Rhizobium

Rhizobium
Rhizobium tropici on an agar plate.
Scientific classification
Kingdom:	Bacteria
Phylum:	Proteobacteria
Class:	Alphaproteobacteria
Order:	Rhizobiales
Family:	Rhizobiaceae
Genus:	Rhizobium Frank 1889
Type species
Rhizobium leguminosarum
Species
R. alamii Berge et al. 2009 R. alkalisoli Lu et al. 2009 R. aggregatum (Hirsch and Müller 1986) Kaur et all. 2011 R. borbori Zhang et al. 2011 R. cellulosilyticum García-Fraile et al. 2007 R. daejeonense Quan et al. 2005 R. endophyticum López-López et al. 2010 R. etli Segovia et al. 1993 R. e. bv. mimosae R. e. bv. phaseoli R. fabae Tian et al. 2008 R. galegae Lindström 1989 R. gallicum Amarger et al. 1997 R. g. bv. gallicum R. g. bv. orientale R. giardinii Amarger et al. 1997 R. grahamii López-López et al. 2011 R. hainanense Chen et al. 1997 R. huautlense Wang et al. 1998 R. indigoferae Wei et al. 2002 R. larrymoorei (Bouzar and Jones 2001) Young 2004 R. leguminosarum (Frank 1879) Frank 1889 R. l. bv. trifolii R. l. bv. viciae R. loessense Wei et al. 2003 R. leucaenae Ribeiro et al. 2011 R. lupini (Schroeter 1886) Eckhardt et al. 1931 R. lusitanum Valverde et al. 2006 R. mediterraneum R. mesoamericanum López-López et al. 2011 R. mesosinicum Lin et al. 2009 R. miluonense Gu et al. 2008 R. mongolense Van Berkum et al. 1998 R. oryzae Peng et al. 2008 R. phaseoli Dangeard 1926 emend. Ramírez-Bahena et al. 2008 R. pisi Ramírez-Bahena et al. 2008 R. radiobacter (Beijerinck and van Delden 1902) Young et al. 2001 R. rhizogenes (Riker et al. 1930) Young et al. 2001 R. rosettiformans Kaur et al. 2011 R. rubi (Hildebrand 1940) Young et al. 2001 R. selenitireducens corrig. Hunter et al. 2008 R. soli Yoon et al. 2010 R. sullae Squartini et al. 2002 R. tibeticum Hou et al. 2009 R. tropici Martínez-Romero et al. 1991 R. tubonense Zhang et al. 2011 R. undicola (de Lajudie et al. 1998) Young et al. 2001 R. vallis Wang et all. 2011 R. vignae Ren et al. 2011 R. vitis (Ophel and Kerr 1990) Young et al. 2001 R. yanglingense Tan et al. 2001

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 compounds made by photosynthesis.

History

Beijerinck in the Netherlands 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 Concept Of Cross Inoculation Grouping (C.I.G)

The cross-inoculation grouping based on the classical studies of Fred, Baldwin and McCoy^{[citation needed]} 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 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 ammonia, 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. [1]

External links

• Current research on Rhizobium leguminosarum at the Norwich Research Park