azospirillum

''Azospirillum'' is a Gram-negative, microaerophilic, non-fermentative and nitrogen-fixing bacterial genus from the family of Rhodospirillaceae. ''Azospirillum'' bacteria can promote plant growth.

Characteristics

The genus ''Azospirillum'' belongs in the alpha-Proteobacteria class of bacteria. ''Azospirillum'' are gram-negative, do not form spores, and have a slightly-twisted oblong-rod shape.  ''Azospirillum'' have at least one flagellum and sometimes multiple flagella, which they use to move rapidly. ''Azospirillum'' are aerobic, but many can also function as microaerobic Diazotroph, diazotrophs, meaning, under low oxygen conditions, they can change inert nitrogen from the air into biologically useable forms. At least three species, ''A. melinis'', ''A. thiophilum'', and ''A. humicireducens'' are Facultative anaerobic organism, facultative anaerobes, and can live, if necessary, without oxygen. Growth of ''Azospirillum'' is possible between 5°C and 42°C and in substrates with a pH of 5 to 9, with optimal growth occurring around 30°C and 7 pH. Microbiologists use nitrogen-free semi-solid media to isolate ''Azospirillum'' from samples. The most commonly used media is called "NFb".

Discovery and reclassification

The first species described in the genus was originally named ''Spirillum lipoferum'' in 1925 by M.W. Beijerinck. In Brazil, during the 1970s, similar strains of this species were found associated with the roots of grain plants by scientists lead by Dr. Johanna Döbereiner. Her group discovered that these bacteria had the ability to fix nitrogen. Due to this discovery, ''Spirillum lipoferum'' was reclassified in 1978 as ''Azospirillum lipoferum'' by Jeffery Tarrand, Noel Krieg, and Döbereiner, who also added ''Azospirillum'' ''brasilense'' to the genus. By 2020, twenty-one species of ''Azospirillum'' had been described, most of which had been discovered after the year 2000.

Origin of name

The prefix “Azo-” comes from the French word “azote”, which means nitrogen.  This prefix is used to denote the ability of the bacteria to Nitrogen fixation, fix atmospheric nitrogen.  The ending “-spirillum” refers to the shape of the bacteria which is similar to spiral-shaped bacteria in the genus ''Spirillum.''

Ecological and agricultural significance

''Azospirillum'' are found in freshwater and soil habitats, especially in close relationships with plant roots. Associations with plants are thought to be largely beneficial. Over 113 species of plants in 35 different plant families have been documented to have benefited from association with a species of ''Azospirillum''. In addition to Vascular plant, vascular plants, the growth of the algae Chlorella vulgaris was positively affected by the presence of ''Azospirillum''. Since the 1970s, ''Azospirillum'' strains have been researched for their effects in improving agricultural yields and improving growth of wild plants. In 2009, the first commercial inoculants containing ''Azospirillum'' came on the market, and by 2018, over 3 million doses were applied annually to crops by farmers, mainly in South America.

Plant growth promotion

''Azospirillum'' promote plant growth through a variety of mechanisms. Many ''Azospirillum'' excrete Plant hormone, plant hormones that alter how the roots of plants grow. Affected roots frequently grow more branches and fine root hairs, which may help the plants acquire water and nutrients more efficiently. In addition to these changes, ''Azospirillum'' can also alter the forms of Plant nutrients in soil, plant nutrients such as nitrogen and phosphorus to make them more available to plants. However, how much nitrogen Azospirillum contribute to crop plants via biological fixation is debated. ''Azospirillum'' also make Antioxidant, antioxidants that protect the plant roots from stresses due to drought and flooding.

Plant growth can also be promoted indirectly by ''Azospirillum'' reducing plant disease. ''Azospirillum'' competes with pathogens on the roots for space and for trace nutrients such as iron. The plants' immune systems can also be primed by ''Azospirillum'' to Plant disease resistance, resist attack by pathogens, a process known as induced systemic resistance.

Known species

''References''

Further reading

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{{Taxonbar, from=Q794056

Rhodospirillales
Bacteria genera