Azospirillum

Azospirillum is a Gram-negative, microaerophilic, non-fermentative and nitrogen-fixing bacterial genus from the family of Rhodospirillaceae.[1][2][3][4][5] Azospirillum bacteria can promote plant growth.[6]

Azospirillum
Scientific classification
Kingdom:
Bacteria
Phylum:
Proteobacteria
Class:
Alphaproteobacteria
Order:
Rhodospirillales
Family:
Rhodospirillaceae
Genus:
Azospirillum

Tarrand et al. 1979[1]
Type species
Azospirillum lipoferum[1]
Species

A. brasilense[1]
A. canadense[1]
A. doebereinerae[1]
A. fermentarium[1]
A. formosense[1]
A. halopraeferens[1]
A. humicireducens[1]
A. irakense[1]
A. largimobile[1]
A. lipoferum[1]
A. melinis[1]
A. oryzae[1]
A. picis[1]
A. rugosum[1]
A. thiophilum[1]
A. zeae[1]

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.[7] [8] 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 diazotrophs, meaning, under low oxygen conditions, they can change inert nitrogen from the air into biologically useable forms.[7][8] At least three species, A. melinis, A. thiophilum, and A. humicireducens are facultative anaerobes, and can live, if necessary, without oxygen.[7] 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. [7] Microbiologists use nitrogen-free semi-solid media to isolate Azospirillum from samples. The most commonly used media is called "NFb".[5]

Discovery and reclassification

The first species described in the genus was originally named Spirillum lipoferum in 1925 by M.W. Beijerinck.[9] 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.[9][5] 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.[10] By 2020, twenty-one species of Azospirillum had been described, most of which had been discovered after the year 2000.[11]

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 fix atmospheric nitrogen.  The ending “-spirillum” refers to the shape of the bacteria which is similar to spiral-shaped bacteria in the genus Spirillum.[8]

Ecological and agricultural significance

Azospirillum are found in freshwater[8] and soil habitats, especially in close relationships with plant roots.[5] 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.[12] In addition to vascular plants, the growth of the algae Chlorella vulgaris was positively affected by the presence of Azospirillum.[13] 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.[14]

Plant growth promotion
Azospirillum promotes growth of fine root hairs.

Azospirillum promote plant growth through a variety of mechanisms. Many Azospirillum excrete 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.[15] In addition to these changes, Azospirillum can also alter the forms of plant nutrients such as nitrogen and phosphorus to make them more available to plants.[15] However, how much nitrogen Azospirillum contribute to crop plants via biological fixation is debated.[16] Azospirillum also make antioxidants that protect the plant roots from stresses due to drought and flooding.[15]

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 resist attack by pathogens, a process known as induced systemic resistance.[15]

Known species
Name[1] Paper that first described species[1] Details about species
Azospirillum agricola Lin et al. 2016 Isolated from agricultural soil in Taiwan[17]
Azospirillum brasilense corrig. Tarrand et al. 1979 Isolated from roots of grasses in South America; One of the best studied species in the genus; Heavily researched for applications in agriculture; Used commercially to promote crop growth, especially in South America.[9]
Azospirillum canadense Mehnaz et al. 2007 Isolated from corn roots in Canada[18]
Azospirillum doebereinerae Eckert et al. 2001 Isolated from Miscanthus grass roots in Germany[19]
Azospirillum fermentarium Lin et al. 2013 Isolated from a fermentation tank in Taiwan[20]
Azospirillum formosense Lin et al. 2012 Isolated from agricultural soil in Taiwan[21]
Azospirillum griseum Yang et al. 2019 Isolated from water from Baiyang Lake in China[22]
Azospirillum halopraeferens Reinhold et al. 1987 Isolated from salt-tolerant Kallar grass in Pakistan;[23] Has been shown

to survive in seawater after experimental inoculation on the roots of mangroves[24]

Azospirillum humicireducens Zhou et al. 2013 Isolated from a microbial fuel cell in China[25]
Azospirillum largimobile corrig. (Skerman et al. 1983) Ben Dekhil et al. 1997 Isolated from lake water in Australia;[5] originally called Conglomeromonas largomobilis[26]
Azospirillum lipoferum Tarrand et al. 1979 First species to be described in the genus; First isolated from garden soil[27]
Azospirillum melinis Peng et al. 2006 Isolated from molasses grass in China[28]
Azospirillum oryzae Xie and Yokota 2005 Isolated from rice roots in Japan[29]
Azospirillum palustre Tikhonova et al. 2019 Isolated from sphagnum peat in Russia; Can use methanol as a food source[30]
Azospirillum picis Lin et al. 2009 Isolated from tar in Taiwan[31]
Azospirillum

ramasamyi

 Anandham et al. 2019 Isolated from bovine fermentation products in Korea[32]
Azospirillum rugosum Young et al. 2008 Isolated from oil contaminated soil in Taiwan[33]
Azospirillum soli Lin et al. 2015 Isolated from agricultural soil in Taiwan[34]
Azospirillum thermophilum Zhao et al. 2020 Isolated from a hot spring in China[35]
Azospirillum

thiophilum

 Lavrinenko et al. 2010 Isolated from a sulfide spring in Russia[36]
Azospirillum zeae Mehnaz et al. 2007 Isolated from corn roots in Canada[37]

References
  1. Parte AC. "Azospirillum". LPSN.
  2. Arora NK (2014). Plant Microbes Symbiosis: Applied Facets. Springer. ISBN 978-81-322-2068-8.
  3. "Azospirillum". www.uniprot.org.
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  5. Cassán FD, Okon Y, Creus CM (2015). Handbook for Azospirillum: Technical Issues and Protocols. Cham: Springer. ISBN 978-3-319-06542-7. OCLC 908335504.
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  22. Yang, Yunzhen; Zhang, RanRan; Feng, Jie; Wang, Chao; Chen, Jifeng (2019-12-01). "Azospirillum griseum sp. nov., isolated from lakewater". International Journal of Systematic and Evolutionary Microbiology. 69 (12): 3676–3681. doi:10.1099/ijsem.0.003460. ISSN 1466-5026. PMID 31135333.
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  32. Anandham, Rangasamy; Heo, Jun; Krishnamoorthy, Ramasamy; SenthilKumar, Murugaiyan; Gopal, Nellaiappan Olgaganathan; Kim, Soo-Jin; Kwon, Soon-Wo (2019-05-01). "Azospirillum ramasamyi sp. nov., a novel diazotrophic bacterium isolated from fermented bovine products". International Journal of Systematic and Evolutionary Microbiology. 69 (5): 1369–1375. doi:10.1099/ijsem.0.003320. ISSN 1466-5026. PMID 30810523.
  33. Young, C. C.; Hupfer, H.; Siering, C.; Ho, M.-J.; Arun, A. B.; Lai, W.-A.; Rekha, P. D.; Shen, F.-T.; Hung, M.-H.; Chen, W.-M.; Yassin, A. F. (2008-04-01). "Azospirillum rugosum sp. nov., isolated from oil-contaminated soil". International Journal of Systematic and Evolutionary Microbiology. 58 (4): 959–963. doi:10.1099/ijs.0.65065-0. ISSN 1466-5026. PMID 18398202.
  34. Lin, Shih-Yao; Hameed, Asif; Liu, You-Cheng; Hsu, Yi-Han; Lai, Wei-An; Shen, Fo-Ting; Young, Chiu-Chung (2015-12-01). "Azospirillum soli sp. nov., a nitrogen-fixing species isolated from agricultural soil". International Journal of Systematic and Evolutionary Microbiology. 65 (Pt_12): 4601–4607. doi:10.1099/ijsem.0.000618. ISSN 1466-5026. PMID 26382036.
  35. Zhao, Zhuo-li; Ming, Hong; Ding, Chen-Long; Ji, Wei-Li; Cheng, Li-Jiao; Niu, Ming-ming; Zhang, Yan-min; Zhang, Ling-Yu; Meng, Xiao-Lin; Nie, Guo-Xing (2020-01-01). "Azospirillum thermophilum sp. nov., isolated from a hot spring". International Journal of Systematic and Evolutionary Microbiology. 70 (1): 550–554. doi:10.1099/ijsem.0.003788. ISSN 1466-5026. PMID 31651377.
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Further reading
  • Okon Y, ed. (1994). Azospirillum/plant associations. Boca Raton: CRC Press. ISBN 0-8493-4925-7.
  • Klingmüller W, ed. (2012). Azospirillum III Genetics · Physiology · Ecology Proceedings of the Third Bayreuth Azospirillum Workshop. Berlin, Heidelberg: Springer Berlin Heidelberg. ISBN 978-3-642-70791-9.
  • Klingmüller W, ed. (2012). Azospirillum IV Genetics · Physiology · Ecology Proceedings of the Fourth Bayreuth Azospirillum Workshop. Berlin, Heidelberg: Springer Berlin Heidelberg. ISBN 978-3-642-73072-6.
  • Garrity GM (2005). Bergey's manual of systematic bacteriology (2nd ed.). New York: Springer. ISBN 0-387-24145-0.
  • Khush GS, Bennett J, eds. (1992). Nodulation and nitrogen fixation in rice : potential and prospects. Manila, Philippines: International Rice Research Institute. ISBN 971-22-0035-3.
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