Bradyrhizobium occurrence in nodules of perennial horsegram.

The introduction of a forage legume into a tropical pasture should decrease the need for N fertilizer, provided biological N2 fixation (BNF) contributes enough to compensate for exported N. Macrotyloma axillare (perennial horsegram) is a suitable legume for composing mixed pastures, and our hypothesis is that the isolation of indigenous rhizobia from roots and rhizosphere is the way of achieving an efficient inoculant to maximize BNF to the legume. Nodules and rhizosphere soil taken from M. axillare grown in a mixed pasture with palisade grass were sampled and used in a trap host assay using Leonard jars containing a mixture of vermiculite and sand. A total of ten bacteria were initially isolated using this technique. The isolates were then used in two experiments to evaluate the inoculation responses on the perennial horsegram in greenhouse conditions to which nodulation, plant growth, and shoot N accumulation were measured. Phylogenetic analyses based on 16S rRNA and recA placed all strains within genus Bradyrhizobium, some of them not previously described. The best strain provided more than 120 nodules and more than 65 mg of nodules per plant. Strain BR14182 was considered the most promising given the high dry matter and N accumulation in plant shoots. This study provides the first analysis of Bradyrhizobium diversity nodulating M. axillare in Brazil and provided evidence of the role of inoculation in incrementing the plant-rhizobium symbiosis in a forage legume.

Land-Use System and Forest Floor Explain Prokaryotic Metacommunity Structuring and Spatial Turnover in Amazonian Forest-to-Pasture Conversion Areas.

Advancing extensive cattle production is a major threat to biodiversity conservation in Amazonia. The dominant vegetation cover has a drastic impact on soil microbial communities, affecting their composition, structure, and ecological services. Herein, we explored relationships between land-use, soil types, and forest floor compartments on the prokaryotic metacommunity structuring in Western Amazonia. Soil samples were taken in sites under high anthropogenic pressure and distributed along a ±800 km gradient. Additionally, the litter and a root layer, characteristic of the forest environment, were sampled. DNA was extracted, and metacommunity composition and structure were assessed through 16S rRNA gene sequencing. Prokaryotic metacommunities in the bulk soil were strongly affected by pH, base and aluminum saturation, Ca + Mg concentration, the sum of bases, and silt percentage, due to land-use management and natural differences among the soil types. Higher alpha, beta, and gamma diversities were observed in sites with higher soil pH and fertility, such as pasture soils or fertile soils of the state of Acre. When taking litter and root layer communities into account, the beta diversity was significantly higher in the forest floor than in pasture bulk soil for all study regions. Our results show that the forest floor's prokaryotic metacommunity performs a spatial turnover hitherto underestimated to the regional scale of diversity.

Biotechnological potential of bacteria from genera Bacillus Paraburkholderia and Pseudomonas to control seed fungal pathogens.

Fungal pathogens are important determinants of plant dynamics in the environment. These pathogens can cause plant death and occasionally yield losses in crops, even at low initial densities in the soil. The objective of this study was to select and evaluate fungal antagonistic bacteria and to determine their biological control capacity in soybean seedlings. A total of 877 strains from the genera Pseudomonas, Bacillus, and Paraburkholderia/Burkholderia were screened, and their antagonistic effects on fungi frequently found in seeds were evaluated using four methods: quadruple plating, paired culture confrontation, strain containment, and inoculation of soybean seeds. The experimental design was completely randomized, with three replications for the first three methods and five replications in a 3 × 9 factorial scheme for the fourth treatment. The strains with the highest biotechnological potential were inoculated into soybean seeds to evaluate the biological control of fungi that attack this crop at germination. Seventy-nine strains presented some type of antagonistic effect on the tested fungi, with two strains presenting a broader antagonistic action spectrum in the seed test. In addition to the antagonistic potential, strains BR 10788 and BR 11793, when simultaneously inoculated or alone, significantly increased the seedling dry matter mass, and promoted the growth of soybean seedlings even in the presence of most fungi. Thus, this study demonstrated the efficiency of the antagonistic activity of these strains in relation to the target fungi, which proved to be potential agents for biological control.

Mimosa caesalpiniifolia Benth. adapts to rhizobia populations with differential taxonomy and symbiotic effectiveness outside of its location of origin.

Mimosa caesalpiniifolia Benth. is a legume native to the semi-arid region of Brazil, in the Northeast. Its successful adaptation to other locations, such as the Atlantic Forest in the Southeast region, may be related to its ability to establish symbiosis with nitrogen-fixing bacteria, especially ß-rhizobia of the genus Paraburkholderia. The objective of this work was to determine whether M. caesalpiniifolia adapted to bacterial symbionts in locals where it was introduced. Bacteria were recovered from nodules of M. caesalpiniifolia and characterized at the genetic level by BOX-PCR and sequencing of the 16S rRNA, recA, nifH, and nodC genes. Their symbiotic effectiveness was assessed under axenic conditions. M. caesalpiniifolia nodulated mainly with Paraburkholderia sabiae and a few strains of Rhizobium in the Southeast. On the other hand, the symbionts found in the Northeast were, predominantly, Paraburkholderia diazotrophica. Regardless of its origin, P. diazotrophica promoted a superior accumulation of plant biomass than other bacterial species. The results presented here demonstrate the ability of M. caesalpiniifolia to adapt to bacterial populations outside its location of origin, and indicate that, in this case, the symbiotic effectiveness was associated with the taxonomical classification of the strains.