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Azotobacter is a genus of usually motile, oval or spherical bacteria that form thick-walled cysts and may produce large quantities of capsular slime. They are aerobic, free-living soil microbes that play an important role in the nitrogen cycle in nature, binding atmospheric nitrogen, which is inaccessible to plants, and releasing it in the form of ammonium ions into the soil. In addition to being a model organism for studying diazotrophs, it is used by humans for the production of biofertilizers, food additives, and some biopolymers. The first representative of the genus, Azotobacter chroococcum, was discovered and described in 1901 by Dutch microbiologist and botanist Martinus Beijerinck. Azotobacter species are Gram-negative bacteria found in neutral and alkaline soils, in water, and in association with some plants.
Paenibacillus polymyxa, also known as Bacillus polymyxa, is a Gram-positive bacterium capable of fixing nitrogen. It is found in soil, plant tissues, marine sediments and hot springs. It may have a role in forest ecosystems and potential future applications as a biofertilizer and biocontrol agent in agriculture.
Pseudomonas stutzeri is a Gram-negative soil bacterium that is motile, has a single polar flagellum, and is classified as bacillus, or rod-shaped. While this bacterium was first isolated from human spinal fluid, it has since been found in many different environments due to its various characteristics and metabolic capabilities. P. stutzeri is an opportunistic pathogen in clinical settings, although infections are rare. Based on 16S rRNA analysis, this bacterium has been placed in the P. stutzeri group, to which it lends its name.
Bradyrhizobium is a genus of Gram-negative 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.
Rhizobacteria are root-associated bacteria that can have a detrimental, neutral or beneficial effect on plant growth. The name comes from the Greek rhiza, meaning root. The term usually refers to bacteria that form symbiotic relationships with many plants (mutualism). Rhizobacteria are often referred to as plant growth-promoting rhizobacteria, or PGPRs. The term PGPRs was first used by Joseph W. Kloepper in the late 1970s and has become commonly used in scientific literature.
Microbial inoculants also known as soil inoculants or bioinoculants are agricultural amendments that use beneficial rhizosphericic or endophytic microbes to promote plant health. Many of the microbes involved form symbiotic relationships with the target crops where both parties benefit (mutualism). While microbial inoculants are applied to improve plant nutrition, they can also be used to promote plant growth by stimulating plant hormone production.
A biofertilizer is a substance which contains living micro-organisms which, when applied to seeds, plant surfaces, or soil, colonize the rhizosphere or the interior of the plant and promotes growth by increasing the supply or availability of primary nutrients to the host plant. Biofertilizers add nutrients through the natural processes of nitrogen fixation, solubilizing phosphorus, and stimulating plant growth through the synthesis of growth-promoting substances. The micro-organisms in biofertilizers restore the soil's natural nutrient cycle and build soil organic matter. Through the use of biofertilizers, healthy plants can be grown, while enhancing the sustainability and the health of the soil. Biofertilizers can be expected to reduce the use of synthetic fertilizers and pesticides, but they are not yet able to replace their use. Since they play several roles, a preferred scientific term for such beneficial bacteria is "plant-growth promoting rhizobacteria" (PGPR).
Azospirillum brasilense is a well studied, nitrogen-fixing (diazotroph), genetically tractable, Gram-negative, alpha-proteobacterium bacterium, first described in Brazil by the group of Johanna Döbereiner and then receiving the name "brasilense". A. brasilense is able to fix nitrogen in the presence of low oxygen levels, making it a microaerobic diazotroph. An isolate from the genus Azospirillum was isolated from nitrogen poor soils in the Netherlands in 1925, however the species A. brasilense was first described in 1978 in Brazil, since this genus is widely found in the rhizospheres of grasses around the world where it confers plant growth promotion. Whether growth promotion occurs through direct nitrogen flux from the bacteria to the plant or through hormone regulation is debated. The two most commonly studied strains are Sp7 and Sp245, both are Brazilian isolates isolated from Tropical grasses from Seropedica, Brazil.
Azospirillum halopraeferens is a species of nitrogen-fixing bacteria associated with the roots of Diplachne fusca and black mangrove. It is microaerophilic and its type strain is Au 4.
Chryseobacterium formosense is a bacterium. It is gram-negative, rod-shaped, non-spore forming and yellow-pigmented. Its type strain is CC-H3-2T. It was first isolated from the rhizosphere of a specimen of Lactuca sativa.
Azospirillum doebereinerae is a species of nitrogen-fixing bacteria associated with the roots of Miscanthus species. Its type strain is GSF71T.
Bradyrhizobium betae is a species of legume-root nodulating, microsymbiotic nitrogen-fixing bacterium first isolated from the roots of Beta vulgaris, hence its name. It is slow-growing an endophytic. The type strain is PL7HG1T.
Gluconacetobacter azotocaptans is a species of acetic acid bacteria first isolated from rhizospheres and rhizoplanes of coffee plants. Its type strain is CFN-Ca54T.
Azospirillum oryzae is a species of nitrogen-fixing bacteria associated with the roots of Oryza sativa. Its type strain is COC8T.
Labrys methylaminiphilus is a Gram-negative and non-motile bacteria from the family Xanthobacteraceae which has been isolated from sediments of a lake in Seattle, Washington, United States.
Xanthobacter flavus is a Gram-negative, nitrogen-fixing and facultatively autotrophic bacteria from the family of Xanthobacteraceae which has been isolated from turf podsol soil in Russia. Xanthobacter flavus has the ability to degrade phenol, oxalate and 1,4-dichlorobenzene.
Sphingomonas azotifigens is a nitrogen-fixing bacteria from the genus of Sphingomonas which has been isolated from the root of a rice plant in Mishima in Japan.
Azospirillum is a Gram-negative, microaerophilic, non-fermentative and nitrogen-fixing bacterial genus from the family of Rhodospirillaceae. Azospirillum bacteria can promote plant growth.
Ciceribacter lividus is a nitrogen fixing, Gram-negative, aerobic and motile bacterium from the genus of Ciceribacter which has been isolated from rhizosphere soil from the plant Cicer arietinum in Kannivadi in India.
Disease suppressive soils function to prevent the establishment of pathogens in the rhizosphere of plants. These soils develop through the establishment of beneficial microbes, known as plant growth-promoting rhizobacteria (PGPR) in the rhizosphere of plant roots. These mutualistic microbes function to increase plant health by fighting against harmful soil microbes either directly or indirectly. As beneficial bacteria occupy space around plant roots they outcompete harmful pathogens by releasing pathogenic suppressive metabolites.
References
- ↑ Mehnaz, S.; Weselowski, B.; Lazarovits, G. (2007). "Azospirillum canadense sp. nov., a nitrogen-fixing bacterium isolated from corn rhizosphere". International Journal of Systematic and Evolutionary Microbiology. 57 (3): 620–624. doi: 10.1099/ijs.0.64804-0 . ISSN 1466-5026. PMID 17329796.
