Bradyrhizobium

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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 Edit this classification
Domain: Bacteria
Phylum: Pseudomonadota
Class: Alphaproteobacteria
Order: Hyphomicrobiales
Family: Nitrobacteraceae
Genus: Bradyrhizobium
Jordan 1982
Type species
Bradyrhizobium japonicum
Species

See text

Synonyms
  • Agromonas Ohta and Hattori 1985[1]
  • "Photorhizobium" Eaglesham et al. 1990[2]
  • "Phytomyxa" Schroeter 1886

Bradyrhizobium is a genus of

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

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]

Taxonomy

Accepted Species

Bradyrhizobium comprises the following species:[6]

Provisional Species

The following species have been published, but not validated according to the

Bacteriological Code.[6]

Phylogeny

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]

Nodulation

Nodule formation

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

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]

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 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]

Genes

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]

Diversity

This genus of bacteria can form either specific or general symbioses;

phenotypic
differences are seen, so not many species have been named.


Some strains are

Oryza breviligulata.[15]

Significance

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]

Notable species

References

  1. PMID 22685107
    .
  2. ^
    ISBN 978-1-4684-6434-4
    .
  3. PMID 25909973
    .
  4. ^
    ISBN 978-0-387-94134-9
    .
  5. PMID 14660374
    .
  6. ^ a b c "List of Prokaryotic names with Standing in Nomenclature —Bradyrhizobium". Retrieved May 23, 2021.
  7. ^
    PMID 33709900
    .
  8. ^
    PMID 20452160
    .
  9. PMID 32373076
    .
  10. ^
    PMID 7737469
    .
  11. doi:10.1016/0038-0717(95)98622-U
    .
  12. ^
    doi:10.1016/S0378-4290(97)00022-1
    .
  13. PMID 16664027
    .
  14. S2CID 43001831
    .
  15. PMID 11097925
    . Retrieved 7 May 2021.
  16. doi:10.1016/j.soilbio.2005.08.018
    .
  17. bioRxiv 10.1101/007187
    .
  18. PMID 11916667
    .
  19. ^
    PMID 19201125
    .
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