Azospirillum
{{Short description|Genus of bacteria}}
{{Taxobox
| name = Azospirillum
| domain = Bacteria
| phylum = Pseudomonadota
| classis = Alphaproteobacteria
| ordo = Rhodospirillales
| familia = Azospirillaceae
| genus = Azospirillum
| genus_authority = Tarrand et al. 1979{{cite journal| vauthors = Parte AC |title=Azospirillum|website=LPSN|url=https://lpsn.dsmz.de/genus/azospirillum}}
| type_species = Azospirillum lipoferum
| subdivision_ranks = Species
| subdivision =
- A. agricola Lin et al. 2016
- A. baldaniorum Dos Santos Ferreira et al. 2020
- A. brasilense corrig. Tarrand et al. 1979 (Approved Lists 1980)
- A. canadense Mehnaz et al. 2007
- A. doebereinerae Eckert et al. 2001
- A. fermentarium Lin et al. 2013
- A. formosense Lin et al. 2012
- A. griseum Yang et al. 2019
- A. halopraeferens Reinhold et al. 1987
- A. humicireducens Zhou et al. 2013
- A. largimobile corrig. (Skerman et al. 1983) Ben Dekhil et al. 1997
- A. lipoferum (Beijerinck 1925) Tarrand et al. 1979 (Approved Lists 1980)
- A. melinis Peng et al. 2006
- "A. oleiclasticum" Wu et al. 2020
- A. oryzae Xie and Yokota 2005
- "A. palatum" Zhou et al. 2009
- A. palustre Tikhonova et al. 2019
- A. picis Lin et al. 2009
- A. ramasamyi Anandham et al. 2019
- A. rugosum Young et al. 2008
- A. soli Lin et al. 2015
- A. thermophilum Zhao et al. 2020
- A. thiophilum Lavrinenko et al. 2010
- A. zeae Mehnaz et al. 2007
| synonyms = * Conglomeromonas Skerman et al. 1983
}}
Azospirillum is a Gram-negative, microaerophilic, non-fermentative and nitrogen-fixing bacterial genus from the family of Rhodospirillaceae.{{cite book| vauthors = Arora NK |title=Plant Microbes Symbiosis: Applied Facets|date=2014|publisher=Springer|isbn=978-81-322-2068-8}}{{cite journal |title=Azospirillum |website=www.uniprot.org |url= https://www.uniprot.org/taxonomy/191 }}{{cite journal | vauthors = Steenhoudt O, Vanderleyden J | title = Azospirillum, a free-living nitrogen-fixing bacterium closely associated with grasses: genetic, biochemical and ecological aspects | journal = FEMS Microbiology Reviews | volume = 24 | issue = 4 | pages = 487–506 | date = October 2000 | pmid = 10978548 | doi = 10.1111/j.1574-6976.2000.tb00552.x | doi-access = free }}{{cite book | first1 = Fabricio Dario | last1 = Cassán | first2 = Yaacov | last2 = Okon | first3 = Cecilia M | last3 = Creus | name-list-style = vanc |title=Handbook for Azospirillum: Technical Issues and Protocols|date=2015|publisher=Springer|isbn=978-3-319-06542-7 |location=Cham|oclc=908335504}} Azospirillum bacteria can promote plant growth.{{cite book | vauthors = Katsy EI |title=Plasticity in plant-growth-promoting and phytopathogenic bacteria|date=2014|publisher=Imprint: Springer|location=New York, NY|isbn=978-1-4614-9203-0}}
Characteristics
The genus Azospirillum belongs in the Alphaproteobacteria class of bacteria. Azospirillum are gram-negative, do not form spores, and have a slightly twisted oblong-rod shape.{{cite book|url=http://link.springer.com/10.1007/978-3-642-30197-1|title=The Prokaryotes: Alphaproteobacteria and Betaproteobacteria|date=2014|publisher=Springer Berlin Heidelberg|isbn=978-3-642-30196-4|editor-last=Rosenberg|editor-first=Eugene |editor-last2=DeLong|editor-first2=Edward F.|editor-last3=Lory|editor-first3=Stephen|editor-last4=Stackebrandt|editor-first4=Erko|editor-last5=Thompson|editor-first5=Fabiano | name-list-style = vanc |location=Berlin, Heidelberg|language=en|doi=10.1007/978-3-642-30197-1|s2cid=12080582 }} {{cite book| vauthors = Madigan MT, Martinko JM, Parker J |url= https://www.worldcat.org/oclc/49558966 |title=Brock biology of microorganisms|date=2003|publisher=Prentice Hall/Pearson Education|isbn=0-13-066271-2|edition=10th|location=Upper Saddle River, NJ|oclc=49558966}} 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 usable forms. At least three species, A. melinis, A. thiophilum, and A. humicireducens are 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.{{cite journal | vauthors = Fukami J, Cerezini P, Hungria M | title = Azospirillum: benefits that go far beyond biological nitrogen fixation | journal = AMB Express | volume = 8 | issue = 1 | pages = 73 | date = May 2018 | pmid = 29728787 | pmc = 5935603 | doi = 10.1186/s13568-018-0608-1 | url = | doi-access = free }} In Brazil, during the 1970s, similar strains of this species were found associated with the roots of grain plants by scientists led 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.{{cite journal | vauthors = Tarrand JJ, Krieg NR, Döbereiner J | title = A taxonomic study of the Spirillum lipoferum group, with descriptions of a new genus, Azospirillum gen. nov. and two species, Azospirillum lipoferum (Beijerinck) comb. nov. and Azospirillum brasilense sp. nov | journal = Canadian Journal of Microbiology | volume = 24 | issue = 8 | pages = 967–80 | date = August 1978 | doi = 10.1139/m78-160 | pmid = 356945 | url = https://cdnsciencepub.com/doi/abs/10.1139/m78-160 | hdl = 10919/54873 | hdl-access = free }} By 2020, twenty-one species of Azospirillum had been described, most of which had been discovered after the year 2000.{{cite web|title=Genus: Azospirillum|url=https://lpsn.dsmz.de/genus/azospirillum|access-date=2020-10-20|website=lpsn.dsmz.de|language=en}}
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.
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.{{Cite journal|last1=Pereg|first1=Lily|last2=de-Bashan|first2=Luz E.|last3=Bashan|first3=Yoav|date=February 2016|title=Assessment of affinity and specificity of Azospirillum for plants|url=http://link.springer.com/10.1007/s11104-015-2778-9|journal=Plant and Soil|language=en|volume=399|issue=1–2|pages=389–414|doi=10.1007/s11104-015-2778-9|bibcode=2016PlSoi.399..389P |s2cid=8547931|issn=0032-079X|url-access=subscription}} In addition to vascular plants, the growth of the algae Chlorella vulgaris was positively affected by the presence of Azospirillum.{{Cite journal|last1=Meza|first1=Beatriz|last2=de-Bashan|first2=Luz E.|last3=Hernandez|first3=Juan-Pablo|last4=Bashan|first4=Yoav|date=2015-06-01|title=Accumulation of intra-cellular polyphosphate in Chlorella vulgaris cells is related to indole-3-acetic acid produced by Azospirillum brasilense|url=http://www.sciencedirect.com/science/article/pii/S0923250815000443|journal=Research in Microbiology|language=en|volume=166|issue=5|pages=399–407|doi=10.1016/j.resmic.2015.03.001|pmid=25797155|issn=0923-2508|doi-access=free}} 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.{{Cite journal|last1=Fukami|first1=Josiane|last2=Cerezini|first2=Paula|last3=Hungria|first3=Mariangela|date=2018-05-04|title=Azospirillum: benefits that go far beyond biological nitrogen fixation|url= |journal=AMB Express|volume=8|issue=1|pages=73|doi=10.1186/s13568-018-0608-1|issn=2191-0855|pmc=5935603|pmid=29728787 |doi-access=free }}
Plant growth promotion
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. 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. However, how much nitrogen Azospirillum contribute to crop plants via biological fixation is debated.{{Citation|last1=Bashan|first1=Yoav|title=Chapter Two - How the Plant Growth-Promoting Bacterium Azospirillum Promotes Plant Growth—A Critical Assessment|date=2010-01-01|url=http://www.sciencedirect.com/science/article/pii/S0065211310080028|work=Advances in Agronomy|volume=108|pages=77–136|editor-last=Sparks|editor-first=Donald L.|publisher=Academic Press|language=en|doi=10.1016/s0065-2113(10)08002-8|access-date=2020-11-03|last2=de-Bashan|first2=Luz E.|url-access=subscription}} Azospirillum also make 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 resist attack by pathogens, a process known as induced systemic resistance.
Known species and genetic diversity
Azospirillum genus harbor over than 20 described species. Despite the remarkable plant growth-promotion properties, less than half of Azospirillum species have the genome sequenced: A. brasilense, A. thiophilum, A. lipoferum, A. oryzae, A. palustre, A. doebereinerae, A. halopraeferens and several undescribed Azospirillum sp. strains. When accessing a phylogenetic tree with all Azospirillum genomes, it is possible to identify two monophyletic groups, one harboring exclusively A. brasilense strains and another the remaining species.{{Cite journal|last1=Rodrigues|first1=Gustavo Lima|last2=Matteoli|first2=Filipe Pereira|display-authors=etal|title=Characterization of cellular, biochemical and genomic features of the diazotrophic plant growth-promoting bacterium Azospirillum sp. UENF-412522, a novel member of the Azospirillum genus |journal=Microbiological Research|volume=254|date=2022-01-01|page=126896 |doi=10.1016/j.micres.2021.126896 |pmid=34715447 |s2cid=239491293 |doi-access=free}} This strongly suggests a higher differentiation of A. brasilense from the remaining strains. The second clade also has very high diversity and not enough resolution to determine strains species only using genetic data.
References
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Further reading
{{Refbegin}}
- {{cite book | veditors = Okon Y |title=Azospirillum/plant associations|date=1994|publisher=CRC Press|location=Boca Raton|isbn=0-8493-4925-7}}
- {{cite book | veditors = Klingmüller W |title=Azospirillum III Genetics · Physiology · Ecology Proceedings of the Third Bayreuth Azospirillum Workshop|date=2012|publisher=Springer Berlin Heidelberg|location=Berlin, Heidelberg|isbn=978-3-642-70791-9}}
- {{cite book | veditors = Klingmüller W |title=Azospirillum IV Genetics · Physiology · Ecology Proceedings of the Fourth Bayreuth Azospirillum Workshop|date=2012|publisher=Springer Berlin Heidelberg|location=Berlin, Heidelberg|isbn=978-3-642-73072-6}}
- {{cite book | vauthors = Garrity GM |title=Bergey's manual of systematic bacteriology.|date=2005|publisher=Springer|location=New York|isbn=0-387-24145-0|edition=2nd|url-access=registration|url=https://archive.org/details/bergeysmanualofs00boon}}
- {{cite book | veditors = Khush GS, Bennett J |title=Nodulation and nitrogen fixation in rice : potential and prospects|date=1992|publisher=International Rice Research Institute|location=Manila, Philippines|isbn=971-22-0035-3}}
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