Bradyrhizobium
Encyclopedia
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 nitrate
s.
. 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 pentose
s as a carbon source.
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 factor
s 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.
, nitrogen fixation
(or uptake) is the second 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.
, 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.
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.
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.
Gram-negative
Gram-negative bacteria are bacteria that do not retain crystal violet dye in the Gram staining protocol. In a Gram stain test, a counterstain is added after the crystal violet, coloring all Gram-negative bacteria with a red or pink color...
soil
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...
bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
, many of which fix nitrogen
Nitrogen fixation
Nitrogen fixation is the natural process, either biological or abiotic, by which nitrogen in the atmosphere is converted into ammonia . This process is essential for life because fixed nitrogen is required to biosynthesize the basic building blocks of life, e.g., nucleotides for DNA and RNA and...
. Nitrogen fixation is an important part of the nitrogen cycle
Nitrogen cycle
The nitrogen cycle is the process by which nitrogen is converted between its various chemical forms. This transformation can be carried out by both biological and non-biological processes. Important processes in the nitrogen cycle include fixation, mineralization, nitrification, and denitrification...
. Plants cannot use atmospheric nitrogen (N2) they must use nitrogen compounds such as nitrate
Nitrate
The nitrate ion is a polyatomic ion with the molecular formula NO and a molecular mass of 62.0049 g/mol. It is the conjugate base of nitric acid, consisting of one central nitrogen atom surrounded by three identically-bonded oxygen atoms in a trigonal planar arrangement. The nitrate ion carries a...
s.
Characteristics
Bradyrhizobium species are Gram-negative bacilli (rod shaped) with a single subpolar or polar flagellumFlagellum
A flagellum is a tail-like projection that protrudes from the cell body of certain prokaryotic and eukaryotic cells, and plays the dual role of locomotion and sense organ, being sensitive to chemicals and temperatures outside the cell. There are some notable differences between prokaryotic and...
. 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
Rhizobia
Rhizobia are soil bacteria that fix nitrogen after becoming established inside root nodules of legumes . Rhizobia require a plant host; they cannot independently fix nitrogen...
, 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
Rhizobium
Rhizobium is a genus of Gram-negative soil bacteria that fix nitrogen. Rhizobium forms an endosymbiotic nitrogen fixing association with roots of legumes and Parasponia....
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 pentose
Pentose
A pentose is a monosaccharide with five carbon atoms. Pentoses are organized into two groups. Aldopentoses have an aldehyde functional group at position 1...
s as a carbon source.
Nodule Formation
NodulesRoot nodule
Root nodules occur on the roots of plants that associate with symbiotic nitrogen-fixing bacteria. Under nitrogen-limiting conditions, capable plants form a symbiotic relationship with a host-specific strain of bacteria known as rhizobia...
are a growth on the roots of leguminous plants where the bacteria reside. The plant roots secrete amino acids and sugars into the rhizosphere
Rhizosphere
The rhizosphere is the narrow region of soil that is directly influenced by root secretions and associated soil microorganisms. Soil which is not part of the rhizosphere is known as bulk soil. The rhizosphere contains many bacteria that feed on sloughed-off plant cells, termed rhizodeposition, and...
. 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 factor
Nod factor
Nodulation factors are signaling molecules produced by bacteria known as rhizobia during the initiation of nodules on the root of legumes. A symbiosis is formed when legumes take up the bacteria...
s 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. NodD is essential for the expression of the other nod genes. There are two different nodD genes nodD1 and nodD2. Research has found that only nodD1 is needed for successful nodulation.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 photosynthesisPhotosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
, nitrogen fixation
Nitrogen fixation
Nitrogen fixation is the natural process, either biological or abiotic, by which nitrogen in the atmosphere is converted into ammonia . This process is essential for life because fixed nitrogen is required to biosynthesize the basic building blocks of life, e.g., nucleotides for DNA and RNA and...
(or uptake) is the second 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.
Genes
Nif and fix are important genes involved in nitrogen fixation among Bradyrizobium species. Nif genes are very similar to genes found in Klebsiella pneumoniaeKlebsiella pneumoniae
Klebsiella pneumoniae is a Gram-negative, non-motile, encapsulated, lactose fermenting, facultative anaerobic, rod shaped bacterium found in the normal flora of the mouth, skin, and intestines....
, a free living diazotroph
Diazotroph
Diazotrophs are bacteria and archaea that fix atmospheric nitrogen gas into a more usable form such as ammonia.A diazotroph is an organism that is able to grow without external sources of fixed nitrogen. Examples of organisms that do this are rhizobia and Frankia and Azospirillum. All diazotrophs...
. 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
Chromosome
A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...
. Cluster I contains most of the nitrogen fixation genes. Cluster II contains three fix genes located near nod genes.
Diversity
This genus of bacteria can form either specific or general symbioses. 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 RNARibosomal RNA
Ribosomal ribonucleic acid is the RNA component of the ribosome, the enzyme that is the site of protein synthesis in all living cells. Ribosomal RNA provides a mechanism for decoding mRNA into amino acids and interacts with tRNAs during translation by providing peptidyl transferase activity...
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.
Significance
Grain legumes are cultivated on about 1.5 million km2 of 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.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. IntercroppingIntercropping
Intercropping is the practice of growing two or more crops in proximity. The most common goal of intercropping is to produce a greater yield on a given piece of land by making use of resources that would otherwise not be utilized by a single crop. Careful planning is required, taking into account...
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.
- Bradyrhizobium elkaniiBradyrhizobium elkaniiBradyrhizobium elkanii is a species of legume-root nodulating, microsymbiotic nitrogen-fixing bacterium originally identified as DNA homology group II strains of B. japonicum ...
, B. liaonigense establish symbiosis with soybeans. - Bradyrhizobium japonicumBradyrhizobium japonicumBradyrhizobium japonicum is a species of legume-root nodulating, microsymbiotic nitrogen-fixing bacterium species. B. japonicum is identified as a DNA homology group .B...
nodulates soybeans, cowpeas, mung beans, and siratro. - Bradyrhizobium yuanmingense nodulates LespedezaLespedezaLespedeza is a genus of some 40 species of flowering plants in the pea family , commonly known as bush clovers or Japanese clovers...
. - Bradyrhizobium canariense nodulates Genistoid legumes endemic to the Canary IslandsCanary IslandsThe Canary Islands , also known as the Canaries , is a Spanish archipelago located just off the northwest coast of mainland Africa, 100 km west of the border between Morocco and the Western Sahara. The Canaries are a Spanish autonomous community and an outermost region of the European Union...
. It has also been found in LupinLupinLupinus, commonly known as Lupins or lupines , is a genus in the legume family . The genus comprises about 280 species , with major centers of diversity in South and western North America , and the Andes and secondary centers in the Mediterranean region and Africa Lupinus, commonly known as Lupins...
and serradella nodules in western AustraliaAustraliaAustralia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
and southern AfricaAfricaAfrica is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
.