Ecological adaptation and phylogenetic analysis of microsymbionts nodulating Polhillia, Wiborgia and Wiborgiella species in the Cape fynbos, South Africa

Nodulation Experiment by Cross-Inoculation of Nitrogen-Fixing Bacteria Isolated from Root Nodules of Several Leguminous Plants

Root-nodule nitrogen-fixing bacteria are known for being specific to particular legumes. This study isolated the endophytic root-nodule bacteria from the nodules of legumes and examined them to determine whether they could be used to promote the formation of nodules in other legumes. Forty-six isolates were collected from five leguminous plants and screened for housekeeping (16S rRNA), nitrogen fixation (nifH), and nodulation (nodC) genes. Based on the 16S rRNA gene sequencing and phylogenetic analysis, the bacterial isolates WC15, WC16, WC24, and GM5 were identified as Rhizobium, Sphingomonas, Methylobacterium, and Bradyrhizobium, respectively. The four isolates were found to have the nifH gene, and the study confirmed that one isolate (GM5) had both the nifH and nodC genes. The Salkowski method was used to measure the isolated bacteria for their capacity to produce phytohormone indole acetic acid (IAA). Additional experiments were performed to examine the effect of the isolated bacteria on root morphology and nodulation. Among the four tested isolates, both WC24 and GM5 induced nodulation in Glycine max. The gene expression studies revealed that GM5 had a higher expression of the nifH gene. The existence and expression of the nitrogen-fixing genes implied that the tested strain had the ability to fix the atmospheric nitrogen. These findings demonstrated that a nitrogen-fixing bacterium, Methylobacterium (WC24), isolated from a Trifolium repens, induced the formation of root nodules in non-host leguminous plants (Glycine max). This suggested the potential application of these rhizobia as biofertilizer. Further studies are required to verify the N2-fixing efficiency of the isolates.

Symbiotic efficiency of Rhizobium leguminosarum sv. trifolii strains originating from the subpolar and temperate climate regions

Red clover (Trifolium pratense L.) is a forage legume cultivated worldwide. This plant is capable of establishing a nitrogen-fixing symbiosis with Rhizobium leguminosarum symbiovar trifolii strains. To date, no comparative analysis of the symbiotic properties and heterogeneity of T. pratense microsymbionts derived from two distinct geographic regions has been performed. In this study, the symbiotic properties of strains originating from the subpolar and temperate climate zones in a wide range of temperatures (10-25 °C) have been characterized. Our results indicate that all the studied T. pratense microsymbionts from two geographic regions were highly efficient in host plant nodulation and nitrogen fixation in a wide range of temperatures. However, some differences between the populations and between the strains within the individual population examined were observed. Based on the nodC and nifH sequences, the symbiotic diversity of the strains was estimated. In general, 13 alleles for nodC and for nifH were identified. Moreover, 21 and 61 polymorphic sites in the nodC and nifH sequences were found, respectively, indicating that the latter gene shows higher heterogeneity than the former one. Among the nodC and nifH alleles, three genotypes (I-III) were the most frequent, whereas the other alleles (IV-XIII) proved to be unique for the individual strains. Based on the nodC and nifH allele types, 20 nodC-nifH genotypes were identified. Among them, the most frequent were three genotypes marked as A (6 strains), B (5 strains), and C (3 strains). Type A was exclusively found in the temperate strains, whereas types B and C were identified in the subpolar strains. The remaining 17 genotypes were found in single strains. In conclusion, our data indicate that R. leguminosarum sv. trifolii strains derived from two climatic zones show a high diversity with respect to the symbiotic efficiency and heterogeneity. However, some of the R. leguminosarum sv. trifolii strains exhibit very good symbiotic potential in the wide range of the temperatures tested; hence, they may be used in the future for improvement of legume crop production.

Seasonal Effect on Bacterial Communities Associated with the Rhizospheres of Polhillia, Wiborgia and Wiborgiella Species in the Cape Fynbos, South Africa

The Cape fynbos biome in South Africa is home to highly diverse and endemic shrub legumes, which include species of Aspalathus, Polhillia, Wiborgia and Wiborgiella. These species play a significant role in improving soil fertility due to their ability to fix N₂. However, information regarding their microbiome is still unknown. Using the 16S rRNA Miseq illumina sequencing, this study assessed the bacterial community structure associated with the rhizospheres of Polhillia pallens, Polhillia brevicalyx, Wiborgia obcordata, Wiborgia sericea and Wiborgiella sessilifolia growing at different locations during the wet and dry seasons in the Cape fynbos. The results showed that the most dominant bacterial phylum was Actinobacteria during both the dry (56.2–37.2%) and wet (46.3–33.3%) seasons. Unclassified bacterial genera (19.9–27.7%) were the largest inhabitants in the rhizospheres of all five species during the two seasons. The other dominant phyla included Bacteroidetes, Acidobacteria, Proteobacteria and Firmicutes. Mycobacterium and Conexibacter genera were the biggest populations found in the rhizosphere soil of all five test species during both seasons, except for W. obcordata soil sampled during the dry season, which had Dehalogenimonas as the major inhabitant (6.08%). In this study plant species and growth season were the major drivers of microbial community structure, with W. obcordata having the greatest influence on its microbiome than the other test species. The wet season promoted greater microbial diversity than the dry season.