Nitrogen fixation ureides

Ammonium, the primary product of nitrogen fixation, is transported to the host cell cytoplasm where it is assimilated into amides and, in some cases, further converted into ureides before being transported to the shoot. Since the physiological environment within the nodule is apparently different from the other parts of the plant, nodule-specific or nodule-abundant forms of several enzymes of the nitrogen and carbon assimilation pathways have evolved, and are induced to improve the efficiency of nitrogen and carbon metabolism in nodules. 

Kohl et al. (1988) estimated that the rate of de novo purine synthesis necessary to support the flux of ureides from the nodule at the peak of nitrogen fixation was 15-20 / mol/hr g nodule fresh weight. This value is extremely... 

Fig. 7. Model for the subcellular localization of reactions of purine synthesis and ureide biogenesis in nodules of ureide-exportlng legumes. The model is based on results of subcellular fractionation and ultrastructural studies. The processes (shown in the hatched boxes) involved in ureide biogenesis (i.e., nitrogen fixation, ammonium assimilation, precursor synthesis, purine synthesis, energy-yielding metabolism, and purine oxidation and catabolism) may occur in more than one subcellular compartment. The location of the enzymes involved in the conversion of IMP to xanthine is not certain. We have proposed that in soybean nodules these reactions [shown in bold-face type with bold arrows] occur in the plastid while in other species such as cowpea these reactions may take place in the ground cytoplasm. In all cases the intermediate exported from the plastid is uncertain. This uncertainty is indicated with the dashed lines and question marks.

Fig. 9. Proposed model for the cellular compartmentalization of the reactions of nitrogen fixation, ammonium assimilation, purine synthesis, and ureide biogenesis in infected and uninfected cells of soybean root nodules. Uncertainty still exists with respect to the nature of the intermediate (e.g., IMP, XMP, xanthine, glutamine ) transported from the infected cell to the uninfected cell as well as the site of purine synthesis. In addition, as discussed in the text the site(s) of PRPP synthesis (plastid and/or cytosolic) and the path and site of synthesis (de novo from the PPP or via salvage) of tibose S-phosphate (R-S-P) are s not defined, lliese uncertainties are indicated with question marks and/or dashed lines. Lb, leghemoglobin.

A similar classification may be appropriate for N2-fixing actinorhizal species. Asparagine is the major product of N2 fixation in Myiica species (32), w hereas the ureide, citrulline, predominates in Alnus species (20, 33). The structures of the difiFerent nitrogenous components transported from N2-fixing plants are given in Figure 3. 

The two- to fourfold variation in sucrose cost per NHj assimilated (Fig. 3B) is not faithfully reflected in respiratory losses of CO2, since dark fixation inputs of CO2 apply to certain compounds (e.g., aspartate and asparagine) but not at all, or to only a minor extent, in the case of others. Net CO2 exchange values therefore range from a net fixation input of 0.58 and 1.08 CO2/NH3 in the case of glutamate and aspartate, respectively, to a net evolution of 0.02 to 0.53 CO2/NH3 for ureide, citrulline, and proline. Based on these calculations the type of products selected by a plant for export of assimilated nitrogen might aJter significantly the apparent respiration status (respiratory quotient) of its assimilatory tissues. 

N2 Fixation The NH3 produced by the bacteroids has first to be exported via bacteroid and host membranes to cytosol of the nodule and then possibly assimilated within various organelles or cell types to form amino acids, amides, or ureides for export in the xylem. Considering the numbers of nitrogen atoms exported per compound, costs of 3, 2, and 1.75 ATP/nitrogen transported would appear to be reasonable estimates of the energy cost for the transport of amino acids, amides, and ureides, respectively. Added to this would be any costs in transmembrane transfers of ammonia from bacteroid to plant. 

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