Nitrogen bacteroids

In symbiosis with Fabales, bacteria live as bacteroids in root nodules inside the plant cells. The plant supplies the bacteroids with nutrients, but it also benefits from the fixed nitrogen that the symbionts make available. 

For an effective symbiotic state, the plant and the microsymbiont must maintain a constant metabolic flow of carbon and nitrogen. While the bacteroids function as an engine for nitrogen fixation the fuel comes from the plant. Dicarboxylic acids are the primary carbon sources fed to the bacteroids by the plant. This unidirectional flow of carbon must be controlled by the PBM. Recently, several specific carbon and amino acid transport systems have been identified in the PBM using isolated peribac-teroid units (PBU Day et al., 1990). Thus, in order for the host plant to house endosymbiotic bacteria and support their metabolic needs, a number of nodulin genes must be induced to support the ontogeny and function of the nodules. 

Sindhu, S.S., Kannenberg, E.L., Brewin, N.J. Lipopolysaccharide maturation in pea and bean bacteroids. In Bothe, de, Newton, B. (eds), Nitrogen Fixation Hundred Years After. Gustav Fischer, Stuttgartand New York (1988), p. 480. 

Tao, H., Noel, D. Lipopolysaccharide epitopes missing from bean bacteroids. In Gresshoff, P.M., Roth, L.E., Stacey, G., Newton, W.E. (eds), Nitrogen Fixation Achievements and Objectives Proceedings of the 8th International Congress on Nitrogen Fixation. Knoxville, Tennessee, USA, May 20-26, 1990. Chapman and Hall, New York and London (1990), p. 276. 

Vedam, V., Haynes, J.G., Kannenberg, E.L., Carlson, R.W., Sherrier, D.J. A Rhizobium leguminosarum lipopolysaccharide lipid A mutant induces nitrogen-fixing nodules with delayed and defective bacteroid formation. Mol Plant-Microbe Interact 17 (2004) 283-291. 

Bergersen BJ and Turner GL. (1975). Leghaemoglobin and the supply of 02 to nitrogen-fixing root nodule bacteroids presence of two oxidase system and ATP production at low free 02 concentration. J. Gen. Microbiol. 91, 345-354. 

Fig. 2. Transmission electron micrograph (16,000x enlargement) through an infected root cell of a soybean plant. The subcellular organelles of the host cell [mitochondria and plasts (a)] are present at the periphery of the cell adjacent to the host cell wall (b). The nitrogen-fixing bacteroids (c) are kept apart from the host cell cytoplasm (the location of leghemoglobin) by the peribacteroid space (d) and the peribacteroid membrane (e), which regulates transport of materials to and from the bacteroids.

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