Nitrogen-fixing symbionts

Pueppke, S.G. et al.. Release of flavonoids by the soybean cultivars McCall and Peking and their perception as signals by the nitrogen-fixing symbiont Sinorhizobium fredii, Plant Physiol., 117, 599, 1998. 

Diazotrophs Nitrogen-fixing bacteria. In the marine environment, some nitrogen fixers live as symbionts in phytoplankton, particularly diatoms. 

Evidence for roles of sulfated oligosaccharides in peptide hormone half-life, symbiont interactions in nitrogen-fixing legumes, and lymphocyte homing. 

Only certain prokaryotes can fix atmospheric nitrogen. These include the cyanobacteria of soils and fresh and salt waters, other kinds of free-living soil bacteria such as Azotobacter species, and the nitrogen-fixing bacteria that live as symbionts in the root nodules of leguminous plants. The first important product of nitrogen fixation is ammonia, which can be used by all organisms either directly or after its conversion to other soluble compounds such as nitrites, nitrates, or amino acids. 

Sawada, H., Kuykendall, L.D., Young, J.M. Changing concepts in the systematics of bacterial nitrogen-fixing legume symbionts. J Gen Appl Microbiol 49 (2003) 155-179. 

In symbiotic systems, like these where the association appears quite intimate or the symbiont population occupies a majority of the host cell volume, the relationship is assumed necessary (Douglas, 1998) and/or beneficial. The benefit of the DDA relationship is not fuUy understood nor characterized, and because N2 fixation has been measured when the DDAs are present, it is presumed that some of the nitrogen fixed by the symbiont is transferred to the host diatom. To date, there are only a few studies that have attempted to understand the nature of the symbiosis between the Richelia symbiont and the host diatom. 

The relationships between bacteria and plants have important economic implications. The activities of the nitrogen-fixing symbiotic bacteria of root nodules are crucial to soil fertility. On the negative side bacterial pathogens are responsible for significant worldwide crop losses. The specificity of bacterial pathogens and symbionts for a particular plant host, the mechanisms involved in the induction of disease symptoms and the triggering of plant host defence mechanisms are all mediated by components of the bacterial cell surface. 

V. Translocation of Nitrogen Compounds between the Symbionts VI, The Relationship of Lichens to Other Nitrogen-Fixing Symbiotic Systems References... 

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. 

In a micro-autography study, field collected Rhizosolenia-Richelia symbioses were incubated with C-labeUed bi-carbonate. Higher density of silver grains localized on the symbiotic Richelia trichomes than on the host Rhizosolenia filaments, suggesting that the Richelia were actively photosynthesizing and the host diatom were inactive (Weare et al., 1974). An equally plausible explanation for less silver grains on the Rhizosolenia host is that some of the fixed and labeled photosynthetic products were transferred to the host from the symbiont. Similar scenarios of carbon, and nitrogen transfer, are well documented in terrestrial... 

Rhizobium occurs free-living, but cannot fix nitrogen when it is existing as a natural member of the soil population. The sequence of events in the infection of roots and formation of an active, nitrogen-flxing nodule reveals a complex series of interactions between symbiont and host. 

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