RESUMO

Tripartite interactions between legumes and their root symbionts (rhizobia and arbuscular mycorrhizal fungi, AMF) are poorly understood, although it is well established that only specific combinations of symbionts lead to optimal plant growth. A classic example in which to investigate such interactions is the Brazilian legume tree Piptadenia gonoacantha (Caesalpinioideae), for which efficient nodulation has been described as dependent on the presence of AMF symbiosis. In this study, we compared the nodulation behaviour of several rhizobial strains with or without AMF inoculation, and performed analyses on nodulation, nodule cytology, N-fixing efficiency, and plant growth response. Nodulation of P. gonoacantha does not rely on the presence of AMF, but mycorrhization was rhizobial strain-dependent, and nodule effectiveness and plant growth were dependent on the presence of specific combinations of rhizobial strains and AMF. The co-occurrence of both symbionts within efficient nodules and the differentiation of bacteroids within nodule cells were also demonstrated. Novel close interactions and interdependency for the establishment and/or functioning of these symbioses were also revealed in Piptadenia, thanks to immunocytochemical analyses. These data are discussed in terms of the evolutionary position of the newly circumscribed mimosoid clade within the Caesalpinioid subfamily and its relative proximity to non-nodulated (but AMF-associated) basal subfamilies.

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RESUMO

Some species of the genus Paraburkholderia that are able to nodulate and fix nitrogen in symbiosis with legumes are called ß-rhizobia and represent a group of ecological and biotechnological importance. We used Mimosa pudica and Phaseolus vulgaris to trap 427 rhizobial isolates from rhizospheric soil of Mimoseae trees in the Brazilian Atlantic Forest. Eighty-four representative strains were selected according to the 16S rRNA haplotypes and taxonomically characterized using a concatenated 16S rRNA-recA phylogeny. Most strains were assembled in the genus Paraburkholderia, including Paraburkholderia sabiae and Pa. nodosa. Mesorhizobium (α-rhizobia) and Cupriavidus (ß-rhizobia) were also isolated, but in smaller proportions. Multilocus sequence analysis and BOX-PCR analyses indicated that six clusters of Paraburkholderia represent potential new species. In the phylogenetic analysis of the nodC gene, the majority of the strains were positioned in the same groups as in the 16S rRNA-recA tree, indicative of stability and vertical inheritance, but we also identified horizontal transfer of nodC in Pa. sabiae. All α- and ß-rhizobial species were trapped by both legumes, although preferences of the host plants for specific rhizobial species have been observed.

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RESUMO

During a survey of root-nodulating symbionts of Mimosoid species in the south-east region of Brazil, eight Paraburkholderia isolates were obtained from nodules of the legume species Piptadenia gonoacantha, either from the field or following a soil trapping method with the same plant host. 16S rRNA gene as well as recA and gyrB phylogenetic markers placed these strains in two new clades within the genus Burkholderia sensu lato. DNA-DNA hybridization values and analyses of average nucleotide identities of the whole genome sequence of selected strains in each clade (STM 7183 and STM 7296) showed that the two clades represented novel species of the genus Paraburkholderia. All eight isolates were further characterized using DNA base content determination, chemotaxonomic and biochemical profiling and symbiotic properties, which allowed to distinguish the novel species from known diazotrophic species of the genus Paraburkholderia. Based on genomic and phenotypic data, the names Paraburkholderia piptadeniae sp. nov. with type strain STM 7183T (=DSM 101189T=LMG 29163T) and Paraburkholderia ribeironis sp. nov. with type strain STM 7296T (=DSM 101188T=LMG 29351T) are proposed.

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RESUMO

The bacterial genus Burkholderia comprises species occupying several habitats, including a group of symbionts of leguminous plants-also called beta-rhizobia-that has been recently ascribed to the new genus Paraburkholderia We used common bean (Phaseolus vulgaris L.) plants to trap rhizobia from an undisturbed soil of the Brazilian Cerrado under the vegetation type 'Cerradão'. Genetic characterization started with the analyses of 181 isolates by BOX-PCR, where the majority revealed unique profiles, indicating high inter- and intra-species diversity. Restriction fragment length polymorphism-PCR of the 16S rRNA of representative strains of the BOX-PCR groups indicated two main clusters, and gene-sequencing analysis identified the minority (27%) as Rhizobium and the majority (73%) as Paraburkholderia Phylogenetic analyses of the 16S rRNA and housekeeping (recA and gyrB) genes positioned all strains of the second cluster in the species P. nodosa, and the phylogeny of a symbiotic gene-nodC-was in agreement with the conserved genes. All isolates were stable vis-à-vis nodulating common bean, but, in general, with a low capacity for fixing N2, although some effective strains were identified. The predominance of P. nodosa might be associated with the edaphic properties of the Cerrado biome, and might represent an important role in terms of maintenance of the ecosystem, which is characterized by acid soils with high saturation of aluminum and low N2 content.

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RESUMO

The legume genus Mimosa has > 500 species, with two major centres of diversity, Brazil (c. 350 spp.) and Mexico (c. 100 spp.). In Brazil most species are nodulated by Burkholderia. Here we asked whether this is also true of native and endemic Mexican species. We have tested this apparent affinity for betaproteobacteria by examining the symbionts of native and endemic species of Mimosa in Mexico, especially from the central highlands where Mimosa spp. have diversified. Nodules were tested for betaproteobacteria using in situ immunolocalization. Rhizobia isolated from the nodules were genetically characterized and tested for their ability to nodulate Mimosa spp. Immunological analysis of 25 host taxa suggested that most (including all the highland endemics) were not nodulated by betaproteobacteria. Phylogenetic analyses of 16S rRNA, recA, nodA, nodC and nifH genes from 87 strains isolated from 20 taxa confirmed that the endemic Mexican Mimosa species favoured alphaproteobacteria in the genera Rhizobium and Ensifer: this was confirmed by nodulation tests. Host phylogeny, geographic isolation and coevolution with symbionts derived from very different soils have potentially contributed to the striking difference in the choice of symbiotic partners by Mexican and Brazilian Mimosa species.

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RESUMO

Burkholderia legume symbionts (also called α-rhizobia) are ancient in origin and are the main nitrogen-fixing symbionts of species belonging to the large genus Mimosa in Brazil. We investigated the extent of the affinity between Burkholderia and species in the tribe Mimoseae by studying symbionts of the genera Piptadenia (P.), Parapiptadenia (Pp.), Pseudopiptadenia (Ps.), Pityrocarpa (Py.), Anadenanthera (A.) and Microlobius (Mi.), all of which are native to Brazil and are phylogenetically close to Mimosa, and which together with Mimosa comprise the "Piptadenia group". We characterized 196 strains sampled from 18 species from 17 locations in Brazil using two neutral markers and two symbiotic genes in order to assess their species affiliations and the evolution of their symbiosis genes. We found that Burkholderia are common and highly diversified symbionts of species in the Piptadenia group, comprising nine Burkholderia species, of which three are new ones and one was never reported as symbiotic (B. phenoliruptrix). However, α-rhizobia were also detected and were occasionally dominant on a few species. A strong sampling site effect on the rhizobial nature of symbionts was detected, with the symbiont pattern of the same legume species changing drastically from location to location, even switching from ß to α-rhizobia. Coinoculation assays showed a strong affinity of all the Piptadenia group species towards Burkholderia genotypes, with the exception of Mi. foetidus. Phylogenetic analyses of neutral and symbiotic markers showed that symbiosis genes in Burkholderia from the Piptadenia group have evolved mainly through vertical transfer, but also by horizontal transfer in two species.

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RESUMO

Rhizobium mesoamericanum STM3625 is a Mimosa pudica symbiont isolated in French Guiana. This strain serves as a model bacterium for comparison of adaptation to mutualism (symbiotic traits, bacterial genetic programs for plant infection) between alpha and beta rhizobial symbionts of Mimosa pudica.

RESUMO

Five strains, JPY461(T), JPY359, JPY389, DPU-3 and STM4206 were isolated from nitrogen-fixing nodules on the roots of Mimosa spp. and their taxonomic positions were investigated using a polyphasic approach. All five strains grew at 15-40 °C (optimum, 30-37 °C), at pH 4.0-8.0 (optimum, pH 6.0-7.0) and with 0-1 % (w/v) NaCl [optimum, 0 % (w/v)]. On the basis of 16S rRNA gene sequence analysis, a representative strain (JPY461(T)) showed 97.2 % sequence similarity to the closest related species Burkholderia acidipaludis SA33(T), a similarity of 97.2 % to Burkholderia terrae KMY02(T), 97.1 % to Burkholderia phymatum STM815(T) and 97.1 % to Burkholderia hospita LMG 20598(T). The predominant fatty acids of the five novel strains were summed feature 2 (comprising C(16 : 1) iso I and/or C(14 : 0) 3-OH), summed feature 3 (comprising C(16 : 1)ω7c and/or C(16 : 1)ω6c), C(16 : 0) , C(16 : 0) 3-OH, C(17 : 0) cyclo, C(18 : 1)ω7c and C(19 : 0) cyclo ω8c. The major isoprenoid quinone was Q-8 and the DNA G+C content of the strains was 63.0-65.0 mol%. The polar lipid profile consisted of a mixture of phosphatidylethanolamine, phosphatidylglycerol, diphosphatidylglycerol, an unidentified aminophospholipid, an unidentified aminolipid and several unidentified phospholipids. The DNA-DNA relatedness of the novel strain with respect to recognized species of the genus Burkholderia was less than 54 %. On the basis of 16S rRNA and recA gene sequence similarities, chemotaxonomic and phenotypic data, the five strains represent a novel species in the genus Burkholderia, for which the name Burkholderia diazotrophica sp. nov. is proposed with the type strain, JPY461(T) ( = LMG 26031(T) = BCRC 80259(T) = KCTC 23308(T)).

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RESUMO

The genetic diversity of 221 Mimosa pudica bacterial symbionts trapped from eight soils from diverse environments in French Guiana was assessed by 16S rRNA PCR-RFLP, REP-PCR fingerprints, as well as by phylogenies of their 16S rRNA and recA housekeeping genes, and by their nifH, nodA and nodC symbiotic genes. Interestingly, we found a large diversity of beta-rhizobia, with Burkholderia phymatum and Burkholderia tuberum being the most frequent and diverse symbiotic species. Other species were also found, such as Burkholderia mimosarum, an unnamed Burkholderia species and, for the first time in South America, Cupriavidus taiwanensis. The sampling site had a strong influence on the diversity of the symbionts sampled, and the specific distributions of symbiotic populations between the soils were related to soil composition in some cases. Some alpha-rhizobial strains taxonomically close to Rhizobium endophyticum were also trapped in one soil, and these carried two copies of the nodA gene, a feature not previously reported. Phylogenies of nodA, nodC and nifH genes showed a monophyly of symbiotic genes for beta-rhizobia isolated from Mimosa spp., indicative of a long history of interaction between beta-rhizobia and Mimosa species. Based on their symbiotic gene phylogenies and legume hosts, B. tuberum was shown to contain two large biovars: one specific to the mimosoid genus Mimosa and one to South African papilionoid legumes.

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RESUMO

Tropical aquatic legumes of the genus Aeschynomene are unique in that they can be stem-nodulated by photosynthetic bradyrhizobia. Moreover, a recent study demonstrated that two Aeschynomene indica symbionts lack canonical nod genes, thereby raising questions about the distribution of such atypical symbioses among rhizobial-legume interactions. Population structure and genomic diversity were compared among stem-nodulating bradyrhizobia isolated from various Aeschynomene species of Central America and Tropical Africa. Phylogenetic analyses based on the recA gene and whole-genome amplified fragment length polymorphism (AFLP) fingerprints on 110 bacterial strains highlighted that all the photosynthetic strains form a separate cluster among bradyrhizobia, with no obvious structuring according to their geographical or plant origins. Nod-independent symbiosis was present in all sampling areas and seemed to be linked to Aeschynomene host species. However, it was not strictly dependent on photosynthetic ability, as exemplified by a newly identified cluster of strains that lacked canonical nod genes and efficiently stem-nodulated A. indica, but were not photosynthetic. Interestingly, the phenotypic properties of this new cluster of bacteria were reflected by their phylogenetical position, as being intermediate in distance between classical root-nodulatingBradyrhizobium spp. and photosynthetic ones. This result opens new prospects about stem-nodulating bradyrhizobial evolution.

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