Characterization of Arabidopsis AtAMT2, a high-affinity ammonium transporter of the plasma membrane

AtAMT2 is an ammonium transporter that is only distantly related to the five members of the AtAMT1 family of high-affinity ammonium transporters in Arabidopsis. The short-lived radioactive ion (13)NH(4)(+) was used to show that AtAMT2, expressed in yeast (Saccharomyces cerevisiae), is a high-affinity transporter with a K(m) for ammonium of about 20 microM. Changes in external pH between 5.0 and 7.5 had little effect on the K(m) for ammonium, indicating that NH(4)(+), not NH(3), is the substrate for AtAMT2. The AtAMT2 gene was expressed in all organs of Arabidopsis and was subject to nitrogen (N) regulation, at least in roots where expression was partially repressed by high concentrations of ammonium nitrate and derepressed in the absence of external N. Although expression of AtAMT2 in shoots responded little to changes in root N status, transcript levels in leaves declined under high CO(2) conditions. Transient expression of an AtAMT2-green fluorescent protein fusion protein in Arabidopsis leaf epidermal cells indicated a plasma membrane location for the AtAMT2 protein. Thus, AtAMT2 is likely to play a significant role in moving ammonium between the apoplast and symplast of cells throughout the plant. However, a dramatic reduction in the level of AtAMT2 transcript brought about by dsRNA interference with gene expression had no obvious effect on plant growth or development, under the conditions tested.

The soybean GmN6L gene encodes a late nodulin expressed in the infected zone of nitrogen-fixing nodules

Previously, we determined the N-terminal amino acid sequences of a number of putative peribacteroid membrane proteins from soybean. Here, we report the cloning of a gene, GmN6L, that encodes one of these proteins. The protein encoded by GmN6L is similar in sequence to MtN6, an early nodulin expressed in Medicago truncatula roots in response to infection by Sinorhizobium meliloti. The GmN6L gene was strongly expressed in mature nodules but not in other plant organs. GmN6L protein was first detected 2 weeks after inoculation with Bradyrhizobium japonicum and was limited to the infected zone of nodules. GmN6L protein was found in symbiosomes isolated from mature soybean nodules, both as a soluble protein and as a peripheral membrane protein bound to the peribacteroid membrane. These data indicate that GmN6L is a late nodulin, which is not involved in the infection process. Homology between GmN6L and FluG, a protein involved in signaling in Aspergillus nidulans, suggests that GmN6L may play a role in communication between the host and microsymbionts during symbiotic nitrogen fixation.

Novel aspects of symbiotic nitrogen fixation uncovered by transcript profiling with cDNA arrays

An array of 2,304 cDNA clones derived from nitrogen-fixing nodules of Lotus japonicus was produced and used to detect differences in relative gene transcript abundance between nodules and uninfected roots. Transcripts of 83 different genes were found to be more abundant in nodules than in roots. More than 50 of these have never before been identified as nodule-induced in any species. Expression of 36 genes was detected in nodules but not in roots. Several known nodulin genes were included among the nodule-induced genes. Also included were genes involved in sucrose breakdown and glycolysis, CO2 recycling, and amino acid synthesis, processes that are known to be accelerated in nodules compared with roots. Genes involved in membrane transport, hormone metabolism, cell wall and protein synthesis, and signal transduction and regulation of transcription were also induced in nodules. Genes that may subvert normal plant defense responses, including two encoding enzymes involved in detoxification of active oxygen species and one that may prohibit phytoalexin synthesis, were also identified. The data represent a rich source of information for hypothesis building and future exploration of symbiotic nitrogen fixation.

GmZIP1 encodes a symbiosis-specific zinc transporter in soybean

The importance of zinc in organisms is clearly established, and mechanisms involved in zinc acquisition by plants have recently received increased interest. In this report, the identification, characterization and location of GmZIP1, the first soybean member of the ZIP family of metal transporters, are described. GmZIP1 was found to possess eight putative transmembrane domains together with a histidine-rich extra-membrane loop. By functional complementation of zrt1zrt2 yeast cells no longer able to take up zinc, GmZIP1 was found to be highly selective for zinc, with an estimated K(m) value of 13.8 microm. Cadmium was the only other metal tested able to inhibit zinc uptake in yeast. An antibody raised against GmZIP1 specifically localized the protein to the peribacteroid membrane, an endosymbiotic membrane in nodules resulting from the interaction of the plant with its microsymbiont. The specific expression of GmZIP1 in nodules was confirmed by Northern blot, with no expression in roots, stems, or leaves of nodulated soybean plants. Antibodies to GmZIP1 inhibited zinc uptake by symbiosomes, indicating that at least some of the zinc uptake observed in isolated symbiosomes could be attributed to GmZIP1. The orientation of the protein in the membrane and its possible role in the symbiosis are discussed.

Legume genomes and discoveries in symbiosis research

A report on the First International Conference on Legume Genomics and Genetics: Translation to Crop Improvement, Minneapolis-St. Paul, USA, 2-6 June 2002.

Structure, function and regulation of ammonium transporters in plants

Ammonium is an important source of nitrogen for plants. It is taken up by plant cells via ammonium transporters in the plasma membrane and distributed to intracellular compartments such as chloroplasts, mitochondria and vacuoles probably via different transporters in each case. Ammonium is generally not used for long-distance transport of nitrogen within the plant. Instead, most of the ammonium transported into plant cells is assimilated locally via glutamine synthetases in the cytoplasm and plastids. Ammonium is also produced by plant cells during normal metabolism, and ammonium transporters enable it to be moved from intracellular sites of production to sites of consumption. Ammonium can be generated de novo from molecular nitrogen (N(2)) by nitrogen-fixing bacteria in some plant cells, such as rhizobia in legume root nodule cells, and at least one ammonium transporter is implicated in the transfer of ammonium from the bacteria to the plant cytoplasm. Plant physiologists have described many of these ammonium transport processes over the last few decades. However, the genes and proteins that underlie these processes have been isolated and studied only recently. In this review, we consider in detail the molecular structure, function and regulation of plant ammonium transporters. We also attempt to reconcile recent discoveries at the molecular level with our knowledge of ammonium transport at the whole plant level.

Localization of H(+)-ATPases in soybean root nodules

The localization of H(+)-ATPases in soybean (Glycine max L. cv. Stevens) nodules was investigated using antibodies against both P-type and V-type enzymes. Immunoblots of peribacteroid membrane (PBM) proteins using antibodies against tobacco and Arabidopsis H(+)-ATPases detected a single immunoreactive band at approximately 100 kDa. These antibodies recognized a protein of similar relative molecular mass in the crude microsomal fraction from soybean nodules and uninoculated roots. The amount of this protein was greater in PBM from mature nodules than in younger nodules. Immunolocalization of P-type ATPases using silver enhancement of colloidal-gold labelling at the light-microscopy level showed signal distributed around the periphery of non-infected cells in both the nodule cortex and nodule parenchyma. In the central nitrogen-fixing zone of the nodule, staining was present in both the infected and uninfected cells. Examination of nodule sections using confocal microscopy and fluorescence staining showed an immunofluorescent signal clearly visible around the periphery of individual symbiosomes which appeared as vesicles distributed throughout the infected cells of the central zone. Electron-microscopic examination of immunogold-labelled sections shows that P-type ATPase antigens were present on the PBM of both newly formed, single-bacteroid symbiosomes just released from infection threads, and on the PBM of mature symbiosomes containing two to four bacteroids. Immunogold labelling using antibody against the B-subunit of V-type ATPase from oat failed to detect this protein on symbiosome membranes. Only a very faint signal with this antibody was detected on Western blots of purified PBM. During nodule development, fusion of small symbiosomes to form larger ones containing multiple bacteroids was observed. Fusion was preceded by the formation of cone-like extensions of the PBM, allowing the membrane to make contact with the adjoining membrane of another symbiosome. We conclude that the major H(+)-ATPase on the PBM of soybean is a P-type enzyme with homology to other such enzymes in plants. In vivo, this enzyme is likely to play a critical role in the regulation of nutrient exchange between legume and bacteroids.

Regulation of GmNRT2 expression and nitrate transport activity in roots of soybean (Glycine max)

A full-length cDNA, GmNRT2, encoding a putative high-affinity nitrate transporter was isolated from a Glycine max (L.) root cDNA library and sequenced. The deduced GmNRT2 protein is 530 amino acids in length and contains 12 putative membrane-spanning domains and a long, hydrophilic C-terminal domain. GmNRT2 is related to high-affinity nitrate transporters in the eukaryotes Chlamydomonas reinhardtii and Aspergillus nidulans, and to putative high-affinity nitrate transporters in barley and tobacco. Southern blot analysis indicated that GmNRT2 is part of a small, multigene family in soybean. Expression of GmNRT2 in roots was regulated by the type of nitrogen source provided to plants: GmNRT2 mRNA levels were barely detectable in ammonium-grown plants, higher in nitrogen-deprived plants, and highest in nitrate-grown plants. Induction of GmNRT2 mRNA levels in roots occurred within 1 h after exposure of plants to nitrate. Nitrate induction of GmNRT2 mRNA levels was accompanied by a fourfold increase in net nitrate uptake by soybean roots at 100 microM external nitrate. The molecular and physiological evidence indicates that GmNRT2 is probably a high-affinity nitrate transporter involved in nitrate uptake by soybean roots.

Characterization of an ammonium transport protein from the peribacteroid membrane of soybean nodules

Nitrogen-fixing bacteroids in legume root nodules are surrounded by the plant-derived peribacteroid membrane, which controls nutrient transfer between the symbionts. A nodule complementary DNA (GmSAT1) encoding an ammonium transporter has been isolated from soybean. GmSAT1 is preferentially transcribed in nodules and immunoblotting indicates that GmSAT1 is located on the peribacteroid membrane. [14C]methylammonium uptake and patch-clamp analysis of yeast expressing GmSAT1 demonstrated that it shares properties with a soybean peribacteroid membrane NH4^{+ channel described elsewhere. GmSAT1 is likely to be involved in the transfer of fixed nitrogen from the bacteroid to the host.}

METABOLITE TRANSPORT ACROSS SYMBIOTIC MEMBRANES OF LEGUME NODULES

Infection of legume roots or stems with soil bacteria of the Rhizobiaceae results in the formation of nodules that become symbiotic nitrogen-fixing organs. Within the infected cells of these nodules, bacteria are enveloped in a membrane of plant origin, called the peribacteroid membrane (PBM), and divide and differentiate to form nitrogen-fixing bacteroids. The organelle-like structure comprised of PBM and bacteroids is termed the symbiosome, and is the basic nitrogen-fixing unit of the nodule. The major exchange of nutrients between the symbiotic partners is reduced carbon from the plant, to fuel nitrogenase activity in the bacteroid, and fixed nitrogen from the bacteroid, which is assimilated in the plant cytoplasm. However, many other metabolites are also exchanged. The metabolic interaction between the plant and the bacteroids is regulated by a series of transporters and channels on the PBM and the bacteroid membrane, and these form the focus of this review.

Isolation and characterization of a cDNA encoding NADP(+)-specific isocitrate dehydrogenase from soybean (Glycine max)

A cDNA that encodes an NADP-specific isocitrate dehydrogenase (IDH) was cloned from a soybean nodule cDNA library by complementation of an Escherichia coli mutant that lacked IDH. DNA sequence analysis showed that the 1583 bp soybean cDNA could encode a protein that shares 63.9% amino acid sequence identity with the Saccharomyces cerevisiae NADP-IDH and long sequences of identity to an IDH from pig. Southern blot analysis suggests that this gene corresponds to a gene family made up of no more than two loci. The IDH cDNA hybridized to a 1.7 kb soybean mRNA and the relative amount of this transcript in soybean leaves, nodules and roots was 1:3.4:7.7. In alfalfa, a 1.7 kb mRNA was also found but the ratios for the corresponding tissues were 1:7.4:7.7. IDH activity was detected in the complemented E. coli strain and the electrophoretic mobility of this activity in nondenaturing polyacrylamide gels was identical to that of an IDH in extracts from soybean cotyledons or nodule cytosol. NADP-IDH specific activity in the E. coli host strain varied with growth phase; the highest rates (ca. 180 nmol/min per mg protein) were observed in late-stationary-phase cells. The enzyme had a broad pH optimum of 8.0 to 9.5 and had an absolute metal cofactor requirement, preferring Mn2+ below pH 8.0 and Mg2+ above pH 8.0. The Km for isocitrate and NADP was 21 microM and 11 microM respectively with Mn2+ as cofactor and 13 microM and 12 microM with Mg2+ as cofactor.

Isolation and characterization of a ntrC mutant of Bradyrhizobium (Parasponia) sp. ANU289

A mutant of Bradyrhizobium (Parasponia) sp. ANU289 affected in the regulation of nitrogen metabolism was isolated. The mutant, designated ANU293, was unable to induce ammonium transport (Amt), nitrate reductase (NR) or glutamine synthetase II (GSII) activities under conditions that induce these activities in the wild-type. However, glutamine synthetase I (GSI), which is expressed constitutively in the wild-type, was present at normal levels in the mutant. The mutant also retained the ability to fix nitrogen in vitro and in planta, although nodule development on siratro (Macroptilium atropurpureum) was retarded. Southern blot analysis showed that ntrC, the product of which is involved in regulation of nitrogen metabolism, is the site of pSUP1021 insertion in ANU293. These results indicate that the transcriptional activator NtrC is required for the expression of Amt, NR and GSII, but not GSI or nitrogenase in Bradyrhizobium (Parasponia) sp. ANU289.

Isolation and analysis of a cDNA clone that encodes an alfalfa (Medicago sativa) aspartate aminotransferase

We have isolated an alfalfa leaf cDNA clone that encodes aspartate aminotransferase (AAT, EC 2.6.1.1) by direct complementation of an Escherichia coli aspartate auxotroph with a plasmid cDNA library. DNA sequence analysis of the recombinant plasmid, pMU1, revealed that a 1514 bp cDNA was inserted in the correct orientation and in-frame with the start of the lacZ coding sequence in the vector, pUC18. The resulting fusion protein is predicted to be 424 amino acids in length with a molecular weight of 46387 Daltons. The cDNA-encoded protein has a characteristic pyridoxal phosphate attachment site motif and has substantial amino acid sequence homology to both animal and bacterial AATs. Plasmid pMU1 encodes an AAT with a Km for aspartate of 3.3 mM, a Km for 2-oxoglutarate of 0.28 mM, and a pH optimum between 8.0 and 8.5. Several lines of evidence including Western blot analysis, the isoelectric point of the encoded protein, and the effect of pH on the activity of the fusion protein, suggest that the cDNA encodes the isozyme AAT-1 rather than AAT-2. Northern blot analysis showed that the aat-1 clone hybridized to a 1.6 kb transcript present in alfalfa leaves, roots and nodules. The relative concentrations of aat-1 mRNA in these tissues were 1:2:5, respectively. Thus, transcription of aat-1 appears to be induced during nodule development. Southern blot analysis suggested that AAT-1 in alfalfa is encoded by either a single-copy gene or a small, multigene family.

Specificity and regulation of the dicarboxylate carrier on the peribacteroid membrane of soybean nodules

Malate and succinate were taken up rapidly by isolated, intact peribacteroid units (PBUs) from soybean (Glycine max (L.) Merr.) root nodules and inhibited each other in a competitive manner. Malonate uptake was slower and was severely inhibited by equimolar malate in the reaction medium. The apparent Km for malonate uptake was higher than that for malate and succinate uptake. Malate uptake by PBUs was inhibited by (in diminishing order of severity) oxaloacetate, fumarate, succinate, phthalonate and oxoglutarate. Malonate and butylmalonate inhibited only slightly and pyruvate,isocitrate and glutamate not at all. Of these compounds, only oxaloacetate, fumarate and succinate inhibited malate uptake by free bacteroids. Malate uptake by PBUs was inhibited severely by the uncoupler carbonylcyanidem-chlorophenyl hydrazone and the respiratory poison KCN, and was stimulated by ATP. We conclude that the peribacteroid membrane contains a dicarboxylate transport system which is distinct from that on the bacteroid membrane and other plant membranes. This system can catalyse the rapid uptake of a range of dicarboxylates into PBUs, with malate and succinate preferred substrates, and is likely to play an important role in symbiotic nitrogen fixation. Energization of both the bacteroid and peribacteroid membranes controls the rate of dicarboxylate transport into peribacteroid units.

Ammonia (C-Methylamine) Transport across the Bacteroid and Peribacteroid Membranes of Soybean Root Nodules

[(14)C]Methylamine (MA; an analog of ammonia) was used to investigate ammonia transport across the bacteroid and peribacteroid membranes (PBM) from soybean (Glycine max) root nodules. Free-living Bradyrhizobium japonicum USDA110 grown under nitrogen-limited conditions showed rapid MA uptake with saturation kinetics at neutral pH, indicative of a carrier. Exchange of accumulated MA for added ammonia occurred, showing that the carrier recognized both NH(4) (+) and CH(3)NH(3) (+). MA uptake by isolated bacteroids, on the other hand, was very slow at low concentrations of MA and increased linearly with increasing MA concentration up to 1 millimolar. Ammonia did not inhibit MA by isolated bacteroids and did not cause efflux of accumulated MA. PBM-enclosed bacteroids (peribacteroid units [PBUs]) were qualitatively similar to free bacteroids with respect to MA transport. The rates of uptake and efflux of MA by PBUs were linearly dependent on the imposed concentration gradient and unaffected by NH(4)Cl. MA uptake by PBUs increased exponentially with increasing pH, confirming that the rate increased linearly with increasing CH(3)NH(2) concentration. The results are consistent with other evidence that transfer of ammonia from the nitrogen-fixing bacteroid to the host cytosol in soybean root nodules occurs solely by simple diffusion of NH(3) across both the bacteroid and peribacteroid membranes.

Electrogenic ATPase Activity on the Peribacteroid Membrane of Soybean (Glycine max L.) Root Nodules

Electrogenic ATPase activity on the peribacteroid membrane from soybean (Glycine max L. cv Bragg) root nodules is demonstrated. Membrane energization was monitored using suspensions of intact peribacteroid membrane-enclosed bacteroids (peribacteroid units; PBUs) and the fluorescent probe for membrane potential (DeltaPsi), bis-(3-phenyl-5-oxoisoxazol-4yl) pentamethine oxonol. Generation of a positive DeltaPsi across the peribacteroid membrane was dependent upon ATP, inhibited by N,N'-dicyclohexyl-carbodiimide and vanadate, but insensitive to N-ethylmaleimide, azide, cyanide, oligomycin, and ouabain. The results suggest the presence of a single, plasma membrane-like, electrogenic ATPase on the peribacteroid membrane. The protonophore, carbonyl-cyanide m-chlorophenyl hydrazone, completely dissipated the established membrane potential. The extent of reduction in the steady state membrane potential upon addition of ions was used to estimate the relative permeability of the peribacteroid membrane to anions. By this criterion, the relative rates of anion transport across the peribacteroid membrane were: NO(3) (-) > NO(2) (-) > Cl(-) > acetate(-) > malate(-). The observation that 10 millimolar NO(3) (-) completely dissipated the membrane potential was of particular interest in view of the fact that NO(3) (-) inhibits symbiotic nitrogen fixation. The possible function of the ATPase in symbiotic nitrogen fixation is discussed.

Stimulation of respiration and nitrogenase in bacteroids of Siratro (Macroptilium atropurpureum) by plant nodule cytosol

Nitrogenase activity (acetylene reduction) of isolated Siratro (Macroptilium atropurpureum) bacteroids was stimulated by addition of plant cytosol fractions which also preserved activity at high (up to 3%) O2 tensions. These effects were not due to leghaemoglobin. Boiling removed some, but not all, of the protective capacity of the cytosol. Heat treated cytosol substantially stimulated the respiration of siratro bacteroids. Of a wide variety of compounds tested, only ascorbate could mimic the cytosol. Ascorbate was present in the cytosol fraction, in significant quantities. The effect of ascorbate was evident at low O2 concentrations and in purified bacteroids, and was inhibited by cyanide. Siratro bacteroids appear to possess an oxidase which could serve a protective role in vivo.