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.
Bacteria are tiny, one-celled organisms – generally 4/100,000 of an inch wide (1 µm) and somewhat longer in length.
Bacteria are especially concentrated in the rhizosphere, the narrow region next to and in the root.
In the future, farmers may be able to inoculate seeds with anti-fungal bacteria, such as P. fluorescens, to ensure that the bacteria reduce pathogens around the seed and root of the crop.
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