Rhizobia

  Rhizobia (from the Greek words rhiza = root and bios = Life) are soil bacteria that fix nitrogen (diazotrophy) after becoming established inside root nodules of legumes (Fabaceae). The rhizobia cannot independently fix nitrogen, and require a plant host. Morphologically they are generally gram negative, motile, non-sporulating rods.

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History

The first species (R. leguminosarum) was identified in 1889, and all further species placed in the Rhizobium genus. However, more advanced methods of analysis have revised this classification and now there are many in other genera. Rhizobium is still sometimes used as the singular of rhizobia. Most research has been done on crop and forage legumes such as clover, beans, and soy. However, recently more work is occurring on indigenous legumes.

Taxonomy

Rhizobia consist of 57 species found in 12 genera.^[1] Most 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 genes responsible for the symbiosis with plants, however, may be closer than the organisms themselves, acquired by horizontal transfer (via bacterial conjugation) rather than from a common ancestor.

Importance in agriculture

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. Nitrogen is the most commonly deficient nutrient in many soils around the world and it is the most commonly supplied plant nutrient. Supply of nitrogen through fertilizers has severe environmental concerns. Nitrogen fixation by Rhizobium is also beneficial to the environment.

Symbiosis

Rhizobia are unique because they live in a symbiotic relationship with legumes. Common crop and forage legumes are peas, beans, clover, and soy. Rhizobia live in the soil where they encounter the root of a legume, and if that bacteria has the correct nod or nodulating genes, symbiosis can occur. The rhizobia enter a root hair and travel down a tube to the relatively anoxic centre of the root hair cell. Here proliferating plant cells form a nodule. The bacteria differentiate morphologically into bacteroids and fix nitrogen from the atmosphere in to a plant usable form, ammonium (NH₄⁺), utilising the enzyme nitrogenase. In return the plant supplies the bacteria with carbohydrates, proteins, and sufficient enough oxygen so as not to interfere with the fixation process.

The legume – Rhizobia 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.

Other diazotrophs

Many other species of bacteria are able to fix nitrogen (diazotrophs), including Frankia which is symbiotic and Azospirillum which is free-living.

References

1. ^ Current taxonomy of rhizobia. Retrieved on 2006-08-07.