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[1]
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. huanghuaihaiense Zhang et al. 2012 B. iriomotense Islam et al. 2010 B. japonicum (Kirchner 1896) Jordan 1982 B. j. bv. genistearum[2] B. j. bv. glycinearum[3] B. jicamae Ramírez-Bahena et al. 2009 B. lablabi Chang et al. 2011 B. liaoningense Xu et al. 1995 B. pachyrhizi Ramírez-Bahena et al. 2009 B. retamae Guerrouj et al. 2013 B. rifense Chahboune et al. 2012 B. yuanmingense Yao et al. 2002 incertae sedis 'Pseudomonas' carboxydohydrogena[4]

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.

Characteristics

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.^[5] Some strains (for example, USDA 6 and CPP) are capable of oxidizing carbon monoxide aerobically.^[6]

Nodulation

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.^[7] NodD is essential for the expression of the other nod genes.^[8] There are two different nodD genes nodD₁ and nodD₂. Research has found that only nodD₁ is needed for successful nodulation.^[7]

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.^[9] The levels of ureide nitrogen in a plant correlate with the amount of fixed nitrogen the plant takes up.^[10]

Genes

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.^[11]

Diversity

This genus of bacteria can form either specific or general symbioses.^[5] 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.^[12]

Significance

Grain legumes are cultivated on about 1.5 million km² of land per year.^[9] The amount of nitrogen fixed annually is about 44–66 million tons worldwide, providing almost half of all nitrogen used in agriculture.^[13]

Application

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.

Species

• Bradyrhizobium betae was isolated from tumor like root deformations on sugar beets; they have an unknown symbiotic status.^[12]
• Bradyrhizobium elkanii, B. liaonigense establish symbiosis with soybeans.^[12]
• Bradyrhizobium japonicum nodulates soybeans, cowpeas, mung beans, and siratro.^[12]
• Bradyrhizobium yuanmingense nodulates Lespedeza.^[12]
• 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.^[12]

References