Context-dependent effects of microbial inoculation on sagebrush seedlings during drought stress

The leaf microbiome interacts with its plant host and can have critical positive and negative effects on plant health. However, the interactions between leaf microbiomes and host plants remain understudied, particularly in reference to context-dependency, e.g., during drought conditions. Plants naturally exist in environments with ever-changing weather and epiphytic foliar microbes are often exposed to extreme conditions. Little is known about how different environmental contexts can alter the effects of phyllosphere microbes on their plant hosts. In this study, we measured the response of seedlings to microbial inoculations and drought conditions. We used Artemisia tridentata subsp. tridentata (hereafter, sagebrush), a foundation species in the vast and critically threatened sagebrush steppe ecosystem. We grew sagebrush plants in growth chambers from sterile seed and after 6 months the seedlings were inoculated in one of four inoculation treatments: (1) no inoculant, (2) sterile water, (3) the microbes washed from the surface of the leaves (a whole natural community), and (4) the single species, Bacillus amyloliquefaciens. We then exposed the seedlings to two environmental treatments: regular watering and a tapered drought. We found context-dependent effects of inoculation on seedling health for both of our inoculation microbial treatments. For example, both inoculants had a negative effect on host plant photosynthesis, which was lessened drought conditions. Furthermore, seedlings inoculated with B. amyloliquefaciens had increased leaf nitrogen levels, and the highest survival under drought conditions. Our study shows that seedling health and survival can be influenced by leaf-associated microbes, and is dependent on environmental context.