Rhizobia (from the Greek words Riza = Root and Bios = Life) are soil bacteria that fixnitrogen (diazotrophy) after becoming established inside root nodules of legumes (Fabaceae). 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.
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:
Soybean root nodules, containing billions of Rhizobium bacteria
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, 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.
Rhizobiumbacteria are mixed with alfalfa seed and deposited into the soil at the same time the alfalfa seed is planted so that the bacteria are placed in the proximity of the developing seedling.
The rhizobiumbacteria could be purchased in an inoculum and mixed with the alfalfa seed in a tub (figure 1) at planting time, or the alfalfa seed could be shipped to a company that applied the rhizobium inoculum and a protective coating to the seed by an industrial process (figure 2).
Rhizobiumbacteria need to be mixed with the alfalfa seed at the time of seeding to permit nodulation of the alfalfa roots by the rhizobiumbacteria.
Rhizobium and the impact of “soluble” phenolic compounds extracted from different root sectors prior to inoculation and at the initial stage of infecting, as well as determination of the content of “insoluble” phenolic compounds forming links with cell structures.
Rhizobium is constrained by certain phases of roots development and the short cells life cycle – targets sensitive for bacteria [3].
Rhizobium at the initial period of the infection is only possible in case of disturbances in the formation of infection threads and bacteria penetration from the latter into the root cells.
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