Ammonia assimilation bacteroids

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. 

The preceding description of nodule initiation, development, and nitrogen-ase clearly demonstrates that the host contribution to symbiosis can be grouped into several functions, including recognition, root hair invasion, infection thread formation, nodule differentiation (e.g., meristems, vascular elements, sclerenchyma), carbon assimilation, organic acid production, ammonia assimilation, maintenance of low O2 by Lb, separation of bacteroids and host cytoplasm by the PBM, senescence, and possibly suppression of host defense responses required for nodule compatibility and/or effectiveness. 

Fig. 7. Proposed pathway for assimilation of ammonia produced by Nj reduction in the bacteroids and synthesis of asparagine by enzymes located in the plant fraction of lupin nodules. The utilization of photosynthate to provide oxaloacetate, NADH and ATP is also indicated. Phosphoenolpyruvate, PEP tricarboxylic acid cycle, TCA. (Based on Scott et al., 1976, and reproduced with permission.)...

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