Home

Research

Strain Collection

F.A.Q.

Glossary

Reading


Personnel

Sponsors

Listserv

News


Dr. Peter H. Graham
439 Borlaug Hall
1991 Upper Buford Circle
St Paul, MN 55406

What are rhizobia? Rhizobia is the common name given to a group of small, rod- shaped, Gram -ve bacteria, which collectively have the ability to produce nodules on the roots (or, in some cases, the stems) of leguminous plants. In early studies with these organisms, it was established that no strain could nodulate all legumes, but that each could nodulate some legumes though not others. This led to the concept of cross-inoculation groups, with organisms grouped according to the hosts they nodulated. For a time this was the basis on which rhizobia were identified. Thus, rhizobia isolated from one species of clover would usually nodulate other species of clover, and were then collectively called Rhizobium trifolii, while rhizobia isolated from medics would also nodulate lucerne and fenugreek, and were called R.meliloti. This specificity is still very important in selecting the appropriate inoculant for your crop or planting, and the wrong choice of inoculant is still a common cause of nodulation failure. Changes in the methods used in bacterial taxonomy (Graham, et al., 1991) and advances in molecular biology have resulted in a rhizobial taxonomy based on a wide range of characters, and to the distinction of new genera and species. Currently six genera and 37+ species are distinguished, but a number of these remain in question (Tighe et al., 2000). While inoculant rhizobia should all be capable of fixing nitrogen within the nodules produced by their host, few have been shown to exhibit this property when growing in pure culture. Azorhizobium caulinodans has this ability, and some Bradyrhizobium strains exhibit low levels of nitrogen fixation activity in older cultures. The genera and species of rhizobia presently distinguished are shown below. The major host for each species is shown in bold. The list of other hosts provided is not intended to be final. Genera and species of root nodule bacteria of legumes Genera/species Principal and other reported hosts Description and Emendments Rhizobium R.etli Phaseolus vulgaris, Mimosa affinis Segovia et al., 1993; Wang et al., 1999 R.galegae Galega orientalis, G.officinalis Lindstrom, 1989; Nick, 1998 R.gallicum Phaseolus vulgaris, Leucaena, Macroptilium, Onobrychis Amarger et al., 1997 R.giardini Phaseolus vulgaris, Leucaena, Macroptilium Amarger et al., 1997 R.hainanense Desmodium sinuatum, Stylosanthes, Vigna, Arachis, Centrosema Chen et al., 1997 R.huautlense Sesbania herbacea Wang et al., 1998 R.indigoferae Indigofera Wei et al., 2002 R.leguminosarum bv trifolii bv viciae bv phaseoli Trifolium Lathyrus, Lens, Pisum, and Vicia Phaseolus vulgaris Dangeard, 1926, Jordan, 1984 R.mongolense Medicago ruthenica, Phaseolus vulgaris van Berkum et al., 1998 R.sullae Hedysarum coronarium Squartini et al., 2002 R.tropici Phaseolus vulgaris, Dalea, Leucaena, Macroptilium, Onobrychis Martinez et al., 1991 Mesorhizobium M.amorphae Amorpha fruticosa Wang et al., 1999 M.chacoense Prosopis alba Velasquez et al., 1998 M.ciceri Cicer arietinum Nour et al., 1994 M.huakuii Astragalus sinicus, Acacia Chen et al., 1991; Jarvis et al., 1997 M.loti Lotus corniculatus Jarvis et al., 1982; Jarvis et al., 1997 M. mediterraneum Cicer arietinum Nour et al., 1995, Jarvis et al., 1997 M.plurifarium Acacia senegal, Prosopis juriflora, Leucaena de Lajudie et al., 1998 M.tianshanense Glycyrrhiza pallidflora, Swansonia, Glycine, Caragana, Sophora Chen et al., 1995 Sinorhizobium S.abri Abrus precatorius Ogasawara et al., 2003 S.americanus Acacia spp. Toledo et al., 2003 S.arboris Acacia senegal, Prosopis chilensis Nick et al., 1999 S.fredi Glycine max Scholla et al., 1984; Chen et al., 1988 S.indiaense Sesbania rostrata Ogasawara et al., 2003 S.kostiense Acacia senegal, Prosopis chilensis Nick et al., 1999 S.kummerowiae Kummerowia stipulacea Wei et al., 2002 S.meliloti Medicago, Melilotus, Trigonella Dangeard, 1926; Lajudie et al., 1994 S.medicae Medicago truncatula, M. polymorpha, M.orbicularis Rome et al., 1996 S.morelense Leucaena leucocephala Wang et al., 2002 S.sahelense Acacia, Sesbania de Lajudie et al., 1994; Boivin and Giraud, 1999 S.terangae Acacia, Sesbania de Lajudie et al., 1994; Lortet et al., 1996 Azorhizobium A.caulinodans Sesbania rostrata Dreyfus et al., 1988 Allorhizobium A.undicola Neptunia natans, Acacia, Faidherbia, Lotus de Lajudie et al., 1998 Bradyrhizobium B.elkanii Glycine max Kuykendall et al., 1992 B.japonicum Glycine max Jordan, 1984 B.liaoningense Glycine max Xu et al., 1995 B.yuanmingense Lespedeza, Medicago, Melilotus Yao et al., 2002 Other genera and species names exist in the literature. Some pre-date the present names, others (e.g., Photobacterium) have not been accepted as valid. Strains of root-nodule bacteria that have not yet been ascribed to any named species are usually identified by the host from which they were isolated, e.g. Rhizobium spp. (Acacia) or Bradyrhizobium spp. (Lupinus).