Biological potential of Ascophyllum nodosum extract on rhizobial diversity in nodules of mothbean Vigna aconitifolia Jacq. via Amplified Ribosomal DNA Restriction Analysis

Aim: The aim of this study was to use Ascophyllum nodosum for potentially increasing the growth and rhizobial diversity in nodulating rhizobia in Vigna aconitifolia. Methodology: Different concentrations of Ascophyllum nodosum extracts (0.01%, 0.02%, 0.05%, 0.10% and 0.50%) were applied via foliar spray and on roots of Vigna aconitifolia. Growth characteristics and Amplified Ribosomal DNA Restriction Analysis were conducted to detect the morphological and molecular changes in rhizobial diversity. The restriction profiles thus obtained were used to study the rhizobial communities via Cluster analysis and Dendrogram using NTSYS-PC program and UPGMA constructed. Results: Roots treated with 0.05% Ascophyllum nodosum extract showed best growth of plants. This concentration not only proved best for the aggregation of nodules but also for obtaining enormous rhizobial diversity. Interpretation: Ascophyllum nodosum is a modern, cheap, non-toxic natural biofertilizer and Amplified Ribosomal DNA Restriction Analysis represents a favorable alternative to culture dependent method for assessing rhizobial diversity in nodulating bacteria.

Dominant associations of Ensifer medicae-Medicage polymorpha and Ensifer meliloti-Medicago lupulina in farmland and natural ecosystem

Aims The nitrogen-fixing rhizobia associated with Medicago polymorpha and M. lupulina in Yunnan, China have been poorly documented. This study aims to analyze the diversity of rhizobia isolated from these two Medicago species and investigate the impact of abiotic (soil properties) and biotic (plant hosts) factors on Medicago-associated rhizobia in this region. Methods 91 rhizobial isolates were characterized by RFLP of 16S rDNA and 16S–23S IGS, BOX-PCR fingerprinting, nodulation assays and phylogeny analyses based on housekeeping and symbiosis genes. The genetic diversity of the rhizobial isolates was assessed by the BOX AIR pattern and Shannon index. Additionally, the correlation of soil properties and rhizobial distribution was determined by the constrained analysis of principle coordinates (CAP) based on Bray-Curtis distance of presence/absence (PA) transformed species data. Results All the tested strains fell in the genus Ensifer and divided into two species, E. medicae and E. meliloti. Both host plants and soil properties contributed to the rhizobial diversity. For either E. meliloti or E. medicae, isolates from native host plants tended to be more genetically diverse than those of the same species from non-native hosts. The soil edaphic factor analysis elucidated that nitrogen, organic matter as well as Ca2+ and Na+ are the key factors to shape the biogeographical distribution of rhizobia. Conclusions This study evidenced the microsymbiont preference of M. polymorpha to E. medicae and M. lupulina to E. meliloti, but also revealed the considerable impacts of both plant hosts and soil factors on the rhizobial diversity and biodistribution.

Different ecological processes drive the assembly of dominant and rare root-associated bacteria in a promiscuous legume

Understanding how plant-associated microbial communities assemble and the roles they play in plant performance are major goals in microbial ecology. For nitrogen-fixing rhizobia, assemblages are mostly determined by filtering by the host as well as abiotic soil conditions. However, for legumes adapted to highly variable environments and nutrient-poor soils, filtering out rhizobial partners may not be an effective strategy to ensure symbiotic benefits. As a consequence, this can lead to net increases in rhizobial diversity driven by stochastic (neutral) assembly processes. Here, we addressed whether symbiotic promiscuity of rooibos (Aspalathus linearis Burm. Dahlgren), reflects increases in rhizobial diversity that are independent of the environmental conditions, following a neutral assembly. We performed a common garden experiment to assess whether root system size and location- and habitat-specific rhizobial propagule pools of rooibos affected rhizobial community diversity and composition. We found a dominance of drift in driving taxonomic turnover in the root nodule communities, but operating at different scales in the dominant Mesorhizobium symbionts and the rest of bacterial taxa. Specifically, drift triggered differentiation between the core rhizobial symbionts of rooibos due to dispersal limitation on a regional scale, whereas it dominated the assembly of non-dominant rhizobial taxa at the root level. This suggests the existence of a significant neutral component in rhizobial community assembly when selectivity by the host plant is low. We conclude that in this promiscuous legume neutral processes govern bacterial community root nodule community assembly, but that these operate at different scales in dominant and rare rhizobial symbionts of the plant.