Bradyrhizobium

Bradyrhizobium
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
Species
B. arachidis Wang et al. 2013 B. betae Rivas et al. 2004 B. canariense Vinuesa et al. 2005 B. cytisi Chahbourne et al. 2011 B. daqingense Wang JY et al. 2012 B. denitrificans (Hirsch and Müller 1986) van Berkum et al. 2011 B. diazoefficiens Delamuta et al 2013 B. elkanii Kuykendall et al. 1993 B. erythrophlei Yao et al. 2015 B. ferriligni Yao et al. 2015 B. ganzhouense Lu et al. 2014 B. huanghuaihaiense Zhang et al. 2012 B. icense Durán et al. 2014 B. ingae da Silva et al. 2014 B. iriomotense Islam et al. 2010 B. japonicum (Kirchner 1896) Jordan 1982 symbiovar genistearum symbiovar glycinearum B. jicamae Ramírez-Bahena et al. 2009 B. lablabi Chang et al. 2011 B. liaoningense Xu et al. 1995 B. lupini Peix et al. 2015 B. manausense Silva et al. 2014 B. neotropicale Zilli et al. 2014 B. oligotrophicum (Ohta and Hattori 1985) Ramírez-Bahena et al. 2013 B. ottawaense Yu et al. 2014 B. pachyrhizi Ramírez-Bahena et al. 2009 B. paxllaeri Durán et al. 2014 B. retamae Guerrouj et al. 2013 B. rifense Chahboune et al. 2012 B. stylosanthis Marçon Delamuta et al. 2016 B. valentinum Durán et al. 2014 B. yuanmingense Yao et al. 2002
Synonyms
Agromonas Ohta and Hattori 1985 "Photorhizobium" (informal)

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 common soil-dwelling micro-organisms that can form symbiotic relationships with leguminous plant species where they fix nitrogen in exchange for carbohydrates from the plant. Like other rhizobia, many members of this genus have the ability to fix atmospheric nitrogen into forms readily available for other organisms to use. Bradyrhizobia are also major components of forest soil microbial communities, where strains isolated from these soils are not typically capable of nitrogen fixation or nodulation. They are slow-growing in contrast to Rhizobium species, which are considered fast-growing rhizobia. In a liquid medium, Bradyrhizobium species take 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 carbon sources. Some strains (for example, USDA 6 and CPP) are capable of oxidizing carbon monoxide aerobically.

Nodules are growths 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.

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