Exploring symbiotic nitrogen fixation and assimilation in pea root nodules by in vivo 15N nuclear magnetic resonance spectroscopy and liquid chromatography-mass spectrometry

Nitrogen (N) fixation and assimilation in pea (Pisum sativum) root nodules were studied by in vivo (15)N nuclear magnetic resonance (NMR) by exposing detached nodules to (15)N(2) via a perfusion medium, while recording a time course of spectra. In vivo (31)P NMR spectroscopy was used to monitor the physiological state of the metabolically active nodules. The nodules were extracted after the NMR studies and analyzed for total soluble amino acid pools and (15)N labeling of individual amino acids by liquid chromatography-mass spectrometry. A substantial pool of free ammonium was observed by (15)N NMR to be present in metabolically active, intact nodules. The ammonium ions were located in an intracellular environment that caused a remarkable change in the in vivo (15)N chemical shift. Alkalinity of the ammonium-containing compartment may explain the unusual chemical shift; thus, the observations could indicate that ammonium is located in the bacteroids. The observed (15)N-labeled amino acids, glutamine/glutamate and asparagine (Asn), apparently reside in a different compartment, presumably the plant cytoplasm, because no changes in the expected in vivo (15)N chemical shifts were observed. Extensive (15)N labeling of Asn was observed by liquid chromatography-mass spectrometry, which is consistent with the generally accepted role of Asn as the end product of primary N assimilation in pea nodules. However, the Asn (15)N amino signal was absent in in vivo (15)N NMR spectra, which could be because of an unfavorable nuclear Overhauser effect. gamma-Aminobutyric acid accumulated in the nodules during incubation, but newly synthesized (15)N gamma-aminobutyric acid seemed to be immobilized in metabolically active pea nodules, which made it NMR invisible.

Rhizobium colonization induced changes in membrane-bound and soluble hydroxyproline-rich glycoprotein composition in pea

Abundance and distribution of plant cell surface proteins of the hydroxyproline-rich glycoprotein (HRGP) class were studied in the pea-Rhizobium symbiosis using immunoblot analysis. The MAC 265-epitope was especially abundant in pea root nodules containing nitrogen-fixing Rhizobium bacteria. A 180-kDa MAC 265-HRGP dominated in pea shoot plasma membranes, while almost no MAC 265-HRGP was detected in root plasma membranes. We show here that a major difference between the plant-derived peribacteroid membrane of the symbiosomes and the root plasma membrane was the presence of a 100-kDa MAC 265-HRGP in the former. Arabinogalactan proteins (AGPs), as recognized by the monoclonal antibodies MAC 207 and JIM 8, were not detected in the peribacteroid membrane, while two isoforms (100 and 220 kDa) were detected in shoot and root plasma membranes. Specific MAC 265-HRGP isoforms were found in the peribacteroid space fraction of the symbiosomes and thus as soluble proteins in the interface between the symbionts. The abundance of the MAC 265-epitope was much reduced in non-nitrogen-fixing nodules when this phenotype resulted from a dicarboxylate transport mutation in Rhizobium. There was no reduction in the abundance of the MAC 265-epitope in non-fixing phenotypes resulting from a mutation in the plant. The results suggest that bacterial signals related to the bacterial ability to fix nitrogen, might be responsible for the regulation of HRGP expression in root nodules.

Ammonia and amino acid transport across symbiotic membranes in nitrogen-fixing legume nodules

Biological nitrogen fixation involves the reduction of atmospheric N2 to ammonia by the bacterial enzyme nitrogenase. In legume-rhizobium symbioses, the nitrogenase-producing bacteria (bacteroids) are contained in the infected cells of root nodules within which they are enclosed by a plant membrane to form a structure known as the symbiosome. The plant provides reduced carbon to the bacteroids in exchange for fixed nitrogen, which is exported to the rest of the plant. This exchange is controlled by plant-synthesised transport proteins on the symbiosome membranes. This review summarises our current understanding of these transport processes, focusing on ammonia and amino acid transport.

Identification of a Transport Mechanism for NH4+ in the Symbiosome Membrane of Pea Root Nodules

Symbiosome membrane vesicles, facing bacteroid-side-out, were purified from pea (Pisum sativum L.) root nodules and used to study NH4+ transport across the membrane by recording vesicle uptake of the NH4+ analog [14C]methylamine (MA). Membrane potentials ([delta][psi]) were imposed on the vesicles using K+ concentration gradients and valinomycin, and the size of the imposed [delta][psi] was determined by measuring vesicle uptake of [14C]tetraphenylphosphonium. Vesicle uptake of MA was driven by a negative [delta][psi] and was stimulated by a low extravesicular pH. Protonophore-induced collapse of the pH gradient indicated that uptake of MA was not related to the presence of a pH gradient. The MA-uptake mechanism appeared to have a large capacity for transport, and saturation was not observed at MA concentrations in the range of 25 [mu]M to 150 mM. MA uptake could be inhibited by NH4+, which indicates that NH4+ and MA compete for the same uptake mechanism. The observed fluxes suggest that voltage-driven channels are operating in the symbiosome membrane and that these are capable of transporting NH4+ at high rates from the bacteroid side of the membrane to the plant cytosol. The pH of the symbiosome space is likely to be involved in regulation of the flux.

[The value of serological examination in neuroborreliosis]

Over a period of 18 months, 155 patients with indications for lumbar puncture (LBP) on account of neurologic symptoms were examined for specific antibodies to Borrelia burgdorferi (BB) in the serum and for production of specific antibodies against BB in the cerebro-spinal fluid (CSF). Positive anti-BB-antibody-production in CSF was found in one patient and the clinical manifestations were consistent with the third stage of neuroborreliosis (NB). Three patients with NB as a tentative diagnosis were found negative for anti-BB-antibody-production in the CSF, and a clinical reevaluation excluded NB as a final diagnosis. The study showed a very high predictive value of the test for anti-BB-antibody-production in the CSF both with positive and negative results. The results suggest further that NB is of a low incidence in Denmark and not often misdiagnosed in favour of more well known diseases.

Products of Dark CO(2) Fixation in Pea Root Nodules Support Bacteroid Metabolism

Products of the nodule cytosol in vivo dark [(14)C]CO(2) fixation were detected in the plant cytosol as well as in the bacteroids of pea (Pisum sativum L. cv "Bodil") nodules. The distribution of the metabolites of the dark CO(2) fixation products was compared in effective (fix(+)) nodules infected by a wild-type Rhizobium leguminosarum (MNF 300), and ineffective (fix(-)) nodules of the R. leguminosarum mutant MNF 3080. The latter has a defect in the dicarboxylic acid transport system of the bacterial membrane. The (14)C incorporation from [(14)C]CO(2) was about threefold greater in the wild-type nodules than in the mutant nodules. Similarly, in wild-type nodules the in vitro phosphoenolpyruvate carboxylase activity was substantially greater than that of the mutant. Almost 90% of the (14)C label in the cytosol was found in organic acids in both symbioses. Malate comprised about half of the total cytosol organic acid content on a molar basis, and more than 70% of the cytosol radioactivity in the organic acid fraction was detected in malate in both symbioses. Most of the remaining (14)C was contained in the amino acid fraction of the cytosol in both symbioses. More than 70% of the (14)C label found in the amino acids of the cytosol was incorporated in aspartate, which on a molar basis comprised only about 1% of the total amino acid pool in the cytosol. The extensive (14)C labeling of malate and aspartate from nodule dark [(14)C]CO(2) fixation is consistent with the role of phosphoenolpyruvate carboxlase in nodule dark CO(2) fixation. Bacteroids from the effective wild-type symbiosis accumulated sevenfold more (14)C than did the dicarboxylic acid transport defective bacteroids. The bacteroids of the effective MNF 300 symbiosis contained the largest proportion of the incorporated (14)C in the organic acids, whereas ineffective MNF 3080 bacteroids mainly contained (14)C in the amino acid fraction. In both symbioses a larger proportion of the bacteroid (14)C label was detected in malate and aspartate than their corresponding proportions of the organic acids and amino acids on a molar basis. The proportion of (14)C label in succinate, 2-oxogultarate, citrate, and fumarate in the bacteroids of the wild type greatly exceeded that of the dicarboxylate uptake mutant. The results indicate a central role for nodule cytosol dark CO(2) fixation in the supply of the bacteroids with dicarboxylic acids.