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The rhizosphere is a narrow region of soil that is directly influenced by root secretions and associated microbial activity [[#References|[7]]]. Plant growth-promoting bacteria (PGPB) occupy the rhizosphere of many plant species and have beneficial effects on the host plant. They may influence the plant in a direct or indirect manner. A direct mechanism would be to increase plant growth by supplying the plant with nutrients and hormones; Indirect mechanisms on the otherhand, include, reduced susceptibility to diseases, and activating a form of defense referred to as induced systematic resistance (ISR) [[#References|[5]]]. Examples of bacteria which have been found to enhance plant growth, include [https://microbewiki.kenyon.edu/index.php/Pseudomonas Pseudomonas], [https://microbewiki.kenyon.edu/index.php/Enterobacter Enterobacter], and [https://microbewiki.kenyon.edu/index.php/Arthrobacter Arthrobacter] [[#References|[6]]].
[[File:Rhizo.jpg|thumb|600px|right|Rhizosphere bacteria on a root. Source: [http://www.indiana.edu/~cres1/biofuel.shtml Microbial Ecology of Sustainable Biofuel Production] ]]
The rhizosphere is a narrow region of soil that is directly influenced by root secretions and associated microbial activity [[#References|[7]]]. Plant growth-promoting bacteria (PGPB) occupy the rhizosphere of many plant species and have beneficial effects on the host plant. They may influence the plant in a direct or indirect manner. A direct mechanism would be to increase plant growth by supplying the plant with nutrients and hormones; Indirect mechanisms on the otherhand, include, reduced susceptibility to diseases, and activating a form of defense referred to as induced systematic resistance (ISR) [[#References|[5]]]. Examples of bacteria which have been found to enhance plant growth, include <i>[https://microbewiki.kenyon.edu/index.php/Pseudomonas Pseudomonas]</i>, <i>[https://microbewiki.kenyon.edu/index.php/Enterobacter Enterobacter]</i>, and <i>[https://microbewiki.kenyon.edu/index.php/Arthrobacter Arthrobacter]</i> [[#References|[6]]].


=Plant Growth Promotion=
=Plant Growth Promotion=


[[File:Root.jpg|thumb|400px|left|Longitudinal cross section of a root and the surrounding rhizosphere. ]]
==Increase Supply of Nutrients==
==Increase Supply of Nutrients==
 
Rhizobacteria have the ability to enhance plant growth in the absence of potentially pathogenic microorganisms. One way in which they can enhance plant growth is by solubilizing normally poorly soluble nutrients with either bacteria [http://en.wikipedia.org/wiki/Siderophores siderophores] or lowering the pH by secreting acidic organic compounds [[#References|[3]]]. Phosphorous is a major macronutrient needed for plants, but is not easily up taken due to it’s reactive nature with iron, aluminum, and calcium; these common reactions result in the precipitation of phosphorous, thus making it unavailable to plants [[#References|[5]]]. Some PGPR can convert phosphorous into a more plant attainable form, such as to orthophosphate [[#References|[6]]]. Iron is also another essential nutrient, but it is scarce in soil. PGPB, can produce compounds called siderophores, which acquire ferric iron (Fe3+), root cells can then take this up by active transport mechanisms [[#References|[6]]].
[[File:Rhizosphere-Bacteria.jpg|thumb|350px|right|]]
Rhizobacteria have the ability to enhance plant growth in the absence of potentially pathogenic microorganisms. One way in which they can enhance plant growth is by solubilizing normally poorly soluble nutrients with either bacteria siderophores or lowering the pH by secreting acidic organic compounds.3 Phosphorous is a major macronutrient needed for plants, but is not easily up taken due to it’s reactive nature with iron, aluminum, and calcium; these common reactions result in the precipitation of phosphorous, thus making it unavailable to plants.5 Some PGPR can convert phosphorous into a more plant attainable form, such as to orthophosphate.6 Iron is also another essential nutrient, but it is scarce in soil. PGPB, can produce compounds called siderophores, which acquire ferric iron (Fe3+), root cells can then take this up by active transport mechanisms.6


==Phytohormones==
==Phytohormones==
[[File:Rhizosphere-Bacteria.jpg|thumb|400px|right|The left side is an example of a root without PGPB and the right side is an example of what the root would be like with PGPB. Source: [http://www.ganeshtree.com/services-provided/understanding-the-science/ Ganesh Tree and Plant Health Care] ]]
Plant growth promotion can also be regulated by the production of hormones and other compounds related to plant development. [http://en.wikipedia.org/wiki/Auxin Auxin] is a class of plant hormones important in the promotion of lateral root formation. Increased lateral root formation leads to an enhanced ability to take up nutrients for the plant [[#References|[3]]]. Other classes of plant hormones includes [http://en.wikipedia.org/wiki/Gibberellins Gibberellins] and [http://en.wikipedia.org/wiki/Cytokinins Cytokinins], which both stimulate shoot development, however their effects on root growth are less well researched [[#References|[3]]].


==Ethylene Levels==
==Ethylene Levels==
[http://en.wikipedia.org/wiki/Ethylene Ethylene] in low levels has been observed to promote growth, but at moderate to high levels it may inhibit root elongation. In plants, 1-aminocyclopropane-1-carboxylate (ACC) and 5’-deoxy-5’methylthioadenosine (MTA) is converted to ACC by ACC synthase [[#References|[9]]]. A number of plant growth-promoting bacteria have been found to contain the enzyme ACC deaminase, which cleaves and sequesters the plant ethylene precursor ACC and thus lowers the level of ethylene in a developing or stressed plant [[#References|[1]]]. The presence of plant growth-promoting bacteria thereby moderates concentration of ACC so that it does not reach a level where it begins to impair root growth.
[[File:Schematic Represenation of plant growth.gif|thumb|400px|left|Plant growth-promoting mechanisms from rhizobacteria. Source: [http://informahealthcare.com/doi/abs/10.1080/07388550902913772?journalCode=bty&& Rajkumar, M. et al. (2009)] ]]


==Nitrogen Fixation==
==Nitrogen Fixation==
Nitrogen availability has become one of the yield-limiting factors in plant growth due to rainfall and mineral leaching into ground water [[#Reference|[8]]]. There are a number of PGPB, which are able to fix atmosphere nitrogen (N<sub>2</sub>) and make it more accessible to plants. Although PGPB have the ability to fix nitrogen, they are not able to provide a sufficient amount to sustain the plants. Due to their effect on shoot elongation and stimulation of nitrate (NO<sub>3</sub><sup>-</sup>) transport systems, they are able to greatly increase the intake of nitrogen by the plants, despite not fixing enough nitrogen on its own for sustanence [[#Reference|[8]]].


[http://en.wikipedia.org/wiki/Groundwater Groundwater] serves as water for more than 50% of people living in North America therefore a significant public resource. To date, major contamination of groundwater in North America are due to the release and use of chlorinated ethenes by industry. Examples of such toxic compounds are perchloroethene (PCE), trichloroethene (TCE). Carbon tetrachloride (CT) is also a major groundwater pollutant [[#References|[4]]]. These compounds were widely used as solvents for dry cleaning and in textile manufacturing. They are sufficiently water soluble and can travel through soil where they reach the groundwater. The relative high concentration of them here can be harmful [[#References|[6]]].
=Induced Systemic Resistance=
Ground water is also contaminated by pollutants that are not highly toxic, but can be utilized or modified by microorganisms to become more toxic. For instance over-fertilization in agriculture leads to an increased nitrate concentration which i.e. can cause the Blue Baby syndrome. This is seen in infants younger than six month old who rely on bacteria to digest their food. Some of these bacteria also convert nitrate, a component of fertilizer, to nitrite. In the blood nitrite reacts with hemoglobin interfering with its ability to carry oxygen. The babies show sign of suffocation and gets a bluish skin [[#References|[2]]].
[[File:Nrmico1129-f1.gif|thumb|400px|right|Interactions between biocontrol plant growth-promoting rhizobacteria (PGPR), plants, pathogens and soil. Source: [http://www.nature.com/nrmicro/journal/v3/n4/full/nrmicro1129.html Haas, D. & Defago, G. (2005)] ]]
 
=Microbial metabolism of groundwater pollutants=
 
==Co-metabolism and degradation of TCE==
 
[[File:Aerobic degradation of TCE.jpg|thumb|400px|right|]]
 
Some dehalorespiring organisms are capable of degrading PCE, TCE and CT into non-toxic compounds. Degradation of PCE is only known to happen through reductive dechlorination and only under anaerobic condition.
TCE is, unlike PCE, able to be degraded under aerobic conditions. This can happen through [http://en.wikipedia.org/wiki/Cometabolism cometabolism]. In co-metabolism a compound is transformed by an organism that doesn’t use the compound as an energy or carbon source and reducing power is not provided. The organism relies on another compound to serve as an energy and carbon source [[#References|[3]]] . Methanotrophic organisms grow on methane as a primary substrate and oxygen but some are also able to degrade TCE as a secondary substrate. This is because of nonspecific enzymatic activity of enzymes (methane monooxygenase, MMO) involved in degradation of the primary substrate. The degradation of TCE serves no beneficial purpose for these organisms. It generates an [http://en.wikipedia.org/wiki/Epoxide epoxide](cf. figure 1) which is transported out of the cell and here other heterotrophic organisms bring about the transformation into non-toxic compounds resulting in the formation of CO2. Several factors inhibit the aerobic degradation of TCE here among the concentration of contamination, the pH and the temperature. Because both TCE and methane bind to the same site in MMO competition between growth substrate and non-growth substrate also seems to limit degradation of TCE [[#References|[3]]].
 
==Dehalogenation==
 
[[File:PDTC complex.gif|thumb|400px|right|]]
 
The bacterium [http://en.wikipedia.org/wiki/Pseudomonas_stutzeri Pseudomonas stutzeri strain KC] can dehalogenate CT into carbon dioxide and chlorine without producing the toxic intermediate chloroform (CCl3H).  This bacterium is originally isolated from an aquifer in Seal Beach in California. It is dependent on anaerobic conditions and in iron-limited media this bacterium produces and secretes a [http://groups.molbiosci.northwestern.edu/holmgren/Glossary/Definitions/Def-C/chelator.html chelator] called pyridine-2,6 (bis)thiocarboxylate (PDTC cf. figure 2.) [[#References|[5]]]. When PDTC is in contact with a broad range of cell components it turns into a reduced form (the iron in the complex is reduced) and this is essential for its extracellular activity. PDCT has to be in a complex with copper in order for the fast turnover rate of CT into CO2.  This complex functions both as a reactant and a catalyst in the reaction. When Pseudomonas stutzeri is in environments were nitrate is present as the electron acceptor a more rapid production of PDTC is observed [[#References|[6]]].
 
==Denitrification==
In many agricultural areas in North America the nitrate concentrations exceed the standards. [http://en.wikipedia.org/wiki/Denitrifying_bacteria Denitrifying organisms] are capable of using nitrate or nitrite as terminal electron acceptors thereby removing the excess of nitrogen from the environment. The organism Methylomirabilis oxyfera is an example of such an organism. This denitrifying bacterium is special in that it doesn’t have the gene encoding nitrous oxide reductase, the protein that converts N2O to N2. Instead they harbor an operon which encodes the complete methane monooxygenase complex. This enables it to oxide methane in an aerobic pathway [[#References|[1]]]. The mechanism takes advances of the oxidation of methane to drive denitrification. They do so by producing oxygen from nitrite via nitrite oxide (thereby bypassing the intermediate nitrous oxide) and then use this oxygen to oxide methane in an anaerobic environment. This is called nitrite dependent anaerobic methane oxidation. The overall redox reaction is 3CH4 + 8NO2- + 8H+ -> 3CO2 + 4N2 + 10 H2O. In this way the organism uses the potent greenhouse gas methane and reduces nitrite thereby contributing to the removal of excess N-compounds in groundwater [[#References|[1]]].


=Treatment technologies=
PGPB are able to control the number of pathogenic bacteria through microbial antagonism, which is achieved by competing with the pathogens for nutrients, producing antibiotics, and the production of anti-fungal metabolites [[#References|[7]]]. Besides antagonism, certain bacteria-plant interactions can induce mechanisms in which the plant can better defend itself against pathogenic bacteria, fungi and viruses [[#References|[2]]]. This is known as induced systemic resistance (ISR) and was first discovered in 1991 by Van Peer et al [[#References|[3]]]. The inducing rhizobacteria triggers a reaction in the roots that creates a signal that spreads throughout the plant which results in the activation of defense mechanisms, such as, reinforcement of plant cell wall, production of anti-microbial phytoalexins, and the synthesis of pathogen related proteins [[#References|[3]]]. Components of bacteria that can activate ISR includes lipopolysaccharides (LPS), flagella, salicylic acid, and sideophores [[#References|[2]]].


[[File:Pumpandtreat.gif|thumb|400px|right|]]
=Biofertilizer=
Agriculturally, the beneficial bacteria can be used as inoculants for crops and plants [[#References|[4]]]. They are given the term [http://en.wikipedia.org/wiki/Biofertilizer biofertilizer], which is a substance that contains living microorganisms and when they are applied to seeds, plant surfaces, or soil, it promotes growth by increasing the supply or availability of primary nutrients to the host plant [[#References|[4]]]. Biofertilizer is different from organic fertilizers, which contains organic compounds that increase soil fertility either directly or as a result of their decay. Not all plant-growth promoting bacteria are considered a biofertilizer; if they control plant growth by control of deleterious organisms, they are instead regarded as biopesticides. Biofertilizers must contain living microoganisms that promote plant growth by improving the nutrient status of the plant.


Contamination of groundwater can lead to severe health problems and environmental changes if left untreated. Bioaugmentation is a widespread biological technique used in the removal of chlorinated compounds. By introducing natural electron donors that are helpful in the removal of halogenated compounds into the groundwater the growth of dehalorespiring organisms can be favored. Optional conditions for dehalogenation are provided without any engineering steps taken [[#References|[7]]].  
=Importance=
Pump and treat method is also one of the most used groundwater remediation techniques. Removal of contaminated groundwater from soil with the use of pumps followed by subsequent remediation at the surface helps overcome the persistence of the pollutants (cf. figure 3). It is typically biological or chemical treatments that remove the pollutants. This method is costly and slow however and some contaminants cannot be removed because they stick to soil and rocks or are not sufficient water soluble [[#References|[4]]].
PGPB has become increasingly important in the agricultural production of certain crops. However the commercialization and utilization of PGPB has been currently limited due to the fact that there have not been consistent responses in different host cultivars and at different field sites [[#References|[4]]]. Additionally, their effects have been used in environmental application, such as promoting re-vegetation in eroded deserts [[#References|[10]]]. Although the use of plant growth-promoting bacteria in agriculture and solving environmental problems seems promising, there is not enough knowledge about these bacteria for them to be put into use. A lot more research needs to be done before they can be proven useful to mankind.


=References=
=References=

Latest revision as of 15:24, 2 October 2015

This student page has not been curated.
Rhizosphere bacteria on a root. Source: Microbial Ecology of Sustainable Biofuel Production

The rhizosphere is a narrow region of soil that is directly influenced by root secretions and associated microbial activity [7]. Plant growth-promoting bacteria (PGPB) occupy the rhizosphere of many plant species and have beneficial effects on the host plant. They may influence the plant in a direct or indirect manner. A direct mechanism would be to increase plant growth by supplying the plant with nutrients and hormones; Indirect mechanisms on the otherhand, include, reduced susceptibility to diseases, and activating a form of defense referred to as induced systematic resistance (ISR) [5]. Examples of bacteria which have been found to enhance plant growth, include Pseudomonas, Enterobacter, and Arthrobacter [6].

Plant Growth Promotion

Longitudinal cross section of a root and the surrounding rhizosphere.

Increase Supply of Nutrients

Rhizobacteria have the ability to enhance plant growth in the absence of potentially pathogenic microorganisms. One way in which they can enhance plant growth is by solubilizing normally poorly soluble nutrients with either bacteria siderophores or lowering the pH by secreting acidic organic compounds [3]. Phosphorous is a major macronutrient needed for plants, but is not easily up taken due to it’s reactive nature with iron, aluminum, and calcium; these common reactions result in the precipitation of phosphorous, thus making it unavailable to plants [5]. Some PGPR can convert phosphorous into a more plant attainable form, such as to orthophosphate [6]. Iron is also another essential nutrient, but it is scarce in soil. PGPB, can produce compounds called siderophores, which acquire ferric iron (Fe3+), root cells can then take this up by active transport mechanisms [6].

Phytohormones

The left side is an example of a root without PGPB and the right side is an example of what the root would be like with PGPB. Source: Ganesh Tree and Plant Health Care

Plant growth promotion can also be regulated by the production of hormones and other compounds related to plant development. Auxin is a class of plant hormones important in the promotion of lateral root formation. Increased lateral root formation leads to an enhanced ability to take up nutrients for the plant [3]. Other classes of plant hormones includes Gibberellins and Cytokinins, which both stimulate shoot development, however their effects on root growth are less well researched [3].

Ethylene Levels

Ethylene in low levels has been observed to promote growth, but at moderate to high levels it may inhibit root elongation. In plants, 1-aminocyclopropane-1-carboxylate (ACC) and 5’-deoxy-5’methylthioadenosine (MTA) is converted to ACC by ACC synthase [9]. A number of plant growth-promoting bacteria have been found to contain the enzyme ACC deaminase, which cleaves and sequesters the plant ethylene precursor ACC and thus lowers the level of ethylene in a developing or stressed plant [1]. The presence of plant growth-promoting bacteria thereby moderates concentration of ACC so that it does not reach a level where it begins to impair root growth.

Plant growth-promoting mechanisms from rhizobacteria. Source: Rajkumar, M. et al. (2009)

Nitrogen Fixation

Nitrogen availability has become one of the yield-limiting factors in plant growth due to rainfall and mineral leaching into ground water [8]. There are a number of PGPB, which are able to fix atmosphere nitrogen (N2) and make it more accessible to plants. Although PGPB have the ability to fix nitrogen, they are not able to provide a sufficient amount to sustain the plants. Due to their effect on shoot elongation and stimulation of nitrate (NO3-) transport systems, they are able to greatly increase the intake of nitrogen by the plants, despite not fixing enough nitrogen on its own for sustanence [8].

Induced Systemic Resistance

Interactions between biocontrol plant growth-promoting rhizobacteria (PGPR), plants, pathogens and soil. Source: Haas, D. & Defago, G. (2005)

PGPB are able to control the number of pathogenic bacteria through microbial antagonism, which is achieved by competing with the pathogens for nutrients, producing antibiotics, and the production of anti-fungal metabolites [7]. Besides antagonism, certain bacteria-plant interactions can induce mechanisms in which the plant can better defend itself against pathogenic bacteria, fungi and viruses [2]. This is known as induced systemic resistance (ISR) and was first discovered in 1991 by Van Peer et al [3]. The inducing rhizobacteria triggers a reaction in the roots that creates a signal that spreads throughout the plant which results in the activation of defense mechanisms, such as, reinforcement of plant cell wall, production of anti-microbial phytoalexins, and the synthesis of pathogen related proteins [3]. Components of bacteria that can activate ISR includes lipopolysaccharides (LPS), flagella, salicylic acid, and sideophores [2].

Biofertilizer

Agriculturally, the beneficial bacteria can be used as inoculants for crops and plants [4]. They are given the term biofertilizer, which is a substance that contains living microorganisms and when they are applied to seeds, plant surfaces, or soil, it promotes growth by increasing the supply or availability of primary nutrients to the host plant [4]. Biofertilizer is different from organic fertilizers, which contains organic compounds that increase soil fertility either directly or as a result of their decay. Not all plant-growth promoting bacteria are considered a biofertilizer; if they control plant growth by control of deleterious organisms, they are instead regarded as biopesticides. Biofertilizers must contain living microoganisms that promote plant growth by improving the nutrient status of the plant.

Importance

PGPB has become increasingly important in the agricultural production of certain crops. However the commercialization and utilization of PGPB has been currently limited due to the fact that there have not been consistent responses in different host cultivars and at different field sites [4]. Additionally, their effects have been used in environmental application, such as promoting re-vegetation in eroded deserts [10]. Although the use of plant growth-promoting bacteria in agriculture and solving environmental problems seems promising, there is not enough knowledge about these bacteria for them to be put into use. A lot more research needs to be done before they can be proven useful to mankind.

References

[1] Glick, B. R. “Modulation of Plant Ethylene Levels by the Bacterial Enzyme ACC Deaminase.” FEMS Microbiology Letters, 2006, DOI: 10.1016/j.femsle.2005.07.030

[2] Lugtenberg, B. and Kamilova, F. “Plant-Growth-Promoting Rhizobacteria.” Annual Review of Microbiology, 2009, DOI: 10.1146/annurev.micro.62.081307.162918

[3] Van Loon, L. C. “Plant Responses to Plant Growth-Promoting Rhizobacteria.” European Journal of Plant Pathology, 2007, DOI: 1007/s10658-007-9165-1

[4] Vessey, J. K. “Plant Growth Promoting Rhizobacteria as Biofertilizers.” Plant and Soil, 2003, DOI: 10.1023/A:1026037216893

[5] Yang, J., Kloepper, J., and Ryu, C. “Rhizosphere Bacteria Helps Plants Tolerate Abiotic Stress.” Trends in Plant Science, 2009, DOI: 10.1016/j.tplants.2008.10.004

[6] Saharan, B. S. and Nehra, V. “Plant Growth Promoting Rhizobacteria: A Critical Review.” Life Sciences and Medicine Research, 2011

[7] Bloemberg, G. V. and Lugtenberg, B. “Molecular Basis of Plant Growth Promotion and Biocontrol by Rhizobacteria.” Current Opinion in Plant Biology, 2001, DOI: 10.1016/S1369-5266(00)00183-7

[8] Mantelin, S. and Touraine, B. “ Plant Growth-Promoting Bacteria and Nitrate Availability: Impacts on Root Development and Nitrate Uptake.” Journal of Experimental Botany, 2003, DOI: 10.1093/jxb/erh010

[9] Glick, B., Cheng, Z., Czarny, J., and Duan, J. “Promotion of Plant Growth by ACC Deaminase-Producing Soil Bacteria.” European Journal of Plant Pathology, 2007, DOI: 10.1007/s10658-007-9162-4

[10] Bashan, Y., Puente, M., de-Bashan, L., and Hernandez, J. P. “Environmental Uses of Plant Growth-Promoting Bacteria.” Plant-Microbe Interactions, 2008

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