Rhizobia are soil bacteria that fix nitrogen after becoming established inside root nodules
of legumes. Rhizobia require a plant host; they cannot independently fix nitrogen. In
general, they are Gram-negative, motile, non-sporulating rods.
History The first known species of rhizobia, Rhizobium
leguminosarum, was identified in 1889, and all further species were initially placed
in the Rhizobium genus. However, more advanced methods of analysis have revised this classification,
and now there are many in other genera. Most research has been done on crop and forage
legumes such as clover, alfalfa, beans, and soy; recently, more work is occurring on North
American legumes. The word rhizobia comes from the Ancient Greek
ῥίζα, rhíza, meaning "root" and βίος, bios, meaning "life". The word rhizobium is
still sometimes used as the singular form of rhizobia
Taxonomy Rhizobia are a paraphyletic group that fall
into two classes of the proteobacteria—the alpha- and beta-proteobacteria. As shown below,
most belong to the order Rhizobiales, but several rhizobia occur in distinct bacterial
orders of the proteobacteria. These groups include a variety of non-symbiotic
bacteria. For instance, the plant pathogen Agrobacterium is a closer relative of Rhizobium
than the Bradyrhizobium that nodulate soybean. The genes responsible for the symbiosis with
plants, however, may be more closely related than the organisms themselves, acquired by
horizontal transfer rather than vertical gene transfer.
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. Symbiotic relationship
Rhizobia are unique in that they are the only nitrogen-fixing bacteria living in a symbiotic
relationship with legumes. Common crop and forage legumes are peas, beans, clover, and
soy. Infection and signal exchange
The symbiotic relationship implies a signal exchange between both partners that leads
to mutual recognition and development of symbiotic structures. Rhizobia live in the soil where
they are able to sense flavonoids secreted by the roots of their host legume plant. Flavonoids
trigger the secretion of nod factors, which in turn are recognized by the host plant and
can lead to root hair deformation and several cellular responses, such as ion fluxes. The
best-known infection mechanism is called intracellular infection, in this case the rhizobia enter
through a deformed root hair in a similar way to endocytosis, forming an intracellular
tube called the infection thread. A second mechanism is called "crack entry"; in this
case, no root hair deformation is observed and the bacteria penetrate between cells,
through cracks produced by lateral root emergence. Later on, the bacteria become intracellular
and an infection thread is formed like in intracellular infections.
The infection triggers cell division in the cortex of the root where a new organ, the
nodule, appears as a result of successive processes.
Infection threads grow to the nodule, infect its central tissue and release the rhizobia
in these cells, where they differentiate morphologically into bacteroids and fix nitrogen from the
atmospheric, elemental N2 into a plant-usable form, ammonium, using the enzyme nitrogenase.
The reaction for all nitrogen-fixing bacteria is:
N2 + 8 H+ + 8 e− → 2 NH3 + H2 In return, the plant supplies the bacteria
with carbohydrates, proteins, and sufficient oxygen so as not to interfere with the fixation
process. Leghaemoglobins, plant proteins similar to human hemoglobins, help to provide oxygen
for respiration while keeping the free oxygen concentration low enough so as not to inhibit
nitrogenase activity. Recently, a Bradyrhizobium strain was discovered to form nodules in Aeschynomene
without producing nod factors, suggesting the existence of alternative communication
signals other than nod factors. 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 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. There are
two competing hypotheses for the mechanism that maintains legume-rhizobium symbiosis.
The sanctions hypothesis suggests the plants police cheating rhizobia. Sanctions could
take the form of reduced nodule growth, early nodule death, decreased carbon supply to nodules,
or reduced oxygen supply to nodules that fix less nitrogen. The partner choice hypothesis
proposes that the plant uses prenodulation signals from the rhizobia to decide whether
to allow nodulation, and chooses only noncheating rhizobia. There is evidence for sanctions
in soybean plants, which reduce rhizobium reproduction in nodules that fix less nitrogen.
Likewise, wild lupine plants allocate fewer resources to nodules containing less-beneficial
rhizobia, limiting rhizobial reproduction inside. This is consistent with the definition
of sanctions just given, although called "partner choice" by the authors. However, other studies
have found no evidence of plant sanctions, and instead support the partner choice hypothesis.
Other diazotrophs Many other species of bacteria are able to
fix nitrogen, but few are able to associate intimately with plants and colonize specific
structures like Legume nodules. Bacteria that do associate with plants include the actinobacteria
Frankia, which form symbiotic root nodules in actinorhizal plants, and several cyanobacteria
associated with aquatic ferns, Cycas and Gunneras. Free-living diazotrophs are often found in
the rhizosphere and in the intercellular spaces of several plants including rice and sugarcane,
but in this case the lack of a specialized structure results in poor nutrient transfer
efficiency compared to legume or actinorhizal nodules.
References ^ "Current taxonomy of rhizobia". Retrieved
2013-12-02. ^ "Bacteria confused with rhizobia, including
Agrobacterium taxonomy". Retrieved 2013-12-02. ^ "Taxonomy of legume nodule bacteria and
agrobacteria". Retrieved 2013-12-02. ^ "What is Rhizobia". Retrieved 2008-07-01.
^ The Nitrogen cycle and Nitrogen fixation, Jim Deacon, Institute of Cell and Molecular
Biology, The University of Edinburgh http:www.biology.ed.ac.ukjdeaconnitrogen.htm ^ Giraud, Eric; et al., L; Vallenet, D; Barbe,
V; Cytryn, E; Avarre, JC; Jaubert, M; Simon, D et al.. "Legumes symbioses: absence of nod
genes in photosynthetic bradyrhizobia". Science 316: 1307–12. doi:10.1126/science.1139548.
PMID 17540897. ^ Denison, R. F.. "Legume sanctions and the
evolution of symbiotic cooperation by rhizobia". American Naturalist 156: 567–576. doi:10.1086/316994.
^ Kiers ET, Rousseau RA, West SA, Denison RF 2003. Host sanctions and the legume–rhizobium
mutualism. Nature 425 : 79-81 ^ Simms et al. 2006. An empirical test of
partner choice mechanisms in a wild legume-rhizobium interaction. Proc. Roy. Soc. B 273:77-81.
^ Heath, K. D.; Tiffin, P.. "Stabilizing mechanisms in legume-rhizobium mutualism". Evolution
63: 652–662. doi:10.1111/j.1558-5646.2008.00582.x. PMID 19087187.
^ Marco, D. E., R. Perez-Arnedo, A. Hidalgo-Perea, J. Olivares, J. E. Ruiz-Sainz, and J. Sanjuan.
2009. A mechanistic molecular test of the plant-sanction hypothesis in legume-rhizobia
mutualism. Acta Oecologica-International Journal of Ecology 35:664-667
Jones, KM; et al., H; Davies, BW; Taga, ME; Walker, GC. "How rhizobial symbionts invade
plants: the Sinorhizobium–Medicago model". Nature Reviews Microbiology 5: 619–33. doi:10.1038/nrmicro1705.
PMC 2766523. PMID 17632573. External links
Legume sanctions maintain Rhizobium mutualism Current list of rhizobia species
Nitrogen Fixation and Inoculation of Forage Legumes