Bacterial diversity in the rhizosphere of Proteaceae species

The Cape Floral Kingdom is an area of unique plant biodiversity in South Africa with exceptional concentrations of rare and endemic species and experiencing drastic habitat loss. Here we present the first molecular study of the microbial diversity associated with the rhizosphere soil of endemic plants of the Proteaceae family (Leucospermum truncatulum and Leucadendron xanthoconus). Genomic DNA was extracted from L. truncatulum rhizosphere soil, L. xanthoconus rhizosphere and non-rhizosphere soil and used as a template for the polymerase chain reaction (PCR) amplification of the 16S ribosomal RNA gene (rDNA). Construction and sequencing of 16S rDNA libraries revealed a high level of biodiversity and led to the identification of several novel bacterial phylotypes. The bacterial community profiles were compared by 16S rDNA denaturing gradient gel electrophoresis (DGGE). Cluster analysis and biodiversity indices revealed that the rhizosphere soil samples were more similar to each other than to non-rhizosphere soil and the rhizosphere soil contained a bacterial diversity that was richer and more equitable compared with non-rhizosphere soil. A Chloroflexus and an Azospirillum genospecies were restricted to the L. xanthoconus rhizosphere soil and Stenotrophomonas genospecies was identified in all rhizosphere soil samples but was not present in the non-rhizosphere soil. Taxon-specific nested PCR and DGGE-identified differences between the Proteaceae plant rhizosphere soil with a Frankia genospecies restricted the L. truncatulum rhizosphere. Archaea-specific rDNA PCR, DGGE and DNA sequencing revealed that Crenarcheote genospecies were excluded from the plant rhizosphere soil and only present in non-rhizosphere soil.