Patterns of Plant Salinity Adaptation Depend on Interactions with Soil Microbes

As plant-microbe interactions are both ubiquitous and critical in shaping plant fitness, patterns of plant adaptation to their local environment may be influenced by these interactions. Identifying the contribution of soil microbes to plant adaptation may provide insight into the evolution of plant traits and their microbial symbioses. To this end, we assessed the contribution of soil microbes to plant salinity adaptation by growing 10 populations of Bromus tectorum, collected from habitats differing in their salinity, in the greenhouse under either high-salinity or nonsaline conditions and with or without soil microbial partners. Across two live soil inoculum treatments, we found evidence for adaptation of these populations to their home salinity environment. However, when grown in sterile soils, plants were slightly maladapted to their home salinity environment. As plants were on average more fit in sterile soils, pathogenic microbes may have been significant drivers of plant fitness herein. Consequently, we hypothesized that the plant fitness advantage in their home salinity may have been due to increased plant resistance to pathogenic attack in those salinity environments. Our results highlight that plant-microbe interactions may partially mediate patterns of plant adaptation as well as be important selective agents in plant evolution.