Specificity in Legume-Rhizobia Symbioses

Most species in the Leguminosae (legume family) can fix atmospheric nitrogen (N2) via symbiotic bacteria (rhizobia) in root nodules. Here, the literature on legume-rhizobia symbioses in field soils was reviewed and genotypically characterised rhizobia related to the taxonomy of the legumes from which they were isolated. The Leguminosae was divided into three sub-families, the Caesalpinioideae, Mimosoideae and Papilionoideae. Bradyrhizobium spp. were the exclusive rhizobial symbionts of species in the Caesalpinioideae, but data are limited. Generally, a range of rhizobia genera nodulated legume species across the two Mimosoideae tribes Ingeae and Mimoseae, but Mimosa spp. show specificity towards Burkholderia in central and southern Brazil, Rhizobium/Ensifer in central Mexico and Cupriavidus in southern Uruguay. These specific symbioses are likely to be at least in part related to the relative occurrence of the potential symbionts in soils of the different regions. Generally, Papilionoideae species were promiscuous in relation to rhizobial symbionts, but specificity for rhizobial genus appears to hold at the tribe level for the Fabeae (Rhizobium), the genus level for Cytisus (Bradyrhizobium), Lupinus (Bradyrhizobium) and the New Zealand native Sophora spp. (Mesorhizobium) and species level for Cicer arietinum (Mesorhizobium), Listia bainesii (Methylobacterium) and Listia angolensis (Microvirga). Specificity for rhizobial species/symbiovar appears to hold for Galega officinalis (Neorhizobium galegeae sv. officinalis), Galega orientalis (Neorhizobium galegeae sv. orientalis), Hedysarum coronarium (Rhizobium sullae), Medicago laciniata (Ensifer meliloti sv. medicaginis), Medicago rigiduloides (Ensifer meliloti sv. rigiduloides) and Trifolium ambiguum (Rhizobium leguminosarum sv. trifolii). Lateral gene transfer of specific symbiosis genes within rhizobial genera is an important mechanism allowing legumes to form symbioses with rhizobia adapted to particular soils. Strain-specific legume rhizobia symbioses can develop in particular habitats.


Introduction
The Leguminosae (Fabaceae, the legume family) is comprised of ca. 19,300 species within 750 genera that occur as herbs, shrubs, vines or trees in mainly terrestrial habitats and are components of most of the world's vegetation types [1][2][3]. Currently, the legume family is divided into three sub-families, the Caesalpinioideae, Mimosoideae and Papilionoideae [3,4]. Members of the Caesalpinioideae are grouped into four tribes, the Caesalpinieae, Cassieae, Cercideae and Detarieae comprising ca. 170 genera and 2250 species. The Mimosoideae are grouped into two tribes, the Ingeae and Mimoseae with ca. 80 genera and 3270 species, while the Papilionoideae consists of 28 tribes with ca. 480 genera and 13,800 species. However, a new classification of the legumes has been proposed with six sub-families based on the plastid matK gene sequences from ca. 20% of all legume species across ca. 90% of all currently recognized genera [5]. The six sub-families proposed are a re-circumscribed Caesalpinioideae, Cercidoideae, Detarioideae, Dialioideae, Duparquetioideae and Papilionoideae. In this system, the currently recognized Mimosoideae is a distinct clade nested within the re-circumscribed Caesalpinioideae. Species within the Cercidoideae, Detarioideae, Dialioideae and Duparquetioideae do not nodulate [5,6].
Most legume species can fix atmospheric nitrogen (N 2 ) via symbiotic bacteria (general term "rhizobia") in root nodules, and this can give them an advantage under low soil nitrogen (N) conditions if other factors are favourable for growth [7,8]. Furthermore, N 2 fixation by legumes can be a major input of N into natural and agricultural ecosystems [9][10][11][12]. Generally, legume nodules can be classified as indeterminate or determinate in growth [13]. Indeterminate nodules maintain meristematic tissue, while determinate nodules have a transient meristem. Nodule type is dependent on host plant, and legume species that can produce both determinate and indeterminate nodules are rare [14,15]. All genera examined in the Caesalpinioideae and Mimosoideae had indeterminate nodules [13]. Within the Papilionoideae, most tribes had indeterminate nodules, but the Desmodieae, Phaseoleae, Psoraleae and some members of the Loteae show "desmodioid" determinate nodules and the Dalbergieae "aeschynomenoid" determinate nodules [13]. Desmodioid nodules have lenticels, and rhizobia "infected" tissue within them also contains uninfected cells. Aeschynomenoid nodules do not have lenticels, have uniform infected tissue and are always associated with lateral or adventitious roots. Where tested, species within the Desmodieae, Phaseoleae and Psoraleae had ureides as the main N-containing compound transported from nodules, but species in the Dalbergieae and Loteae transported amides/amino acids [13]. Indeterminate nodules have a single or branched apical meristem, and a few genera, such as Lupinus (Genisteae) and Listia (Crotalaria), have "lupinoid" nodules with two or more lateral meristems, which in some cases completely surround the subtending root [16,17]. Generally, indeterminate nodules have a mixture of infected and uninfected cells in the central nodule tissue, but lupinoid nodules, as for aeschynomenoid nodules (Dalbergiae), have uniformly-infected cells.
The nodulation process for almost all legumes studied is initiated by the legume production of a mix of compounds, mainly flavonoids, which induce the synthesis of NodD protein in rhizobia [18,19]. Different legumes produce different types/mixes of compounds. The NodD protein activates the transcription of other genes involved in the nodulation process, including those required to produce Nod factors, the signal molecules produced by the rhizobia and detected by the plant, which induce nodule organogenesis [20]. The nodABC genes encode for the proteins required to make the core Nod factor structure [18]. Nod factors from different rhizobia have a similar structure of a chitin-like N-acetyl glucosamine oligosaccharide backbone with a fatty acyl chain at the non-reducing end, but differ in their length of N-acetyl glucosamine oligosaccharide backbone and the length and saturation of the fatty acid chain. The Nod-factor core is modified by species-specific proteins, which results in various substitutions, including acetylation, glycosylation, methylation and sulfation. Perception of the Nod-factor signal in legumes is mediated by Nod factor receptors, which are plasma membrane localized serine/threonine receptor kinases in the case of the model legumes Lotus japonicus and Medicago truncatula [18,19].
The available data indicate that rhizobia enter the roots of most legume species via root hair infection [13]. Here, rhizobia enter root hairs, and host cell wall material grows around the developing "infection", forming an infection thread, which grows through the cortex of the root, branching repeatedly. Generally, rhizobia are released from the tips of these infection threads into membrane-bound structures within host cells called symbiosomes where they differentiate into their N 2 -fixing form known as bacteroids. However, all species examined in the Caesalpinioideae, except herbaceous Chamaecrista spp. and a few species in the Papilionoideae, retain their rhizobia within infection threads [2,13]. Bacteroids vary greatly in their level of differentiation and viability depending on the legume host [13,21]. The process of root hair infection can lead to the formation of either indeterminate or desmodioid determinate nodules. A second mode of rhizobial infection occurs with species in the Dalbergiae (aeschynemonoid nodules) where rhizobia enter roots at the sites of lateral root emergence ("crack" entry), and infection threads are not involved in the infection process [22,23]. Table 1. Legume-rhizobia symbioses in the legume sub-family Mimosoideae. All species have indeterminate nodules. Table 2. Legume-rhizobia symbioses in the inverted repeat lacking clade (IRLC) of the legume sub-family Papilionoideae. All species in the IRLC have indeterminate nodules.  Table 3. Legume-rhizobia symbioses of species in the sub-family Papilionoideae with indeterminate nodules excluding the inverted repeat lacking clade. Table 3. Legume-rhizobia symbioses of species in the sub-family Papilionoideae with indeterminate nodules excluding the inverted repeat lacking clade. Table 4. Legume-rhizobia symbioses of species in the sub-family Papilionoideae with determinate nodules. Table 4. Cont.
Here, the literature on legume-rhizobia symbioses in field soils was reviewed and genotypically characterised rhizobia related to the taxonomy of the legumes from which they were isolated. The objectives of the work were to collate data on legume rhizobia symbioses and then assess to what extent legume specificity for rhizobial symbionts is related to legume taxonomy.

Framework and Assumptions of Study
The general classification of the Leguminosae follows Lewis et al., 2005 [1], with updates [3,4]. The sub-families Caesalpinioideae, Mimosoideae and Papilionoideae are considered separately. The Papilionoideae is split into those that show indeterminate nodules and those that show determinate nodules. Those that show indeterminate nodules are further split into the IRLC and all other clades.
Nodulating bacteria were classified at the genus level, on the basis of sequences of the 16S rRNA gene (almost all cases), and/or the 16S-23S DNA intergenic spacer region, and/or common house-keeping genes, and/or DNA-DNA hybridisations, and these results are presented in the tables. Sequences for nif and nod genes are considered in the text. Rhizobial genus and species names validated in the International Journal of Systematic and Evolutionary Microbiology were used with one exception: Burkholderia was retained as opposed to using Paraburkholderia [301,302], as a case to reinstate Burkholderia is being prepared by workers in the field. The term symbiovar (sv.) is used when describing rhizobial strains within the same species that differ with respect to the legume species they effectively nodulate [303].
A comprehensive collation of published legume-rhizobia symbioses up until 30 September 2016 was carried out. Articles were collected by searching the Institute for Scientic Information (ISI) Web of Science using each legume genus partnered with each of the rhizobia, Bradyrhizobium, Burkholderia, Cupriavidus, Ensifer, Mesorhizobium, Rhizobium and Sinorhizobium as the keywords. Further searches were carried out on the literature quoted in the selected papers and those listed as quoting the selected papers in the ISI Web of Science. Only data for plants sampled under field conditions, or for plants grown in soils taken from the field, or supplied field soil extracts were used. Bacteria isolated from legume nodules were accepted as rhizobia if they were shown to produce functional (N 2 fixing) nodules on inoculation of their original legume host or a species within the original host legume genus under axenic conditions. The range of measurements and visual assessments used as evidence of the occurrence of N 2 fixation were accepted. These were acetylene reduction activity, red/pink nodules (evidence of leghaemoglobin and, hence, nodules assumed to be active), increased total plant or shoot dry matter or N content, visually greener (increased chlorophyll) and increased plant vigour. However, it is acknowledged that in some cases, greater growth, vigour and/or greenness could have been caused by plant hormone production by the bacterium [304]. All data obtained for all species are presented with three exceptions. Representative data are presented for Glycine max, Phaseolus vulgaris and Vigna unguiculata due to the large number of publications on these three species.

Caesalpinioideae-Rhizobia Symbioses
Of the three legume sub-families, the Caesalpinioideae contains the smallest proportion of nodulated genera with nodulation confirmed for Campsiandra, Chidlowia, Dimorphandra, Erythrophleum, Jacqueshuberia, Melanoxylon, Moldenhauwera and Tachigali in the tribe Caesalpinieae and Chamaecrista in the tribe Cassieae [2,13]. Only two studies have genotypically characterised bacteria confirmed as rhizobia of Caesalpinioideae species. Firstly, five rhizobial isolates from Dimorphandra wilsonii and one from Dimorphandra jorgei sampled in the Cerrado biome in Brazil were Bradyrhizobium [305]. Secondly, 166 rhizobial isolates from Erythrophleum fordii sampled at four sites in the Guangdong and Guangsii Provinces of the southern sub-tropical region of China were also all Bradyrhizobium [306]. In both studies, core and symbiosis gene sequences indicated that there were diverse and novel strains amongst the isolates.
Data are available for bacterial isolates from nodules of other Caesalpinioideae species, but their ability to produce N 2 fixing nodules on their legume host under axenic conditions was not tested.
Specifically, three isolates from Tachigali versicolor sampled on Barro Colorado Island, Panama, which were not tested on their original host plant, but were shown to nodulate Macroptilium atropurpureum, were Bradyrhizobium [307]. Similarly, strain STM934, stated to be confirmed as Bradyrhizobium, was isolated from nodules of Erythrophleum guineensis growing in natural forests of the Ziama reservation in southeast Guinea and shown to produce functional nodules on Macroptilium atropurpureum [308]. In this case, a re-inoculation experiment was carried out on the original host, but the substrate was non-sterile forest soil. Bradyrhizobium was isolated from and shown to nodulate Chamaecrista sampled in Kakadu National Park, Northern Territory, Australia, but N 2 fixation was not reported [309]. Furthermore, there are several reports that Bradyrhizobium inoculum can increase nodulation of Chamaecrista spp. under field conditions in Australia and China [310][311][312]. Thus, the available evidence indicates that Bradyrhizobium spp. are the dominant, possibly exclusive, rhizobial symbionts of legumes in the Caesalpinioideae, but data are limited, and the degree of specificity between legumes in the Caesalpinioideae and their rhizobial symbionts cannot be assessed without further work.

Mimosoideae-Rhizobia Symbioses
Rhizobia have been characterized from 15 species across seven genera in the tribe Ingeae and ca. 120 species from 13 genera in the tribe Mimoseae within the sub-family Mimosoideae (Table 1). Bradyrhizobium, Ensifer, Mesorhizobium and Rhizobium were each reported to nodulate species in the Ingeae and the Mimoseae. Furthermore, Ochrobactrum was reported to nodulate Acacia mangium (Ingeae); Allorhizobium and Devosia were reported to nodulate Neptunia natans (Mimoseae); and there are many reports that Cupriavidus and Burkholderia nodulate Mimosa spp. and related species (Mimoseae) ( Table 1). In addition, excepting Acacia auriculiformis (Ingeae) and Mimosa pigra (Mimoseae), all species that were examined in three or more separate studies, Acacia mangium, Acacia saligna, Calliandra grandiflora and Senegalia senegal (Ingeae), Leucaena leucocephala, Mimosa diplotricha, Mimosa pudica, Parapiptadenia rigida, Prosopis alba and Vachellia tortilis (Mimoseae), were nodulated by at least three different rhizobial genera. Thus, a range of rhizobial genera, including both αand β-proteobacteria, can nodulate legume species across the two Mimosoideae tribes, and generally, where tested over different studies, species within the Ingeae and Mimoseae tribes were promiscuous with respect to their rhizobial symbionts.
The Mimosa species examined across studies were the pan-tropical invasive Mimosa diplotricha and Mimosa pudica, and findings for these species appear not to reflect the situation with most Mimosa spp., which are endemic with a restricted range. The evidence indicates that most Mimosa spp. show specificity towards the rhizobial genus depending on their distribution with Burkholderia, Rhizobium/Ensifer and Cupriavidus, the main rhizobial symbiont of endemic Mimosa spp. in central and southern Brazil, central Mexico and southern Uruguay, respectively [73,74,79]. The 16S rRNA and housekeeping gene sequences were diverse for Mimosa Burkholderia symbionts in Brazil, Rhizobium/Ensifer in Mexico and Cupriavidus in Uruguay. For Burkholderia in Brazil and Rhizobium/Ensifer in Mexico, the symbiosis gene sequences were largely congruent with the 16S rRNA and housekeeping gene sequences, indicating that these genes diverged over a long period within Burkholderia without substantial horizontal gene transfer between species [73,74]. For Cupriavidus rhizobia in Uruguay, the nodA gene sequences were not congruent with the housekeeping gene sequences, but grouped together in a cluster [79]. This is strong evidence that the various Cupriavidus species obtained their symbiosis genes via within group lateral gene transfer [32, 79,313]. It is not known if endemic Mimosa spp. nodulated by a particular rhizobia genus can form N 2 -fixing symbioses with Mimosa rhizobia of different genera from outside their region.

The IRLC
Data are available for 103 species from 27 genera/five tribes in the IRLC with Ensifer, Mesorhizobium and Rhizobium, commonly, and Bradyrhizobium, Neorhizobium and Phyllobacterium, rarely, reported to nodulate species within this clade ( Table 2). There are no reports of Burkholderia or Cupriavidus symbionts within the IRLC. Previously, Galega officinalis, Galega orientalis and Hedysarum coronarium within the IRLC clade were reported to only form effective nodules with their respective symbionts Neorhizobium galegeae sv. officinalis, Neorhizobium galegeae sv. orientalis and Rhizobium sullae [120,145,296,297]. The data in Table 2 indicate two other specific relationships between IRLC legumes and rhizobia. Firstly, eight separate studies on Cicer arietinum carried out over different countries and continents reported Mesorhizobium as the only symbiont. The 16S rRNA and housekeeping gene sequences indicated that strains of Mesorhizobium ciceri and M. mediterraneum were common, but not exclusive Mesorhizobium symbionts of Cicer arietinum in most studies outside China, with M. muleiense the main symbiont in Northwest China [100]. Across studies where tested, nifH and nodC gene sequences were similar for all Mesorhizobium isolates shown to produce functional nodules on Cicer arietinum, indicating their specificity towards this legume species and that lateral transfer of these genes had occurred between the different Mesorhizobium spp. [97,99,100].
Secondly, for the tribe Fabeae, seventeen studies across five Lathyrus species, Lens culinaris, Pisum sativum and eleven Vicia spp. reported Rhizobium as the only symbiont. Across these studies, Rhizobium leguminosarum (and where tested, R. leguminosarum sv. viciae) was the most common symbiont with some varieties of Pisum sativum, such as cv. Afghanistan, only nodulated by specific strains of Rhizobium leguminosarum sv. viciae, which occur in soils in their native range in Afghanistan/Turkey [103]. Furthermore, in a study of 154 isolates of 18 Vicia species grown in 16 Chinese provinces, only 17 representative Rhizobium Leguminosarum sv. viciae isolates, from a wide range of potential rhizobia, produced fully-developed, effective ("colour red") nodules [115]. Thus, a highly specific relationship has developed between species in the Fabeae and R. leguminosarum sv. viciae, but it is not an exclusive relationship, as R. fabeae [116], R. multihospitium [105], R. pisi [111], R. laguerreae [314] and R. anhuiense [108] have been reported to effectively nodulate Fabeae species. However, the nifH and nodC gene sequences of all of these rhizobia showed high similarity, indicating their specificity towards the Fabeae species and that, in this case, lateral gene transfer had occurred between different Rhizobium spp. [32,108].
Within the tribe Trifolieae, 14 out of 16 Medicago/Melilotus spp. had Ensifer as symbiont, but in four cases not exclusively. The most studied species, Medicago sativa (lucerne), was commonly nodulated by Ensifer meliloti, which is the recommended inoculum for this legume crop, but it also had Neorhizobium and Rhizobium symbionts. However, Medicago laciniata and Medicago rigiduloides were found to only nodulate with Ensifer strains sampled in their native range in the Mediterranean Basin. These strains were formally described as Ensifer meliloti sv. medicaginis and Ensifer meliloti sv. rigiduloides, respectively [148,153]. Similarly, different Trifolium spp. have different compatibility with different strains of Rhizobium leguminosarum sv trifolii, which is the recommended inoculum for most Trifolium crops. Trifolium ambiguum, in particular, has been highlighted as only forming N 2 -fixing nodules with strains of Rhizobium leguminosarum sv trifolii specific to its region of origin (the Caucasus and Eastern Europe) [315].

Clades with Indeterminate Nodules, Excluding the IRLC
Data are shown for 113 species from 33 genera across 13 Papilionoideae tribes with indeterminate nodules that do not show the IRLC mutation (Table 3). Azorhizobium, Bradyrhizobium, Burkholderia, Ensifer, Mesorhizobium, Methylobacterium, Microvirga, Neorhizobium, Ochrobactrum, Pararhizobium, Phyllobacterium and Rhizobium were all reported to nodulate species within this group. Amorpha fruticosa and Dalea purpurea (Amorpheae), Retama sphaerocarpa and Spartium junceum (Genisteae), Coronilla varia (Loteae), Tephrosia falciformis and Tephrosia villosa (Millettiea), Gliricidia sepium and Robinia pseudoacacia (Robineae) and Sesbania sericea and Sesbania virgata (Sesbanieae) were nodulated by two rhizobial genera. Aspalathus linearis and Crotalaria pallida (Crotalarieae), Tephrosia purpurea (Millettiea), Sesbania cannabina, Sesbania punicea, Sesbania rostrata and Sesbania sesban (Sesbanieae), Sophora alopecuroides and Sophora flavescens (Sophoreae) and Ammopiptanthus nanus and Ammopiptanthus mongolicus (Thermopsideae) were all nodulated by at least three different rhizobial genera. Thus, generally, where tested, Papilionoideae species with indeterminate nodules excluding the IRLC were promiscuous in relation to rhizobial symbiont. Within the Genisteae, Bradyrhizobium was the only symbiont reported for nine Cytisus spp. across ten separate studies and three Genista spp. across three separate studies (Table 3). Furthermore, eight out of 10 Lupinus spp. (Genisteae) across 11 separate studies were nodulated by Bradyrhizobium. These results indicate that Bradyrhizobium may be the main symbionts of Genisteae species, but further work is required to confirm this. Generally, 16S rRNA and housekeeping gene sequences indicate that diverse Bradyrhizobium spp. form N 2 -fixing nodules on Cytisus spp. and Lupinus spp., but the diversity of their symbiosis genes is dependent on the geographical origin of the legumes. For example, most Bradyrhizobium isolates from native Lupinus spp. in Europe form a distinct lineage, 'clade 11', on the basis of their nodA gene sequences [180,195]. Similarly, different Bradyrhizobium spp. associated with native Cytisus villosus in Morocco all showed similar nodC and nifH sequences, which were closely related to those of Bradyrhizobium japonicum sv. genistearum [175]. In contrast, rhizobia sampled from invasive Cytisus scoparius sampled in six states in the United States, differed with respect to housekeeping and symbiosis gene sequences [174]. Specifically, one group of isolates had both housekeeping and symbiosis gene sequences similar to a Bradyrhizobium clade from native legumes in Western North America, but two clades had nifD, nifH and nodC sequences highly similar or identical to a Cytisus scoparius strain isolated in Spain, while their housekeeping genes were similar to American Bradyrhizobium clades. Thus, it appears that Bradyrhizobium ancestrally associated with native North American legumes have acquired symbiosis genes from European Cytisus scoparius Bradyrhizobium symbionts via lateral gene transfer.
The two exceptions to Bradyrhizobium as the rhizobial symbiont of Lupinus spp. were Ochrobactrum [181] and Microvirga [166], both of which are rare as rhizobial symbionts of legumes. The nodD sequence for Ochrobactrum and the nodA sequence for Microvirga indicated that both bacteria obtained their nod genes via horizontal gene transfer from more common rhizobial genera. Microvirga is also the only bacterial genus shown to form N 2 -fixing nodules on Listia angolensis [17]. Similarly, Listia bainesii has been found to only produce N 2 -fixing nodules with pink pigmented Methylobacterium [17]. It was suggested that the seasonally waterlogged habitat of Listia spp. may have resulted in the selection of unusual rhizobial symbionts adapted to their environments [17].
In one study in the Cape Floristic Region (CFR) of South Africa, Burkholderia was reported to be the exclusive symbiont of ten Cyclopia spp., Podalyria calyptera and Virgilia oroboides, all species in the Podalyrieae plus three Hypocalyptus spp. (Hypocalypteae) [191]. Burkholderia was confirmed to nodulate Podolyria calyptrata and Virgilia oroboides in the CFR [136,193,197]. The majority of Burkholderia isolates had unique nifH and nodA gene sequences, and the specificity of these symbioses needs testing.
Previously, Sesbania sesban was reported to be highly promiscuous with respect to rhizobial symbionts [31], and the data here indicate that this could be a genus level trait (Table 3). However, the reports that Sophora alopecuroides and Sophora flavescens sampled in China are nodulated by Ensifer, Mesorhizobium, Phyllobacterium and Rhizobium with a wide range of symbiosis gene sequences [207,208] contrasts with the finding that New Zealand (NZ) native Sophora spp. were exclusively nodulated by Mesorhizobium spp. with almost identical unique nodA and nodC gene sequences [210,316,317]. This emphasises that species within the same genus can vary greatly with respect to their specificity for rhizobial symbionts.

Clades with Determinate Nodules
The Dalbergieae are almost exclusively of tropical/sub-tropical distribution and show an aeschynomenoid determinate nodule structure [2]. Rhizobia have been characterised for 23 species from seven genera in the Dalbergieae, Adesmia, Aeschynomene, Arachis, Centrolobium, Dalbergia, Pterocarpus and Zornia (Table 4). Bradyrhizobium was found to nodulate all species, except Adesmia bicolor (Rhizobium), with Rhizobium also reported for Arachis hypogaea in two studies. Thus, on the data available, the Dalbergieae appear to be primarily nodulated by Bradyrhizobium. For Arachis hypogaea, twelve separate studies reported Bradyrhizobium as a rhizobial symbiont (Table 4). Across these studies, both core and symbiosis gene sequences indicated that Arachis hypogaea was nodulated by a diverse range of Bradyrhizobium spp. and are promiscuous with respect to Bradyrhizobium spp. Excepting Arachis hypogaea, data are limited for rhizobial symbionts of species in the Dalbergieae. However, the unusual ability of specific Bradyrhizobium strains that lack canonical nodABC genes to form N 2 -fixing nodules on roots and/or stems of particular Aeschynomene spp. is highlighted [34,215].
The closely related tribes Desmodieae, Phaseoleae and Psoraleae are also mainly of tropical/sub-tropical distribution, and with rare exceptions, species within these tribes showed a desmodioid nodule structure [2,15]. Rhizobia have been characterized for 25 species from three genera, Desmodium, Kummerowia and Lespedeza, in the Desmodieae (Table 4). Species from all three genera, Desmodium microphyllum, Desmodium racemosum, Desmodium sequax, Kummerowia striata, Lespedeza bicolor and Lespedeza daurica, were nodulated by rhizobia from three separate genera. Similarly, for 28 species across 14 genera within the Phaseoleae, there was no strong evidence for high specificity for rhizobial symbiont (Table 4). Phaseolus vulgaris and Vigna unguiculata have been highlighted as being promiscuous with respect to their rhizobial symbionts under field conditions. Data in Table 4 show that both species can be nodulated by different rhizobial genera in the α-proteobacteria, as well as Burkholderia in the β-proteobacteria. Across three studies, Phaseolus lunatus was reported to be nodulated by Bradyrhizobium and Rhizobium, while Vigna angularis, Vigna radiata and Vigna subterranea were reported to be nodulated by three separate rhizobial genera. Data are limited for other genera/species within the Phaseoleae with the exception of Glycine max, which is the main grain/oil seed legume grown worldwide, and Glycine soja. Both Glycine spp. were nodulated by Bradyrhizobium, Ensifer and Rhizobium. In the one case where separate studies were carried out on one species within the Psoraleae, Psoralea pinnata was nodulated by Bradyrhizobium, Burkholderia and Mesorhizobium [136,193,287]. Thus, where tested, species within the Desmodieae, Phaseoleae and Psoraleae were promiscuous with respect to their rhizobial symbionts.
Species in the Loteae, which show a desmodoid nodule structure, are of temperate distribution [2]. Data are available for 16 Lotus spp. within the Loteae across 13 separate studies. For all species examined in two or more studies, at least two rhizobia genera were reported as symbionts. Overall, the available data indicate that legume species with a desmodioid determinate nodule structure are promiscuous with respect to their rhizobia symbionts.

Legume Specificity for Rhizobial Symbionts
The objectives of the work were to collate data on legume rhizobia symbioses and assess the extent that legume specificity for rhizobial symbiont is related to legume taxonomy. Bradyrhizobium spp. were the exclusive rhizobial symbionts of species in the Caesalpinioideae; but, rhizobia were characterised for only three legume species over two studies, and the degree of specificity between legumes in the Caesalpinioideae and their rhizobial symbionts cannot be assessed without further work. Generally, species within the two Mimosoideae tribes, Ingeae and Mimoseae were promiscuous with respect to their rhizobial symbionts, but Mimosa spp. show specificity towards the rhizobia genus depending on their distribution, with Burkholderia, Rhizobium/Ensifer and Cupriavidus the main rhizobial symbiont of endemic Mimosa spp. in central and southern Brazil, central Mexico and southern Uruguay, respectively [73,74,79]. Papilionoideae species with indeterminate nodules were split into the IRLC and all other clades. A range of species within both groups nodulated with different rhizobia genera, but there was also strong evidence that some species within both groups showed specificity for rhizobial genus or species/symbiovar. Specificity for rhizobial genus appears to hold at the tribe level for the Fabeae (Rhizobium), the genus level for Cytisus (Bradyrhizobium), Lupinus (Bradyrhizobium) and NZ native Sophora spp. (Mesorhizobium) and the species level for Cicer arietinum (Mesorhizobium), Listia bainesii (Methylobacterium) and Listia angolensis (Microvirga). Specificity for rhizobial species/symbiovar appears to hold for Galega officinalis (Neorhizobium galegeae sv. officinalis), Galega orientalis (Neorhizobium galegeae sv. orientalis), Hedysarum coronarium (Rhizobium sullae), Medicago laciniata (Ensifer meliloti sv. medicaginis), Medicago rigiduloides (Ensifer meliloti sv. rigiduloides) and Trifolium ambiguum (Rhizobium leguminosarum sv. trifolii). For Papilionoideae with determinate nodules, the Dalbergieae (aeschynomenoid nodules) were primarily nodulated by Bradyrhizobium, while those in the Desmodieae, Phaseoleae, Psoraleae and Loteae (desmodioid nodules) were promiscuous with respect to rhizobial genus. Thus, on the data available, species in the Papilionoideae that show specificity for rhizobial genus, species or symbiovar have indeterminate nodules and are generally (but not exclusively) of temperate distribution. However, many temperate legumes with indeterminate nodules are promiscuous with respect to the rhizobial genus indicating that high specificity for rhizobial symbiont only occurs under specific conditions [318].
For Mimosa spp., specificity towards rhizobial genus depending on distribution is likely to be at least in part related to the relative occurrence of the potential symbionts in soils of the different regions [74,79]. For example, evidence indicates that Mimosa Cupriavidus symbionts are absent from soils in central and southern Brazil, while Mimosa Burkholderia symbionts are absent from soils in southern Uruguay. This has been related to soil characteristics with low pH favouring Burkholderia over Cupriavidus in Brazil, but high heavy metal content favouring Cupriavidus over Burkholderia in Uruguay [73,74,79]. It was proposed that native Mimosa in the different regions have selected symbiotic bacteria adapted to local conditions, which resulted in the development of highly specific associations [74,79]. However, for this to occur, such rhizobia must be available in the soil. The within genus 16S rRNA and housekeeping gene sequences were diverse for Mimosa symbionts in all regions. For Burkholderia in Brazil and Rhizobium/Ensifer in Mexico, the symbiosis gene sequences were largely congruent with the 16S rRNA and housekeeping gene sequences [73,74]. For Cupriavidus rhizobia in Uruguay, the nodA gene sequences were not congruent with the housekeeping gene sequences, but grouped together in a cluster, indicating that the various species within the group obtained their symbiosis genes via within group lateral gene transfer [79]. Lateral gene transfer is a mechanism whereby rhizobia and non-rhizobial bacteria adapted to local soil conditions could become specific rhizobial symbionts of legumes growing in these soils. The evidence described above indicates that lateral gene transfer of symbiosis genes has been important within the Papilionoideae in relation to the development of specific relationships between the Fabeae and Rhizobium spp., Cytisus and Bradyrhizobium spp., Lupinus and Bradyrhizobium spp., NZ native Sophora and Mesorhizobium spp. and Cicer arietinum and Mesorhizobium spp. Data are presented for Mesorhizobium isolates from Cicer arietinum and NZ native Sophora spp. to emphasise this point.
Firstly, 20 Mesorhizobium isolates from Cicer arietinum, sampled across three countries in five separate studies, showed diverse 16S rRNA sequences, but highly similar nodC sequences ( Figure 1A,B). Here, evidence is strong that native Mesorhizobium muleiense adapted to alkaline soils in Gansu and Xinjiang Provinces of China obtained its Cicer arietinum-specific symbiotic genes from Mesorhizobium ciceri or Mesorhizobium mediterraneum introduced together with Cicer arietinum used as a crop [100]. Secondly, 48 isolates from four NZ native Sophora spp. sampled at eight different field sites separated into eight groups and three individual isolates on the basis of their concatenated recA, gln11 and rpoβ gene sequences, but showed almost identical nodC (and nodA [210]) sequences (Figure 2A,B). Seven of the groups have been formally identified as new species [316,317]. This relationship between NZ native Sophora spp. and Mesorhizobium spp. with specific symbiosis gene sequences is highly specific as none of twenty rhizobial isolates from common weed and crop legumes in NZ produced functional nodules on the NZ native Sophora microphylla [190]. Furthermore, Mesorhizobium isolates from Carmichaelia, Clianthus and Montigena, the only other NZ native legume genera, did not nodulate NZ native Sophora microphylla or Sophora tetraptera [127]. Mesorhizobium isolates from Carmichaelia, Clianthus and Montigena had unique nodC sequences different from those of Sophora Mesorhizobium isolates ( Figure 2B). However, in some cases, their 16S rRNA, recA and gln11 sequences were similar to those of Sophora Mesorhizobium isolates, emphasizing the importance of the specific symbiosis genes in the NZ Sophora Mesorhizobium symbiosis [137]. Generally, Sophora isolates from the same field site grouped together on concatenated recA, gln11 and rpoβ gene sequences ( Figure 2B). This apparent link between housekeeping gene sequences and field site is compatible with the proposal that lateral transfer of symbiosis genes to Mesorhizobium strains adapted to local soil conditions has occurred.      Some varieties of Pisum sativum, such as cv. Afghanistan, are only nodulated by specific strains of Rhizobium leguminosarum sv. viciae, which occur in soils in their native range in Afghanistan/Turkey [103]. The ability of these strains to nodulate Pisum sativum cv. Afghanistan is controlled by a single recessive gene, sym2 in the plant. The sym2 allele interacts with a specific gene, nodX, present in R. leguminosarum sv. viciae strains able to nodulate cv. Afghanistan [319]. The nodX gene product acetylates a Nod factor, which mediates a specific compatible interaction with this cultivar [320,321]. Furthermore, Medicago laciniata and Medicago rigiduloides were found to only nodulate with Ensifer meliloti strains (E. meliloti sv. medicaginis and E. meliloti sv. rigiduloides, respectively) sampled in their native range in the Mediterranean Basin [148,153], and Trifolium ambiguum only forms N 2 -fixing nodules with strains of Rhizobium leguminosarum sv. trifolii specific to its region of origin, the Caucasus and Eastern Europe [315]. The mechanisms of these highly specific relationships are not fully understood, but the nodA gene sequences of E. meliloti sv. medicaginis and nodA, nodB and nodC gene sequences of E. meliloti sv. rigiduloides diverged from those of E. meliloti strains, which nodulated other Medicago spp. [148,153], while the ability of certain strains of Rhizobium leguminosarum sv. trifolii to effectively nodulate Trifolium ambiguum, but not Trifolium repens (white clover) appears to be linked to a 111-bp insertion in their nifH/fixA intergenic region [315]. The factors that have resulted in these highly specific relationships are not known. Furthermore, it is not known if these highly specific relationships reflect adaptation of the symbioses and result in greater rates of N 2 fixation or more efficient N 2 fixation, as would be expected on theoretical grounds [322], and this warrants further study.

Conclusions
Overall, the data indicate that lateral gene transfer of specific symbiosis genes within rhizobial genera is an important mechanism allowing legumes to form symbioses with rhizobia adapted to particular soils. It also maintains specificity between legume species and rhizobia species with specific symbiosis genes. Strain-specific legume rhizobia symbioses can develop in particular habitats.