Invasive Robinia pseudoacacia in China is nodulated by Mesorhizobium and Sinorhizobium species that share similar nodulation genes with native American symbionts

The aim of this work is to describe the diversity of potentially symbiotic bacteria associated with the invasive introduced legume Robinia pseudoacacia in China. Thirty-three isolates from 33 separate trees and nodules were characterized using restriction length fragment polymorphism and sequencing of 16S rRNA, nodA, nodC and nifH genes. Their 16S rRNA gene patterns and sequences placed them in three clades: 85% of isolates were related to the Mesorhizobium mediterraneum/temperatum group, whereas the remaining were similar either to Mesorhizobium amorphae or to Sinorhizobium meliloti . However, despite their diverse taxonomic positions, the nodA, nodC and nifH genes' phylogenies indicated that these R. pseudoacacia symbionts share similar symbiosis genes, implying gene transfers and a degree of host specificity. Comparison of R. pseudoacacia symbiotic diversity in native and other invaded areas suggests that most Chinese symbionts may not have arrived with the seed but were local bacteria that acquired specific symbiotic genes from native American rhizobia.     

Diversity of N-acyl homoserine lactone-producing and -degrading bacteria in soil and tobacco rhizosphere

In Gram-negative bacteria, quorum-sensing (QS) communication is mostly mediated by N-acyl homoserine lactones (N-AHSL). The diversity of bacterial populations that produce or inactivate the N-AHSL signal in soil and tobacco rhizosphere was investigated by restriction fragment length polymorphism (RFLP) analysis of amplified 16S DNA and DNA sequencing. Such analysis indicated the occurrence of N-AHSL-producing strains among the alpha-, beta- and gamma-proteobacteria, including genera known to produce N-AHSL (Rhizobium, Sinorhizobium and Pseudomonas) and novel genera with no previously identified N-AHSL-producing isolates (Variovorax, Sphingomonas and Massilia). The diversity of N-AHSL signals was also investigated in relation to the genetic diversity of the isolates. However, N-AHSL-degrading strains isolated from soil samples belonged to the Bacillus genus, while strains isolated from tobacco rhizospheres belonged to both the Bacillus genus and to the alpha subgroup of proteobacteria, suggesting that diversity of N-AHSL-degrading strains may be modulated by the presence of the tobacco plant. Among these rhizospheric isolates, novel N-AHSL-degrading genera have been identified (Sphingomonas and Bosea). As the first simultaneous analysis of both N-AHSL-degrading and -producing bacterial communities in a complex environment, this study revealed the coexistence of bacterial isolates, belonging to the same genus or species that may produce or degrade N-AHSL.     

Nitrogen-fixing nodules from rose wood legume trees (Dalbergia spp.) endemic to Madagascar host seven different genera belonging to alpha- and beta-Proteobacteria

Although legume biodiversity is concentrated in tropical regions, the majority of studies on legume nodulating bacteria (LNB) are focused on cultivated leguminous plants from temperate regions. However, recent works on tropical regions tend to indicate that the actual diversity of LNB is largely underestimated. In this study, we report the isolation and characterization of 68 nitrogen-fixing root nodule bacteria collected from eight endemic tree species of Dalbergia in Madagascar. The isolates were characterized by (i) restriction fragment length polymorphism (RFLP) analysis of 16S-IGS rDNA, (ii) 16S rDNA gene sequencing and (iii) nodulation tests. Results revealed a wide diversity of bacteria present in the nodules of Dalbergia. Among the 68 isolated bacteria, 65 belonged to Bradyrhizobium, Mesorhizobium, Rhizobium, Azorhizobium and Phyllobacterium from the alpha-class of Proteobacteria, and three isolates belonged to Burkholderia and Ralstonia from the beta-class of Proteobacteria. Our results also show for the first time that a strain belonging to the Burkholderia cepacia complex is able to induce efficient nodules on a legume plant.     

Cupriavidus necator isolates are able to fix nitrogen in symbiosis with different legume species

The aim of the present study was to identify a collection of 35 Cupriavidus isolates at the species level and to examine their capacity to nodulate and fix N(2). These isolates were previously obtained from the root nodules of two promiscuous trap species, Phaseolus vulgaris and Leucaena leucocephala, inoculated with soil samples collected near Sesbania virgata plants growing in Minas Gerais (Brazil) pastures. Phenotypic and genotypic methods applied for this study were SDS-PAGE of whole-cell proteins, and 16S rRNA and gyrB gene sequencing. To confirm the ability to nodulate and fix N(2), the presence of the nodC and nifH genes was also determined, and an experiment was carried out with two representative isolates in order to authenticate them as legume nodule symbionts. All 35 isolates belonged to the betaproteobacterium Cupriavidus necator, they possessed the nodC and nifH genes, and two representative isolates were able to nodulate five different promiscuous legume species: Mimosa caesalpiniaefolia, L. leucocephala, Macroptilium atropurpureum, P. vulgaris and Vigna unguiculata. This is the first study to demonstrate that C. necator can nodulate legume species.     

Genetic diversity of bradyrhizobial populations from diverse geographic origins that nodulate Lupinus spp. and Ornithopus spp

The genetic diversity of 45 bradyrhizobial isolates that nodulate several Lupinus and Ornithopus species in different geographic locations was investigated by 16S rDNA PCR-RFLP and sequence analysis, 16S-23S rDNA intergenic spacer (IGS) PCR-RFLP analysis, and ERIC-PCR genomic fingerprinting. Reference strains of Bradyrhizobium japonicum, B. liaoningense and B. elkanii and some Canarian isolates from endemic woody legumes in the tribe Genisteae were also included. The 16S rDNA-RFLP analysis resolved 9 genotypes of lupin isolates, a group of fourteen isolates presented restriction-genotypes identical or very similar to B. japonicum, while another two main groups of isolates (69%) presented genotypes that clearly separated them from the reference species of soybean. 16S rDNA sequencing of representative strains largely agreed with restriction analysis, except for a group of six isolates, and showed that all the lupin isolates are relatives of B. japonicum, but different lineages were observed. The 16S-23S IGS-RFLP analysis showed a high resolution level, resolving 19 distinct genotypes among 30 strains analysed, and so demonstrating the heterogeneity of the 16S-RFLP groups. ERIC-PCR fingerprint analysis showed an enormous genetic diversity producing a different pattern for each but two of the isolates. Phylogeny of nodC gene was independent from the 16S rRNA phylogeny, and showed a tight relationship in the symbiotic region of the lupin isolates with isolates from Canarian genistoid woody legumes, and in concordance, cross-nodulation was found. We conclude that Lupinus is a promiscuous host legume that is nodulated by rhizobia with very different chromosomal genotypes, which could even belong to several species of Bradyrhizobium. No correlation among genomic background, original host plant and geographic location was found, so, different chromosomal genotypes could be detected at a single site and in a same plant species, on the contrary, an identical genotype was detected in very different geographical locations and plants.     

Unexpectedly diverse Mesorhizobium strains and Rhizobium leguminosarum nodulate native legume genera of New Zealand, while introduced legume weeds are nodulated by Bradyrhizobium species

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.     

Diverse rhizobia associated with woody legumes Wisteria sinensis, Cercis racemosa and Amorpha fruticosa grown in the temperate zone of China

Fifty-nine bacterial isolates from root nodules of the woody legumes Wisteria sinensis, Cercis racemosa and Amorpha fruticosa grown in the central and eastern regions of China were characterized with phenotypic analysis, PCR-based 16S and 23S rRNA gene RFLP, Box PCR and 16S rRNA gene sequencing. Seven main phena were defined in numerical taxonomy, which corresponded to distinct groups within the genera Agrobacterium, Bradyrhizobium, Mesorhizobium and Rhizobium in 16S and 23S rRNA gene PCR-RFLP. The phylogenetic relationships of the 16S rRNA genes supported the grouping results of PCR-RFLP. Most of the isolates from Amorpha fruticosa were classified into two groups closely related to Mesorhizobium amorphae. Seventeen of the 21 isolates from Wisteria sinensis were identified as two groups related to Rhizobium and Agrobacterium. Six out of 10 isolates from Cercis racemosa were identified as a group related to Bradyrhizobium. Our results indicated that each of the investigated legumes nodulated mainly with one or two rhizobial groups, although isolates from different plants intermingled in some small bacterial groups. In addition, correlation between geographic origin and grouping results was found in the isolates from Amorpha fruticosa. These results revealed that the symbiotic bacteria might have been selected by both the legume hosts and the geographic factors.     

Diverse bacteria isolated from root nodules of Phaseolus vulgaris and species within the genera Campylotropis and Cassia grown in China

Eighty bacterial isolates from root nodules of the leguminous plants Phaseolus vulgaris, Campylotropis spp. and Cassia spp. grown in China were classified into five groups by phenotypic analyses, SDS-PAGE of whole-cell proteins, PCR-based 16S rRNA gene restriction-fragment-length-polymorphism and sequencing. Thirty-three isolates from the three plant genera were identified as Agrobacterium tumefaciens because they are closely related to the type strain of A. tumefaciens. Fourteen isolates from P. vulgaris grown in Yunnan and Inner Mongolia were classified as R. leguminosarum bv. phaseoli based on their close relationship with the type strain in numerical taxonomy and in 16S rDNA phylogeny. Twenty-seven isolates from Campylotropis delavayi, P. vulgaris and four species of Cassia grown in the central zones of China were classified into three groups within the genus Bradyrhizobium. One of these three groups could be defined as Bradyrhizobium japonicum. Our results demonstrated that P. vulgaris and the species of Campylotropis and Cassia could form nodules with diverse rhizobia in Chinese soils, including novel lineages associated with P. vulgaris. These results also offered information about the convergent evolution between rhizobia and legumes since the rhizobial populations associated with P. vulgaris in Chinese soils were completely different from those in Mexico, the original cite of this plant. Some rhizobial species could be found in all of the three leguminous genera.     

Genetic diversity and phylogeny of rhizobia isolated from Caragana microphylla growing in desert soil in Ningxia, China

Rhizobia are soil bacteria with the capacity to induce nitrogen-fixing nodules on the roots or stems of legume plants. A total of 40 bacterial isolates from the root nodules of Caragana microphylla growing in desert soil in Ningxia, China, were analyzed for genetic diversity and phylogenetic position. These isolates were classified into 7 types of 16S ribosomal DNA (rDNA) using polymerase chain reaction-restriction fragment length polymorphism analysis. They were grouped into 4 clades, Rhizobium-Agrobacterium, Sinorhizobium, Phyllobacterium, and Bradyrhizobium, when the phylogenies of 16S rDNA, recA, and atpD genes were applied. Phylogenetic analysis showed that the tree generated from the 16S rDNA sequencing agreed with that produced from the recA and atpD genes. By analyzing phylogenetic relationship using the 3 loci, the isolates in the branches of Phyllobacterium and Sinorhizobium could be identified as P. brassicacearum and S. meliloti. The isolates in the branch of Rhizobium-Agrobacterium were the most abundant microsymbiont of C. microphylla and were designated R. leguminosarum, R. galegae, R. alamii, and A. tumefaciens. Two isolates with low sequence similarity to the known species of Bradyrhizobium might be novel species in this genus.     

Phenotypic and genotypic diversity of Genista saharae microsymbionts from the infra-arid region of Tunisia

Aims: Genista saharae, indigenous of Sahara, is a spontaneous shrub that plays an important ecological role for the preservation and fertility of poor and eroded soils. This legume has not been examined for its root nodule bacteria. The taxonomic diversity of bacteria from root nodules of G. saharae growing in the infra-arid region of Tunisia was investigated. Methods and Results: A total of 28 bacterial strains isolated from root nodules of G. saharae grown in Tunisian soil were characterized using a polyphasic approach including phenotypic characteristics, PCR-RFLP of 16S rDNA and 16S rRNA gene sequencing. It was found that new isolates are diverse and affiliated to Ensifer (75%), Rhizobium (10%) and Phyllobacterium (15%). The Phyllobacterium isolates lacked the capacity for nodule formation on this plant. Conclusions: Genista saharae formed nodules with diverse rhizobia in Tunisian soils. Furthermore, our results support the presence of non-nodulating commensal strains (Phyllobacterium) in legumes nodule. Significance and Impact of the Study: This study is the first report on the characterization of G. saharae microsymbionts in Tunisia.     

Genetic markers for analysing symbiotic relationships and lateral gene transfer in Neotropical bradyrhizobia

Assays with seven sets of lineage-specific polymerase chain reaction (PCR) primers in the ribosomal RNA region were performed on 96 isolates of the Bradyrhizobium sp. nodule bacteria from Barro Colorado Island, Panama. The isolates were derived from 10 legume host species in six genera (Centrosema, Desmodium, Dioclea, Inga, Machaerium and Vigna). The PCR assays differentiated 13 composite genotypes, and sequencing of a 5' 23S rRNA region indicated that all but one had a unique sequence. The most common genotype (seen in 44% of the isolates) was associated with all six legume host genera, and had a marker profile and 5' 23S rRNA sequence identical to a Bradyrhizobium lineage associated with several other legume genera in Panama and Costa Rica. Another 46% of the isolates had genotypes found to be associated with two to three legume genera. Bradyrhizobium strains with low host specificity thus appear to be prevalent in this tropical forest. Based on 16S rRNA and 5' 23S rRNA markers, most of the isolates had clear affinities to either B. japonicum or B. elkanii. However, one strain (Cp5-3) with a B. elkanii-type 16S rRNA marker had a 5' 23S rRNA region resembling B. japonicum. A partition homogeneity test indicated that relationships of strain Cp5-3 were significantly discordant for 16S rRNA vs. 23S rRNA sequences, and a runs test detected significant mosaic structure across the rRNA region. Lateral gene transfer events have therefore played a role in the evolution of symbiotic bacteria in this environment.     

Phylogenetic diversity of non-nodulating Rhizobium associated with pine ectomycorrhizae

Most Rhizobium species described are symbionts that form nodules on legume roots; however, non-nodulating strains of Rhizobium are also widespread in nature. Unfortunately, knowledge of non-nodulating Rhizobium is quite limited compared with nodulating Rhizobium . Here, we studied the phylogenetic diversity of Rhizobium species that inhabit Japanese red pine roots ( Pinus densiflora ). Because fine roots of pine trees are usually colonized by ectomycorrhizal fungi in nature, we mainly used ectomycorrhizal root tips for bacterial isolation. Out of 1195 bacteria isolated from 75 independent root samples from the field and greenhouse experiments, 102 isolates were confirmed to be Rhizobium following partial 16S rRNA gene analysis. Rhizobium species were occasionally dominant in culturable bacterial communities, whereas no Rhizobium species were isolated from the soil itself. Molecular phylogenetic analyses using 16S rRNA, atpD , and recA gene sequences revealed that isolated Rhizobium strains were phylogenetically diverse and that several were distantly related to known Rhizobium species. Considering that a single species of pine is associated with unique and phylogenetically diverse Rhizobium populations, we should pay more attention to non-nodulating strains to better understand the diversity, ecology, and evolution of the genus Rhizobium and plant– Rhizobium associations.     

Diversity of endophytic bacteria within nodules of the Sphaerophysa salsula in different regions of Loess Plateau in China

A total of 115 endophytic bacteria were isolated from root nodules of the wild legume Sphaerophysa salsula grown in two ecological regions of Loess Plateau in China. The genetic diversity and phylogeny of the strains were revealed by restriction fragment length polymorphism and sequencing of 16S rRNA gene and enterobacterial repetitive intergenic consensus-PCR. Their symbiotic capacity was checked by nodulation tests and analysis of nifH gene sequence. This is the first systematic study on endophytic bacteria associated with S. salsula root nodules. Fifty of the strains found were symbiotic bacteria belonging to eight putative species in the genera Mesorhizobium, Rhizobium and Sinorhizobium, harboring similar nifH genes; Mesorhizobium gobiense was the main group and 65 strains were nonsymbiotic bacteria related to 17 species in the genera Paracoccus, Sphingomonas, Inquilinus, Pseudomonas, Serratia, Mycobacterium, Nocardia, Streptomyces, Paenibacillus, Brevibacillus, Staphylococcus, Lysinibacillus and Bacillus, which were universally coexistent with symbiotic bacteria in the nodules. Differing from other similar studies, the present study is the first time that symbiotic and nonsymbiotic bacteria have been simultaneously isolated from the same root nodules, offering the possibility to accurately reveal the correlation between these two kinds of bacteria. These results provide valuable information about the interactions among the symbiotic bacteria, nonsymbiotic bacteria and their habitats.     

Assessing Genotypic Diversity and Symbiotic Efficiency of Five Rhizobial Legume Interactions Under Cadium Stress for Soil Phytoremediation

In the framework of soil phytoremediation using local legume plants coupled with their native root-nodulating bacteria to increase forage yields and preserve contaminated soils in arid regions of Tunisia, we investigated the diversity of bacteria from root nodules of Lathyrus sativus, Lens culinaris, Medicago marina, M. truncatula, and M. minima and the symbiotic efficiency of these five legume symbiosis under Cadmium stress. Fifty bacterial strains were characterized using physiological and biochemical features such heavy metals resistant, and PCR-RFLP of 16S rDNA. Taxonomically, the isolates nodulating L. sativus, and L. culinaris are species within the genera Rhizobium and the ones associated to Medicago sp, within the genera Sinorhizobium. The results revealed also that the cadmium tolerance of the different legumes-rhizobia interaction was as follows: M. minima<M. truncatula<M. marina<L. sativus<L. culinaris indicating that the effect of Cadmium on root nodulation and biomass production is more deleterious on M. minima-S. meliloti and M. truncatula-S. meliloti than in other symbiosis. Knowledge on genetic and functional diversity of M. marina, L. sativus and L. culinaris microsymbiotes is very useful for inoculant strain selection and can be selected to develop inoculants for soil phytoremediation.     

Bradyrhizobium sp. nodulating the Mediterranean shrub Spanish broom (Spartium junceum L.)

AIMS: The molecular diversity of 25 strains of rhizobia, isolated in Sicily from root nodules of the Mediterranean shrubby legume Spanish broom (Spartium junceum L.), is presented in relation to the known rhizobial reference strains. METHODS AND RESULTS: Our approach to the study of the S. junceum rhizobial diversity combined the information given by the 16S and the intergenic spacer (IGS) 16S-23S rDNA polymorphic region by obtaining them in a single polymerase chain reaction (PCR) step. The PCR fragment size of the S. junceum isolates was 2400-2500 bp and that of the reference strains varied from 2400 in Bradyrhizobium strains to 2800 in Sinorhizobium strains. Inter- and intrageneric length variability was found among the reference strains. Restriction fragment length polymorphisms (RFLP) analysis allowed us to identify eight genotypes among the S. junceum rhizobia that were clustered into two groups, both related to the Bradyrhizobium lineage. Sequencing of representative strains of the two clusters confirmed these data. The 16S-IGS PCR-RFLP approach, when applied to rhizobial reference strains, allowed very close species (i.e. Rhizobium leguminosarum/R. tropici) to be separated with any of the three enzymes used; however, cluster analysis revealed inconsistencies with the 16S-based phylogenesis of rhizobia. CONCLUSIONS: Rhizobia nodulating S. junceum in the Mediterranean region belong to the Bradyrhizobium lineage. Our results confirm the resolution power of the 16S-23S rDNA in distinguishing among rhizobia genera and species, as well as the usefulness of the PCR-RFLP method applied to the entire 16S-IGS region for a rapid tracking of the known relatives of new isolates. SIGNIFICANCE AND IMPACT OF THE STUDY: The present paper is, to our knowledge, the first report on rhizobia nodulating a Mediterranean wild woody legume.     

Divergent Bradyrhizobium symbionts on Tachigali versicolor from Barro Colorado Island, Panama

Relationships of root-nodule bacteria from the tree Tachigali versicolor (legume subfamily Caesalpinioideae) were analyzed for 20 isolates sampled from juvenile plants growing on Barro Colorado Island (BCI), Panama. Bacterial genetic diversity appeared to be low. In the highly polymorphic 5' intervening sequence region of 23S rRNA, all isolates had the same length variant. A 472 bp segment spanning this region was sequenced in four isolates, and all proved to be identical at every nucleotide position. RFLP analysis of a 868 bp fragment of the nitrogenase alpha-subunit gene likewise indicated that all 20 isolates shared an identical set of restriction sites. Phylogenetic analysis of both partial 23S rRNA and nearly full-length 16S rRNA sequences showed that bacterial symbionts of T. versicolor fall into the genus Bradyrhizobium. However, they are divergent from the bradyrhizobia associated with other BCI legumes, as well as from other currently known bacteria in this genus. Inoculation tests with two promiscuously-nodulating legumes showed that bacteria from T. versicolor were unable to form nodules on Vigna unguiculata, but did nodulate Macroptilium atropurpureum, although the nodules lacked nitrogen fixation activity. The association of Tachigali with a divergent lineage of Bradyrhizobium is noteworthy in view of this plant's position within a clade of the mostly non-nodulating "primitive" legume subfamily Caesalpinioideae that gave rise to the predominantly nodulating subfamily Mimosoideae.     

Mercury-resistant rhizobial bacteria isolated from nodules of leguminous plants growing in high Hg-contaminated soils

A survey of symbiotic bacteria from legumes grown in high mercury-contaminated soils (Almadén, Spain) was performed to produce a collection of rhizobia which could be well adapted to the environmental conditions of this region and be used for restoration practices. Nineteen Hg-tolerant rhizobia were isolated from nodules of 11 legume species (of the genera Medicago, Trifolium, Vicia, Lupinus, Phaseolus, and Retama) and characterized. Based on their growth on Hg-supplemented media, the isolates were classified into three susceptibility groups. The minimum inhibitory concentrations (MICs) and the effective concentrations that produce 50% mortality identified the patterns of mercury tolerance and showed that 15 isolates were tolerant. The dynamics of cell growth during incubation with mercury showed that five isolates were unaffected by exposure to Hg concentrations under the MICs. Genetic analyses of the 16S rRNA gene assigned ten strains to Rhizobium leguminosarum, six to Ensifer medicae, two to Bradyrhizobium canariense, and one to Rhizobium radiobacter. Inoculation of host plants and analysis of the nodC genes revealed that most of them were symbiotically effective. Finally, three isolates were selected for bioremediation processes with restoration purposes on the basis of their levels of Hg tolerance, their response to high concentrations of this heavy metal, and their genetic affiliation and nodulation capacity.     

Genetic characterization of fast-growing rhizobia able to nodulate Prosopis alba in North Spain

Prosopis is a Mimosaceae legume tree indigenous to South America and not naturalized in Europe. In this work 18 rhizobial strains nodulating Prosopis alba roots were isolated from a soil in North Spain that belong to eight different randomly amplified polymorphic DNA groups phylogenetically related to Sinorhizobium medicae, Sinorhizobium meliloti and Rhizobium giardinii according to their intergenic spacer and 16S rRNA gene sequences. The nodC genes of isolates close to S. medicae and S. meliloti were identical to those of S. medicae USDA 1,037(T) and S. meliloti LMG 6,133(T) and accordingly all these strains were able to nodulate both alfalfa and Prosopis. These nodC genes were phylogenetically divergent from those of the isolates close to R. giardinii that were identical to that of R. giardinii H152(T) and therefore all these strains formed nodules in common beans and Prosopis. The nodC genes of the strains isolated in Spain were phylogenetically divergent from that carried by Mesorhizobium chacoense Pr-5(T) and Sinorhizobium arboris LMG 1,4919(T) nodulating Prosopis in America and Africa, respectively. Therefore, Prosopis is a promiscuous host which can establish symbiosis with strains carrying very divergent nodC genes and this promiscuity may be an important advantage for this legume tree to be used in reforestation.     

Phylogenetic diversity of culturable bacteria from Antarctic sandy intertidal sediments

The diversity of culturable bacteria associated with sandy intertidal sediments from the coastal regions of the Chinese Antarctic Zhongshan Station on the Larsemann Hills (Princess Elizabeth Land, East Antarctica) was investigated. A total of 65 aerobic heterotrophic bacterial strains were isolated at 4°C. Microscopy and 16S rRNA gene sequence analysis indicated that the isolates were dominated by Gram-negative bacteria, while only 16 Gram-positive strains were isolated. The bacterial isolates fell in five phylogenetic groups: Alpha- and Gammaproteobacteria, Bacteroidetes, Actinobacteria and Firmicutes. Based on phylogenetic trees, all the 65 isolates were sorted into 29 main clusters, corresponding to at least 29 different genera. Based on sequence analysis (<98% sequence similarity), the Antarctic isolates belonged to at least 37 different bacterial species, and 14 of the 37 bacterial species (37.8%) represented potentially novel taxa. These results indicated a high culturable diversity within the bacterial community of the Antarctic sandy intertidal sediments.     

Symbiotic Relationships of Legumes and Nodule Bacteria on Barro Colorado Island,Panama: A Review

New data on 129 bacterial isolates were analyzed together with prior samples to characterize community-level patterns of legume–rhizobial symbiosis on Barro Colorado Island (BCI), Panama. Nodules have been sampled from 24 BCI legume species in 18 genera, representing about one quarter of the legume species and one half of the genera on the island. Most BCI legumes associated exclusively with nodule symbionts in the genus Bradyrhizobium, which comprised 86.3% of all isolates (315 of 365). Most of the remaining isolates (44 of 365) belonged to the β-proteobacterial genus Burkholderia; these were restricted to two genera in the legume subfamily Mimosoideae. Multilocus sequence analysis indicated that BCI Bradyrhizobium strains were differentiated into at least eight lineages with deoxyribonucleic acid divergence of the same magnitude as found among currently recognized species in this bacterial genus. Two of these lineages were widely distributed across BCI legumes. One lineage was utilized by 15 host species of diverse life form (herbs, lianas, and trees) in 12 genera spanning two legume subfamilies. A second common lineage closely related to the taxon B. elkanii was associated with at least five legume genera in four separate tribes. Thus, BCI legume species from diverse clades within the family frequently share interaction with a few common lineages of nodule symbionts. However, certain host species were associated with unique symbiont lineages that have not been found on other coexisting BCI legumes. More comprehensive sampling of host taxa will be needed to characterize the overall diversity of nodule bacteria and the patterns of symbiont sharing among legumes in this community.