Rhizobia are soil bacteria that enter into a symbiosis with leguminous plants, producing root outgrowths (nodules) within which the bacteria convert atmospheric nitrogen into ammonia, that is made available to the plants. Legume nodules develop as a result of signal exchange between the symbiotic partners and the rhizobia infect the nodules via specialized infection threads. The bacteria are released into plant cells within the nodule and develop into a specialized form which is primarily dedicated to the production of ammonia via the action of an enzyme called nitrogenase.

Keywords: nitrogen fixation; nodules; legume; nitrogenase

Figure 1.

(a) The ancient Chinese pictographic character ‘shu’ representing soybean. The symbol on the left is thought to represent a plant with a branched shoot and a root with three tear‐drop lines thought to represent nodules. (b) Photographs of determinate Phaseolus vulgaris bean root nodules (upper image) and indeterminate pea root nodules (lower image).

Figure 2.

Structure of a Nod factor made by Sinorhizobium meliloti, showing four β 1–4‐linked N‐acetyl glucosamine residues carrying the C16:2 acyl chain, acetate and sulfate substituents that determine host‐specific nodulation characteristics.

Figure 3.

Light micrograph of a section of a nodule stained to show an infection thread infecting a young developing root nodule on vetch.

Figure 4.

Electron microscope image of a thin section of three adjacent cells within a pea nodule, one showing a transverse section of an infection thread (bottom right). Each plant cell contains many symbiosomes consisting of differentiated rhizobia (bacteroids), each surrounded by a plant‐made peribacteroid membrane.



Brewin NJ (2004) Plant cell wall remodelling in the rhizobium–legume symbiosis. Critical Reviews in Plant Sciences 23: 293–316.

Chen WM, Moulin L, Bontemps C et al. (2003) Legume symbiotic nitrogen fixation by beta‐proteobacteria is widespread in nature. Journal of Bacteriology 185: 7266–7272.

Dixon R and Kahn D (2004) Genetic regulation of biological nitrogen fixation. Nature Reviews Microbiology 2: 621–631.

Downie JA (2005) Legume haemoglobins: symbiotic nitrogen fixation needs bloody nodules. Current Biology 15: R196–R198.

Doyle JJ (1998) Phylogenetic perspectives on nodulation: evolving views of plants and symbiotic bacteria. Trends in Plant Science 3: 473–478.

Fisher RF and Long SR (1993) Interactions of NodD at the Nod box – NodD binds to two distinct sites on the same face of the helix and induces a bend in the DNA. Journal of Molecular Biology 233: 336–348.

Gage DJ (2004) Infection and invasion of roots by symbiotic, nitrogen‐fixing rhizobia during nodulation of temperate legumes. Microbiology and Molecular Biology Reviews 68: 280.

Herder GD, Schroeyers K, Holsters M and Goormachtig S (2006) Signaling and gene expression for water‐tolerant legume nodulation. Critical Reviews in Plant Sciences 25: 367–380.

Lodwig EM, Hosie AHF, Bordes A et al. (2003) Amino‐acid cycling drives nitrogen fixation in the legume – Rhizobium symbiosis. Nature 422: 722–726.

Mergaert P, Uchiumi T, Alunni B et al. (2006)Eukaryotic control on bacterial cell cycle and differentiation in the Rhizobium‐legume symbiosis. Proceedigns of the National Academy of Sciences of the USA 103: 5230–5235.

Oldroyd GED and Downie JA (2004) Calcium, kinases and nodulation signalling in legumes. Nature Reviews Molecular Cell Biology 5: 566–576.

van Rhijn P, Fujishige NA, Lim PO and Hirsch AM (2001) Sugar‐binding activity of pea lectin enhances heterologues infection of transgenic alfalfa plants by Rhizobium leguminosarum biovar viciae. Plant Physiology 126: 133–144.

Sawada H, Kuykendall LD and Young JM (2003) Changing concepts in the systematics of bacterial nitrogen‐fixing legume symbionts. Journal of General and Applied Microbiology 49: 155–179.

Spaink HP (2000) Root nodulation and infection factors produced by rhizobial bacteria. Annual Review of Microbiology 54: 257–288.

Sullivan JT and Ronson CW (1998) Evolution of rhizobia by acquisition of a 500‐kb symbiosis island that integrates into a phe‐tRNA gene. Proceedigns of the National Academy of Sciences of the USA 95: 5145–5149.

Further Reading

Broughton WJ, Jabbouri S and Perret X (2000) Keys to symbiotic harmony. Journal of Bacteriology 182: 5641–5652.

Diaz CL, Melchers LS, Hooykaas PJJ, Lugtenberg BJJ and Kijne JW (1989) Root lectin as a determinant of host‐plant specificity in the Rhizobium‐legume symbiosis. Nature 338: 579–581.

Downie JA (1998) Functions of rhizobial nodulation genes.In: Kondorosi A, Spaink HP and Hooykaus PJJ (eds) The Rhizobiaceae, pp. 387–402. The Netherlands: Kluwer

Fraysse N, Couderc F and Poinsot V (2003) Surface polysaccharide involvement in establishing the rhizobium–legume symbiosis. European Journal of Biochemistry 270: 1365–1380.

Galibert F, Finan TM, Long SR et al. (2001) The composite genome of the legume symbiont Sinorhizobium meliloti. Science 293: 668–672.

Gonzalez V, Santamaria RI, Bustos P et al. (2006) The partitioned Rhizobium etli genome: genetic and metabolic redundancy in seven interacting replicons. Proceedigns of the National Academy of Sciences of the USA 103: 3834–3839.

Hymowitz T (1970) Domestication of soybean. Economic Botany 24: 408.

Kaneko T, Nakamura Y, Sato S et al. (2000) Complete genome structure of the nitrogen‐fixing symbiotic bacterium Mesorhizobium loti. DNA Research 7: 331–338.

Kaneko T, Nakamura Y, Sato S et al. (2002) Complete genomic sequence of nitrogen‐fixing symbiotic bacterium Bradyrhizobiumjaponicum USDA110. DNA Research 9: 189–197.

Perret X, Staehelin C and Broughton WJ (2000) Molecular basis of symbiotic promiscuity. Microbiology and Molecular Biology Reviews 64: 180.

Spaink HP, Kondorosi A and Hooykaas PJJ (1998) The Rhizobiaceae. Dordrecht: Kluwer Academic Publishers.

Sprent JI (2001) Nodulation in Legumes. London: Cromwell Press Ltd.

Young JPW, Crossman LC, Johnston AWB et al. (2006) The genome of Rhizobium leguminosarum has recognizable core and accessory components. Genome Biology 7.

Contact Editor close
Submit a note to the editor about this article by filling in the form below.

* Required Field

How to Cite close
Allan Downie, J(Jul 2007) Rhizobia. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0020120]