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 ?
Rhizobia authenticity for host specificity was studied as demonstrated by Engelkeand his coworker (9).
The phenotypic characteristics studied shows that these rhizobacterial strains are outside the traditional rhizobia and/or belong to fast growing rhizobia; may represent new genospecies that need further characterization to assess for their taxonomical status.
To date, no study has been done on the range of morphological and genetic diversity of indigenous rhizobia from agricultural soils of lower eastern Kenya.
Exoplolysaccharides produced by rhizobia act as signaling molecules and play an important role in the formation of nodules [38].
The exploitation of pastoral systems in a more sustainable way, with less use of mineral inputs without damage to yields can be obtained by using nitrogen-fixing rhizobia, which action can promote growth of cultivated grasses.
Before planting, soils were sampled for assessment of the native rhizobia population density (approximate number g [soil.sup.-1], CFU) in a greenhouse experiment.
(2012) also observed the same results to cowpea, where plants inoculated with a commercial rhizobia strain showed more efficiency on gas exchanges than the control plants.
Elia and now-retired ARS microbiologist Peter van Berkum developed a DNA sequencing technique that can genetically identify different strains of Bradyrhizobium, the rhizobia that are symbiotic with soybeans.
However, there is a low risk of inoculation where a legume has been used frequently and the soil is not hostile to rhizobia. Rhizobial inoculations applied in furrow sowing are placed deeper in the soil and will have a better chance of survival.
These are collectively called rhizobia. These rhizobia enter the roots of the legume and form a nodule, a portion of the root that is swollen.