Intracellular distribution of enzymes associated with nitrogen assimilation in roots

Exploration of nitrate-to-glutamate assimilation in non-photosynthetic roots of higher plants by studies of 15N-tracing, enzymes involved, reductant supply, and nitrate signaling: A review and synthesis

Roots of the higher plants can assimilate inorganic nitrogen by an enzymatic reduction of the most oxidized form (+6) nitrate to the reduced form (-2) glutamate. For such reactions, the substrates (originated from photosynthates) must be imported to supply energy through the reductant-generating systems within the root cells. Intensive studies over last 70 years (reviewed here) revealed the precise mechanisms of nitrate-to-glutamate transformation in roots with elaborate searches of ¹⁵N-tracing, enzymes involved, the reductant-supplying system, and nitrate signaling. In the 1970s, the tracing of ¹⁵N-labeled nitrate and ammonia in the roots demonstrated the sequential reduction and assimilation of nitrate to nitrite, ammonia, glutamine amide, and then glutamate. These reactions involve nitrate reductase (NADH-NR, EC 1.7.1.1) in the cytosol, nitrite reductase (ferredoxin [Fd]-NiR, EC 1.7.7.1), glutamine synthetase (GS2, EC 6.3.1.2), and glutamate synthase (Fd-GOGAT, EC 1.4.7.1) in the plastids. NADH for NR is generated by glycolysis in the cytosol, and NADPH for Fd-NIR and Fd-GOGAT are produced by the oxidative pentose phosphate pathway (OPPP). Electrons from NADPH are conveyed to reduce NIR and Fd-GOGAT through Fd-NADP⁺ reductase (FNR, EC 1.6.7.1) specifically in the roots. Physiological and molecular analyses showed the parallel inductions of NR, NIR, GS2, Fd-GOGAT, OPPP enzymes, FNR, and Fd in response to a short-term nitrate supply. Recent studies proposed a molecular mechanism of nitrate-induction of these genes and proteins. Roots can also assimilate the reduced form of inorganic ammonia by the combination of cytosolic GS1 and plastidic NADH-GOGAT.

Signaling by glutamate dehydrogenase in response to pesticide treatment and nitrogen fertilization of peanut (Arachis hypogaea L.)

The responses of glutamate dehydrogenase (GDH) to NH(4)(+) and herbicides offer a new approach for probing the effects of NH(4)(+)-pesticide interactions at the whole-plant level. Although pesticides and fertilizers have greatly enhanced food production, their combined biochemical effects are not known in detail. Peanut plants were treated with different rates of Basagran (3-(1-methylethyl)-1H-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide), Bravo 720 (tetrachloroiso-phthalonitrile), and Sevin XLR Plus (1-naphthyl N-methylcarbamate), with and without 25 mM NH(4)Cl fertilization. Isoelectric focusing, followed by native 7.5% polyacrylamide gel electrophoresis (PAGE) fractionated the peanut seed GDH fully to its isoenzyme population patterns. The pesticide treatments induced positive skewing of the GDH isoenzymes, but NH(4)Cl-pesticide cotreatments induced a negatively skewed distribution. Basagran, Sevin, and Bravo increased the amination activities of GDH from 30.0 +/- 2.8 units in the control assay to 479.0 +/- 20.7, 63.0 +/- 5.8, and 35.2 +/- 2.2 units, respectively, therefore indicating a direct GDH-pesticide interaction. Neither the NH(4)(+) nor the pesticides increased the peanut seed protein yields above the threshold of 3.8 +/- 0.7 g per pot. But in the GDH combination of the signals from a pesticide and NH(4)(+), at least 70% of the pesticide signal was overridden by NH(4)(+) with concomitant increases in peanut seed protein yields to 7.0 +/- 1.8 g per pot. Basagran, Sevin, and Bravo possess different pesticidal properties, but their effects on GDH activity were related in the decreasing order of their nucleophilicity, viz. Basagran > Sevin > Bravo.

Quantitative intercellular localization of NADH-dependent glutamate synthase protein in different types of root cells in rice plants

The quantitative analysis with immunogold-electron microscopy using a single-affinity-purified anti-NADH-glutamate synthase (GOGAT) immunoglobulin G (IgG) as the primary antibody showed that the NADH-GOGAT protein was present in various forms of plastids in the cells of the epidermis and exodermis, in the cortex parenchyma, and in the vascular parenchyma of root tips (<10 mm) of rice (Oryza sativa) seedlings supplied with 1 mM NH4+ for 24 h. The values of the mean immunolabeling density of plastids were almost equal among these different cell types in the roots. However, the number of plastids per individual cell type was not identical, and some parts of the cells in the epidermis and exodermis contained large numbers of plastids that were heavily immunolabeled. Although there was an indication of labeling in the mitochondria using the single-affinity-purified anti-NADH-GOGAT IgG, this was not confirmed when a twice-affinity-purified IgG was used, indicating an exclusively plastidial location of the NADH-GOGAT protein in rice roots. These results, together with previous work from our laboratory (K. Ishiyama, T. Hayakawa, and T. Yamaya [1998] Planta 204: 288-294), suggest that the assimilation of exogeneously supplied NH4+ ions is primarily via the cytosolic glutamine synthetase/plastidial NADH-GOGAT cycle in specific regions of the epidermis and exodermis in rice roots. We also discuss the role of the NADH-GOGAT protein in vascular parenchyma cells.

Structure and regulation of ferredoxin-dependent glutamase synthase from Arabidopsis thaliana. Cloning of cDNA expression in different tissues of wild-type and gltS mutant strains, and light induction

Ferredoxin (Fd)-dependent glutamate synthase is present in green leaves, etiolated leaves, shoots and roots of Arabidopsis thaliana (ecotype Columbia). In photosynthetic green leaves and shoots, Fd-dependent glutamate synthase accounts for more than 96% of the total glutamate synthase activity in vitro with the remaining activity derived from an enzyme that uses NADH as the electron donor. In etiolated leaves and roots, Fd-dependent glutamate synthase is 3-4-fold less active than in green leaves, but represents 70-85% of the total glutamate synthase activity in these tissues. Fd-dependent glutamate synthase is detected as a single peptide of 165 kDa on a western blot of green leaf and shoot tissues, and this Fd-dependent glutamate synthase polypeptide is 3-4-fold less abundant in etiolated leaves and roots. In these non-photosynthetic tissues, there is a higher activity of NADH-dependent glutamate synthase. The A. thaliana gltS mutant (strain CS254) contains only 1.7% and 17.5% of the wild-type Fd-dependent glutamate synthase activity in leaves and roots, respectively. Western blots indicate that the Fd-dependent glutamate synthase peptide of 165 kDa is absent from leaves and roots of the gltS mutant. In contrast, NADH-dependent glutamate synthase activity in leaves and roots is unaffected. During illumination of wild-type dark-grown leaves for 72 h, the levels of Fd-dependent glutamate synthase protein and its activity increased threefold to levels equivalent to those in green leaves. In contrast, NADH-dependent glutamate synthase activity decrease twofold during illumination. The complete nucleotide sequence of the complementary DNA for A. thaliana Fd-dependent glutamate synthase has been determined. Analysis of the amino acid sequence deduced from the complete cDNA sequence (5178 bp) has revealed that A. thaliana Fd-dependent glutamate synthase is synthesized as a 1648-amino-acid precursor protein (180090 Da) which consists of a 131-amino-acid transit peptide (14603 Da) and a 1517-amino-acid mature peptide (165487 Da). The A. thaliana Fd-dependent glutamate synthase has a high similarity to maize Fd-dependent glutamate synthase (83%) and to the analogous region of NADH-dependent glutamate synthase (42%) and NADPH-dependent glutamate synthases (40-43%) from different organisms. The A. thaliana Fd-dependent glutamate synthase contains the purF-type glutamine-amido-transfer domain as well as flavin and iron-sulfur-cluster-binding domains. The deduced primary structures of A. thaliana Fd-dependent glutamate synthase and of glutamate synthases from other organisms indicate that Fd-dependent glutamate synthase may have evolved from bacterial NADPH-dependent glutamate synthase. The cDNA hybridized to RNA of about 5.3 kb from different tissues of A. thaliana. A high steady-state level of Fd-dependent glutamate synthase mRNA is found in photosynthetic green leaves and shoots, and roots contain less mRNA for Fd-dependent glutamate synthase. In the gltS mutant, there are twofold and fourfold lower levels of Fd-dependent glutamate synthase mRNA in leaves and roots, respectively, relative to those in wild-type A. thaliana. Under continuous illumination of dark-grown leaves, the Fd-dependent glutamate synthase mRNA is induced twofold to a level equivalent to that in green leaves.

Latent nitrate reductase activity is associated with the plasma membrane of corn roots

Latent nitrate reductase activity (NRA) was detected in corn (Zea mays L., Golden Jubilee) root microsome fractions. Microsome-associated NRA was stimulated up to 20-fold by Triton X-100 (octylphenoxy polyethoxyethanol) whereas soluble NRA was only increased up to 1.2-fold. Microsome-associated NRA represented up to 19% of the total root NRA. Analysis of microsomal fractions by aqueous two-phase partitioning showed that the membrane-associated NRA was localized in the second upper phase (U2). Analysis with marker enzymes indicated that the U2 fraction was plasma membrane (PM). The PM-associated NRA was not removed by washing vesicles with up to 1.0 M NACl but was solubilized from the PM with 0.05% Triton X-100. In contrast, vanadate-sensitive ATPase activity was not solubilized from the PM by treatment with 0.1% Triton X-100. The results show that a protein capable of reducing nitrate is embedded in the hydrophobic region of the PM of corn roots.

Action of light, nitrate and ammonium on the levels of NADH- and ferredoxin-dependent glutamate synthases in the cotyledons of mustard seedlings

In mustard (Sinapis alba L.) cotyledons, NADH-dependent glutamate synthase (NADH-GOGAT, EC 1.4.1.14) is only detectable during early seedling development with a peak of enzyme activity occurring between 2 and 2.5 d after sowing. With the beginning of plastidogenesis at approximately 2 d after sowing, ferredoxindependent glutamate synthase (Fd-GOGAT, EC 1.4.7.1) appears while NADH-GOGAT drops to a very low level. The enzymes were separated by anion exchange chromatography. Both enzymes are stimulated by light operating through phytochrome. However, the extent of induction is much higher in the case of Fd-GOGAT than in the case of NADH-GOGAT. Moreover, NADH-GOGAT is inducible predominantly by red light pulses, while the light induction of Fd-GOGAT operates predominantly via the high irradiance response of phytochrome. The NADH-GOGAT level is strongly increased if mustard seedlings are grown in the presence of nitrate (15 mM KNO3,15 mM NH4NO3) while the Fd-GOGAT level is only slightly affected by these treatments. No effect on NADH-GOGAT level was observed by growing the seedlings in the presence of ammonium (15 mM NH4Cl) instead of water, whereas the level of Fd-GOGAT was considerably reduced when seedlings were grown in the presence of NH4Cl. Inducibility of NADH-GOGAT by treatment with red light pulses or by transferring water-grown seedlings to NO 3 (-) -containing medium follows a temporal pattern of competence. The very low Fd-GOGAT level in mustard seedlings grown under red light in the presence of the herbicide Norflurazon, which leads to photooxidative destruction of the plastids, indicates that the enzyme is located in the plastids. The NADH-GOGAT level is, in contrast, completely independent of plastid integrity which indicates that its location is cytosolic. It is concluded that NADH-GOGAT in the early seedling development is mainly concerned with metabolizing stored glutamine whereas Fd-GOGAT is involved in ammonium assimilation.

Cytosolic glutamine synthetase in higher plants. A comparative immunological study

Cytosolic glutamine synthetase (GS1) was purified to homogeneity from etiolated barley leaves by DEAE-Sephacel and hydroxyapatite chromatography, gel filtration and polyacrylamide gel electrophoresis. Specific antibodies against the purified protein were raised by the immunization of rabbits. Immunoprecipitation experiments demonstrated that cytosolic glutamine synthetases isolated from the leaves of different plant species were very similar proteins. Good recognition of other cytosolic glutamine synthetases from roots, root nodular tissue and seeds by barley GS1 antibodies was obtained, suggesting that they too are all quite similar proteins. In contrast, chloroplast glutamine synthetase (GS2) was considered to be a different protein in view of its low level of recognition by barley GS1 antibodies.

Cellular and subcellular organization of pathways of ammonia assimilation and ureide synthesis in nodules of cowpea (Vigna unguiculata L. Walp.)

Fractionation of cell organelles of nitrogen-fixing nodules of cowpea (Vigna unguiculata L. Walp) by discontinuous and continuous sucrose density centrifugation indicated that starch-containing plastids possessed the complete pathway for purine nucleotide synthesis together with significant activities of some other enzymes associated with the provision of substrates in purine synthesis; triosephosphate isomerase (EC 5.3.1.1), NADH-glutamate synthase (EC 2.6.1.53), aspartate aminotransferase (EC 2.6.1.1), phosphoglycerate oxidoreductase (EC 1.1.1.95), and methylene tetrahydrofolate oxidoreductase (EC 1.5.1.5). Enzymes of purine oxidation, xanthine oxidoreductase (EC 1.2.3.2), and urate oxidase (EC 1.7.3.3) were recovered in the soluble fraction; glutamine synthetase (EC 6.3.1.2) occurred in bacteroids and in the cytosol. Intact, infected (bacteroid-containing) and uninfected cells were prepared by enzymatic maceration of the central zone of the nodule and partially separated by centrifugation on discontinuous sucrose gradients. Glutamine synthetase was largely restricted to infected cells whereas plastid enzymes, de novo purine synthesis, and urate oxidase were present in both cell types. Although the levels of all enzymes assayed were higher in infected cells, both cell types possessed the necessary enzyme complement for ureide formation. A model for the cellular and subcellular organization of nitrogen metabolism and the transport of nitrogenous solutes in cowpea nodules is proposed.

The supply of reducing power for nitrite reduction in plastids of seedling pea roots (Pisum sativum L.)

Plastids were separated from extracts of pea (Pisum sativum L.) roots by sucrose-density-gradient centrifugation. The incubation of roots of intact pea seedlings in solutions containing 10 mM KNO3 resulted in increased plastid activity of nitrite reductase and to a lesser extent glutamine synthetase. There were also substantial increases in the activity of glucose-6-phosphate and 6-phosphogluconate dehydrogenases. No other plastid-located enzymes of nitrate assimilation or carbohydrate oxidation showed evidence of increased activity in response to the induction of nitrate assimilation. Studies with [1-(14)C]-and [6-(14)C]glucose indicated that there was an increased flow of carbon through the plastid-located pentose-phosphate pathway concurrent with the induction of nitrate assimilation. It is suggested that there is a close interaction through the supply and demand for reductant between the pathway of nitrite assimilation and the pentose-phosphate pathway located in the plastid.

Subcellular organization of ureide biogenesis from glycolytic intermediates and ammonium in nitrogen-fixing soybean nodules

Subcellular organelle fractionation of nitrogen-fixing nodules of soybean (Glycine max (L.) Merr.) indicates that a number of enzymes involved in the assimilation of ammonia into amino acids and purines are located in the proplastids. These include asparagine synthetase (EC 6.3.1.1), phosphoribosyl amidotransferase (EC 2.4.2.14), phosphoglycerate dehydrogenase (EC 1.1.1.95), serine hydroxymethylase (EC 2.1.2.1), and methylene-tetrahydrofolate dehydrogenase (EC 1.5.1.5). Of the two isoenzymes of asparate aminotransferase (EC 2.6.1.1) in the nodule, only one was located in the proplastid fraction. Both glutamate synthase (EC 1.4.1.14) and triosephosphate isomerase (EC 5.3.1.1) were associated at least in part with the proplastids. Glutamine synthetase (EC 6.3.1.2) and xanthine dehydrogenase (EC 1.2.1.37) were found in significant quantities only in the soluble fraction. Phosphoribosylpyrophosphate synthetase (EC 2.7.6.1) was found mostly in the soluble fraction, although small amounts of it were detected in other organelle fractions. These results together with recent organelle fractionation and electron microscopic studies form the basis for a model of the subcellular distribution of ammonium assimilation, amide synthesis and uredie biogenesis in the nodule.