| Rhizobia |
Articles about Rhizobia |
Information AboutRhizobia |
| CATEGORIES ABOUT RHIZOBIA | |
| rhizobiales | |
| symbiosis | |
| nitrogen metabolism | |
| soil biology | |
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There are several different genera of rhizobia. All of them belong to the Rhizobiales , a probably- Monophyletic group of Proteobacteria . Within that group, however, they are scattered among several different families: These groups also include a variety of other bacteria. For instance, the plant pathogen '' Agrobacterium '' is a closer relative of ''Rhizobium'' than the rhizobia that nodulate soybean (and may not really be a separate genus). The Gene s responsible for the Symbiosis with plants, however, may be closer than the organisms themselves, acquired by Horizontal Transfer rather than from a Common Ancestor . The legume-rhizobium Symbiosis is a classic example of Mutualism -- rhizobia supply ammonia or amino acids to the plant and in return receive organic acids (principally as the dicarboxylic acids malate and succinate) as a carbon and energy source, -- but its evolutionary persistence is actually somewhat surprising. Because several unrelated strains infect each individual plant, any one strain could redirect resources from Nitrogen Fixation to its own reproduction without killing the host plant upon which they all depend. But this form of cheating should be equally tempting for all strains, a classic Tragedy Of The Commons . It turns out that legume plants guide the evolution of rhizobia towards greater mutualism by reducing the Oxygen supply to nodules that fix less nitrogen, thereby reducing the frequency of cheaters in the next generation. '' Frankia '' and '' Azospirillum '', are genera of similar Bacteria that do not live on legumes. EXTERNAL LINKS
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