Rhizobia


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Related to Rhizobia: Bacteroids, Leghaemoglobin

Rhizobia

 

(nodule bacteria), bacteria of the genus Rhizobium, which form small nodules on the roots of certain legumes, symbiotically fix molecular nitrogen, and produce in the process a number of physiologically active substances with beneficial effects on the legumes.

Rhizobia play an important part in enriching soils with nitrogen. The young bacteria, which measure 0.5–0.9 X 1.2–3 microns, are nonsporiferous, motile, gram-negative, and aerobic. Entering a root hair, they bring about the active division of the root cells. This leads to the appearance of a small nodule. The rhizobia grow in the nodule, turning into thickened and branched forms called bacteroids. These actively fix molecular nitrogen. Rhizobia also assimilate ammonium salts, nitrates, and amino acids. Their sources of carbon can be monosaccharides, disaccharides, certain polysaccharides, organic acids, and alcohols. On smooth culture media, rhizobia form round, colorless, transparent, mucoid colonies, which grow well at 25°C. The nodules formed by active rhizobia contain the pigment leghemoglobin, which gives them a rosy color.

Different rhizobia cause the dvelopment of nodules in specificlegumes; Rhizobium meliloti in alfalfa and melilot; Rh. leguminosarum in vetch, peas, and fodder beans; Rh. trifolii inclover, and Rh. japonicum in soy. When the nodules are destroyed the rhizobia survive in the soil as saprophytes. The seeds of legumes are infected with rhizobia in order to promote nodule formation; this is an aspect of what is known as bacterial fertilization.

A. A. IMSHENETSKII

References in periodicals archive ?
Aouani ME, Mhamdi R, Jebara M, Amarger N (2001) Characterization of rhizobia nodulating chickpea in Tunisia.
With funding from the Australian Centre for International Agricultural Research, CSIRO scientists have worked with colleagues in China on a revegetation project using Australian legumes and rhizobia.
They are specifically high in arabinose, galacturonic acid, galactose, and rhamnose; features that may facilitate bacterial penetration and that have been shown to be strikingly different from other dicotyledonous and monocotyledonous species that do not harbor Rhizobia (Mort & Grover, 1988).
In addition to exploring how the bacteria form stem nodules, Hardy is trying to discover what gives this rhizobium its special characteristics, whether other photosynthetic rhizobia exist and when such organisms appeared in evolution.
The aims of this study were to evaluate the symbiotic efficiency and the host range of Rhizobia of some leguminous trees and the cross-nodulation patterns with local Acacia trees grown in the central region of Saudi Arabia.
Can catalase and exopolysaccharides producing rhizobia ameliorate drought stress in wheat?
The production of EPS is specific to rhizobia, which gives a viscosity to bacterial colonies when they are cultivated on a solid medium [30].
That way, besides inserting the atmospheric N in the soil-plant system by symbiotic fixation, the symbiotic rhizobia of native legumes and adapted to soil and climatic conditions of RS and SC can directly stimulate cultivated grasses that may be offered to animals in intercropping/succession to these legumes.
Before planting, soils were sampled for assessment of the native rhizobia population density (approximate number g [soil.
However, this ability is genotype-dependent because the climate may affect directly the non-native rhizobia and restrict the survival and efficiency of the root set in nodulating plants.