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Cross section though a soybean (*Glycine max* 'Essex') root nodule. The Rhizobacteria, *Bradyrhizobium japonicum*, colonizes the roots and establishes a nitrogen fixing symbiosis. This high magnification image shows part of a cell with single bacteroids within their host plant. In this image, endoplasmic reticulum, dictysome and cell wall can be seen.

Rhizobacteria are root-colonizing bacteria that form symbiotic relationships with many plants. The name comes from the Greek rhiza, meaning root. Though parasitic varieties of rhizobacteria exist, the term usually refers to bacteria that form a relationship beneficial for both parties (mutualism). They are an important group of microorganisms used in biofertilizer. Biofertilization accounts for approximately 65% of the nitrogen supply to crops worldwide. 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.^[1] PGPRs have different relationships with different species of host plants. The two major classes of relationships are rhizospheric and endophytic. Rhizospheric relationships consist of the PGPRs that colonize the surface of the root, or superficial intercellular spaces of the host plant, often forming root nodules. The dominant species found in the rhizosphere is a microbe from the genus Azospirillum.^[2] Endophytic relationships involve the PGPRs residing and growing within the host plant in the apoplastic space.^[1]

Nitrogen fixation

Nitrogen fixation is one of the most beneficial processes performed by rhizobacteria. Nitrogen is a vital nutrient to plants and gaseous nitrogen (N₂) is not available to them due to the high energy required to break the triple bonds between the two molecules.^[3] Rhizobacteria through nitrogen fixation are able to convert gaseous nitrogen (N₂) to ammonia (NH₃) making it an available nutrient to the host plant which can support and enhance plant growth. The host plant provides the bacteria with amino acids so they do not need to assimilate ammonia.^[4] The amino acids are then shuttled back to the plant with newly fixed nitrogen. Nitrogenase is an enzyme involved in nitrogen fixation and requires anaerobic conditions. Membranes within root nodules are able to provide these conditions. The rhizobacteria require oxygen to metabolize therefore oxygen is provided by a hemoglobin protein called leghemoglobin which is produced within the nodules.^[3] Legumes are well known nitrogen fixing crops and have been utilized for centuries in crop rotation to maintain the health of the soil.

Symbiotic relationships

The symbiotic relationship between rhizobacteria and its host plant is not without costs. For the plant to be able to benefit from the added available nutrients provided by the rhizobacteria, it needs to provide a place and the proper conditions for the rhizobacteria to live. Creating and maintaining root nodules for rhizobacteria can cost between 12%-25% of the plants total photosynthetic output. Legumes are often able to colonize early successional environments due to the unavailability of nutrients. Once colonized though the rhizobacteria make the soil surrounding the plant more nutrient rich, which in turn can lead to competition with other plants. The symbiotic relationship in short can lead to increased competition.^[3]

PGPRs increase the availability of nutrients through the solubilization of unavailable forms of nutrients and by the production of siderophores which aids in the facilitating of nutrient transport. Phosphorus, a limiting nutrient for plant growth, can be plentiful in soil but is most commonly found in insoluble forms. Organic acids and phosphotases released by rhizobacteria found in plant rhizospheres facilitate the conversion of insoluble forms of phosphorus to plant available forms such as H₂PO₄^-. PGPR bacteria include Pseudomonas putida, Azospirillum fluorescens, and Azospirillum lipoferum and notable nitrogen fixing bacteria associated with legumes includes Allorhizobium, Azorhizobium, Bradyrhizobium, and Rhizobium.^[4]

Though microbial inoculants can be beneficial for crops, they are not widely used in industrial agriculture, as large-scale application techniques have yet to become economically viable. A notable exception is the use of rhizobial inoculants for legumes such as peas. Inoculation with PGPRs ensure efficient nitrogen fixation, and they have been employed in North American agriculture for over 100 years.

Plant growth-promoting rhizobacteria

Plant growth-promoting rhizobacteria (PGPR) was first defined by Kloepper and Schroth ^[5] to describe soil bacteria that colonize the roots of plants following inoculation onto seed and that enhance plant growth. The following are implicit in the colonization process: ability to survive inoculation onto seed, to multiply in the spermosphere (region surrounding the seed) in response to seed exudates, to attach to the root surface, and to colonize the developing root system.^[6] The ineffectiveness of PGPR in the field has often been attributed to their inability to colonize plant roots.^[7]^[8] A variety of bacterial traits and specific genes contribute to this process, but only a few have been identified. These include motility, chemotaxis to seed and root exudates, production of pili or fimbriae, production of specific cell surface components, ability to use specific components of root exudates, protein secretion, and quorum sensing. The generation of mutants altered in expression of these traits is aiding our understanding of the precise role each one plays in the colonization process.^[9]^[10]

Progress in the identification of new, previously uncharacterized genes is being made using nonbiased screening strategies that rely on gene fusion technologies. These strategies employ reporter transposons ^[11] and in vitro expression technology (IVET) ^[12] to detect genes expressed during colonization.

Using molecular markers such as green fluorescent protein or fluorescent antibodies it is possible to monitor the location of individual rhizobacteria on the root using confocal laser scanning microscopy.^[8]^[13]^[14] This approach has also been combined with an rRNA-targeting probe to monitor the metabolic activity of a rhizobacterial strain in the rhizosphere and showed that bacteria located at the root tip were most active ^[15]

Mechanisms of action

PGPR enhance plant growth by direct and indirect means, but the specific mechanisms involved have not all been well-characterized,^[6] Direct mechanisms of plant growth promotion by PGPR can be demonstrated in the absence of plant pathogens or other rhizosphere microorganisms, while indirect mechanisms involve the ability of PGPR to reduce the harmful effects of plant pathogens on crop yield. PGPR have been reported to directly enhance plant growth by a variety of mechanisms: fixation of atmospheric nitrogen that is transferred to the plant^[16], production of siderophores that chelate iron and make it available to the plant root, solubilization of minerals such as phosphorus, and synthesis of phytohormones. Direct enhancement of mineral uptake due to increases in specific ion fluxes at the root surface in the presence of PGPR has also been reported . PGPR strains may use one or more of these mechanisms in the rhizosphere. Molecular approaches using microbial and plant mutants altered in their ability to synthesize or respond to specific phytohormones have increased our understanding of the role of phytohormone synthesis as a direct mechanism of plant growth enhancement by PGPR ^[17] PGPR that synthesize auxins and cytokinins or that interfere with plant ethylene synthesis have been identified.

Pathogenic roles

Studies conducted on sugar beet crops found that some root-colonizing bacteria were deleterious rhizobacteria (DRB). It was found that sugar beet seeds inoculated with DRB had reduced germination rates, root lesions, reduced root elongation, root distortions, increased fungi infection and decreased plant growth. In one trial the sugar beet yield was reduced by 48%.^[18]

Six strains of rhizobacteria have been identified as being DRB. The strains are in the genera Enterobacter, Klebsiella, Citrobacter, Flavobacterium, Achromobacter, Arthrobacter. Due to a large number of taxonomic species yet to be described complete characterization has not been possible as DRB are highly variable.^[18]

The presence of PGPRs has proven to reduce and inhibit the colonization of DRB on sugar beet roots. Plots inoculated with PGPRs and DRBs had an increase in production of 39% while plots only treated with DRbs had a reduction in production of 30%.^[18]

Biocontrol

Rhizobacteria are also able to control plant diseases that are caused by other bacteria and fungi. Disease is suppressed through induced systematic resistance and through the production of anti-fungal metabolites. Pseudomonas Biocontrol strains have been genetically modified to improve plant growth and improve the disease resistance of agricultural crops. In agriculture, inoculant bacteria are often applied to the seed coat of seeds prior to being sown. Inoculated seeds are more likely to establish large enough rhizobacteria populations within the rhizosphere to produce notable beneficial effects on the crop.^[1]

References

^ ^a ^b ^c Vessy, J. Kevin (August 2003). "Plant Growth Promoting Rhizobacteria as Biofertilizers". Plant and Soil. 255 (2): 571–586.
^ Bloemberg, Guido V.; Lugtenberg, Ben J. (August 2001). "Molecular basis of plant growth promotion and biocontrol by rhizobacteria". Current Opinion in Plant Biology. 4 (4): 343–350.
^ ^a ^b ^c Cain, Michael L.; Bowman, William D.; Hacker, Sally D. (2011). "Chapter 16: Change in Communities". Ecology. Sinauer Associates. pp. 359–362. ISBN 978-0-87893-445-4.
^ ^a ^b Willey, Joanne M.; Sherwood, Linda M.; Woolverton, Christopher J. (2011). "Chapter 29: Microorganisms in Terrestrial Ecosystems". Prescott's Microbiology. McGraw-Hill. pp. 703–706. ISBN 978-0-07-131367-4.
^ Kloepper, Joseph W.; Schroth, Milton N. (1978). "Plant growth-promoting rhizobacteria on radishes" (PDF). Proceedings of the 4th International Conference on Plant Pathogenic Bacteria. 2. Angers, France: Station de Pathologie Végétale et Phytobactériologie, INRA: 879–882. {{cite journal}}: Unknown parameter |publicationplace= ignored (|publication-place= suggested) (help)
^ ^a ^b Kloepper, Joseph W. (1993). "Plant growth-promoting rhizobacteria as biological control agents". In Metting, F. Blaine, Jr. (ed.). Soil Microbial Ecology: Applications in Agricultural and Environmental Management. New York, USA: Marcel Dekker Inc. pp. 255–274. {{cite book}}: Unknown parameter |publicationplace= ignored (|publication-place= suggested) (help)CS1 maint: multiple names: editors list (link)
^ Benizri, E.; Baudoin, E.; Guckert, A. (2001). "Root colonization by inoculated plant growth promoting rhizobacteria". Biocontrol Science and Technology. 11 (5): 557–574.
^ ^a ^b Bloemberg, Guido V.; Lugtenberg, Ben J. J. (2001). "Molecular basis of plant growth promotion and biocontrol by rhizobacteria". Current Opinion in Plant Biology. 4 (4): 343–350.
^ Lugtenberg, Ben J. J.; Dekkers, Linda; Bloemberg, Guido V. (2001). "Molecular determinants of rhizosphere colonization by Pseudomonas". Annual Review of Phytopathology. 39: 461–490.
^ Persello-Cartieaux, F.; Nussaume, L.; Robaglia, C. (2003). "Tales from the underground: Molecular plant-rhizobacteria interactions". Plant, Cell & Environment. 26 (2): 189–199.
^ Roberts, Daniel P.; Yucel, Irem; Larkin, Robert P. (1998). "Genetic approaches for analysis and manipulation of rhizosphere colonization by bacterial biocontrol agents". In Boland, Greg J.; Kuykendall, L. David (eds.). Plant-Microbe interactions and Biological Control. Books in Soils, Plants, and the Environment. Vol. 63. New York, USA: Marcel Dekker Inc. pp. 415–431. {{cite book}}: Unknown parameter |publicationplace= ignored (|publication-place= suggested) (help)
^ Rainey, P. B. 1999. Adaptation of Pseudomonas fluorescens to the plant rhizosphere. Environ. Microbiol. 1:243-257.
^ Bloemberg, Guido V.; Wijfjes, André H. M.; Lamers, Gerda E. M.; Stuurman, Nico; Lugtenberg, Ben J. J. (2000). "Simultaneous imaging of Pseudomonas fluorescens WCS365 populations expressing three different autofluorescent proteins in the rhizosphere: New perspectives for studying microbial communities". Molecular Plant-Microbe Interactions. 13 (11): 1170–1176.
^ Sørensen, Jan; Jensen, Linda E.; Nybroe, Ole (2001). "Soil and rhizosphere as habitats for Pseudomonas inoculants: New knowledge on distribution, activity and physiological state derived from micro-scale and single-cell studies". Plant and Soil. 232 (1–2): 97–108.
^ Lübeck, Peter S.; Hansen, Michael; Sørensen, Jan (2000). "Simultaneous detection of the establishment of seed-inoculated Pseudomonas fluorescens strain DR54 and native soil bacteria on sugar beet root surfaces using fluorescence antibody and in situ hybridization techniques". FEMS Microbiology Ecology. 33 (1): 11–19.
^ Zakry, F.A.A.; Shamsuddin, Z.H.; Khairuddin, A.R.; Zakaria, Z.Z.; Anuar, A.R. (2012). "Inoculation of Bacillus sphaericus UPMB-10 to young oil palm and measurement of its uptake of fixed nitrogen using the ¹⁵N isotope dilution technique". Microbes and Environments. 27 (3): 257–262. doi:10.1264/jsme2.ME11309.
^ Glick, Bernard R. (1995). "The enhancement of plant growth by free-living bacteria". Canadian Journal of Microbiology. 41 (2): 109–117.
^ ^a ^b ^c Suslow, Trevor V.; Schroth, Milton N. (January 1982). "Role of Deleterious Rhizobacteria as Minor Pathogens in Reducing Crop Growth". Phytopathology. 72 (1): 111–115.

[Vessy_2003-1] Vessy, J. Kevin (August 2003). "Plant Growth Promoting Rhizobacteria as Biofertilizers". Plant and Soil. 255 (2): 571–586.

[Bloemberg_2001-2] Bloemberg, Guido V.; Lugtenberg, Ben J. (August 2001). "Molecular basis of plant growth promotion and biocontrol by rhizobacteria". Current Opinion in Plant Biology. 4 (4): 343–350.

[Cane_2011-3] Cain, Michael L.; Bowman, William D.; Hacker, Sally D. (2011). "Chapter 16: Change in Communities". Ecology. Sinauer Associates. pp. 359–362. ISBN 978-0-87893-445-4.

[Willey_2011-4] Willey, Joanne M.; Sherwood, Linda M.; Woolverton, Christopher J. (2011). "Chapter 29: Microorganisms in Terrestrial Ecosystems". Prescott's Microbiology. McGraw-Hill. pp. 703–706. ISBN 978-0-07-131367-4.

[5] Kloepper, Joseph W.; Schroth, Milton N. (1978). "Plant growth-promoting rhizobacteria on radishes" (PDF). Proceedings of the 4th International Conference on Plant Pathogenic Bacteria. 2. Angers, France: Station de Pathologie Végétale et Phytobactériologie, INRA: 879–882. {{cite journal}}: Unknown parameter |publicationplace= ignored (|publication-place= suggested) (help)

[Kloepper,_J._W_1993._Pages_255-274-6] Kloepper, Joseph W. (1993). "Plant growth-promoting rhizobacteria as biological control agents". In Metting, F. Blaine, Jr. (ed.). Soil Microbial Ecology: Applications in Agricultural and Environmental Management. New York, USA: Marcel Dekker Inc. pp. 255–274. {{cite book}}: Unknown parameter |publicationplace= ignored (|publication-place= suggested) (help)CS1 maint: multiple names: editors list (link)

[7] Benizri, E.; Baudoin, E.; Guckert, A. (2001). "Root colonization by inoculated plant growth promoting rhizobacteria". Biocontrol Science and Technology. 11 (5): 557–574.

[Bloemberg,_G._V._2001-8] Bloemberg, Guido V.; Lugtenberg, Ben J. J. (2001). "Molecular basis of plant growth promotion and biocontrol by rhizobacteria". Current Opinion in Plant Biology. 4 (4): 343–350.

[9] Lugtenberg, Ben J. J.; Dekkers, Linda; Bloemberg, Guido V. (2001). "Molecular determinants of rhizosphere colonization by Pseudomonas". Annual Review of Phytopathology. 39: 461–490.

[10] Persello-Cartieaux, F.; Nussaume, L.; Robaglia, C. (2003). "Tales from the underground: Molecular plant-rhizobacteria interactions". Plant, Cell & Environment. 26 (2): 189–199.

[11] Roberts, Daniel P.; Yucel, Irem; Larkin, Robert P. (1998). "Genetic approaches for analysis and manipulation of rhizosphere colonization by bacterial biocontrol agents". In Boland, Greg J.; Kuykendall, L. David (eds.). Plant-Microbe interactions and Biological Control. Books in Soils, Plants, and the Environment. Vol. 63. New York, USA: Marcel Dekker Inc. pp. 415–431. {{cite book}}: Unknown parameter |publicationplace= ignored (|publication-place= suggested) (help)

[12] Rainey, P. B. 1999. Adaptation of Pseudomonas fluorescens to the plant rhizosphere. Environ. Microbiol. 1:243-257.

[13] Bloemberg, Guido V.; Wijfjes, André H. M.; Lamers, Gerda E. M.; Stuurman, Nico; Lugtenberg, Ben J. J. (2000). "Simultaneous imaging of Pseudomonas fluorescens WCS365 populations expressing three different autofluorescent proteins in the rhizosphere: New perspectives for studying microbial communities". Molecular Plant-Microbe Interactions. 13 (11): 1170–1176.

[14] Sørensen, Jan; Jensen, Linda E.; Nybroe, Ole (2001). "Soil and rhizosphere as habitats for Pseudomonas inoculants: New knowledge on distribution, activity and physiological state derived from micro-scale and single-cell studies". Plant and Soil. 232 (1–2): 97–108.

[15] Lübeck, Peter S.; Hansen, Michael; Sørensen, Jan (2000). "Simultaneous detection of the establishment of seed-inoculated Pseudomonas fluorescens strain DR54 and native soil bacteria on sugar beet root surfaces using fluorescence antibody and in situ hybridization techniques". FEMS Microbiology Ecology. 33 (1): 11–19.

[16] Zakry, F.A.A.; Shamsuddin, Z.H.; Khairuddin, A.R.; Zakaria, Z.Z.; Anuar, A.R. (2012). "Inoculation of Bacillus sphaericus UPMB-10 to young oil palm and measurement of its uptake of fixed nitrogen using the ¹⁵N isotope dilution technique". Microbes and Environments. 27 (3): 257–262. doi:10.1264/jsme2.ME11309.

[17] Glick, Bernard R. (1995). "The enhancement of plant growth by free-living bacteria". Canadian Journal of Microbiology. 41 (2): 109–117.

[Suslow_1982-18] Suslow, Trevor V.; Schroth, Milton N. (January 1982). "Role of Deleterious Rhizobacteria as Minor Pathogens in Reducing Crop Growth". Phytopathology. 72 (1): 111–115.

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@@ Line 1: / Line 1: @@
 [[Image:Root-nodule01.jpg|thumb|right|300px|Cross section though a soybean (''Glycine max'' 'Essex') root nodule. The Rhizobacteria, ''[[Bradyrhizobium|Bradyrhizobium japonicum]]'', colonizes the roots and establishes a nitrogen fixing symbiosis. This high magnification image shows part of a cell with single bacteroids within their host plant. In this image, endoplasmic reticulum, dictysome and cell wall can be seen.]]
-'''Rhizobacteria''' are root-colonizing [[bacteria]] that form symbiotic relationships with many plants. The name comes from the [[Greek language|Greek]] ''rhiza'', meaning ''root''. Though parasitic varieties of rhizobacteria exist, the term usually refers to bacteria that form a relationship beneficial for both parties ([[Mutualism (biology)|mutualism]]).  They are an important group of microorganisms used in [[biofertilizer]].  Biofertilization accounts for approximately 65% of the nitrogen supply to crops worldwide.  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.<ref name="Vessy_2003"/> PGPRs have different relationships with different species of host plants.  The two major classes of relationships are rhizospheric and [[endophytic]].  Rhizospheric relationships consist of the PGPRs that colonize the surface of the root, or superficial intercellular spaces of the host plant, often forming [[root nodules]].  The dominant species found in the rhizosphere is a microbe from the genus ''Azospirillum''.<ref name="Bloemberg_2001">Bloemberg, GV., Lugtenberg, BJ. (August 2001). "Molecular basis of plant growth promotion and biocontrol by rhizobacteria". Current Opinion in Plant Biology 4 (4): 343-350.</ref>  Endophytic relationships involve the PGPRs residing and growing within the host plant in the apoplastic space.<ref name="Vessy_2003">Vessy, J.K. (August 2003). "Plant Growth Promoting Rhizobacteria as Biofertilizers". Plant and Soil 255: pp 571-586.</ref>
+'''Rhizobacteria''' are root-colonizing [[bacteria]] that form symbiotic relationships with many plants. The name comes from the [[Greek language|Greek]] ''rhiza'', meaning ''root''. Though parasitic varieties of rhizobacteria exist, the term usually refers to bacteria that form a relationship beneficial for both parties ([[Mutualism (biology)|mutualism]]). They are an important group of microorganisms used in [[biofertilizer]]. Biofertilization accounts for approximately 65% of the nitrogen supply to crops worldwide. 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.<ref name="Vessy_2003"/> PGPRs have different relationships with different species of host plants. The two major classes of relationships are rhizospheric and [[endophytic]]. Rhizospheric relationships consist of the PGPRs that colonize the surface of the root, or superficial intercellular spaces of the host plant, often forming [[root nodules]]. The dominant species found in the rhizosphere is a microbe from the genus ''Azospirillum''.<ref name="Bloemberg_2001">{{cite journal |last1=Bloemberg |first1=Guido V. |last2= Lugtenberg |first2= Ben J. |date=August 2001 |title=Molecular basis of plant growth promotion and biocontrol by rhizobacteria |journal=Current Opinion in Plant Biology |volume=4 |issue=4 |pages=343–350}}</ref> Endophytic relationships involve the PGPRs residing and growing within the host plant in the apoplastic space.<ref name="Vessy_2003">{{cite journal |last1=Vessy |first1=J. Kevin |date=August 2003 |title=Plant Growth Promoting Rhizobacteria as Biofertilizers |journal=Plant and Soil |volume=255 |issue=2 |pages=571–586}}</ref>
 ==Nitrogen fixation==
-[[Nitrogen fixation]] is one of the most beneficial processes performed by rhizobacteria.  Nitrogen is a vital nutrient to plants and gaseous nitrogen (N<sub>2</sub>) is not available to them due to the high energy required to break the triple bonds between the two molecules.<ref name="Cane_2011"/>  Rhizobacteria through nitrogen fixation are able to convert gaseous nitrogen (N<sub>2</sub>) to ammonia (NH<sub>3</sub>) making it an available nutrient to the host plant which can support and enhance plant growth.  The host plant provides the bacteria with amino acids so they do not need to assimilate ammonia.<ref name="Willey_2011">Willey, J., Sherwood, L., Woolverton, C. (2011). "Chapter 29: Microorganisms in Terrestrial Ecosystems". Prescott's Microbiology. McGraw-Hill. pp. 703–706. ISBN 978-0-07-131367-4.</ref>  The amino acids are then shuttled back to the plant with newly fixed nitrogen.  [[Nitrogenase]] is an enzyme involved in nitrogen fixation and requires anaerobic conditions.  Membranes within root nodules are able to provide these conditions.  The rhizobacteria require oxygen to metabolize therefore oxygen is provided by a hemoglobin protein called [[leghemoglobin]] which is produced within the nodules.<ref name="Cane_2011">Cain, ML., Bowman, WD., Hacker, SD. (2011). "Chapter 16: Change in Communities". Ecology. Sinauer Associates, Inc. pp. 359–362. ISBN 9780878934454.</ref>  Legumes are well known nitrogen fixing crops and have been utilized for centuries in crop rotation to maintain the health of the soil.
+[[Nitrogen fixation]] is one of the most beneficial processes performed by rhizobacteria. Nitrogen is a vital nutrient to plants and gaseous nitrogen (N<sub>2</sub>) is not available to them due to the high energy required to break the triple bonds between the two molecules.<ref name="Cane_2011"/> Rhizobacteria through nitrogen fixation are able to convert gaseous nitrogen (N<sub>2</sub>) to ammonia (NH<sub>3</sub>) making it an available nutrient to the host plant which can support and enhance plant growth. The host plant provides the bacteria with amino acids so they do not need to assimilate ammonia.<ref name="Willey_2011">{{cite book |last1=Willey |first1=Joanne M. |last2=Sherwood |first2=Linda M. |last3=Woolverton |first3=Christopher J. |year=2011 |chapter=Chapter 29: Microorganisms in Terrestrial Ecosystems |title=Prescott's Microbiology |publisher=McGraw-Hill |pages=703–706 |isbn= 978-0-07-131367-4}}</ref> The amino acids are then shuttled back to the plant with newly fixed nitrogen. [[Nitrogenase]] is an enzyme involved in nitrogen fixation and requires anaerobic conditions. Membranes within root nodules are able to provide these conditions. The rhizobacteria require oxygen to metabolize therefore oxygen is provided by a hemoglobin protein called [[leghemoglobin]] which is produced within the nodules.<ref name="Cane_2011">{{cite book |last1=Cain |first1=Michael L. |last2=Bowman |first2=William D. |last3=Hacker |first3=Sally D. |year=2011 |chapter=Chapter 16: Change in Communities |title=Ecology |publisher=Sinauer Associates |pages=359–362 |isbn=978-0-87893-445-4}}</ref> Legumes are well known nitrogen fixing crops and have been utilized for centuries in crop rotation to maintain the health of the soil.
 ==Symbiotic relationships==
-The symbiotic relationship between rhizobacteria and its host plant is not without costs.  For the plant to be able to benefit from the added available nutrients provided by the rhizobacteria, it needs to provide a place and the proper conditions for the rhizobacteria to live.  Creating and maintaining root nodules for rhizobacteria can cost between 12%-25% of the plants total photosynthetic output.  Legumes are often able to colonize early successional environments due to the unavailability of nutrients.  Once colonized though the rhizobacteria make the soil surrounding the plant more nutrient rich, which in turn can lead to competition with other plants.  The symbiotic relationship in short can lead to increased competition.<ref name="Cane_2011"/>
+The symbiotic relationship between rhizobacteria and its host plant is not without costs. For the plant to be able to benefit from the added available nutrients provided by the rhizobacteria, it needs to provide a place and the proper conditions for the rhizobacteria to live. Creating and maintaining root nodules for rhizobacteria can cost between 12%-25% of the plants total photosynthetic output. Legumes are often able to colonize early successional environments due to the unavailability of nutrients. Once colonized though the rhizobacteria make the soil surrounding the plant more nutrient rich, which in turn can lead to competition with other plants. The symbiotic relationship in short can lead to increased competition.<ref name="Cane_2011"/>
-PGPRs increase the availability of nutrients through the solubilization of unavailable forms of nutrients and by the production of [[siderophores]] which aids in the facilitating of nutrient transport.  [[Phosphorus]], a limiting nutrient for plant growth, can be plentiful in soil but is most commonly found in insoluble forms.  Organic acids and phosphotases released by rhizobacteria found in plant rhizospheres facilitate the conversion of insoluble forms of phosphorus to plant available forms such as H<sub>2</sub>PO<sub>4</sub><sup>-</sup>.  PGPR bacteria include ''[[Pseudomonas putida]]'', ''Azospirillum fluorescens'', and ''Azospirillum lipoferum'' and notable nitrogen fixing bacteria associated with legumes includes ''Allorhizobium'', ''Azorhizobium'', ''Bradyrhizobium'', and ''Rhizobium''.<ref name="Willey_2011"/>
+PGPRs increase the availability of nutrients through the solubilization of unavailable forms of nutrients and by the production of [[siderophores]] which aids in the facilitating of nutrient transport. [[Phosphorus]], a limiting nutrient for plant growth, can be plentiful in soil but is most commonly found in insoluble forms. Organic acids and phosphotases released by rhizobacteria found in plant rhizospheres facilitate the conversion of insoluble forms of phosphorus to plant available forms such as H<sub>2</sub>PO<sub>4</sub><sup>-</sup>. PGPR bacteria include ''[[Pseudomonas putida]]'', ''Azospirillum fluorescens'', and ''Azospirillum lipoferum'' and notable nitrogen fixing bacteria associated with legumes includes ''Allorhizobium'', ''Azorhizobium'', ''Bradyrhizobium'', and ''Rhizobium''.<ref name="Willey_2011"/>
 Though [[Soil inoculant|microbial inoculants]] can be beneficial for crops, they are not widely used in [[industrial agriculture]], as large-scale application techniques have yet to become economically viable. A notable exception is the use of rhizobial inoculants for [[legumes]] such as peas. Inoculation with PGPRs ensure efficient nitrogen fixation, and they have been employed in North American agriculture for over 100 years.
 ==Plant growth-promoting rhizobacteria==
-'''Plant growth-promoting rhizobacteria''' '''''(PGPR)''''' was first defined by Kloepper and Schroth <ref>Kloepper, J. W., and Schroth, M. N. 1978. Plant growth-promoting rhizobacteria on radishes. Pages 879-882 in: Proc. of the 4th Internat. Conf. on Plant Pathogenic Bacter. Vol. 2, Station de Pathologie Vegetale et Phytobacteriologie, INRA, Angers, France.</ref> to describe soil [[bacteria]] that colonize the roots of plants following inoculation onto [[seed]] and that enhance [[plant growth]]. The following are implicit in the colonization process: ability to survive inoculation onto seed, to multiply in the spermosphere (region surrounding the seed) in response to seed [[exudate]]s, to attach to the root surface, and to colonize the developing [[root system]].<ref name="Kloepper, J. W 1993. Pages 255-274">Kloepper, J. W. 1993. Plant growth-promoting rhizobacteria as biological control agents. Pages 255-274 in: Soil Microbial Ecology: Applications in Agricultural and Environmental Management. F. B. Metting, Jr., ed. Marcel Dekker Inc., New York, USA.</ref> The ineffectiveness of PGPR in the field has often been attributed to their inability to colonize plant roots.<ref>Benizri, E., Baudoin, E., and Guckert, A. 2001. Root colonization by inoculated plant growth promoting rhizobacteria. Biocontrol Sci. Technol. 11:557-574.</ref><ref name="Bloemberg, G. V. 2001">Bloemberg, G. V., and Lugtenberg, B. J. J. 2001. Molecular basis of plant growth promotion and biocontrol by rhizobacteria. Curr. Opin. Plant Biol. 4:343-350.</ref> A variety of bacterial traits and specific [[gene]]s contribute to this process, but only a few have been identified. These include [[motility]], [[chemotaxis]] to seed and [[Rhizosphere#Secretions|root exudates]], production of [[Pilus|pili]] or [[fimbria (bacteriology)|fimbriae]], production of specific cell surface components, ability to use specific components of root exudates, protein secretion, and [[quorum sensing]]. The generation of [[mutant]]s altered in expression of these traits is aiding our understanding of the precise role each one plays in the colonization process.<ref>Lugtenberg, B. J. J., Dekkers, L., and Bloemberg, G. V. 2001. Molecular determinants of rhizosphere colonization by Pseudomonas. Ann. Rev. Phytopathol. 38:461-490.</ref><ref>Persello-Cartieaux, F., Nussaume, L., and Robaglia, C. 2003. Tales from the underground: Molecular plant-rhizobacteria interactions. Plant Cell Environ. 26:189-199.</ref> Progress in the identification of new, previously uncharacterized genes is being made using nonbiased screening strategies that rely on [[gene fusion]] technologies. These strategies employ reporter [[transposon]]s <ref>Roberts, D. P., Yucel, I., and Larkin, R. P. 1998. Genetic approaches for analysis and manipulation of rhizosphere colonization by bacterial biocontrol agents. Pages 415-431 in: Plant-Microbe interactions and Biological Control. G. J. Boland and L. D. Kuykendall, eds. Books in Soils, Plants, and the Environment, vol. 63. Marcel Dekker Inc., New York, USA.</ref> and in vitro expression technology (IVET) <ref>Rainey, P. B. 1999. Adaptation of Pseudomonas fluorescens to the plant rhizosphere. Environ. Microbiol. 1:243-257.</ref> to detect genes expressed during colonization.
+Plant growth-promoting rhizobacteria (PGPR) was first defined by Kloepper and Schroth <ref>{{cite journal |last1=Kloepper |first1=Joseph W. |last2=Schroth |first2=Milton N. |year=1978 |title=Plant growth-promoting rhizobacteria on radishes |pages=879–882 |journal=Proceedings of the 4th International Conference on Plant Pathogenic Bacteria |volume=2 |publisher=Station de Pathologie Végétale et Phytobactériologie, INRA |publicationplace=Angers, France |url=http://www.bashanfoundation.org/kloepper/kloepperradish.pdf}}</ref> to describe soil [[bacteria]] that colonize the roots of plants following inoculation onto [[seed]] and that enhance [[plant growth]]. The following are implicit in the colonization process: ability to survive inoculation onto seed, to multiply in the spermosphere (region surrounding the seed) in response to seed [[exudate]]s, to attach to the root surface, and to colonize the developing [[root system]].<ref name="Kloepper, J. W 1993. Pages 255-274">{{cite book |last1=Kloepper |first1=Joseph W. |year=1993 |chapter=Plant growth-promoting rhizobacteria as biological control agents |pages=255–274 |title=Soil Microbial Ecology: Applications in Agricultural and Environmental Management |editor1-last=Metting |editor1-first=F. Blaine, Jr. |publisher=Marcel Dekker Inc.|publicationplace=New York, USA}}</ref> The ineffectiveness of PGPR in the field has often been attributed to their inability to colonize plant roots.<ref>{{cite journal |last1=Benizri |first1=E. |last2=Baudoin |first2=E. |last3=Guckert |first3=A. |year=2001 |title=Root colonization by inoculated plant growth promoting rhizobacteria |journal=Biocontrol Science and Technology |volume=11 |issue=5 |pages=557–574}}</ref><ref name="Bloemberg, G. V. 2001">{{cite journal |last1=Bloemberg |first1=Guido V. |last2=Lugtenberg |first2=Ben J. J. |year=2001 |title=Molecular basis of plant growth promotion and biocontrol by rhizobacteria |journal=Current Opinion in Plant Biology |volume=4 |issue=4 |pages=343–350}}</ref> A variety of bacterial traits and specific [[gene]]s contribute to this process, but only a few have been identified. These include [[motility]], [[chemotaxis]] to seed and [[Rhizosphere#Secretions|root exudates]], production of [[Pilus|pili]] or [[fimbria (bacteriology)|fimbriae]], production of specific cell surface components, ability to use specific components of root exudates, protein secretion, and [[quorum sensing]]. The generation of [[mutant]]s altered in expression of these traits is aiding our understanding of the precise role each one plays in the colonization process.<ref>{{cite journal |last1=Lugtenberg |first1=Ben J. J. |last2=Dekkers |first2=Linda |last3=Bloemberg |first3=Guido V. |year=2001 |title=Molecular determinants of rhizosphere colonization by Pseudomonas |journal=Annual Review of Phytopathology |volume=39 |pages=461–490}}</ref><ref>{{cite journal |last1=Persello-Cartieaux |first1=F. |last2=Nussaume |first2=L. |last3=Robaglia |first3=C. |year=2003 |title=Tales from the underground: Molecular plant-rhizobacteria interactions |journal=Plant, Cell & Environment |volume=26 |issue=2 |pages=189–199}}</ref>
+Progress in the identification of new, previously uncharacterized genes is being made using nonbiased screening strategies that rely on [[gene fusion]] technologies. These strategies employ reporter [[transposon]]s <ref>{{cite book |last1=Roberts |first1=Daniel P. |last2=Yucel |first2=Irem |last3=Larkin |first3=Robert P. |year=1998 |chapter=Genetic approaches for analysis and manipulation of rhizosphere colonization by bacterial biocontrol agents |pages=415–431 |title=Plant-Microbe interactions and Biological Control |editor1-last=Boland |editor1-first=Greg J. |editor2-last=Kuykendall |editor2-first=L. David |series=Books in Soils, Plants, and the Environment |volume=63 |publisher=Marcel Dekker Inc. |publicationplace=New York, USA}}</ref> and in vitro expression technology (IVET) <ref>Rainey, P. B. 1999. Adaptation of Pseudomonas fluorescens to the plant rhizosphere. Environ. Microbiol. 1:243-257.</ref> to detect genes expressed during colonization.
-Using [[molecular marker]]s such as green [[fluorescent]] [[protein]] or fluorescent [[Antibody|antibodies]] it is possible to monitor the location of individual rhizobacteria on the root using [[confocal laser scanning microscopy]].<ref name="Bloemberg, G. V. 2001"/><ref>Bloemberg, G. V., Wijfjes, A. H. M., Lamers, G. E. M., Stuurman, N., and Lugtenberg, B. J. J. 2000. Simultaneous imaging of Pseudomonas fluorescens WCS365 populations expressing three different autofluorescent proteins in the rhizosphere: New perspectives for studying microbial communities. Mol. Plant- Microbe Interact. 13:1170-1176.</ref><ref>Sorensen, J., Jensen, L. E., and Nybroe, O. 2001. Soil and rhizosphere as habitats for Pseudomonas inoculants: New knowledge on distribution, activity and physiological state derived from micro-scale and single-cell studies. Plant Soil 232:97-108.</ref> This approach has also been combined with an [[rRNA]]-targeting probe to monitor the [[metabolic activity]] of a rhizobacterial strain in the [[rhizosphere]] and showed that bacteria located at the root tip were most active <ref>Lubeck, P. S., Hansen, M., and Sorensen, J. 2000. Simultaneous detection of the establishment of seed-inoculated Pseudomonas fluorescens strain DR54 and native soil bacteria on sugar beet root surfaces using fluorescence antibody and in situ hybridization techniques. FEMS Microbiol. Ecol. 33:11-19.</ref>
+Using [[molecular marker]]s such as green [[fluorescent]] [[protein]] or fluorescent [[Antibody|antibodies]] it is possible to monitor the location of individual rhizobacteria on the root using [[confocal laser scanning microscopy]].<ref name="Bloemberg, G. V. 2001"/><ref>{{cite journal |last1=Bloemberg |first1=Guido V. |last2=Wijfjes |first2=André H. M. |last3=Lamers |first3=Gerda E. M. |last4=Stuurman |first4=Nico |last5=Lugtenberg |first5=Ben J. J. |year=2000 |title=Simultaneous imaging of ''Pseudomonas fluorescens'' WCS365 populations expressing three different autofluorescent proteins in the rhizosphere: New perspectives for studying microbial communities |journal=Molecular Plant-Microbe Interactions |volume=13 |issue=11 |pages=1170–1176}}</ref><ref>{{cite journal |last1=Sørensen |first1=Jan |last2=Jensen |first2=Linda E. |last3=Nybroe |first3=Ole |year=2001 |title=Soil and rhizosphere as habitats for ''Pseudomonas'' inoculants: New knowledge on distribution, activity and physiological state derived from micro-scale and single-cell studies |journal=Plant and Soil |volume=232 |issue=1-2 |pages=97–108}}</ref> This approach has also been combined with an [[rRNA]]-targeting probe to monitor the [[metabolic activity]] of a rhizobacterial strain in the [[rhizosphere]] and showed that bacteria located at the root tip were most active <ref>{{cite journal |last1=Lübeck |first1=Peter S. |last2=Hansen |first2=Michael |last3=Sørensen |first3=Jan |year=2000 |title=Simultaneous detection of the establishment of seed-inoculated ''Pseudomonas fluorescens'' strain DR54 and native soil bacteria on sugar beet root surfaces using fluorescence antibody and in situ hybridization techniques |journal=FEMS Microbiology Ecology |volume=33 |issue=1 |pages=11–19}}</ref>
 === Mechanisms of action===
-PGPR enhance plant growth by direct and indirect means, but the specific mechanisms involved have not all been well-characterized,<ref name="Kloepper, J. W 1993. Pages 255-274"/> Direct mechanisms of plant growth promotion by PGPR can be demonstrated in the absence of plant [[pathogen]]s or other rhizosphere [[microorganism]]s, while indirect mechanisms involve the ability of PGPR to reduce the harmful effects of plant pathogens on [[crop yield]]. PGPR have been reported to directly enhance plant growth by a variety of mechanisms: fixation of atmospheric [[nitrogen]] that is transferred to the plant<ref>Zakry, F.A.A., Shamsuddin, Z.H., Khairuddin, A.R., Zakaria, Z.Z., and Anuar, A.R. (2012). Inoculation of Bacillus sphaericus UPMB-10 to young oil palm and measurement of its uptake of fixed nitrogen using the 15N isotope dilution technique. Microbes and Environments 27(3): 257-262.</ref>, production of [[siderophore]]s that [[Chelation|chelate]] [[iron]] and make it available to the plant root, [[solubilization]] of minerals such as [[phosphorus]], and synthesis of [[phytohormone]]s. Direct enhancement of [[mineral uptake]] due to increases in specific ion fluxes at the root surface in the presence of PGPR has also been reported . PGPR strains may use one or more of these mechanisms in the rhizosphere. Molecular approaches using microbial and plant mutants altered in their ability to synthesize or respond to specific phytohormones have increased our understanding of the role of phytohormone synthesis as a direct mechanism of plant growth enhancement by PGPR <ref>Glick, B. R. 1995. The enhancement of plant growth by free-living bacteria. Can. J. Microbiol. 41:109-117.</ref> PGPR that synthesize [[auxin]]s and [[cytokinin]]s or that interfere with plant [[ethylene]] synthesis have been identified.
+PGPR enhance plant growth by direct and indirect means, but the specific mechanisms involved have not all been well-characterized,<ref name="Kloepper, J. W 1993. Pages 255-274"/> Direct mechanisms of plant growth promotion by PGPR can be demonstrated in the absence of plant [[pathogen]]s or other rhizosphere [[microorganism]]s, while indirect mechanisms involve the ability of PGPR to reduce the harmful effects of plant pathogens on [[crop yield]]. PGPR have been reported to directly enhance plant growth by a variety of mechanisms: fixation of atmospheric [[nitrogen]] that is transferred to the plant<ref>
+{{cite journal |last1=Zakry |first1=F.A.A. |last2=Shamsuddin |first2=Z.H. |last3=Khairuddin |first3=A.R. |last4=Zakaria |first4=Z.Z. |last5=Anuar |first5=A.R. |year=2012 |title=Inoculation of ''Bacillus sphaericus'' UPMB-10 to young oil palm and measurement of its uptake of fixed nitrogen using the <sup>15</sup>N isotope dilution technique |journal=Microbes and Environments |volume=27 |issue=3 |pages=257–262|doi=10.1264/jsme2.ME11309}}
+</ref>, production of [[siderophore]]s that [[Chelation|chelate]] [[iron]] and make it available to the plant root, [[solubilization]] of minerals such as [[phosphorus]], and synthesis of [[phytohormone]]s. Direct enhancement of [[mineral uptake]] due to increases in specific ion fluxes at the root surface in the presence of PGPR has also been reported . PGPR strains may use one or more of these mechanisms in the rhizosphere. Molecular approaches using microbial and plant mutants altered in their ability to synthesize or respond to specific phytohormones have increased our understanding of the role of phytohormone synthesis as a direct mechanism of plant growth enhancement by PGPR <ref>{{cite journal |last1=Glick |first1=Bernard R. |year=1995 |title=The enhancement of plant growth by free-living bacteria |journal=Canadian Journal of Microbiology |volume=41 |issue=2 |pages=109–117}}</ref> PGPR that synthesize [[auxin]]s and [[cytokinin]]s or that interfere with plant [[ethylene]] synthesis have been identified.
 ==Pathogenic roles==
-Studies conducted on [[sugar beet]] crops found that some root-colonizing bacteria were deleterious rhizobacteria (DRB).  It was found that sugar beet seeds inoculated with DRB had reduced germination rates, root lesions, reduced root elongation, root distortions, increased fungi infection and decreased plant growth.  In one trial the sugar beet yield was reduced by 48%.<ref name="Suslow_1982">Suslow, TV., Schroth, MN. (January 1982). "Role of Deleterious Rhizobacteria as Minor Pathogens in Reducing Crop Growth". Phytopathology 72 (1): 111-115.</ref>
+Studies conducted on [[sugar beet]] crops found that some root-colonizing bacteria were deleterious rhizobacteria (DRB). It was found that sugar beet seeds inoculated with DRB had reduced germination rates, root lesions, reduced root elongation, root distortions, increased fungi infection and decreased plant growth. In one trial the sugar beet yield was reduced by 48%.<ref name="Suslow_1982">{{cite journal |last1=Suslow |first1=Trevor V. |last2=Schroth |first2=Milton N. |date=January 1982 |title=Role of Deleterious Rhizobacteria as Minor Pathogens in Reducing Crop Growth |journal=Phytopathology |volume=72 |issue=1 |pages=111–115}}</ref>
-Six strains of rhizobacteria have been identified as being DRB.  The strains are in the genera ''[[Enterobacter]]'', ''[[Klebsiella]]'', ''[[Citrobacter]]'', ''[[Flavobacterium]]'', ''[[Achromobacter]]'', ''[[Arthrobacter]]''.  Due to a large number of taxonomic species yet to be described complete characterization has not been possible as DRB are highly variable.<ref name="Suslow_1982"/>
+Six strains of rhizobacteria have been identified as being DRB. The strains are in the genera ''[[Enterobacter]]'', ''[[Klebsiella]]'', ''[[Citrobacter]]'', ''[[Flavobacterium]]'', ''[[Achromobacter]]'', ''[[Arthrobacter]]''. Due to a large number of taxonomic species yet to be described complete characterization has not been possible as DRB are highly variable.<ref name="Suslow_1982"/>
-The presence of PGPRs has proven to reduce and inhibit the colonization of DRB on sugar beet roots.  Plots inoculated with PGPRs and DRBs had an increase in production of 39% while plots only treated with DRbs had a reduction in production of 30%.<ref name="Suslow_1982"/>
+The presence of PGPRs has proven to reduce and inhibit the colonization of DRB on sugar beet roots. Plots inoculated with PGPRs and DRBs had an increase in production of 39% while plots only treated with DRbs had a reduction in production of 30%.<ref name="Suslow_1982"/>
 ==Biocontrol==
-Rhizobacteria are also able to control plant diseases that are caused by other bacteria and fungi.  Disease is suppressed through induced systematic resistance and through the production of anti-fungal metabolites.  ''Pseudomonas'' [[Biocontrol]] strains have been genetically modified to improve plant growth and improve the disease resistance of agricultural crops.  In agriculture, [[inoculant]] bacteria are often applied to the seed coat of seeds prior to being sown.  Inoculated seeds are more likely to establish large enough rhizobacteria populations within the rhizosphere to produce notable beneficial effects on the crop.<ref name="Vessy_2003"/>
+Rhizobacteria are also able to control plant diseases that are caused by other bacteria and fungi. Disease is suppressed through induced systematic resistance and through the production of anti-fungal metabolites. ''Pseudomonas'' [[Biocontrol]] strains have been genetically modified to improve plant growth and improve the disease resistance of agricultural crops. In agriculture, [[inoculant]] bacteria are often applied to the seed coat of seeds prior to being sown. Inoculated seeds are more likely to establish large enough rhizobacteria populations within the rhizosphere to produce notable beneficial effects on the crop.<ref name="Vessy_2003"/>
 == References ==