Rhizobia
|
Soybean root nodules, each containing billions of Bradyrhizobium bacteria |
Rhizobia (from the
Greek words
riza = 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.
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.
Rhizobia consist of 55 species found in 12 genera
. 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 rather than from a
common ancestor.
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 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, 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 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. In return the plant feeds the bacteria with sugars, proteins, and oxygen.
The legumeā"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.
Many other species of bacteria are able to fix nitrogen (
diazotrophs), including
Frankia which is symbiotic (but not in nodules), and
Azospirillum which is free-living.
*
Legume sanctions maintain Rhizobium mutualism*
Current list of rhizobia species*
Nitrogen Fixation and Inoculation of Forage Legumes
