low levels of N fertilization. In the two cultivars examined, we found a general decrease of the metabolic and enzyme markers occurred during leaf ageing, and that this decrease was enhanced when plants were N-limited. Both correlation studies and principal components analysis (PCA) showed that there was a strong relationship among total N, chlorophyll, soluble protein, ammonium, amino acids and glutamine synthetase (GS) activity. The use of a marker such as GS activity to predict the N status of wheat, as a function of both plant development and N availability, is discussed with the aim of selecting wheat genotypes with better N-use efficiency.