Contrasting Microbial Taxonomic and Functional Colonisation Patterns in Wild Populations of the Pan-Palaeotropical C4 Grass, Themeda triandra

The interactions between native plants and soil microbiota are not well characterised, despite growing recognition of their importance for host plant fitness and ecological functioning. We used shotgun metagenomics to examine microbial taxonomic and functional colonisation patterns in wild populations of the pan-palaeotropical C4 grass, Themeda triandra, across a globally representative aridity gradient (aridity index 0.318–0.903). We investigated these patterns through the two-step selection process whereby microbes are recruited from bulk soils into rhizospheres (soil on the root surface), and root interiors (endospheres). We provide clear evidence of this process through decreasing microbial taxonomic diversity from bulk soil to T. triandra roots. Surprisingly, microbial functional potential showed the opposite trend: the diversity of potential functions (exponent of Shannon's diversity) increased from bulk soil to the rhizosphere and endosphere, but functional richness did not. Finally, we found that increasing aridity was associated with rhizospheres that were more compositionally similar, yet remained highly diverse in functional potential. Overall, aridity is strongly associated with the root-associated microbiome of T. triandra, selecting for microbiota that likely support plant resilience under dry conditions. Furthermore, microbial functional potential closely tracks taxonomic composition and aridity trends, highlighting how native plants can shape their microbial communities.