Molecular identification and characterization of cytosolic isoforms of glutamine synthetase in maize roots

Pine has two glutamine synthetase paralogs, GS1b.1 and GS1b.2, exhibiting distinct biochemical properties

The enzyme glutamine synthetase (EC 6.3.1.2) is mainly responsible for the incorporation of inorganic nitrogen into organic molecules in plants. In the present work, a pine (Pinus pinaster) GS1 (PpGS1b.2) gene was identified, showing a high sequence identity with the GS1b.1 gene previously characterized in conifers. Phylogenetic analysis revealed that the presence of PpGS1b.2 is restricted to the genera Pinus and Picea and is not found in other conifers. Gene expression data suggest a putative role of PpGS1b.2 in plant development, similar to other GS1b genes from angiosperms, suggesting evolutionary convergence. The characterization of GS1b.1 and GS1b.2 at the structural, physicochemical, and kinetic levels has shown differences even though they have high sequence homology. GS1b.2 had a lower optimum pH (6 vs. 6.5) and was less thermally stable than GS1b.1. GS1b.2 exhibited positive cooperativity for glutamate and substrate inhibition for ammonium. However, GS1b.1 exhibited substrate inhibition behavior for glutamate and ATP. Alterations in the kinetic characteristics produced by site-directed mutagenesis carried out in this work strongly suggest an implication of amino acids at positions 264 and 267 in the active center of pine GS1b.1 and GS1b.2 being involved in affinity toward ammonium. Therefore, the amino acid differences between GS1b.1 and GS1b.2 would support the functioning of both enzymes to meet distinct plant needs.

Glutamine Synthetase Contributes to the Regulation of Growth, Conidiation, Sclerotia Development, and Resistance to Oxidative Stress in the Fungus Aspergillus flavus

The basic biological function of glutamine synthetase (Gs) is to catalyze the conversion of ammonium and glutamate to glutamine. This synthetase also performs other biological functions. However, the roles of Gs in fungi, especially in filamentous fungi, are not fully understood. Here, we found that conditional disruption of glutamine synthetase (AflGsA) gene expression in Aspergillus flavus by using a xylose promoter leads to a complete glutamine deficiency. Supplementation of glutamine could restore the nutritional deficiency caused by AflGsA expression deficiency. Additionally, by using the xylose promoter for the downregulation of AflgsA expression, we found that AflGsA regulates spore and sclerotic development by regulating the transcriptional levels of sporulation genes abaA and brlA and the sclerotic generation genes nsdC and nsdD, respectively. In addition, AflGsA was found to maintain the balance of reactive oxygen species (ROS) and to aid in resisting oxidative stress. AflGsA is also involved in the regulation of light signals through the production of glutamine. The results also showed that the recombinant AflGsA had glutamine synthetase activity in vitro and required the assistance of metal ions. The inhibitor molecule L-α-aminoadipic acid suppressed the activity of rAflGsA in vitro and disrupted the morphogenesis of spores, sclerotia, and colonies in A. flavus. These results provide a mechanistic link between nutrition metabolism and glutamine synthetase in A. flavus and suggest a strategy for the prevention of fungal infection.

Assembly status transition offers an avenue for activity modulation of a supramolecular enzyme

Nature has evolved many supramolecular proteins assembled in certain, sometimes even seemingly oversophisticated, morphological manners. The rationale behind such evolutionary efforts is often poorly understood. Here, we provide atomic-resolution insights into how the dynamic building of a structurally complex enzyme with higher order symmetry offers amenability to intricate regulation. We have established the functional coupling between enzymatic activity and protein morphological states of glutamine synthetase (GS), an old multi-subunit enzyme essential for cellular nitrogen metabolism. Cryo-EM structure determination of GS in both the catalytically active and inactive assembly states allows us to reveal an unanticipated self-assembly-induced disorder-order transition paradigm, in which the remote interactions between two subcomplex entities significantly rigidify the otherwise structurally fluctuating active sites, thereby regulating activity. We further show in vivo evidences that how the enzyme morphology transitions could be modulated by cellular factors on demand. Collectively, our data present an example of how assembly status transition offers an avenue for activity modulation, and sharpens our mechanistic understanding of the complex functional and regulatory properties of supramolecular enzymes.

Transcriptomic and Biochemical Analysis Reveal Integrative Pathways Between Carbon and Nitrogen Metabolism in Guzmania monostachia (Bromeliaceae) Under Drought

Most epiphytes are found in low-nutrient environments with an intermittent water supply. To deal with water limitation, many bromeliads perform crassulacean acid metabolism (CAM), such as Guzmania monostachia, which shifts from C₃ to CAM and can recycle CO₂ from the respiration while stomata remain closed during daytime and nighttime (CAM-idling mode). Since the absorbing leaf trichomes can be in contact with organic (urea) and inorganic nutrients (NO₃ ^-, NH₄ ⁺) and the urea hydrolysis releases NH₄ ⁺ and CO₂, we hypothesized that urea can integrate the N and C metabolism during periods of severe drought. Under this condition, NH₄ ⁺ can be assimilated into amino acids through glutamine synthetase (GS), while the CO₂ can be pre-fixated by phosphoenolpyruvate carboxylase (PEPC). In this context, we evaluated the foliar transcriptome of G. monostachia to compare the relative gene expression of some genes involved with CAM and the N metabolism when bromeliads were submitted to 7days of drought. We also conducted a controlled experiment with an extended water deficit period (21days) in which bromeliads were cultivated in different N sources (urea, NH₄ ⁺, and NO₃ ^-). Our transcriptome results demonstrated an increment in the expression of genes related to CAM, particularly those involved in the carboxylation metabolism (PEPC1, PPCK, and NAD-MDH), the movement of malate through vacuolar membrane (ALMT9), and the decarboxylation process (PEPCK). Urea stimulated the expression of PEPC1 and ALMT9, while Urease transcripts increased under water deficit. Under this same condition, GS1 gene expression increased, indicating that the NH₄ ⁺ from urea hydrolysis can be assimilated in the cytosol. We suggest that the link between C and N metabolism occurred through the supply of carbon skeleton (2-oxoglutarate, 2-OG) by the cytosolic isocitrate dehydrogenase since the number of NADP-ICDH transcripts was also higher under drought conditions. These findings indicate that while urea hydrolysis provides NH₄ ⁺ that can be consumed by glutamine synthetase-cytosolic/glutamate synthase (GS1/GOGAT) cycle, the CO₂ can be used by CAM, maintaining photosynthetic efficiency even when most stomata remain closed 24h (CAM-idling) as in the case of a severe water deficit condition. Thus, we suggest that urea could be used by G. monostachia as a strategy to increase its survival under drought, integrating N and C metabolism.

An Overview of Important Enzymes Involved in Nitrogen Assimilation of Plants

Nitrogen (N) is a macro-nutrient that is essential for growth development and resistance against biotic and abiotic stresses of plants. Nitrogen is a constituent of amino acids, proteins, nucleic acids, chlorophyll, and various primary and secondary metabolites. The atmosphere contains huge amounts of nitrogen but it cannot be taken up directly by plants. Plants can take up nitrogen in the form of nitrate, ammonium, urea, nitrite, or a combination of all these forms. In addition, in various leguminous rhizobia, bacteria can convert atmospheric nitrogen to ammonia and supply it to the plants. The form of nitrogen nutrition is also important in plant growth and resistance against pathogens. Nitrogen content has an important function in crop yield. Nitrogen deficiency can cause reduced root growth, change in root architecture, reduced plant biomass, and reduced photosynthesis. Hence, understanding the function and regulation of N metabolism is important. Several enzymes and intermediates are involved in nitrogen assimilation. Here we provide an overview of the important enzymes such as nitrate reductase, nitrite reductase, glutamine synthase, GOGAT, glutamate dehydrogenase, and alanine aminotransferase that are involved in nitrogen metabolism.

Characteristics and influence factors of NO2 exchange flux between the atmosphere and P. nigra

Nitrogen dioxide (NO₂) is an important substance in atmospheric photochemical processes and can also be absorbed by plants. NO₂ fluxes between the atmosphere and P. nigra seedlings were investigated by a double dynamic chambers method in Beijing from June 15 to September 3, 2017. The range of NO₂ exchange fluxes between P. nigra seedlings and the atmosphere was from -14.6 to 0.8 nmol/(m²·sec) (the positive data represent NO₂ emission from trees, while the negative values indicate absorption). Under ambient concentrations, the mean NO₂ flux during the fast-growing stage (Jun. 15-Aug. 4) was -3.0 nmol/(m²·sec), greater than the flux of -1.5 nmol/(m²·sec) during the later growth stage (Aug. 8-Sept. 3). The daily exchange fluxes of NO₂ obviously fluctuated. The fluxes were largest in the morning and decreased gradually over time. Additionally, the NO₂ fluxes were larger under high light intensities than under low light intensities during the whole growth period. The effects of temperature on NO₂ fluxes were different under two growth periods. The NO₂ exchange fluxes were larger in a range of temperatures close to 44°C in the fast-growing stage, whereas there were no evident differences in NO₂ exchange fluxes under widely differing temperatures in the later growth stage. Under polluted conditions, the uptake ability of NO₂ was weakened. Additionally, the compensation point of NO₂ was 5.6 ppb in the fast-growing stage, whereas it was 1.4 ppb in the later growth stage. The deposition velocities of NO₂ were between 0.3 and 2.4 mm/sec.

Roles of Cytosolic Glutamine Synthetases in Arabidopsis Development and Stress Responses

Glutamine (Gln) has as a central role in nitrogen (N) and carbon (C) metabolism. It is synthesized during assimilation of ammonium by cytosolic and plastidial glutamine synthetases (GS; EC 6.1.1.3). Arabidopsis thaliana has five cytosolic GS (GS1) encoding genes designated as GLN1;1-GLN1;5 and one plastidial GS (GS2) gene. In this report that concerns cytosolic GS, we show by analyzing single, double and triple mutants that single genes were dispensable for growth under laboratory conditions. However, loss of two or three GS1 isoforms impacted plant form, function and the capacity to tolerate abiotic stresses. The loss of GLN1;1, GLN1;2 and GLN1;3 resulted in a significant reduction of vegetative growth and seed size. In addition, we infer that GLN1;4 is essential for pollen viability but only in the absence of GLN1;1 and GLN1;3. Transcript profiling revealed that expression of GLN1;1, GLN1;2, GLN1;3 and GLN1;4 was repressed by salinity and cold stresses. Among all single gln1 mutants, growth of gln1;1 seedlings showed an enhanced sensitivity to the GS inhibitor phosphinothricin (PPT), as well as to cold and salinity treatments, suggesting a non-redundant role for GLN1;1. Furthermore, the increased sensitivity of gln1;1 mutants to methyl viologen was associated with an accelerated accumulation of reactive oxygen species (ROS) in the thylakoid of chloroplasts. Our data demonstrate, for the first time, an involvement of the cytosolic GS1 in modulating ROS homeostasis in chloroplasts. Collectively, the current study establishes a link between cytosolic Gln production and plant development, ROS production and stress tolerance.

Gluconeogenesis and nitrogen metabolism in maize

Two pathways can be used by gluconeogenesis in plants: one employs phosphoenolpyruvate carboxykinase (PEPCK) and the other pyruvate orthophosphate dikinase (PPDK). The occurrence-location of these enzymes was determined in developing kernels of maize. PPDK was much more abundant than PEPCK in extracts of whole kernels. However, their location within the kernel was different. PPDK was particularly abundant in the peripheral endosperm (in which alanine is abundant), whereas PEPCK was localised in the pedicel and basal endosperm transfer cells (where asparagine is metabolised). The abundance of these enzymes was also determined in maize roots where there was a massive increase in abundance of PEPCK and a small increase in abundance of PPDK when they were fed ammonium; PEPCK was located in the pericycle and various cell types associated with the vasculature. On the other hand, there was a large increase in abundance of PPDK in roots subjected to anoxia (which induces an accumulation of alanine), whereas the abundance of PEPCK was decreased. These results show: firstly, that gluconeogenesis can potentially occur in many different tissues of maize. Secondly, within one organ PPDK can be abundant in some tissues and PEPCK in others. Thirdly, the abundance of PPDK and PEPCK is often associated with the metabolism of certain nitrogenous compounds and can be dramatically altered by factors related to nitrogen metabolism. In maize roots and developing kernels PPDK was associated with alanine metabolism. By contrast, the presence of PEPCK in maize roots and kernels was associated with either ammonium or asparagine metabolism. We propose that gluconeogenesis is often a component of a widespread mechanism that is used in coordinating the import/mobilisation of nitrogenous compounds with their utilisation. Further, potentially component of this mechanism may have provided building blocks that were used in the evolution of processes such as C₄ photosynthesis, Crassulacean acid metabolism, stomatal metabolism and the biochemical pH stat.

Cytosolic Glutamine Synthetase Isozymes Play Redundant Roles in Ammonium Assimilation Under Low-Ammonium Conditions in Roots of Arabidopsis thaliana

Ammonium is a major nitrogen source for plants; it is assimilated into glutamine via a reaction catalyzed by glutamine synthetase (GLN). Arabidopsis expresses four cytosolic GLN genes, GLN1; 1, GLN1; 2, GLN1; 3 and GLN1; 4, in roots. However, the function and organization of these GLN1 isozymes in ammonium assimilation in roots remain unclear. In this study, we aimed to characterize the four GLN1 isozymes. The levels of growth of the wild type and gln1 single and multiple knockout lines were compared in a hydroponic culture at ammonium concentrations of 0.1 and 3 mM. Under the low-ammonium concentration, in single mutants for each GLN1 gene, there was little effect on growth, whereas the triple mutant for GLN1; 1, GLN1; 2 and GLN1; 3 grew slowly and accumulated ammonium. Under the high-ammonium concentration, the single mutant for GLN1; 2 showed 50% decreases in fresh weight and glutamine, whereas the other gln1 single mutants did not show notable changes in the phenotype. The double mutant for GLN1; 1 and GLN1; 2 showed less growth and a lower glutamine concentration than the single mutant for GLN1; 2. Promoter analysis indicated an overlapping expression of GLN1; 1 with GLN1; 2 in the surface layers of the roots. We thus concluded that: (i) at a low concentration, ammonium was assimilated by GLN1; 1, GLN1; 2 and GLN1; 3, and they were redundant; (ii) low-affinity GLN1; 2 could contribute to ammonium assimilation at concentrations ranging from 0.1 to 3 mM; and (iii) GLN1; 1 supported GLN1; 2 within the outer cell layers of the root.

Combined acid rain and lanthanum pollution and its potential ecological risk for nitrogen assimilation in soybean seedling roots

Rare earth elements (REEs) are used in various fields, resulting in their accumulation in the environment. This accumulation has affected the survival and distribution of crops in various ways. Acid rain is a serious global environmental problem. The combined effects on crops from these two types of pollution have been reported, but the effects on crop root nitrogen assimilation are rarely known. To explore the impact of combined contamination from these two pollutants on crop nitrogen assimilation, the soybean seedlings were treated with simulated environmental pollution from acid rain and a representative rare earth ion, lanthanum ion (La3+), then the indexes related to plant nitrogen assimilation process in roots were determined. The results showed that combined treatment with pH 4.5 acid rain and 0.08 mM La3+ promoted nitrogen assimilation synergistically, while the other combined treatments all showed inhibitory effects. Moreover, acid rain aggravated the inhibitory effect of 1.20 or 0.40 mM La3+ on nitrogen assimilation in soybean seedling roots. Thus, the effects of acid rain and La3+ on crops depended on the combination levels of acid rain intensity and La3+ concentration. Acid rain increases the bioavailability of La3+, and the combined effects of these two pollutants were more serious than that of either pollutant alone. These results provide new evidence in favor of limiting overuse of REEs in agriculture. This work also provides a new framework for ecological risk assessment of combined acid rain and REEs pollution on soybean crops.

Cloning, molecular characterization, and phylogeny of two evolutionary distinct glutamine synthetase isoforms in the green microalga Haematococcus pluvialis (Chlorophyceae)

Haematococcus pluvialis (Chlorophyta) is a widely used microalga of great economic potential, yet its molecular genetics and evolution are largely unknown. We present new detailed molecular and phylogenetic analysis of two glutamine synthetase (GS) enzymes and genes (gln) under the Astaxanthin-inducing conditions of light- and nitrogen-stress. Structure analysis identified key residues and confirmed two decameric GS₂ holoenzymes, a cytoplasmic enzyme, termed GS_2c , and a plastidic form, termed GS_2p , due to chloroplast-transit peptides at its N-terminus. Gene expression analysis showed dissociation of mRNA, protein, and enzyme activity levels for both GS₂ under different growth conditions, indicating the strong post-transcriptional regulation. Data-mining identified novel and specified published gln genes from Prasinophyceae, Chlorophyta, Trebouxiophyceae, Charophyceae, Bryophyta, Lycopodiophyta, Spermatophyta, and Rhodophyta. Phylogenetic analysis found homologues to the cytosolic GS_2c of H. pluvialis in all other photo- and non-photosynthetic Eukaryota. The chloroplastic GS_2p was restricted to Chlorophyta, Bryophyta, some Proteobacteria and Fungii; no homologues were identified in Spermatophyta or other Eukaryota. This indicates two independent prokaryotic donors for these two gln genes in H. pluvialis. Combined phylogenetic analysis of GS, chl-b synthase, elongation factor, and light harvesting complex homologues project a newly refined model of Viridiplantae evolution. Herein, a GS₁ evolved into the cytosolic GS_2c and was passed on to all Eukaryota. Later, the chloroplastic GS_2p entered the Archaeplastida lineage via a horizontal gene transfer at the divergence of Chlorophyta and Rhodophyta lineages. GS_2p persisted in Chlorophyta and Bryophyta, but was lost during Spermatophyta evolution. These data suggest the revision of GS classification and nomenclature, and extend our understanding of the photosynthetic Eukaryota evolution.

Mineral nitrogen sources differently affect root glutamine synthetase isoforms and amino acid balance among organs in maize

BACKGROUND

Glutamine synthetase (GS) catalyzes the first step of nitrogen assimilation in plant cell. The main GS are classified as cytosolic GS1 and plastidial GS2, of which the functionality is variable according to the nitrogen sources, organs and developmental stages. In maize (Zea mays L.) one gene for GS2 and five genes for GS1 subunits are known, but their roles in root metabolism are not yet well defined. In this work, proteomic and biochemical approaches have been used to study root GS enzymes and nitrogen assimilation in maize plants re-supplied with nitrate, ammonium or both.

RESULTS

The plant metabolic status highlighted the relevance of root system in maize nitrogen assimilation during both nitrate and ammonium nutrition. The analysis of root proteomes allowed a study to be made of the accumulation and phosphorylation of six GS proteins. Three forms of GS2 were identified, among which only the phosphorylated one showed an accumulation trend consistent with plastidial GS activity. Nitrogen availabilities enabled increments in root total GS synthetase activity, associated with different GS1 isoforms according to the nitrogen sources. Nitrate nutrition induced the specific accumulation of GS1-5 while ammonium led to up-accumulation of both GS1-1 and GS1-5, highlighting co-participation. Moreover, the changes in thermal sensitivity of root GS transferase activity suggested differential rearrangements of the native enzyme. The amino acid accumulation and composition in roots, xylem sap and leaves deeply changed in response to mineral sources. Glutamine showed the prevalent changes in all nitrogen nutritions. Besides, the ammonium nutrition was associated with an accumulation of asparagine and reducing sugars and a drop in glutamic acid level, significantly alleviated by the co-provision with nitrate.

CONCLUSION

This work provides new information about the multifaceted regulation of the GS enzyme in maize roots, indicating the involvement of specific isoenzymes/isoforms, post-translational events and biochemical factors. For the first time, the proteomic approach allowed to discriminate the individual contribution of the GS1 isoforms, highlighting the participation of GS1-5 in nitrate metabolism. Moreover, the results give new insights about the influence of amino acid metabolism in plant C/N balance.

Expression, purification, and characterization of recombinant mangrove glutamine synthetase

To expand our knowledge about the relationship of nitrogen use efficiency and glutamine synthetase (GS) activity in the mangrove plant, a cytosolic GS gene from Avicennia marina has been heterologously expressed in and purified from Escherichia coli. Synthesis of the mangrove GS enzyme in E. coli was demonstrated by functional genetic complementation of a GS deficient mutant. The subunit molecular mass of GSI was ~40 kDa. Optimal conditions for biosynthetic activity were found to be 35 °C at pH 7.5. The Mg(2+)-dependent biosynthetic activity was strongly inhibited by Ni(2+), Zn(2+), and Al(3+), whereas was enhanced by Co(2+). The apparent K m values of AmGLN1 for the substrates in the biosynthetic assay were 3.15 mM for glutamate, and 2.54 mM for ATP, 2.80 mM for NH4 (+) respectively. The low affinity kinetics of AmGLN1 apparently participates in glutamine synthesis under the ammonium excess conditions.

Root phosphoenolpyruvate carboxylase and NAD-malic enzymes activity increase the ammonium-assimilating capacity in tomato

Plant ammonium tolerance has been associated with the capacity to accumulate large amounts of ammonium in the root vacuoles, to maintain carbohydrate synthesis and especially with the capacity of maintaining high levels of inorganic nitrogen assimilation in the roots. The tricarboxylic acid cycle (TCA) is considered a cornerstone in nitrogen metabolism, since it provides carbon skeletons for nitrogen assimilation. The hypothesis of this work was that the induction of anaplerotic routes of phosphoenolpyruvate carboxylase (PEPC), malate dehydrogenase (MDH) and malic enzyme (NAD-ME) would enhance tolerance to ammonium nutrition. An experiment was established with tomato plants (Agora Hybrid F1) grown under different ammonium concentrations. Growth parameters, metabolite contents and enzymatic activities related to nitrogen and carbon metabolism were determined. Unlike other tomato cultivars, tomato Agora Hybrid F1 proved to be tolerant to ammonium nutrition. Ammonium was assimilated as a biochemical detoxification mechanism, thus leading to the accumulation of Gln and Asn as free amino acids in both leaves and roots as an innocuous and transitory store of nitrogen, in addition to protein synthesis. When the concentration of ammonium in the nutrient solution was high, the cyclic operation of the TCA cycle seemed to be interrupted and would operate in two interconnected branches to provide α-ketoglutarate for ammonium assimilation: one branch supported by malate accumulation and by the induction of anaplerotic PEPC and NAD-ME in roots and MDH in leaves, and the other branch supported by stored citrate in the precedent dark period.

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.

The 5' untranslated region of the soybean cytosolic glutamine synthetase β(1) gene contains prokaryotic translation initiation signals and acts as a translational enhancer in plants

Glutamine synthetase (GS) catalyzes the synthesis of glutamine from glutamate and ammonia. In plants, it occurs as two major isoforms, a cytosolic form (GS(1)) and a nuclear encoded chloroplastic form. The focus of this paper is to determine the role of the 5'UTR of a GS(1) gene. GS(1) gene constructs with and without its 5' and 3' UTRs, driven by a constitutive promoter, were agroinfiltrated into tobacco leaves and the tissues were analyzed for both transgene transcript and protein accumulation. The constructs were also tested in an in vitro transcription/translation system and in Escherichia coli. Our results showed that while the 3'UTR functioned in the destabilization of the transcript, the 5'UTR acted as a translation enhancer in plant cells but not in the in vitro translation system. The 5'UTR of the GS(1) gene when placed in front of a reporter gene (uidA), showed a 20-fold increase in the level of GUS expression in agroinfiltrated leaves when compared to the same gene construct without the 5'UTR. The 5'UTR-mediated translational enhancement is probably another step in the regulation of GS in plants. The presence of the GS(1) 5'UTR in front of the GS(1) coding region allowed for its translation in E. coli suggesting the commonality of the translation initiation mechanism for this gene between plants and bacteria.

The cytosolic glutamine synthetase GLN1;2 plays a role in the control of plant growth and ammonium homeostasis in Arabidopsis rosettes when nitrate supply is not limiting

Glutamine synthetase (EC 6.3.1.2) is a key enzyme of ammonium assimilation and recycling in plants where it catalyses the synthesis of glutamine from ammonium and glutamate. In Arabidopsis, five GLN1 genes encode GS1 isoforms. GLN1;2 is the most highly expressed in leaves and is over-expressed in roots by ammonium supply and in rosettes by ample nitrate supply compared with limiting nitrate supply. It is shown here that the GLN1;2 promoter is mainly active in the minor veins of leaves and flowers and, to a lower extent, in the parenchyma of mature leaves. Cytoimmunochemistry reveals that the GLN1;2 protein is present in the companion cells. The role of GLN1;2 was determined by examining the physiology of gln1;2 knockout mutants. Mutants displayed lower glutamine synthetase activity, higher ammonium concentration, and reduced rosette biomass compared with the wild type (WT) under ample nitrate supply only. No difference between mutant and WT can be detected under limiting nitrate conditions. Despite total amino acid concentration was increased in the old leaves of mutants at high nitrate, no significant difference in nitrogen remobilization can be detected using (15)N tracing. Growing plants in vitro with ammonium or nitrate as the sole nitrogen source allowed us to confirm that GLN1;2 is induced by ammonium in roots and to observe that gln1;2 mutants displayed, under such conditions, longer root hair and smaller rosette phenotypes in ammonium. Altogether the results suggest that GLN1;2 is essential for nitrogen assimilation under ample nitrate supply and for ammonium detoxification.

14-3-3 proteins fine-tune plant nutrient metabolism

14-3-3 Proteins regulate many cellular processes by binding to phosphorylated proteins. Previous findings suggest a connection between three 14-3-3 isoforms and plant nutrient signaling. To better understand how these 14-3-3s regulate metabolism in response to changes in plant nutrient status, putative new targets involved in nitrogen (N) and sulfur (S) metabolisms have been identified. The interactions between these 14-3-3s and multiple proteins involved in N and S metabolism and altered activity of the target proteins were confirmed in planta. Using a combination of methods, this work elucidates how 14-3-3s function as modulators of plant N and S metabolic pathways.

Molecular cloning and characterisation of cytoplasmic glutamine synthetase gene BcGS1 from non-heading Chinese cabbage

BACKGROUND

Glutamine synthetase (GS; EC 6.3.1.2) is a key enzyme of nitrogen (N) assimilation, catalysing the synthesis of glutamine from ammonium and glutamate. Plants have two types of GS isoenzyme that are localised in different compartments: one in the cytosol (GS1) and the other in the chloroplast (GS2). GS1 is the major form of GS in plant roots and directly converts ammonium taken up by plant roots to glutamine.

RESULTS

The GS1 gene cDNA of non-heading Chinese cabbage (Brassica campestrisssp. chinensis Makino) cultivar 'Suzhouqing' was isolated by RT-PCR (real-time polymerase chain reaction) and (5'/3')-RACE (rapid amplification of cDNA ends) techniques. It was classified as GS1 by sequence alignment and motif search and named B. campestris ssp. chinensis Makino GS1 (BcGS1). Subcellular localisation analysis showed that BcGS1 was distributed in the cytoplasm of cells. BcGS1 was expressed in all parts, but mainly in the roots, which was verified by northern blotting analysis. Additionally, its expression was influenced by the N source concentration.

CONCLUSION

These results suggest that BcGS1 is a novel member of the GS family in plants. BcGS1 was significantly related to N assimilation in non-heading Chinese cabbage, demonstrating that this gene plays an important role in plant growth and development.

Effectiveness and mode of action of phosphonate inhibitors of plant glutamine synthetase

BACKGROUND

Aiming at the rational design of new herbicides, the availability of the three-dimensional structure of the target enzyme greatly enhances the optimisation of lead compounds and the design of derivatives with increased activity. Among the most widely exploited herbicide targets is glutamine synthetase. Recently, the structure of a cytosolic form of the maize enzyme has been described, making it possible to verify whether steric, electronic and hydrophobic features of a compound are in agreement with inhibitor-protein interaction geometry.

RESULTS

Three series of compounds (aminophosphonates, hydroxyphosphonates and aminomethylenebisphosphonates) were evaluated as possible inhibitors of maize glutamine synthetase. Aminomethylenebisphosphonate derivatives substituted in the phenyl ring retained the inhibitory potential, whereas variations in the scaffold, i.e. the replacement of the second phosphonate moiety with a hydroxyl or an amino residue, resulted in a significant loss of activity. A kinetic characterisation showed a non-competitive mechanism against glutamate and an uncompetitive mechanism against ATP. A docking analysis suggested the mode of bisphosphonate binding to the active site.

CONCLUSION

Results made it possible to define the features required to maintain or enhance the biological activity of these compounds, which represent lead structures to be further exploited for the design of new substances endowed with herbicidal activity.

The importance of cytosolic glutamine synthetase in nitrogen assimilation and recycling

Glutamine synthetase assimilates ammonium into amino acids, thus it is a key enzyme for nitrogen metabolism. The cytosolic isoenzymes of glutamine synthetase assimilate ammonium derived from primary nitrogen uptake and from various internal nitrogen recycling pathways. In this way, cytosolic glutamine synthetase is crucial for the remobilization of protein-derived nitrogen. Cytosolic glutamine synthetase is encoded by a small family of genes that are well conserved across plant species. Members of the cytosolic glutamine synthetase gene family are regulated in response to plant nitrogen status, as well as to environmental cues, such as nitrogen availability and biotic/abiotic stresses. The complex regulation of cytosolic glutamine synthetase at the transcriptional to post-translational levels is key to the establishment of a specific physiological role for each isoenzyme. The diverse physiological roles of cytosolic glutamine synthetase isoenzymes are important in relation to current agricultural and ecological issues.

Light modulates activity and expression of glutamine synthetase isoforms in maize seedling roots

In maize roots, continuous illumination inhibits chloroplastic glutamine synthetase (GS2) activity, which decreased in light from 72.8% in 4-day-old to 26% in 10-day-old plants. In dark-adapted plants transferred to light for 6 days, GS2 activity declined from 100% to 41%, but in light-adapted plants transferred to darkness, it increased to the level of the dark control. Changes of cytosolic (GS1) activity were minor, with a similar trend. Quantitative RT-PCR revealed that light/dark treatments moderately affected only transcription of GS1 isoforms, with the exception of GS1-2, which was dramatically induced by darkness and repressed by light.

Effect of chilling and acclimation on the activity of glutamine synthetase isoforms in maize seedlings

Effects of chilling and acclimation on the activity of cytosolic (GS1) and plastidic (GS2) isoforms of glutamine synthetase (E.C. 6.3.1.2) were studied in chilling-sensitive and acclimation-responsive maize inbred G50. Glutamine synthetase activity in mesocotyls and roots of chilled (7 d/4?C) and rewarmed (1 d/27?C) etiolated plants was "1/3 that of controls. In coleoptiles+leaves of light-grown plants, GS1 was reduced to 75%, and GS2 to 50%. Acclimation (3 d/14?C) increased GS activity and alleviated the effects of chilling. Exposure to H2O2 or menadione also reduced GS activity. Since chilling causes oxidative stress in maize, acclimation probably preserves GS activity by protecting GS from oxidative inactivation. .

The three-dimensional structure of an eukaryotic glutamine synthetase: Functional implications of its oligomeric structure

The structure of the prokaryotic glutamine synthetases type I (GS-I), key enzymes in nitrogen metabolism, was determined several years ago by X-ray diffraction, and consists of a double hexameric ring. The structure of the eukaryotic GS from the plant Phaseolus vulgaris (Glutamine synthetase type II; GS-II) has now been determined at low-resolution using electron microscopy and image processing, and consists of an octamer composed of two tetramers placed back-to-back and rotated 90 degrees with respect to each other. The oligomeric structure possesses a twofold symmetry, very suggestive of each tetramer being composed of two dimers. This is reinforced by the fact that dimers are isolated as a stable albeit non-functional species during the purification procedure. Given the fact that the active site of all types of GS is formed by highly conserved residues located in the interface of two interacting monomers, the geometry of the reconstructed tetramer suggests that it only contains two functional active sites, i.e., an active site per dimer. This is supported by biochemical data, which reveal that while the octamer binds eight ATP molecules, it only binds four molecules of the transition state analogue and GS inhibitor methionine-(S)-sulfoximine-P (MetSox-P). All this suggests for the GS-II enzyme an oligomeric structure containing four active sites and four possible regulatory sites, which might point to a complex regulatory behavior.

Two cytosolic glutamine synthetase isoforms of maize are specifically involved in the control of grain production

The roles of two cytosolic maize glutamine synthetase isoenzymes (GS1), products of the Gln1-3 and Gln1-4 genes, were investigated by examining the impact of knockout mutations on kernel yield. In the gln1-3 and gln1-4 single mutants and the gln1-3 gln1-4 double mutant, GS mRNA expression was impaired, resulting in reduced GS1 protein and activity. The gln1-4 phenotype displayed reduced kernel size and gln1-3 reduced kernel number, with both phenotypes displayed in gln1-3 gln1-4. However, at maturity, shoot biomass production was not modified in either the single mutants or double mutants, suggesting a specific impact on grain production in both mutants. Asn increased in the leaves of the mutants during grain filling, indicating that it probably accumulates to circumvent ammonium buildup resulting from lower GS1 activity. Phloem sap analysis revealed that unlike Gln, Asn is not efficiently transported to developing kernels, apparently causing reduced kernel production. When Gln1-3 was overexpressed constitutively in leaves, kernel number increased by 30%, providing further evidence that GS1-3 plays a major role in kernel yield. Cytoimmunochemistry and in situ hybridization revealed that GS1-3 is present in mesophyll cells, whereas GS1-4 is specifically localized in the bundle sheath cells. The two GS1 isoenzymes play nonredundant roles with respect to their tissue-specific localization.

Molecular analysis of two mutants from Lotus japonicus deficient in plastidic glutamine synthetase: functional properties of purified GLN2 enzymes

Two photorespiratory mutants from Lotus japonicus, namely Ljgln2-1 and Ljgln2-2, deficient in plastidic glutamine synthetase (GLN2), were analysed at the molecular level. Both mutants showed normal levels of Gln2 mRNA, indicating that they were affected post-transcriptionally. Complete sequencing of full-length Gln2 cDNAs revealed the presence of a single point mutation on each mutant, leading to G85R and L278H amino acid replacements, respectively. Different types of experimental approaches, including heterologous expression and complementation tests in Escherichia coli, showed that both GLN2 mutant proteins completely lacked of biosynthetic and transferase enzyme activities. Moreover, it was also shown that while GLN2-1 mutant protein was assembled into a less stable inactive octamer, GLN2-2 mutant protein was unable to acquire a proper quaternary structure and was rapidly degraded. Therefore, the mutations analysed are the first of their type affecting the stability and/or the quaternary structure of the GLN2 enzyme. The kinetic parameters of purified recombinant GLN2 were determined. The enzyme showed positive cooperativity towards ammonium and Mg(2+). Thiol compounds stimulated by twofold the biosynthetic activity but not the transferase activity of recombinant GLN2 and were able to alter the kinetics towards glutamate of the enzyme. Moreover, the biosynthetic activity of recombinant GLN2 was stimulated by more than tenfold by the presence of free Mg(2+).

Interactions between nitrogen and cytokinin in the regulation of metabolism and development

Inorganic nitrogen is a substrate for nitrogen assimilation and also functions as a signal triggering widespread changes in gene expression that modulate metabolism and development. To integrate the actions of the nitrogen signal at the whole plant level, plants use multiple signaling routes that communicate internal and external nitrogen status. One route depends on nitrate itself and one uses cytokinin as a messenger. Recent genome-wide research has shown that the nitrate-specific signal regulates a wide variety of metabolic processes including nitrogen and carbon metabolism, and cytokinin biosynthesis. Cytokinin-mediated signaling is related to the control of development, protein synthesis and acquisition of macronutrients. The coordination and interaction of both regulatory pathways is important for normal plant growth under variable nitrogen supply conditions.

Atomic structure of plant glutamine synthetase: a key enzyme for plant productivity

Plants provide nourishment for animals and other heterotrophs as the sole primary producer in the food chain. Glutamine synthetase (GS), one of the essential enzymes for plant autotrophy catalyzes the incorporation of ammonia into glutamate to generate glutamine with concomitant hydrolysis of ATP, and plays a crucial role in the assimilation and re-assimilation of ammonia derived from a wide variety of metabolic processes during plant growth and development. Elucidation of the atomic structure of higher plant GS is important to understand its detailed reaction mechanism and to obtain further insight into plant productivity and agronomical utility. Here we report the first crystal structures of maize (Zea mays L.) GS. The structure reveals a unique decameric structure that differs significantly from the bacterial GS structure. Higher plants have several isoenzymes of GS differing in heat stability and catalytic properties for efficient responses to variation in the environment and nutrition. A key residue responsible for the heat stability was found to be Ile-161 in GS1a. The three structures in complex with substrate analogues, including phosphinothricin, a widely used herbicide, lead us to propose a mechanism for the transfer of phosphate from ATP to glutamate and to interpret the inhibitory action of phosphinothricin as a guide for the development of new potential herbicides.

Gln49 and Ser174 residues play critical roles in determining the catalytic efficiencies of plant glutamine synthetase

Two essential residues playing critical roles in determining the substrate specificities of cytosolic glutamine synthetase (GS1) have been identified from the alignment of high-affinity (GLN1;1 and GLN1;4) and low-affinity (GLN1;2 and GLN1;3) GS1 isoenzymes in Arabidopsis, and confirmed by site-directed mutagenesis. The results indicated that either K49Q or A174S mutation is sufficient to increase the catalytic efficiencies of GLN1;3 by decreasing its Km values for ammonium. In contrast, replacement of Gln49 and Ser174 by lysine and alanine, respectively, was detrimental to glutamine synthetic activities in GLN1;4. The results suggested that Gln49 and Ser174 in the high-affinity GS1 isoenzymes are interchangeable with Lys49 and Ala174 in the low-affinity variants at the corresponding positions.

Biochemical background and compartmentalized functions of cytosolic glutamine synthetase for active ammonium assimilation in rice roots

Rice plants in paddy fields prefer to utilize ammonium as a major nitrogen source. Glutamine synthetase (GS) serves for assimilation of ammonium in rice root, and ameliorates the toxic effect of ammonium excess. Among the three isoenzymes of the cytosolic GS1 gene family in rice, OsGLN1;1 and OsGLN1;2 were abundantly expressed in roots. Analysis of the purified enzymes showed that OsGLN1;1 and OsGLN1;2 can be classified into high-affinity subtypes with relatively high V(max) values, as compared with the major high-affinity isoenzyme, GLN1;1, in Arabidopsis. Low-affinity forms of GS1 comparable to those in Arabidopsis (GLN1;2 and GLN1;3) were absent in rice roots. The OsGLN1;1 and OsGLN1;2 transcripts showed reciprocal responses to ammonium supply in the surface cell layers of roots. OsGLN1;1 accumulated in dermatogen, epidermis and exodermis under nitrogen-limited condition. By contrast, OsGLN1;2 was abundantly expressed in the same cell layers under nitrogen-sufficient conditions, replenishing the loss of OsGLN1;1 following ammonium treatment. Within the central cylinder of elongating zone, OsGLN1;1 and OsGLN1;2 were both induced by ammonium, which was distinguishable from the response observed in the surface cell layers. The high-capacity Gln synthetic activities of OsGLN1;1 and OsGLN1;2 facilitate active ammonium assimilation in specific cell types in rice roots.

Kinetic properties and ammonium-dependent regulation of cytosolic isoenzymes of glutamine synthetase in Arabidopsis

Glutamine synthetase (GS; EC 6.3.1.2) is a key enzyme of nitrogen assimilation, catalyzing the synthesis of glutamine from ammonium and glutamate. In Arabidopsis, cytosolic GS (GS1) was accumulated in roots when plants were excessively supplied with ammonium; however, the GS activity was controlled at a constant level. The discrepancy between the protein content and enzyme activity of GS1 was attributable to the kinetic properties and expression of four distinct isoenzymes encoded by GLN1;1, GLN1;2, GLN1;3 and GLN1;4, genes that function complementary to each other in Arabidopsis roots. GLN1;2 was the only isoenzyme significantly up-regulated by ammonium, which correlated with the rapid increase in total GS1 protein. GLN1;2 was localized in the vasculature and exhibited low affinities to ammonium (Km = 2450 +/- 150 microm) and glutamate (Km = 3.8 +/- 0.2 mm). The expression of the counterpart vascular tissue-localizing low affinity isoenzyme, GLN1;3, was not stimulated by ammonium; however, the enzyme activity of GLN1;3 was significantly inhibited by a high concentration of glutamate. By contrast, the high affinity isoenzyme, GLN1;1 (Km for ammonium < 10 microm; Km for glutamate = 1.1 +/- 0.4 mm) was abundantly accumulated in the surface layers of roots during nitrogen limitation and was down-regulated by ammonium excess. GLN1;4 was another high affinity-type GS1 expressed in nitrogen-starved plants but was 10-fold less abundant than GLN1;1. These results suggested that dynamic regulations of high and low affinity GS1 isoenzymes at the levels of mRNA and enzyme activities are dependent on nitrogen availabilities and may contribute to the homeostatic control of glutamine synthesis in Arabidopsis roots.

Distinctive properties and expression profiles of glutamine synthetase from a plant symbiotic fungus

The nucleotide sequences reported in this paper have been submitted to the GenBank(R)/EBI Nucleotide Sequence Databases with accession numbers AF462037 (glutamine synthetase) and AF462032 (glutamate synthase). Nitrogen retrieval and assimilation by symbiotic ectomycorrhizal fungi is thought to play a central role in the mutualistic interaction between these organisms and their plant hosts. Here we report on the molecular characterization of the key N-assimilation enzyme glutamine synthetase from the mycorrhizal ascomycete Tuber borchii (TbGS). TbGS displayed a strong positive co-operativity ( n =1.7+/-0.29) and an unusually high S(0.5) value (54+/-16 mM; S(0.5) is the substrate concentration value at which v =(1/2) V (max)) for glutamate, and a correspondingly low sensitivity towards inhibition by the glutamate analogue herbicide phosphinothricin. The TbGS mRNA, which is encoded by a single-copy gene in the Tuber genome, was up-regulated in N-starved mycelia and returned to basal levels upon resupplementation of various forms of N, the most effective of which was nitrate. Both responses were accompanied by parallel variations of TbGS protein amount and glutamine synthetase activity, thus indicating that TbGS levels are primarily controlled at the pre-translational level. As revealed by a comparative analysis of the TbGS mRNA and of the mRNAs for the metabolically related enzymes glutamate dehydrogenase and glutamate synthase, TbGS is not only the sole messenger that positively responds to N starvation, but also the most abundant under N-limiting conditions. A similar, but even more discriminating expression pattern, with practically undetectable glutamate dehydrogenase mRNA levels, was observed in fruitbodies. The TbGS mRNA was also found to be expressed in symbiosis-engaged hyphae, with distinctively higher hybridization signals in hyphae that were penetrating among and within root cells.

Functional expression of two pine glutamine synthetase genes in bacteria reveals that they encode cytosolic holoenzymes with different molecular and catalytic properties

Two glutamine synthetase isogenes, GS1a and GS1b, isolated from pine have been functionally expressed in E. coli and the characteristics of individual gene products compared. When bacteria were grown at 37 degrees C most pine GS1 protein was found in the insoluble fraction but lowering of the expression temperature increased yield of both GS1 polypeptide and activity in the soluble fraction. High levels of functionally active GS1a (309 + or - 35 nkat mg(-1)) and GS1b (1,166 + or - 65 nkat mg(-1)) enzymes were obtained by decreasing the expression temperature to 10 degrees C. Purification and characterization of recombinant products showed that pine GS1 polypeptides are assembled in octameric GS holoenzymes showing structural and kinetic differences. The results are discussed with regard to the specific localization of GS1a and GS1b in different cell types of pine seedlings. The isoform GS1a may control the assimilation of the high levels of ammonium released in photosynthetic tissues, whereas GS1b enzyme could mitigate oscillations in glutamate availability providing a constant flux of glutamine for nitrogen transport in vascular cells.

Molecular and enzymatic analysis of ammonium assimilation in woody plants

Ammonium is assimilated into amino acids through the sequential action of glutamine synthetase (GS) and glutamate synthase (GOGAT) enzymes. This metabolic pathway is driven by energy, reducing power and requires the net supply of 2-oxoglutarate that can be provided by the reaction catalysed by isocitrate dehydrogenase (IDH). Most studies on the biochemistry and molecular biology of N-assimilating enzymes have been carried out on annual plant species and the available information on woody models is far more limited. This is in spite of their economic and ecological importance and the fact that nitrogen is a common limiting factor for tree growth. GS, GOGAT and IDH enzymes have been purified from several woody species and their kinetic and molecular properties determined. A number of cDNA clones have also been isolated and characterized. Although the enzymes are remarkably well conserved along the evolutionary scale, major differences have been found in their compartmentation within the cell between angiosperms and conifers, suggesting possible adaptations to specific functional roles. The analysis of the gene expression patterns in a variety of biological situations such as changes in N nutrition, development, biotic or abiotic stresses and senescence, suggest that cytosolic GS plays a central and pivotal role in ammonium assimilation and metabolism in woody plants. The modification of N assimilation efficiency has been recently approached in trees by overexpression of a cytosolic pine GS in poplar. The results obtained, suggest that an increase in cytosolic GS might lead to a global effect on the synthesis of nitrogenous compounds in the leaves, with enhanced vegetative growth of transgenic trees. All these data suggest that manipulation of cytosolic GS may have consequences for plant growth and biomass production.

Constitutive overexpression of cytosolic glutamine synthetase (GS1) gene in transgenic alfalfa demonstrates that GS1 may be regulated at the level of RNA stability and protein turnover

Glutamine synthetase (GS) catalyzes the ATP-dependent condensation of NH4+ with glutanate to yield glutamine. Gene constructs consisting of the cauliflower mosaic virus (CaMV) 35S promoter driving a cytosolic isoform of GS (GS1) gene have been introduced into alfalfa (Medicago sativa). Although transcripts for the transgene were shown to accumulate to high levels in the leaves, they were undetectable in the nodules. However, significant amounts of beta-glucuronidase activity could be detected in nodules of plants containing the CaMV 35S promoter-beta-glucuronidase gene construct, suggesting that the transcript for the GS1 transgene is not stable in the root nodules. Leaves of alfalfa plants with the CaMV 35S promoter-GS1 gene showed high levels of accumulation of the transcript for the transgene when grown under low-nitrogen conditions and showed a significant drop in the level of GS1 transcripts when fed with high levels of NO3-. However, no increase in GS activity or polypeptide level was detected in the leaves of transgenic plants. The results suggest that GS1 is regulated at the level of RNA stability and protein turnover.

Two genes encoding distinct cytosolic glutamine synthetases are closely linked in the pine genome

The major isoenzyme of glutamine synthetase found in leaves of angiosperms is the chloroplastic form. However, pine seedlings contain two cytosolic glutamine synthetases in green cotyledons: GS1a, the predominant isoform, and GS1b, a minor enzyme whose relative amount is increased following phosphinotricin treatment. We have cloned a GS1b cDNA, and comparison with the previously reported GS1a cDNA sequence indicated that they correspond to separate cytosolic GS genes encoding distinct protein products. Phylogenetic analysis showed that the newly reported sequence is closer to cytosolic angiosperm GS than to GS1a, suggesting therefore that GS1a could be a divergent gymnospermous GS1 gene. Gene mapping using a F2 family of maritime pine showed co-localization of both GS genes on group 2 of the genetic linkage map. This result supports the proposed origin of different members of the GS1 family by adjacent gene duplication. The implications for gymnosperm genome organization are discussed.

Complementary DNA cloning and characterization of ferredoxin localized in bundle-sheath cells of maize leaves

In maize (Zea mays L.) two leaf-specific ferredoxin (Fd) isoproteins, Fd I and Fd II, are distributed differentially in mesophyll and bundle-sheath cells. A novel cDNA encoding the precursor of Fd II (pFD2) was isolated by heterologous hybridization using a cDNA for Fd I (pFD1) as a probe. The assignment of the cDNAs to the Fds was verified by capillary liquid-chromatography/electrospray ionization-mass spectrometry. RNA-blot analysis demonstrated that transcripts for Fd I and Fd II accumulated specifically in mesophyll and bundle-sheath cells, respectively. The mature regions of pFD1 and pFD2 were expressed in Escherichia coli as functional Fds. Fd I and Fd II had similar redox potentials of -423 and -406 mV, respectively, but the Km value of Fd-NADP+ reductase for Fd II was about 3-fold larger than that for Fd I. Asparagine at position 65 of Fd II is a unique residue compared with Fd I and other Fds from various plants, which have aspartic acid or glutamic acid at the corresponding position as an electrostatic interaction site with Fd-NADP+ reductase. Substitution of asparagine-65 with aspartic acid increased the affinity of Fd II with Fd-NADP+ reductase to a level comparable to that of Fd I. These structural and functional differences of Fd I and Fd II may be related to their cell-specific expression in the leaves of a C4 plant.