The picture above shows a well-nodulated subclover plant (Trifolium subterraneum).
2. TAXONOMIC DESCRIPTION
All rhizobium are root nodulating bacteria and are gram negative, non spore forming medium sized rod shaped cells that contain the enzyme complex called nitrogenase and are typically motile. Their cellular morphology and biochemical characteristics are very similar to those of nonsymbiotic nitrogen-fixing bacteria called Azotobacter. The primary distinguishing characteristic of Rhizobium species is their ability to nodulate leguminous plants. Rhizobia are predominantly aerobic chemoorganotrophs and grow well in the presence of oxygen and utilize a wide range of relatively simple carbohydrates and amino compounds. (With the exception of a few strains, they have not been found to fix N in the free-living form except under special conditions.) Optimal growth of most strains occurs at a temperature range of 25-30ÃÂÃÂ° C and at a pH of 6.0-7.0. Despite their usual aerobic metabolism, many strains are able to grow well under microaerophillic conditions at oxygen tensions of less than 0.1 atm. Rhizobium is surrounded by a slimy capsule made of exopolysaccharide, which protects it from drying out. And also helps the bacterium stick to root hairs during various stages of its life cycle.
There are three species of Rhizobium. They include Rhizobium leguminosarum, R. meliloti, and R. loti. The species R. leguminosarum is further divided into three biovars.
The above picture shows the inside of a Rhizobium nodule with individual bacteroids under electron microscopy (15000 X).
3. ISOLATION AND ECOLOGY
Members of the genus Rhizobium commonly inhibit the soil. Identification of Rhizobia is relatively easy if isolated from the host plant nodules, but difficult if isolated from the soil. The Rhizobia do not grow well on the peptone media used routinely for many bacteria but do so on various complex extracts of plant origin. Yeast mannitol (YM) is the most generally suitable for their growth. Mannitol is often used as the carbon source. Various chemical additives are used in the media to suppress or indicate the growth of Rhizobium. Bromthymol blue acts as a pH indicator when added to the standard YM medium. Fast-growing Rhizobia produce an acid reaction in this media while slow growers produce alkaline reactions. Congo red can sometimes assist the recognition of Rhizobia amongst other kinds of bacteria. In general Rhizobia absorbs the dye weakly whereas many other bacteria take it up strongly. Media in which the yeast extract is replaced with inorganic combined nitrogen and or one to several amino acids, and with one or more vitamins, can be used to determine more specifically the organisms nutritive requirements, metabolic behavior and growth products.
Rhizobium is a type of nitrogen fixing bacteria that lives in soil and forms a
symbiotic association with the root cells of leguminous plants. Legumes are herbaceous
woody plants that produce seeds in pods; examples of legumes include peas, beans, alfalfa,
vetches, clovers, lupines, trefoils, locust, and mimosa. This symbiotic relationship
between the bacteria and the root tissue is mutulistic, meaning that both organisms
benefit from the association. The plant's root cells provide nutrients and carbohydrates
for energy for the bacteria and the bacteria reciprocate by supplying the plant with fixed
nitrogen compounds. The Rhizobium appears to be host specific; in other words, one
species of Rhizobium seems able to nodulate only one species of legume. It is
common practice before planting the seeds of leguminous plants to inoculate with Rhizobia
because not all soils contain the right species of Rhizobium for optimal symbiosis
with a given legume. It has been proven that plant productivity increase when the Rhizobia
are present. Rhizobium provides the major biological source of fixed nitrogen in
agricultural soils. Rhizobium is responsible for a significant amount of nitrogen
fixation; this species can fix up to 220 pounds of N2 per agricultural acre per
year. The majority of the research and work done on nitrogen fixation is done using this
organism--Rhizobium. It is because of its agricultural importance in leguminous
plants that it has been so extensively studied.
The above picture shows R. meliloti on mannitol salt agar.
4. ADDITIONAL SOURCES OF INFORMATION
Vincent, J.M. 1970. A Manual for the Practical study of the Root-Nodule Bacteria. Burgess and Son Ltd. Great Britain. 45pp.
Postgate, John. 1978. Nitrogen Fixation. The Camelot Press Ltd. Great Britain. 44-48pp.
H. J. Hoben, P. Somasegaran. Handbook for Rhizobia. Springer-Verlag. New York. 1-6pp.
Holt, J.G., N.R. Krieg, P.H.A. Sneath, J.T. Staley, and S.T. Williams. 1994. Bergey's Manual of Systematic Bacteriology. Williams and Wilkins. Baltimore, Maryland. 235-242 pp.
5. LINKS TO OTHER SITES ON Rhizobium
Microbe Zoo-Rhizobium in Soil
Microbe Zoo-Rhizobium on Root Hair Tip
Microbe Zoo-Rhizobium on Clover Root Hair
Microbe Zoo-Aquatic Root Nodules
Another cool site on Rhizobium and Nitrogen Fixaton
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