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Cross section though a soybean (Glycine max 'Essex') root nodule.Bradyrhizobium japonicum infects the roots and establishes a nitrogen fixing symbiosis. This high magnification image shows part of a cell with single bacteroids within their symbiosomes
Scientific classification
Kingdom: Bacteria
Phylum: Proteobacteria
Class: Alphaproteobacteria
Order: Rhizobiales
Family: Bradyrhizobiaceae
Genus: Bradyrhizobium
Jordan 1982
Type species
Bradyrhizobium japonicum
  • B. betae
  • B. canariense
  • B. elkanii
  • B. japonicum
  • B. liaoningense
  • B. yuanmingense
incertae sedis

Bradyrhizobium is a genus of Gram-negative soil bacteria, many of which fix nitrogen. Nitrogen fixation is an important part of the nitrogen cycle. Plants cannot use atmospheric nitrogen (N2) they must use nitrogen compounds such as nitrates.



Bradyrhizobium species are Gram-negative bacilli (rod shaped) with a single subpolar or polar flagellum. They are a common soil dwelling microorganism that can form symbiotic relationships with leguminous plant species where they fix nitrogen in exchange for carbohydrates from the plant. Like other rhizobia, they have the ability to fix atmospheric nitrogen into forms readily available for other organisms to use. They are slow growing in contrast to Rhizobium species, which are considered fast growing rhizobia. In a liquid media broth, it takes Bradyrhizobium species 3-5 days to create a moderate turbidity and 6-8 hours to double in population size. They tend to grow best with pentoses as a carbon source.[1]



Nodule Formation

Nodules are a growth on the roots of leguminous plants where the bacteria reside. The plant roots secrete amino acids and sugars into the rhizosphere. The rhizobia move toward the roots and attach to the root hairs. The plant then releases flavanoids, which induce the expression of nod genes within the bacteria. The expression of these genes results in the production of enzymes called Nod factors that initiate root hair curling. During this process, the rhizobia are curled up with the root hair. The rhizobia penetrate the root hair cells with an infection thread that grows through the root hair into the main root. This causes the infected cells to divide and form a nodule. The rhizobia can now begin nitrogen fixation.

Nod Genes

There are over 55 genes known to be associated with nodulation.[2] NodD is essential for the expression of the other nod genes.[3] There are two different nodD genes nodD1 and nodD2. Research has found that only nodD1 is needed for successful nodulation.[2]

Nitrogen fixation

Bradyrhizobium and other rhizobia take atmospheric nitrogen and fix it into ammonia (NH3) or ammonium (NH4+). Plants cannot use atmospheric nitrogen; they must use a combined or fixed form of the element. After photosynthesis, nitrogen fixation (or uptake) is the second most important process for the growth and development of plants.[4] The levels of ureide nitrogen in a plant correlate with the amount of fixed nitrogen the plant takes up.[5]


Nif and fix are important genes involved in nitrogen fixation among Bradyrizobium species. Nif genes are very similar to genes found in Klebsiella pneumoniae, a free living diazotroph. The genes found in Bradyrhizobia have similar function and structure to the genes found in K. pneumoniae. Fix genes are important for symbiotic nitrogen fixation and were first discovered in rhizobia species. The nif and fix genes are found in at least two different clusters on the chromosome. Cluster I contains most of the nitrogen fixation genes. Cluster II contains three fix genes located near nod genes.[6]


This genus of bacteria can form either specific or general symbiosis.[1] This means that one species of Bradyrhizobium may only be able to nodulate one legume species, whereas other Bradyrhizobium species may be able to nodulate several legume species. Ribosomal RNA is highly conserved in this group of microbes, making Bradyrhizobium extremely difficult to use as an indicator of species diversity. DNA-DNA hybridizations have been used instead and show more diversity. However, there are few phenotypic differences and therefore not many species have been named.[7]


Grain legumes are cultivated on about 1.5 million km2 of land per year.[4] The amount of nitrogen fixed annually is about 44-66 million tons worldwide, providing almost half of all nitrogen used in agriculture.[8]


Bradyrhizobium fix more nitrogen that the plant can use. The excess nitrogen is left in the soil and available for other plants or later crops. Intercropping with a legume has the potential to decrease the need for applied fertilizer. There are commercial inoculants of Bradyrhizobium available, these can be applied as a peat or liquid directly to the seed before planting.


  • Bradyrhizobium betae was isolated from tumor like root deformations on sugar beets; they have an unknown symbiotic status.[7]
  • Bradyrhizobium elkanii, B. liaonigense establish symbiosis with soybeans.[7]
  • Bradyrhizobium japonicum nodulates soybeans, cowpeas, mung beans, and siratro.[7]
  • Bradyrhizobium yuanmingense nodulates Lespedeza.[7]
  • Bradyrhizobium canariense nodulates Genistoid legumes endemic to the Canary Islands. It has also been found in Lupin and serradella nodules in western Australia and southern Africa.[7]


  1. ^ a b P. Somasegaran (1994). Handbook for rhizobia: methods in legume-rhizobium technology. New York: Springer-Verlag. pp. 1–6, 167. ISBN 0387941347.  
  2. ^ a b Stacey, Gary (1995). "Bradyrhizobium japonicumnodulation genetics". FEMS Microbiology Letters 127: 1. doi:10.1111/j.1574-6968.1995.tb07441.x.  
  3. ^ Stacey, G (1995). "Signal exchange in the Bradyrhizobium-soybean symbiosis". Soil Biology and Biochemistry 27: 473. doi:10.1016/0038-0717(95)98622-U.  
  4. ^ a b Caetanoanolles, G (1997). "Molecular dissection and improvement of the nodule symbiosis in legumes". Field Crops Research 53: 47. doi:10.1016/S0378-4290(97)00022-1.  
  5. ^ van Berkum, P.; Sloger, C.; Weber, D. F.; Cregan, P. B.; Keyser, H. H. (1985). "Relationship between ureide N and N2 fixation, aboveground N accumulation, acetylene reduction, and nodule mass in greenhouse and field studies with Glycine max L.". Plant Physiol. 77 (1): 53–58. doi:10.1104/pp.77.1.53. PMID 16664027. PMC 1064455.  
  6. ^ Hennecke, H (1990). "Nitrogen fixation genes involved in the Bradyrhizobium japonicum-soybean symbiosis.". FEBS letters 268 (2): 422–6. doi:10.1016/0014-5793(90)81297-2. PMID 2200721.  
  7. ^ a b c d e f Rivas, Raul; Martens, Miet; De Lajudie, Philippe; Willems, Anne (2009). "Multilocus sequence analysis of the genus Bradyrhizobium☆". Systematic and Applied Microbiology 32: 101. doi:10.1016/j.syapm.2008.12.005.  
  8. ^ Alberton, O; Kaschuk, G; Hungria, M (2006). "Sampling effects on the assessment of genetic diversity of rhizobia associated with soybean and common bean". Soil Biology and Biochemistry 38: 1298. doi:10.1016/j.soilbio.2005.08.018.  


Up to date as of January 23, 2010

From Wikispecies


Main Page
Superregnum: Bacteria
Regnum: Bacteria
Phylum: Proteobacteria
Classis: Alpha Proteobacteria
Ordo: Rhizobiales
Familia: Bradyrhizobiaceae
Genus: Bradyrhizobium
Spieces: Bradyrhizobium japonicum - Bradyrhizobium sp. BTAi1 - Bradyrhizobium sp. ORS278



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