Rhizobium
Encyclopedia
Rhizobium is a genus
of Gram-negative
soil
bacteria
that fix nitrogen
. Rhizobium forms an endosymbiotic
nitrogen fixing
association with roots of legumes and Parasponia.
The bacteria colonize plant cells within root nodules; here the bacteria convert atmospheric nitrogen to ammonia
and then provide organic nitrogenous compounds such as glutamine
or ureides to the plant. The plant provides the bacteria with organic compound
s made by photosynthesis
.
was the first to isolate and cultivate a microorganism
from the nodules of legumes in 1888. He named it Bacillus radicicola which is now placed in Bergey's Manual of Determinative Bacteriology under the genus Rhizobium.
The principle of cross-inoculation grouping is based on the ability of an isolate of Rhizobium to form nodules in a limited number of species
of legumes related to one another.
All rhizobia that could form nodules on roots of certain legume types have been collectively taken as a species. This system of classification has provided a workable basis for the agricultural practice of legume inoculation. Under this scheme, seven species are generally recognized.
The system of cross-inoculation grouping of rhizobia is not perfect since bacteria have been found to cross-infect or interchange between groups. However, until a better system of classification has been perfected, it appears as if we have to be content with the cross-inoculation grouping as a convenient and workable method of classifying root nodule bacteria into species.
The combined results of both somatic and flagellar reactions have served to distinguish strains within a cross-inoculation group. Serological methods can be used as a means of obtaining information on the distribution of strains that can be recognised within an area, on widely separated areas, on the plant or within a nodule. Serologically, it is known that a single nodule contains a homogeneous population of a single strain of Rhizobium, although it is not uncommon to find more than one strain on the same plant.
, which acts as a natural fertilizer for the plants. Current research is being conducted by Agricultural Research Service microbiologists to discover a way to utilize Rhizobium’s biological nitrogen fixation
. This research involves the genetic mapping of various Rhizobium species with its respective symbiotic plant species, like alfalfa or soybean. The goal of this research is to increase the plants’ productivity without using fertilizers. http://www.ars.usda.gov/is/AR/archive/jan10/microbe0110.htm
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
of Gram-negative
Gram-negative
Gram-negative bacteria are bacteria that do not retain crystal violet dye in the Gram staining protocol. In a Gram stain test, a counterstain is added after the crystal violet, coloring all Gram-negative bacteria with a red or pink color...
soil
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...
bacteria
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 fix nitrogen
Nitrogen fixation
Nitrogen fixation is the natural process, either biological or abiotic, by which nitrogen in the atmosphere is converted into ammonia . This process is essential for life because fixed nitrogen is required to biosynthesize the basic building blocks of life, e.g., nucleotides for DNA and RNA and...
. Rhizobium forms an endosymbiotic
Endosymbiont
An endosymbiont is any organism that lives within the body or cells of another organism, i.e. forming an endosymbiosis...
nitrogen fixing
Nitrogen fixation
Nitrogen fixation is the natural process, either biological or abiotic, by which nitrogen in the atmosphere is converted into ammonia . This process is essential for life because fixed nitrogen is required to biosynthesize the basic building blocks of life, e.g., nucleotides for DNA and RNA and...
association with roots of legumes and Parasponia.
The bacteria colonize plant cells within root nodules; here the bacteria convert atmospheric nitrogen to ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
and then provide organic nitrogenous compounds such as glutamine
Glutamine
Glutamine is one of the 20 amino acids encoded by the standard genetic code. It is not recognized as an essential amino acid but may become conditionally essential in certain situations, including intensive athletic training or certain gastrointestinal disorders...
or ureides to the plant. The plant provides the bacteria with organic compound
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...
s made by photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
.
History
Beijerinck in the NetherlandsNetherlands
The Netherlands is a constituent country of the Kingdom of the Netherlands, located mainly in North-West Europe and with several islands in the Caribbean. Mainland Netherlands borders the North Sea to the north and west, Belgium to the south, and Germany to the east, and shares maritime borders...
was the first to isolate and cultivate a microorganism
Microorganism
A microorganism or microbe is a microscopic organism that comprises either a single cell , cell clusters, or no cell at all...
from the nodules of legumes in 1888. He named it Bacillus radicicola which is now placed in Bergey's Manual of Determinative Bacteriology under the genus Rhizobium.
The Concept Of Cross Inoculation Grouping (C.I.G)
The cross-inoculation grouping based on the classical studies of Fred, Baldwin and McCoy is being generally followed.The principle of cross-inoculation grouping is based on the ability of an isolate of Rhizobium to form nodules in a limited number of species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
of legumes related to one another.
All rhizobia that could form nodules on roots of certain legume types have been collectively taken as a species. This system of classification has provided a workable basis for the agricultural practice of legume inoculation. Under this scheme, seven species are generally recognized.
The system of cross-inoculation grouping of rhizobia is not perfect since bacteria have been found to cross-infect or interchange between groups. However, until a better system of classification has been perfected, it appears as if we have to be content with the cross-inoculation grouping as a convenient and workable method of classifying root nodule bacteria into species.
The combined results of both somatic and flagellar reactions have served to distinguish strains within a cross-inoculation group. Serological methods can be used as a means of obtaining information on the distribution of strains that can be recognised within an area, on widely separated areas, on the plant or within a nodule. Serologically, it is known that a single nodule contains a homogeneous population of a single strain of Rhizobium, although it is not uncommon to find more than one strain on the same plant.
Research
Rhizobium forms a symbiotic relationship with certain plants such as legumes. The Rhizobium fixes nitrogen from the air into ammoniaAmmonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
, which acts as a natural fertilizer for the plants. Current research is being conducted by Agricultural Research Service microbiologists to discover a way to utilize Rhizobium’s biological nitrogen fixation
Nitrogen fixation
Nitrogen fixation is the natural process, either biological or abiotic, by which nitrogen in the atmosphere is converted into ammonia . This process is essential for life because fixed nitrogen is required to biosynthesize the basic building blocks of life, e.g., nucleotides for DNA and RNA and...
. This research involves the genetic mapping of various Rhizobium species with its respective symbiotic plant species, like alfalfa or soybean. The goal of this research is to increase the plants’ productivity without using fertilizers. http://www.ars.usda.gov/is/AR/archive/jan10/microbe0110.htm