Azospirillum brasilense

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Classification

Bacteria; Proteobacteria; Alphaproteobacteria; Rhodospirillales; Rhodospirillaceae [3]

Azospirillum brasilense

Description and Significance

Azospirillum brasilense is one of the most well-studied plant growth promoting bacteria. It is considered a free-living soil bacterium that has the ability to affect the growth of numerous agricultural crops worldwide through the excretion of various hormones and the bacteria’s ability of nitrogen fixation. [1] Many countries use bacterial inoculants containing A.brasilense alone or in concert with other plant growth promoting bacteria. [1] Within the phylum of Proteobacteria there are multiple subgroups; A.brasilense belongs to the alpha subclass of Proteobacteria, Alphaproteobacteria. [3] The bacterium belongs to group IV of Alphaproteobacteria, Rhodospirillales. [3] The phenotypic differentiation of A.brasilense from other diazotrophic members of group IV is based upon the size and shape of the bacterial cell, and the mode of nitrogen fixation among other things. [3] Within its family Azospirillum can be distinguished from other members by lack of phototropy, the inability to form root and stem hypertrophies, and G+C content. [3] A.brasilense can be distinguished from other Azosprillum species based upon the ability of utilization of ribose and mannose. [3]

Structure, Metabolism, and Life Cycle

Azosprillium brasilense has a vibroid shape with single polar flagellum attached to the cell making it motile. [2] This flagellum is important in the adsorption step of plant root attachment. [4] The genome consists of multiple and mini chromosomes instead of a single circular chromosome. The cell also contains plasmids which encode for genes involved in the interaction of the bacterium with plant roots. [2] A.brasilense is capable of promoting growth of plants through the secretion of phytohormones, the most common being Indole Acetic Acid (IAA). A.brasilense produces IAA by utilizing the amino acid tryptophan as a precursor. IAA along with various other phytohormones alter the metabolism and the morphology of the plant room system in favor of better mineral and water adsorption [1] A.brasilense is capable of fixing nitrogen through assimilation of ammonium and the activity of nitrogenase. [1] Nitrogen fixation occurs under microaerobic conditions where nitrogen in limited. [1]

Ecology and Pathogenesis

Azosprillum brasilense is routinely found in the plant rhizosphere of crop plants and agricultural lands as well as from grasses and cereals. The bacterium is considered free-living, but appears to have preference for proximity to plant roots instead of open soil. A.brasilense colonizes the surface of plant roots through a two step process: attachment and anchoring. [1] A glycoprotein is used for attachment and an unidentified polysachharide is used to anchor the bacterium to the plant root hair. [4]

References

[1] Bashan, Yoav, Gina Holguin, and Luz E. De-Bashan. "Azospirillum-plant Relationships: Physiological, Moecular, Agricultural, and Environmental Advances 1997-2003)." Canadian Journal of Microbiology 50 (2004): 521-7y7. Web. July-Aug. 2013.

[2] Holguin, G., C. L. Patten, and B. R. Glick. "Genetics and Molecular Biology of Azospirillum." Biology and Fertility of Soils 29.1 (1999): 10-23. Print.

[3] Okon, Yaacov. Azospirillum/Plant Associations. Boca, Raton: CRC, 1994. Google. Web. 21 July 2013.

[4] Steenhoudt, O., and J. Vanderlayden. "Result Filters." National Center for Biotechnology Information. U.S. National Library of Medicine, 24 Oct. 2004. Web. 21 July 2013.

Author

Page authored by Elizabeth Fleege, student of Mandy Brosnahan, Instructor at the University of Minnesota-Twin Cities, MICB 3301/3303: Biology of Microorganisms.