Summary: Nitrogen‐fixing symbiosis is globally important in ecosystem functioning and agriculture, yet the evolutionary history of nodulation remains the focus of considerable debate. Recent evidence suggesting a single origin of nodulation followed by massive parallel evolutionary losses raises questions about why a few lineages in the N₂‐fixing clade retained nodulation and diversified as stable nodulators, while most did not. Within legumes, nodulation is restricted to the two most diverse subfamilies, Papilionoideae and Caesalpinioideae, which show stable retention of nodulation across their core clades. We characterize two nodule anatomy types across 128 species in 56 of the 152 genera of the legume subfamily Caesalpinioideae: fixation thread nodules (FTs), where nitrogen‐fixing bacteroids are retained within the apoplast in modified infection threads, and symbiosomes, where rhizobia are symplastically internalized in the host cell cytoplasm within membrane‐bound symbiosomes (SYMs). Using a robust phylogenomic tree based on 997 genes from 147 Caesalpinioideae genera, we show that losses of nodulation are more prevalent in lineages with FTs than those with SYMs. We propose that evolution of the symbiosome allows for a more intimate and enduring symbiosis through tighter compartmentalization of their rhizobial microsymbionts, resulting in greater evolutionary stability of nodulation across this species‐rich pantropical legume clade.

The genus Methylobacterium , when first proposed by Patt et al. in 1976, was a monospecific genus created to accommodate a single pink pigmented facultatively methylotrophic bacterium. The genus now has over 50 validly published species, however, the percentage 16S rRNA sequence divergence within Methylobacterium questions whether or not they can still be accommodated within one genus. Additionally, several strains are described as belonging to Methylobacterium , but nodulate legumes and in some cases are unable to utilize methanol as a sole carbon source. This study reviews and discusses the current taxonomic status of Methylobacterium . Based on 16S rRNA gene, multi-locus sequence analysis, genomic and phenotypic data, the 52 Methylobacterium species can no longer be retained in one genus. Consequently, a new genus, Methylorubrum gen. nov., is proposed to accommodate 11 species previously held in Methylobacterium . The reclassified species names are proposed as: Methylorubrum aminovorans comb. nov. (type strain TH-15T=NCIMB 13343T=DSM 8832T), Methylorubrum extorquens comb. nov. (type strain NCIMB 9399T=DSM 1337T), Methylorubrum podarium comb. nov. (type strain FM4T=NCIMB 14856T=DSM 15083T), Methylorubrum populi comb. nov. (type strain BJ001T=NCIMB 13946T=ATCC BAA-705T), Methylorubrum pseudosasae comb. nov. (type strain BL44T=ICMP 17622T=NBRC 105205T), Methylorubrum rhodesianum comb. nov. (type strain NCIMB 12249T=DSM 5687T), Methylorubrum rhodinum comb. nov. (type strain NCIMB 9421T=DSM 2163T), Methylorubrum salsuginis comb. nov. (type strain MRT=NCIMB 14847T=NCCB 100140T), Methylorubrum suomiense comb. nov. (type strain F20T=NCIMB 13778T=DSM 14458T), Methylorubrum thiocyanatum comb. nov. (type strain ALL/SCN-PT=NCIMB 13651T=DSM 11490T) and Methylorubrum zatmanii comb. nov. (type strain NCIMB 12243T=DSM 5688T). The taxonomic position of several remaining species is also discussed.

Herein the members of the Subcommittee on Taxonomy of Rhizobia and Agrobacteria of the International Committee on Systematics of Prokaryotes review recent developments in rhizobial and agrobacterial taxonomy and propose updated minimal standards for the description of new species (and genera) in these groups. The essential requirements (minimal standards) for description of a new species are (1) a genome sequence of at least the proposed type strain and (2) evidence for differentiation from other species based on genome sequence comparisons. It is also recommended that (3) genetic variation within the species is documented with sequence data from several clearly different strains and (4) phenotypic features are described, and their variation documented with data from a relevant set of representative strains. Furthermore, it is encouraged that information is provided on (5) nodulation or pathogenicity phenotypes, as appropriate, with relevant gene sequences. These guidelines supplement the current rules of general bacterial taxonomy, which require (6) a name that conforms to the International Code of Nomenclature of Prokaryotes, (7) validation of the name by publication either directly in the International Journal of Systematic and Evolutionary Microbiology or in a validation list when published elsewhere, and (8) deposition of the type strain in two international culture collections in separate countries.

Strains of Gram-negative, rod-shaped, non-spore-forming bacteria were isolated from nitrogen-fixing nodules of the native legumes Listia angolensis (from Zambia) and Lupinus texensis (from Texas, USA). Phylogenetic analysis of the 16S rRNA gene showed that the novel strains belong to the genus Microvirga , with ≥96.1 % sequence similarity with type strains of this genus. The closest relative of the representative strains Lut6^T and WSM3557^T was Microvirga flocculans TFB^T, with 97.6–98.0 % similarity, while WSM3693^T was most closely related to Microvirga aerilata 5420S-16^T, with 98.8 % similarity. Analysis of the concatenated sequences of four housekeeping gene loci (dnaK, gyrB, recA and rpoB) and cellular fatty acid profiles confirmed the placement of Lut6^T, WSM3557^T and WSM3693^T within the genus Microvirga . DNA–DNA relatedness values, and physiological and biochemical tests allowed genotypic and phenotypic differentiation of Lut6^T, WSM3557^T and WSM3693^T from each other and from other Microvirga species with validly published names. The nodA sequence of Lut6^T was placed in a clade that contained strains of Rhizobium , Mesorhizobium and Sinorhizobium , while the 100 % identical nodA sequences of WSM3557^T and WSM3693^T clustered with Bradyrhizobium , Burkholderia and Methylobacterium strains. Concatenated sequences for nifD and nifH show that the sequences of Lut6^T, WSM3557^T and WSM3693^T were most closely related to that of Rhizobium etli CFN42^T nifDH. On the basis of genotypic, phenotypic and DNA relatedness data, three novel species of Microvirga are proposed: Microvirga lupini sp. nov. (type strain Lut6^T = LMG 26460^T = HAMBI 3236^T), Microvirga lotononidis sp. nov. (type strain WSM3557^T = LMG 26455^T = HAMBI 3237^T) and Microvirga zambiensis sp. nov. (type strain WSM3693^T = LMG 26454^T = HAMBI 3238^T).

Ensifer (Sinorhizobium) medicae is an effective nitrogen fixing microsymbiont of a diverse range of annual Medicago (medic) species. Strain WSM419 is an aerobic, motile, non-spore forming, Gram-negative rod isolated from a M. murex root nodule collected in Sardinia, Italy in 1981. WSM419 was manufactured commercially in Australia as an inoculant for annual medics during 1985 to 1993 due to its nitrogen fixation, saprophytic competence and acid tolerance properties. Here we describe the basic features of this organism, together with the complete genome sequence, and annotation. This is the first report of a complete genome sequence for a microsymbiont of the group of annual medic species adapted to acid soils. We reveal that its genome size is 6,817,576 bp encoding 6,518 protein-coding genes and 81 RNA only encoding genes. The genome contains a chromosome of size 3,781,904 bp and 3 plasmids of size 1,570,951 bp, 1,245,408 bp and 219,313 bp. The smallest plasmid is a feature unique to this medic microsymbiont.