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Rhizobium, Agrobacterium and Allorhizobium are genera within the bacterial family Rhizobiaceae, together with Sinorhizobium. The species of Agrobacterium, Agrobacterium tumefaciens (syn. Agrobacterium radiobacter), Agrobacterium rhizogenes, Agrobacterium rubi and Agrobacterium vitis, together with Allorhizobium undicola, form a monophyletic group with all Rhizobium species, based on comparative 16S rDNA analyses. Agrobacterium is an artificial genus comprising plant-pathogenic species. The monophyletic nature of Agrobacterium, Allorhizobium and Rhizobium and their common phenotypic generic circumscription support their amalgamation into a single genus, Rhizobium. Agrobacterium tumefaciens was conserved as the type species of Agrobacterium, but the epithet radiobacter would take precedence as Rhizobium radiobacter in the revised genus. The proposed new combinations are Rhizobium radiobacter, Rhizobium rhizogenes, Rhizobium rubi, Rhizobium undicola and Rhizobium vitis.Keywords : Rhizobiaceae, phenetic, phylogenetic, polyphasic, taxonomy INTRODUCTIONNitrogen-fixing bacteria that form symbiotic associations with members of the Leguminosae, and related pathogenic bacteria, have been ascribed to the genera Agrobacterium Conn 1942, Allorhizobium de Lajudie et al. 1998b, Azorhizobium Dreyfus et al. 1988, Bradyrhizobium Jordan 1982, Mesorhizobium Jarvis et al. 1997, Phyllobacterium Kno$ sel 1984, Rhizobium Frank 1889 and Sinorhizobium Chen et al. 1988. For some time, it has been clear that the nomenclature of Rhizobium and related genera does not accurately describe their natural classification and is in need of Abbreviations : ITS, internal transcribed spacer ; LBP, local bootstrap probability ; ME, minimum-evolution ; ML, maximum-likelihood ; MP, maximum-parsimony ; NJ, neighbour-joining.revision (Kersters & De Ley, 1984 ;Willems & Collins, 1993 ;Sawada et al., 1993b ;de Lajudie et al., 1998b). In this paper, the relevant literature is reviewed and a nomenclature is proposed that aims to reflect, as closely as possible, the natural polyphasic and phenetic relationships of these taxa. The original species nomenclatureThe nomenclature of Rhizobium species was originally shaped by the belief that a natural classification could be based on the specificity of symbiotic plant range of bacterial strains and species. The recognition that nodulation and specificity were characters of strains carrying particular Sym plasmids, reviewed by Martı! nez- Romero & Palacios (1990), and therefore were taxonomically unreliable, has led to the aban- J. M. Young and others donment of this concept. In Rhizobium species, nonsymbiotic strains have been reported for different species (Jarvis et al., 1989 ; Laguerre et al., 1993 ;Segovia et al., 1991 ;Sobero! n-Cha! vez & Na! jera, 1989), indicating that symbiotic plasmids can be lost in nature and are not essential for bacterial survival. Furthermore, functional plasmids can be transferred between members of Rhizobium and Agrobacterium species (Abe et al., 1998 ; Hooykaas et al., 1977 ;Martı...

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As of February 2003, bacteria that form nitrogen-fixing symbiotic associations with legumes have been confirmed in 44 species of 12 genera. Phylogenies of these taxa containing legume symbionts based on the comparative analysis of 16S rDNA sequences show that they are not clustered in one lineage but are distributed in the classes Alphaproteobacteria and Betaproteobacteria, and dispersed over the following nine monophyletic groups, being intermingled with other taxa that do not contain legume symbionts (shown in parentheses below): Group 1, which comprises Rhizobium and Allorhizobium species containing legume symbionts (intermingled with Agrobacterium and Blastobacter species, which are nonsymbionts); Group 2, Sinorhizobium and Ensifer species (with unclassified nonsymbionts); Group 3, Mesorhizobium species (with nonsymbiotic Aminobacter and Pseudaminobacter species); Group 4, Bradyrhizobium species and Blastobacter denitrificans (with nonsymbiotic Agromonas, Nitrobacter, Afipia, and Rhodopseudomonas species); Group 5, 'Methylobacterium nodulans" (with nonsymbiotic Methylobacterium species); Group 6, Azorhizobium species (with nonsymbiotic Xanthobacter and Aquabacter species); Group 7, 'Devosia neptuniae" (with nonsymbiotic Devosia species and unclassified nonsymbionts); Group 8, symbiotic Burkholderia strains (with nonsymbiotic Burkholderia species); and Group 9, Ralstonia taiwanensis (with nonsymbiotic Ralstonia species). For Groups 5, 8, and 9, the present classification, in which 'each monophyletic group comprises one genus wherein legume symbionts and nonsymbionts are intermingled with each other, " is considered to be retained as is because they are clearly separated from other genera at high bootstrap values and have already been sufficiently characterized based on polyphasic taxonomy. As for the remaining six monophyletic groups, on the other hand, there are currently three options for emending their current classification (definitions and circumscriptions) at the generic level: A) the current classification shall be retained as is; B) all the genera within each monophyletic group shall be amalgamated into one single genus in conformity with the results of phylogenetic analysis; or C) each subordinate lineage in each monophyletic group shall be proposed as a genus. It is considered that research and discussions will be continuously conducted for emending the classification of these monophyletic groups based chiefly on Options B and C as preferable candidates.

The New Zealand native legume flora are represented by four genera, Sophora, Carmichaelia, Clianthus, and Montigena. The adventive flora of New Zealand contains several legume species introduced in the 19th century and now established as serious invasive weeds. Until now, nothing has been reported on the identification of the associated rhizobia of native or introduced legumes in New Zealand. The success of the introduced species may be due, at least in part, to the nature of their rhizobial symbioses. This study set out to address this issue by identifying rhizobial strains isolated from species of the four native legume genera and from the introduced weeds: Acacia spp. (wattles), Cytisus scoparius (broom), and Ulex europaeus (gorse). The identities of the isolates and their relationship to known rhizobia were established by comparative analysis of 16S ribosomal DNA, atpD, glnII, and recA gene sequences. Maximum-likelihood analysis of the resultant data partitioned the bacteria into three genera. Most isolates from native legumes aligned with the genus Mesorhizobium, either as members of named species or as putative novel species. The widespread distribution of strains from individual native legume genera across Mesorhizobium spp. contrasts with previous reports implying that bacterial species are specific to limited numbers of legume genera. In addition, four isolates were identified as Rhizobium leguminosarum. In contrast, all sequences from isolates from introduced weeds aligned with Bradyrhizobium species but formed clusters distinct from existing named species. These results show that native legume genera and these introduced legume genera do not have the same rhizobial populations.Rhizobia are soil-inhabiting bacteria that form symbiotic relationships with plant legume species in root nodules. The bacteria fix nitrogen from the atmosphere to form ammonia, which is assimilated by the plant. This relationship has been exploited by agriculture to enhance legume crop growth without the addition of nitrogen-containing fertilizers. For this reason, the majority of research in this field has focused on the herbaceous crop legumes of agricultural significance. In contrast, few studies have been made of rhizobial associations among noncrop legumes, despite the fact that they may be important in ecological interactions.Worldwide, there are an estimated 17,000 to 19,000 legume species (19). However, symbiotic bacterial species have only been identified for a small proportion of these. To date, 45 symbiotic nodulating bacterial species have been identified in 10 genera: Azorhizobium, Blastobacter, Bradyrhizobium, Burkholderia, Devosia, Mesorhizobium, Methylobacterium, Ralstonia, Rhizobium, and Sinorhizobium (28). Most of the species are in the genera Rhizobium, Azorhizobium, Bradyrhizobium, Mesorhizobium, and Sinorhizobium and are related to one another in the order Rhizobiales, with Rhizobium and Sinorhizobium in the family Rhizobiaceae, and Mesorhizobium in the Phyllobacteriaceae (28).The symbiotic relationships o...