Plant-growth promoting rhizobacteria
Encyclopedia
Plant growth-promoting rhizobacteria (PGPR) was first defined by Kloepper and Schroth 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
. The ineffectiveness of PGPR in the field has often been attributed to their inability to colonize plant roots,. 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 root exudates, production of pili
or fimbria
e, 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,. 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 and in vitro expression technology (IVET) 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
,,. 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
s or other rhizosphere microorganism
s, while indirect mechanisms involve the ability of PGPR to reduce the deleterious 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, production of siderophore
s 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 PGPR that synthesize auxin
s and cytokinin
s or that interfere with plant ethylene
synthesis have been identified.
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
that colonize the roots of plants following inoculation onto seed
Seed
A seed is a small embryonic plant enclosed in a covering called the seed coat, usually with some stored food. It is the product of the ripened ovule of gymnosperm and angiosperm plants which occurs after fertilization and some growth within the mother plant...
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
Exudate
An exudate is any fluid that filters from the circulatory system into lesions or areas of inflammation. It can apply to plants as well as animals. Its composition varies but generally includes water and the dissolved solutes of the main circulatory fluid such as sap or blood...
s, to attach to the root surface, and to colonize the developing root system
Root system
In mathematics, a root system is a configuration of vectors in a Euclidean space satisfying certain geometrical properties. The concept is fundamental in the theory of Lie groups and Lie algebras...
. The ineffectiveness of PGPR in the field has often been attributed to their inability to colonize plant roots,. A variety of bacterial traits and specific gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
s contribute to this process, but only a few have been identified. These include motility
Motility
Motility is a biological term which refers to the ability to move spontaneously and actively, consuming energy in the process. Most animals are motile but the term applies to single-celled and simple multicellular organisms, as well as to some mechanisms of fluid flow in multicellular organs, in...
, chemotaxis
Chemotaxis
Chemotaxis is the phenomenon in which somatic cells, bacteria, and other single-cell or multicellular organisms direct their movements according to certain chemicals in their environment. This is important for bacteria to find food by swimming towards the highest concentration of food molecules,...
to seed and root exudates, production of pili
Pilus
right|thumb|350px|Schematic drawing of bacterial conjugation. 1- Donor cell produces pilus. 2- Pilus attaches to recipient cell, brings the two cells together. 3- The mobile plasmid is nicked and a single strand of DNA is then transferred to the recipient cell...
or fimbria
Fimbria
A fimbria is a Latin word that literally means "fringe." It is commonly used in science and medicine, with its meaning depending on the field of study or the context...
e, production of specific cell surface components, ability to use specific components of root exudates, protein secretion, and quorum sensing
Quorum sensing
Quorum sensing is a system of stimulus and response correlated to population density. Many species of bacteria use quorum sensing to coordinate gene expression according to the density of their local population. In similar fashion, some social insects use quorum sensing to determine where to nest...
. The generation of mutant
Mutant
In biology and especially genetics, a mutant is an individual, organism, or new genetic character, arising or resulting from an instance of mutation, which is a base-pair sequence change within the DNA of a gene or chromosome of an organism resulting in the creation of a new character or trait not...
s altered in expression of these traits is aiding our understanding of the precise role each one plays in the colonization process,. 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
Transposon
Transposable elements are sequences of DNA that can move or transpose themselves to new positions within the genome of a single cell. The mechanism of transposition can be either "copy and paste" or "cut and paste". Transposition can create phenotypically significant mutations and alter the cell's...
s and in vitro expression technology (IVET) to detect genes expressed during colonization.
Using molecular markers such as green fluorescent protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
or fluorescent antibodies
Antibody
An antibody, also known as an immunoglobulin, is a large Y-shaped protein used by the immune system to identify and neutralize foreign objects such as bacteria and viruses. The antibody recognizes a unique part of the foreign target, termed an antigen...
it is possible to monitor the location of individual rhizobacteria
Rhizobacteria
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...
on the root using confocal laser scanning microscopy
Confocal laser scanning microscopy
Confocal laser scanning microscopy is a technique for obtaining high-resolution optical images with depth selectivity. The key feature of confocal microscopy is its ability to acquire in-focus images from selected depths, a process known as optical sectioning...
,,. This approach has also been combined with an rRNA-targeting probe to monitor the metabolic activity of a rhizobacterial strain in the rhizosphere
Rhizosphere
The rhizosphere is the narrow region of soil that is directly influenced by root secretions and associated soil microorganisms. Soil which is not part of the rhizosphere is known as bulk soil. The rhizosphere contains many bacteria that feed on sloughed-off plant cells, termed rhizodeposition, and...
and showed that bacteria located at the root tip were most active
Mechanisms of Action
PGPR enhance plant growth by direct and indirect means, but the specific mechanisms involved have not all been well-characterized, Direct mechanisms of plant growth promotion by PGPR can be demonstrated in the absence of plant pathogenPathogen
A pathogen gignomai "I give birth to") or infectious agent — colloquially, a germ — is a microbe or microorganism such as a virus, bacterium, prion, or fungus that causes disease in its animal or plant host...
s or other rhizosphere microorganism
Microorganism
A microorganism or microbe is a microscopic organism that comprises either a single cell , cell clusters, or no cell at all...
s, while indirect mechanisms involve the ability of PGPR to reduce the deleterious effects of plant pathogens on crop yield
Crop yield
In agriculture, crop yield is not only a measure of the yield of cereal per unit area of land under cultivation, yield is also the seed generation of the plant itself...
. PGPR have been reported to directly enhance plant growth by a variety of mechanisms: fixation of atmospheric nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
that is transferred to the plant, production of siderophore
Siderophore
Siderophores are small, high-affinity iron chelating compounds secreted by grasses and microorganisms such as bacteria and fungi...
s that chelate
Chelation
Chelation is the formation or presence of two or more separate coordinate bonds between apolydentate ligand and a single central atom....
iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
and make it available to the plant root, solubilization
Solubilization
Solubilization, according to an IUPAC definition, is a short form for micellar solubilization, a term used in colloidal and surface chemistry....
of minerals such as phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
, and synthesis of phytohormones. Direct enhancement of mineral uptake
Mineral uptake
In plants, mineral uptake is the process in which minerals enter the cellular material, typically following the same pathway as water. The most normal entrance portal for mineral uptake is through plant roots. Some mineral ions diffuse in-between the cells...
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 PGPR that synthesize auxin
Auxin
Auxins are a class of plant hormones with some morphogen-like characteristics. Auxins have a cardinal role in coordination of many growth and behavioral processes in the plant's life cycle and are essential for plant body development. Auxins and their role in plant growth were first described by...
s and cytokinin
Cytokinin
Cytokinins are a class of plant growth substances that promote cell division, or cytokinesis, in plant roots and shoots. They are involved primarily in cell growth and differentiation, but also affect apical dominance, axillary bud growth, and leaf senescence...
s or that interfere with plant ethylene
Ethylene
Ethylene is a gaseous organic compound with the formula . It is the simplest alkene . Because it contains a carbon-carbon double bond, ethylene is classified as an unsaturated hydrocarbon. Ethylene is widely used in industry and is also a plant hormone...
synthesis have been identified.