Rhizosphere bacterial communities of Namib Desert plant species: Evidence of specialised plant-microbe associations

Rhizosphere microbial communities are intimately associated with plant root surfaces. The rhizosphere microbiome is recruited from the surrounding soil and is known to impact positively on the plant host via enhanced resistance to pathogens, increased nutrient availability, growth stimulation and increased resistance to desiccation. Desert ecosystems harbour a diversity of perennial and annual plant species, generally exhibiting considerable physiological adaptation to the low-water environment. In this study, we explored the rhizosphere bacterial microbiomes associated with selected desert plant species. The rhizosphere bacterial communities of 11 plant species from the central Namib Desert were assessed using 16S rRNA gene-dependent phylogenetic analyses. The rhizosphere microbial community of each host plant species was compared with control soils collected from their immediate vicinity, and with those of all other host plants. Rhizosphere and control soil bacterial communities differed significantly and were influenced by both location and plant species. Rhizosphere-associated genera included 67 known plant growth-promoting taxa, including Rhizobium, Bacillus, Microvirga, Kocuria and Paenibacillus. Other than Kocuria, these genera constituted the 'core' rhizosphere bacterial microbiome, defined as being present in > 90 % of the rhizosphere communities. Nine of the 11 desert plant species harboured varying numbers and proportions of species-specific microbial taxa. Predictive analyses of functional pathways linked to rhizosphere microbial taxa showed that these were significantly enriched in the biosynthesis or degradation of a variety of substances such as sugars, secondary metabolites, phenolic compounds and antimicrobials. Overall, our data suggest that plant species in the Namib Desert recruit unique taxa to their rhizosphere bacterial microbiomes that may contribute to their resilience in this extreme environment.