Bradyrhizobium

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.^[3] 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.^[4] Some strains (for example, USDA 6 and CPP) are capable of oxidizing carbon monoxide aerobically.^[5]

Bradyrhizobium comprises the following species:^[6]

The following species have been published, but not validated according to the Bacteriological Code.^[6]

The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature (LPSN).^[6] The phylogeny is based on whole-genome analysis.^[9]

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 flavonoids, 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.

Over 55 genes are known to be associated with nodulation.^[10] NodD is essential for the expression of the other nod genes.^[11] The two different nodD genes are: nodD₁ and nodD₂. Only nodD₁ is needed for successful nodulation.^[10]

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 most important process for the growth and development of plants.^[12] The levels of ureide nitrogen in a plant correlate with the amount of fixed nitrogen the plant takes up.^[13]

Nif and fix are important genes involved in nitrogen fixation among Bradyrhizobium 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.^[14]

This genus of bacteria can form either specific or general symbioses;^[4] 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, few phenotypic differences are seen, so not many species have been named.

Some strains are photosynthetic, these Bradyrhizobium often form nodules in the stems of semi-aquatic Aeschynomene legumes, and have also been found in the nodal roots of African wild rice Oryza breviligulata.^[15]

Grain legumes are cultivated on about 1.5 million km² of land per year.^[12] The amount of nitrogen fixed annually is about 44–66 million tons worldwide, providing almost half of all nitrogen used in agriculture.^[16] Commercial inoculants of Bradyrhizobium are available.

Bradyrhizobium has also been identified as a contaminant of DNA extraction kit reagents and ultrapure water systems, which may lead to its erroneous appearance in microbiota or metagenomic datasets.^[17] The presence of nitrogen-fixing bacteria as contaminants may be due to the use of nitrogen gas in ultrapure water production to inhibit microbial growth in storage tanks.^[18]

• Bradyrhizobium betae was isolated from tumor-like root deformations on sugar beets; they have an unknown symbiotic status.^[19]
• Bradyrhizobium elkanii, Bradyrhizobium diazoefficiens, and Bradyrhizobium liaoningense establish symbiosis with soybeans.^[19]
• Bradyrhizobium japonicum nodulates soybeans, cowpeas, mung beans, and siratro.^[19]
• Bradyrhizobium yuanmingense nodulates Lespedeza.^[19]
• 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.^[19]