Taxonomy of rhizobia Frédéric Zakhia, Philippe de Lajudie To cite this version: Frédéric Zakhia, Philippe de Lajudie. Taxonomy of rhizobia. Agronomie, EDP Sciences, 2001, 21 (6-7), pp.569-576. 10.1051/agro:2001146. hal-00886134 HAL Id: hal-00886134 https://hal.archives-ouvertes.fr/hal-00886134 Submitted on 1 Jan 2001 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Agronomie 21 (2001) 569–576 569 © INRA, EDP Sciences, 2001 Review article Taxonomy of rhizobia Frédéric ZAKHIA, Philippe de LAJUDIE* Laboratoire des Symbioses Tropicales et Méditerranéennes, UMR 113 INRA/AGRO-M/CIRAD/IRD, TA 10 / J, Campus International de Baillarguet, 34398 Montpellier Cedex 5, France (Received 15 January 2001; revised 8 March 2001; accepted 13 March 2001) Abstract – Rhizobia are the bacteria that form nitrogen-fixing symbioses with legumes. Based on their characterisation by polypha- sic taxonomy, their classification has undergone great changes in recent years. The current six rhizobium genera and 28 recognised species are reviewed here. Rhizobium / taxonomy Résumé – Taxonomie des rhizobia. Les rhizobia sont les bactéries qui forment des symbioses fixatrices d’azote avec des plantes de la famille des légumineuses. Suite à l’adoption de la taxonomie polyphasique comme critère de caractérisation, leur classification a subi de nombreux remaniements ces dernières années. Une revue de cette classification en six genres et 28 espèces actuellement reconnus est présentée ici. Rhizobium / taxonomie 1. INTRODUCTION Rhizobia are diverse and, during recent years, their classification has undergone great changes due to new Nitrogen fixation, that is to say the reduction of phylogenetic and polyphasic data leading to the descrip- atmospheric nitrogen to ammonia, is the basic phenome- tion of new taxa [73]. Still it is an expanding field non making nitrogen available for the biosphere. Among because more and more rhizobia are isolated and charac- living organisms, only some prokaryotes have this abili- terised every day, especially from the tropical and the ty. They are spread over many of the major bacterial mediterranean zones, where diversity is still poorly doc- phyla. Among these, some soil bacteria, commonly and umented. collectively called rhizobium or rhizobia, can form sym- Until the early 1980s all symbiotic nitrogen-fixing bioses with leguminous plants. They induce the forma- bacteria from leguminous plants were classified in the tion of specialised organs, called nodules, where they fix single genus Rhizobium, including six species R. legumi- nitrogen to the benefit of the plant. These symbioses nosarum, R. meliloti, R. trifolii, R. phaseoli, R. lupini have major environmental and agricultural importance and R. japonicum. This taxonomy matched the cross- since they are responsible for most of the biological inoculation groups, defined as the set of host-plants atmospheric nitrogen fixation on earth. nodulated by the same bacterial strains. For a long time Communicated by Jean-Jacques Drevon (Montpellier, France) * Correspondence and reprints [email protected] 570 F. Zakhia, P. de Lajudie the genera Rhizobium, Agrobacterium, and field of application, dependent on the addressed ques- Phyllobacterium were grouped in the family tion, the particular conditions, the number and the type Rhizobiaceae mainly based on their common functional of strains. The level of discrimination of a technique may characteristic of inducing cell proliferations on plants. vary depending on the studied bacterial taxon. In a polyphasic approach various complementary techniques Taxonomy based on the concept of cross-inoculation with different discriminating levels are chosen to charac- groups failed because of the many exceptions to this terise the strains to be classified. The conclusion should rule. Moreover, at the same time, it was also widely rec- be a consensus with minimal contradictions, drawing ognized that Rhizobium classification should adjust to nomenclatural consequences and emphasising fast and general bacterial taxonomy, and include a panel of reliable features for identification (preferably monopha- genomic, phenotypic and phylogenetic features instead sic). Vandamme et al. [57] last reformulated the phylo- of the sole nodulation properties. Molecular techniques genetic concept of the bacterial species as the assem- have developed considerably during the past 20 years blage of isolates which originated from a common and were made available for characterisation of rhizobia, ancestor population in which a steady generation of leading to considerable changes in their classification. genetic diversity resulted in clones with different degrees of recombination and characterised by a certain degree of phenotypic consistency, by a significant degree of 2. POLYPHASIC TAXONOMY DNA/DNA hybridisation and by over 97% 16S rDNA sequence homology. Taxonomy studies the natural relationships existing between organisms and leads to their classification, a pre-requisite for the identification of any organism. 3. CURRENT RHIZOBIUM TAXONOMY Taxonomy [23] comprises (i) characterisation of the organisms, (ii) classification on the basis of similarity, Rhizobia are rod bacteria, Gram-negative, aerobic, not (iii) nomenclature to give names to groups and (iv) iden- sporulated [29]. Phylogenetically, they belong to the tification of unknown organisms, to determine if they alpha subdivision of Proteobacteria. When it was recog- belong or not to one of the units classified and/or named. nised that the host spectrum was not the sole criterium to The central concept of bacterial taxonomy is the bacteri- take into account for rhizobium classification, the al species. Unlike animals and plants, bacteria have sim- Rhizobium species were first reclassified into two gen- ple morphological and physiological characteristics, lim- era, the revised Rhizobium genus including the fast ited and insufficient for taxonomic descriptions [68]. It is growing strains, and the new genus Bradyrhizobium, cre- now recognised that bacterial classification must reflect ated for the slow growing ones [28]. Since then, isolation the phylogenetic relationships between bacteria, main of rhizobia from an increasing number of plant species witnesses of which are the sequences of ribosomal RNA. around the world and their characterisation by modern In 1987, a group of taxonomists proposed a phylogenetic polyphasic taxonomy [57] has led to the description of definition of the bacterial species based on various additional new genera and species (Tab. I). Based on methodologies including the study of 16S ribosomal 16S ribosomal DNA sequences, the symbionts of legu- RNA and DNA/DNA hybridisations, but still recom- minous plants belong to three main separate phylogenet- mending laying out discriminating phenotypic character- ic branches (Fig. 1). istics to name a new species [62]. Previously Colwell [9] founded the basis of modern bacterial taxonomy by rec- ommending the integration of all pieces of information 3.1. Rhizobium/Agrobacterium branch obtained at various levels of the cell - DNA, RNA, phe- notypic and expressed features (proteins and their func- A first branch consists of several subbranches, each tions, fatty acids, chimiotaxonomic markers) and by var- corresponding to four rhizobial genera, i.e. Rhizobium, ious techniques contributing to a polyphasic taxonomy Sinorhizobium, Mesorhizobium and Allorhizobium, inter- likely to lead to a more stable classification. During the mixed with other genera, some plant-related bacteria last twenty years, many techniques of bacterial charac- (Agrobacterium, Phyllobacterium), soil bacteria terisation have been developed at each level of informa- (Mycoplana), and clinical bacteria (Brucella, tion, and more computer facilities are becoming every Ochrobactrum, Bartonella). The first subbranch corre- day available for numerical analysis of data. sponds to the s.s. Rhizobium genus and includes R. legu- Each technique used in taxonomy has (i) its own dis- minosarum (type species), R. tropici [37], R. etli [50], criminating power varying from the individual or species R. gallicum [2], R. mongolense [56] and Agrobacterium levels to the genus, family and higher levels, and (ii) its biovar 2. The second subbranch corresponds to the Taxonomy of rhizobia 571 Table I. Classification of nitrogen fixing bacteria forming symbioses with legume plants. Species Host-plant References Rhizobium 22 R. leguminosarum 22, 29 biovar viciae Pisum sativum, Vicia, Lathyrus, Lens 22, 29 biovar trifolii Trifolium pratense 22, 29 biovar phaseoli Phaseolus vulgaris 22, 29 R. tropici Type II A Phaseolus vulgaris, Leucaena leucocephala 37 Type II B Phaseolus vulgaris, Leucaena leucocephala 37 R. etli 50 biovar phaseoli Phaseolus vulgaris 24, 50 biovar mimosae Mimosa affinis, Leucaena leucocephala, Phaseolus vulgaris L. 60 R. hainanense Desmodium sinuatum & other plants of arid regions 8 R. gallicum 2 biovar gallicum Phaseolus vulgaris 2 biovar phaseoli Phaseolus vulgaris 2 R. mongolense Medicago ruthenica 56 R.