Plants affiliate with a diverse consortium of microbial symbionts that contribute in fundamental ways to growth, reproduction, and stress tolerance. These relationships can be optimized to local conditions in ways that enhance local adaptation of plant partners. We used observational and experimental approaches to explore the role of microbes in plant local adaptation to limestone barrens (alvars). Alvars are characterized by thin soils that overlay limestone bedrock; consequently, they have shallow rooting depths, high pH, and are especially drought-prone. For this project we asked two main questions: 1) Do root-associated microbial communities differ between alvars and surrounding old-field habitats? 2) Is plant adaptation to local conditions contingent upon local soil microbial communities? We hypothesized that root microbiome composition impacts plant fitness by contributing to nutrient acquisition and drought tolerance and by inducing changes in root morphology. Furthermore, we predicted that the root microbiome composition will have a greater impact on plant fitness in alvar environments, where water, nutrients, and space are limiting. We collected samples of a common plant, Hypericum perforatum, which grows on alvars and in the surrounding habitat, and used high-throughput tagged amplicon sequencing of SSU-rRNA genes (for bacteria) and ITS1 (for fungi) to characterize its root microbiome. We also conducted field reciprocal transplant experiments in 2015 and 2016 to explore how microbes affect plant germination, survival, and growth.
Results/Conclusions
Fungal and bacterial root communities were significantly different between alvar and old-field habitats. Reciprocal transplant experiments indicated that alvar seeds had higher probability of germination when transplanted into home live soils, but only when transplanted into their home habitat. Alvar seedlings had a higher probability of survival when transplanted into their home live soils, regardless of transplant site. Our results support our prediction that microbial mutualists are more important for plants growing in marginal alvar habitats, where they contribute to local adaptation. Understanding the involvement of microbes in plant local adaptation is important for understanding basic questions in ecology, such as what determines patterns of plant distribution and abundance, but also for broader applied topics, such as predicting plant responses to human-mediated changes to the environment.