summary Cucumber (Cucumis sativus L.) plants grown in PVC tubes with a partially sterilized soil‐sand mixture were inoculated with the vesicular‐arbuscular mycorrhizal fungus Glomus fasciculatum (Thaxter) Gerdemann & Trappe emend. Walker & Koske or left uninoculated. The soil column of each PVC tube was divided into a root and a hyphal compartment by a mesh bag (60 μm), which retained the roots but allowed external hyphae to pass. Inoculated plants rapidly became infected and an extensive mycelium developed. Three weeks after seedling emergence plants were labelled with 14CO2 for 16 h. The distribution of 14C within the plants and the 14C flow into external hyphae and soil were measured during an 80 h chase period. Below‐ground respiration in mycorrhizal plants accounted for 27% of the photo assimilated 14C. Organic 14C in the soil represented 3˙1 % of the fixed 14C, and 26 % of this was located in external hyphae. Based on conservative assumptions concerning dry weight of internal mycorrhizal infection and growth yield of the fungus, it was estimated that mycorrhizal events consumed 20 % of photoassimilated 14C. The specific incorporation of C by the external mycelium in the hyphal compartment was 41 μg C mg−1 dry wt. d−1. The importance of external VA mycorrhizal hyphae for the distribution of plant‐derived C in the soil volume and as a substrate source for the soil biota is discussed.

Products of the nodule cytosol in vivo dark [14C]C02 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 CO2 fixation products was compared in effective (fix+) nodules infected by a wild-type Rhizobium leguminosarum (MNF 300), and ineffective (fix-) nodules of the R. Ieguminosarum mutant MNF 3080. The latter has a defect in the dicarboxylic acid transport system of the bacterial membrane.The 14C incorporation from ['4C]CO2 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 14C 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 14C was contained in the amino acid fraction of the cytosol in both symbioses. More than 70% of the 14C 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 14C labeling of malate and aspartate from nodule dark (14CJCO2 fixation is consistent with the role of phosphoenolpyruvate carboxiase in nodule dark CO2 fixation. Bacteroids from the effective wild-type symbiosis accumulated sevenfold more 14C than did the dicarboxylic acid transport defective bacteroids. The bacteroids of the effective MNF 300 symbiosis contained the largest proportion of the incorporated 14C in the organic acids, whereas ineffective MNF 3080 bacteroids mainly contained 14C in the amino acid fraction. In both symbioses a larger proportion of the bacteroid 14C 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 14C 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 CO2 fixation in the supply of the bacteroids with dicarboxylic acids.Symbiotic N2 fixation in legume root nodules is fueled by carbon sources supplied by the host plant. Substantial evi-'This research was initiated via a fellowship to L.

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.

No abstract

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. ␥-Aminobutyric acid accumulated in the nodules during incubation, but newly synthesized 15 N ␥-aminobutyric acid seemed to be immobilized in metabolically active pea nodules, which made it NMR invisible.Symbiotic nitrogen (N) fixation, the process whereby N 2 -fixing bacteria enter into associations with plants, provides the major source of N for the biosphere. Nitrogenase, a bacterial enzyme, catalyzes the reduction of atmospheric dinitrogen to ammonium. In rhizobia-leguminous plant symbioses, a widely accepted and simple model of N transfer from the symbiotic form of the bacterium, called a bacteroid, to the plant implies that nitrogenase-generated ammonia diffuses across the bacteroid membrane and is assimilated into amino acids in the plant compartment of the nodule tissue. However, the transport of symbiotically fixed N across the membranes surrounding the bacteroid and the form in which this occurs has been a matter of controversy.Until recently, it has been generally accepted that Rhizobium bacteroids do not assimilate ammonium into amino acids to a great extent during symbiosis, but more recent results challenge this view and suggest a possible involvement of amino acids as the form of fixed N delivered from the bacteroid to the plant (for review, see Poole and Allaway, 2000; Day et al., 2001). At present, consensus has not been reached as to whether substantial N assimilation takes place in the nodule bacteroid compartment, and new investigations of nodule N metabolism are required.The...