Increasing aridity strengthens core bacterial rhizosphere associations in the pan-palaeotropical C4 grass, Themeda triandra

Interactions between soil microbiota and keystone plant species in grassland ecosystems are poorly understood, yet these interactions are fundamental to grassland ecosystem dynamics (e.g., responses to climate change). Here, we used 16S rRNA amplicon sequencing to characterise microbiota in rhizospheres and bulk soils associated with the keystone grass species, Themeda triandra, a pan-palaeotropical perennial C4 grass with known soil-microbial interactions related to growth promotion and drought tolerance. We applied this method to eight populations across a 3-fold aridity gradient (annual aridity index range = 0.293 to 0.903) in southern Australia. By examining the relative contributions of climatic, soil abiotic, ecological, and host-specific phenotypic traits, we identified the ecological drivers of core T. triandra-associated microbiota. We show that aridity had the strongest effect on shaping these core microbiotas, and report greater abundance of differentially-expressed bacterial taxa from the core rhizospheres in more arid T. triandra populations. These results suggest that T. triandra naturally growing under semi-arid conditions is more reliant on a greater diversity of rhizosphere core taxa than plants growing in wetter conditions. Our study underscores the likely importance of targeted recruitment of bacteria into the rhizosphere by grassland keystone species, such as T. triandra, when growing in arid conditions. This recruitment is expected to become even more important under climate change.