Rhizobia
Rhizobium, Bradyrhizobium, Mesorhizobium, Sinorhizobium, and Azorhizobium - known as rhizobia (from the Greek words Riza = Root and Bios = Life) - are symbiotic nitrogen fixers that can be found in the roots of plants and especially in legume plants. They are responsible for the worlds largest portion of fixed atmospheric nitrogen. (Nitrogen-fixation by organisms provides about 65% of the the biosphere's available nitrogen (Lodwig et al. 2003).) Bradyrhizobium japonicum has been used since 1957 in molecular genetics, physiology, and ecology due to its exellent ability in symbiotic nitrogen fixation.
The rhizobia can not independently fix nitrogen, and require a plant host. Although much of the nitrogen is removed when protein-rich grain or hay is harvested, significant amounts can remain in the soil for future crops. This is especially important when nitrogen fertilizer is not used, as in organic rotation schemes or some less-industrialized countries.
Rhizobia are Gram-negative, nitrogen-fixing bacteria that form nodules on host plants. They also have symbiotic relationships with legume plants, which can't live without these bacteria's essential nitrogen-fixing processes. In nodules, the rhizobia bacteriods use carbon and energy from the plant in the form of dicarboxylic acids. Recent studies have suggested that the bacteroids do more than just provide the plant with ammonium (through nitrogen fixation). It was shown that a more complex amino-acid cycle is needed for Rhizobium to successfully fix nitrogen in pea nodules. Rhizobium can use the amino acids from the plant to shut down their ammonium assimilation; however, the bacteria must provide the plant with ammonium in order to obtain the amino acids. This alone would mean that the plant could regulate the amount of dicarboxylate that the bacteroids use by amino acid supply and dominate the relationship. This is not the case, however, because the bacteroids "act like plant organelles to cycle amino acids back to the plant for asparagine synthesis," making the plant dependent on them (Lodwig et al. 2003). This system creates mutualism between the bacteria and the plant.
However, nitrogen fixation is an energy expensive process that requires up to 22% of the plants net photosynthate. In addition, at least 25% of the electron flux through the nitrogenase goes towards reducing protons into hydrogen gas. This process of nitrogenase-dependent hydrogen production is a major factor in the efficiency of symbiotic nitrogen fixation. To have more efficient energy use, some Rhizobium and many Bradyrhizobium strains recycle the hydrogen produced by nitrogenase in nodule bacteroids that have a hydrogen uptake system (Hup). However, Sinorhizobium meliloti, M. ciceri, and R. leguminosarum by. viciae UML2 strains have poor expression of the hup system (Palacios et al. 2000).
Classification
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:
| Family | Rhizobia |
| Rhizobiaceae | Rhizobium (including Allorhizobium), Sinorhizobium |
| Bradyrhizobiaceae | Bradyrhizobium |
| Hyphomicrobiaceae | Azorhizobium |
| Phyllobacteriaceae | Mesorhizobium |
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 genes 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.
Ecology
Rhizobia can be found in the roots, or rhizosphere, of other types of plants where they cause the formation of nodules. For example, Bradyrhizobium japonicum was first isolated from a soybean nodule in Florida in 1957. Rhizobium sp. NGR234 has a host range of more than 112 genera of legumes (Viprey et al. 2000). These symbiotic relationships occur when rhizobia penetrate their hosts with centripetally-developing infection threads. The bacterium induces the meristem at the cortex of the plant roots where nodules then develop. Meanwhile, the infection threads make their way into the nodule cells and release rhizobia into the cytoplasm of infected cells. The rhizobia, which act as symbiosomes, enlarge and differentiate into nitrogen-fixing bacteroids, which have low free-oxygen levels. The symbiotic development comes from an exchange of chemical signals between the plant and the bacteria. One of the first signals in this continuous exchange are called flavonoids and are released by the legume roots. They actually activate the expression of nodulation genes (nod, noe, and nol) by interacting with rhizobial regulators of the NodD family. Most of these nodulations genes then help synthesis and secrete a family of lipochito-oligosaccharide molecules that help the bacteria get into the root hairs (Viprey et al. 2000).
Frankia and Azospirillum, are genera of similar bacteria that do not live on legumes.