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

Abstract

Polhillia, Wiborgia and Wiborgiella species are shrub legumes endemic to the Cape fynbos of South Africa. They have the ability to fix atmospheric N2 when in symbiosis with soil bacteria called ‘rhizobia’. The aim of this study was to assess the morpho-physiological and phylogenetic characteristics of rhizobia associated with the nodulation of Polhillia, Wiborgia and Wiborgiella species growing in the Cape fynbos. The bacterial isolates from root nodules consisted of a mixture of fast and intermediate growers that differed in colony shape and size. The isolates exhibited tolerance to salinity (0.5–3% NaCl) and pH (pH 5–10) and different antibiotic concentrations, and could produce 0.51 to 51.23 µg mL−1 of indole-3-acetic acid (IAA), as well as solubilize tri-calcium phosphate. The ERIC-PCR results showed high genomic diversity in the rhizobial population and grouped them into two major clusters. Phylogenetic analysis based on 16S rRNA, atpD, glnII, gyrB, nifH and nodC gene sequences revealed distinct and novel evolutionary lineages related to the genus Rhizobium and Mesorhizobium, with some of them being very close to Mesorhizobium australicum. However, the phylogenetic analysis of glnII and nifH genes of some isolates showed incongruency.

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

“…The fact that this genus was absent in the microbial community during the dry season could suggest that this group of bacteria is highly endophytic and reside mainly inside root nodules, and available in the rhizosphere soils in very small numbers (˂0.5%), especially during the unfavorable dry seasons. Similarly, Mesorhizobium, Rhizobium , Agrobacterium and Pseudomonas bacteria, which were isolated from root nodules of these five shrub legumes [ 8 , 13 ], were also absent in this study, which is further evidence of the endophytic nature of these root-nodule bacteria.…”

“…The Actinobacteria phylum plays a major role in plant growth promotion through processes such as biological N 2 fixation, production of IAA, antibiotics and siderophores, as well as the solubilization of P, K and Zn [ 37 , 38 , 39 ]. In fact, antibiotic resistance, the production of IAA and the solubilization of P are common functional attributes of bacterial symbionts associated with Polhillia , Wiborgia and Wiborgiella species [ 13 ].…”

“…Using partial chromosomal and symbiotic gene sequences, Mpai et al [ 13 ] found a marked diversity of Bradyrhizobium bacteria present in the rhizosphere soil of Wiborgia obcordata at Bushmans Kloof. In fact, Bradyrhizobium was present in soil collected during the wet season (2.6%) but absent in soil from the dry season.…”

“…Similar to other shrub legumes in the Cape fynbos [ 5 , 11 , 12 ]; Polhillia , Wiborgia and Wiborgiella species are able to nodulate with a diversity of soil bacteria, including Mesorhizobium and Rhizobium species [ 13 ], indicating their potential in fynbos biodiversity improvement. Furthermore, they are reported to derive over 61% of their N nutrition from biological nitrogen fixation [ 8 ], which means they depend on soil microbes for survival.…”

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

“…The neighbor-joining phylogenies of the nifH and nodC genes were consistent with the 16S rRNA gene-based phylogeny, which suggested that these symbiotic genes were coevolved [ 27 , 32 , 35 - 36 ]. Incongruencies in the nifH and nodC phylogenies have been previously reported [ 37 , 38 ], and a higher sequence similarity of the nodC gene was reported in Bradyrhizobium and Mesorhizobium spp. [ 39 ].…”