Bradyrhizobium

Bradyrhizobiumis a genus ofGram-negativesoil bacteria, many of which fix nitrogen.Nitrogen fixationis an important part of thenitrogen cycle. Plants cannot use atmospheric nitrogen (N2); they must use nitrogen compounds such asnitrates.
Bradyrhizobiumspecies are Gram-negative bacilli (rod-shaped) with a single subpolar or polarflagellum. They are common soil-dwelling micro-organisms that can form symbiotic relationships withleguminousplant species where they fix nitrogen in exchange forcarbohydratesfrom the plant. Like otherrhizobia, 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 toRhizobiumspecies, which are considered fast-growing rhizobia. In a liquid medium,Bradyrhizobiumspecies take 3–5 days to create a moderate turbidity and 6–8 hours to double in population size. They tend to grow best withpentosesas carbon sources.Some strains (for example, USDA 6 and CPP) are capable of oxidizing carbon monoxide aerobically.
Bradyrhizobiumcomprises the following species:
The following species have been published, but not validated according to theBacteriological Code.
The currently accepted taxonomy is based on theList of Prokaryotic names with Standing in Nomenclature(LPSN).The phylogeny is based on whole-genome analysis.
Nodulesare growths on the roots of leguminous plants where the bacteria reside. The plant roots secreteamino acidsand sugars into therhizosphere. The rhizobia move toward the roots and attach to theroot hairs. The plant then releasesflavonoids, which induce the expression ofnodgenes within the bacteria. The expression of these genes results in the production of enzymes calledNod factorsthat initiateroot 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.NodDis essential for the expression of the othernodgenes.The two differentnodDgenes are:nodD1andnodD2. OnlynodD1is needed for successful nodulation.
Bradyrhizobiumand 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. Afterphotosynthesis,nitrogen fixation(or uptake) is the most important process for the growth and development of plants.The levels of ureide nitrogen in a plant correlate with the amount of fixed nitrogen the plant takes up.
Nifandfixare important genes involved in nitrogen fixation amongBradyrhizobiumspecies.Nifgenes are very similar to genes found inKlebsiella pneumoniae, a free-livingdiazotroph. The genes found in bradyrhizobia have similar function and structure to the genes found inK. pneumoniae.Fixgenes are important for symbiotic nitrogen fixation and were first discovered in rhizobia species. Thenifandfixgenes are found in at least two different clusters on thechromosome. Cluster I contains most of the nitrogen fixation genes. Cluster II contains threefixgenes located nearnodgenes.
This genus of bacteria can form either specific or general symbioses;one species ofBradyrhizobiummay only be able to nodulate one legume species, whereas otherBradyrhizobiumspecies may be able to nodulate several legume species.Ribosomal RNAis highly conserved in this group of microbes, makingBradyrhizobiumextremely difficult to use as an indicator of species diversity.DNA–DNA hybridizationshave been used instead and show more diversity. However, fewphenotypicdifferences are seen, so not many species have been named.
Some strains arephotosynthetic, theseBradyrhizobiumoften form nodules in the stems of semi-aquaticAeschynomenelegumes, and have also been found in the nodal roots of African wild riceOryza breviligulata.
Grain legumes are cultivated on about 1.5 million kmof land per year.The amount of nitrogen fixed annually is about 44–66 million tons worldwide, providing almost half of all nitrogen used in agriculture.Commercial inoculants ofBradyrhizobiumare available.
Bradyrhizobiumhas also been identified as a contaminant of DNA extraction kit reagents andultrapure watersystems, which may lead to its erroneous appearance in microbiota or metagenomic datasets.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.