MICROBE-MEDIATED ADAPTATION IN PLANTS

Plant-associated microbes affect a wide-variety of plant functional traits, and thus they likely affect patterns of plant local adaptation. However, the role of microbes in plant local adaptation is rarely tested. In a survey of the plant local adaptation literature, I found that the vast majority of studies that report local adaptation (94%) performed transplants into microbe-containing soils and measured traits that are microbe-mediated. In these studies, microbe-mediated effects are confounded with plant genotype effects. For my dissertation research, I sought to describe the phenomenon of microbe-mediated adaptation and provide a definitive test using a modified reciprocal transplant experiments that moved seeds and microbes into two habitats. Here I present my dissertation work in three chapters: In my first chapter, I propose two ways in which microbes facilitate patterns of adaptation for plants, microbe-mediated local adaptation and microbe-mediated adaptive plasticity. Microbe-mediated local adaptation is when plant genotypes have higher fitness at their home site because of a genotype-specific affiliation with locally important microbe(s). Microbe-mediated adaptive plasticity is when a plant genotype has higher fitness in multiple habitats because it demonstrates plasticity in associating with locally important microbes. In my second chapter, I describe soil conditions, plant phenotype, and microbial communities in two habitats. I used high-throughput tagged amplicon sequencing of SSU-rRNA genes (for bacteria) and ITS1 (for fungi) to characterize the root microbiome of Saint John’s wort, Hypericum perforatum. At each root sampling location, I quantified soil characteristics to evaluate habitat variables that are correlated with root microbial communities. I also collected information on plant growth and fecundity for each focal plant to identify microbes that are correlated with plant phenotype. We found that bacterial and fungal communities were significantly different between habitats. Community differences were correlated with soil nitrogen and pH, and two taxa in the phylum Actinobacteria were significantly correlated with plant height. In my third chapter, I test for microbe-mediated adaptation using the two habitats, limestone barrens (alvars) and old-fields, described in chapter two. I conducted reciprocal transplant experiments in two years, exploring microbe effects on plant germination, survival, and growth. I found that alvar seeds had higher probability of germination when transplanted into home soils with microbes, but only when transplanted into their home habitat. Similarly, alvar seedlings had a higher probability of survival when transplanted into their home live soils, but here transplant site was unimportant. I also found local microbes inhibited local adaptation for old-field seedlings and facilitated local adaptation for alvar seedlings. My results indicate that microbial mutualists were important for plants growing in marginal alvar habitats, and that microbes changed patterns of local adaptation