Arid Soil Bacterial Legacies Improve Drought Resilience of the Keystone Grass, <scp><i>Themeda triandra</i></scp>

Plant-microbe interactions are critical to ecosystem functioning and impact soil legacies, where plants exert a lasting influence on the microbial and physicochemical conditions of the soils in which they grow. These soil legacies can affect subsequent plant growth and fitness. Specifically, biotic soil legacies can influence microbially associated plant fitness through the movement of soil microbiota in a two-step selection process: Microbes are recruited from bulk soil into the rhizosphere (the space around roots) and then into the endosphere (within plant roots). Furthermore, these endosphere root microbiota can also influence plant behaviour, shaping bulk soil communities over time. However, the potential of these soil legacies to provide host plant drought tolerance remains poorly understood. In a drought stress greenhouse trial, we show that arid soil legacies increased the biomass of the keystone grass Themeda triandra under both drought and control conditions. We report strong positive associations between T. triandra biomass and bacterial alpha diversity across soils, rhizospheres and endospheres. These findings show that bacterial soil legacies have an important but underappreciated role in grassland species resilience to drought and could be better harnessed to support resilient grassland restoration efforts.