Drought legacy in rhizosphere bacterial communities alters subsequent plant performance

Rhizosphere bacterial communities (‘rhizobacteria’) can mediate plant-soil feedbacks that enhance plant drought tolerance, but the composition and ecological characteristics of these populations are poorly understood. We investigated the impact of drought on rhizobacteria communities and the effect of propagating drought-impacted communities on the drought tolerance of naive plants in a two-phase experiment. We tested whether propagating rhizosphere soils from drought-stressed shrub willow (Salix purpurea), or continuously watered controls, affected the performance of a subsequent generation of droughted plants. The impacts of drought on rhizobacteria and plant fitness were evaluated using 16S rRNA gene sequencing and measures of plant growth (root mass and stem length etc.) and stress (abscisic acid, osmolality). Uninoculated reference plants were used to isolate the effects of inoculation. Drought had a significant and lasting impact on the structure of rhizobacterial communities, characterized by increased populations of Actinobacteria (Acidimicrobiia, Thermoleophilia and Micrococcaceae). Foliar osmolality was significantly higher in plants receiving drought-selected communities relative to uninoculated controls, but no significant differences in plant growth were observed. Inoculation with rhizosphere soil from watered controls significantly reduced plant growth. The rhizosphere of watered control plants was dominated by Proteobacteria and Bacteroidetes. Our findings demonstrate that plants are affected by the legacy effects of drought on the rhizosphere microbiome. This drought legacy was propagated and persisted throughout nine weeks of plant growth, independent of prevailing water stress. Drought-impacted rhizospheres had larger populations of desiccation-tolerant (ex. Arthrobacter) and putatively endophytic taxa (ex. Rhizobium) with established plant growth promoting capabilities.