Characterizing Successional Dynamics and Host Impacts on the Big Sagebrush Leaf Microbiome

The leaf microbiome contributes to the functioning of Earth’s ecosystems through interactions with host plants, modifying their health, for better or for worse. The leaf microbiome contains bacteria, fungi, and other microorganisms that interact with each other and are subject to the processes governing community assembly. Microbial community assembly on the leaf is influenced by host plant characteristics, interactions between the different community members, the selective and dispersal-mediated effects of the weather, and time. To study the effects of a leaf microbiome on its host plant it is first important to characterize it; observing the species present and how they are affected by the host plant, weather, and time. For the first chapter of my dissertation, I characterized the fungal leaf microbiome of basin big sagebrush (Artemisia tridentata subsp. tridentata) leaves over the course on one year, measuring the factors influencing community structure throughout time. I found that the primary factors structuring the sagebrush leaf microbiome are air temperature and the age of the leaf. I discovered that there was a set of fungi living consistently on sagebrush leaves, potentially co-evolving with their host plant over an indeterminate period of time. Additionally, I isolated cultures of microbial species with known effects on the health of other plants, such as the growth promoter Bacillus amyloliquefaciens. For my second chapter, I grew sagebrush seedlings in a growth chamber and inoculated them with both a whole natural community of microbes from sagebrush leaves and a single species inoculant of the B. amyloliquefaciens isolated from sagebrush in the field. We included two environmental treatments, a tapered drought vs. regular watering, to test the interactive effects of inoculation and stress on sagebrush seedling growth, photosynthesis, and nutrient content. I observed that sagebrush seedlings were susceptible to fungal pathogens during regular watering in a growth chamber, leading to decreased photosynthetic activity and biomass, and that these effects were lessened in the drought conditions. Additionally, B. amyloliquefaciens inoculation led to increased percent Nitrogen in the leaves. I noted both positive and negatives effects in our different microbial inoculant groups and that the interactions between microbial inoculation and drought had a positive effect on plant photosynthetic rates. My third chapter was a perspective article. It argues that the potential of leaf microbes to impact entire ecosystems is too great to ignore and should be a consideration when developing management strategies for conservation. I proposed a framework for developing research and application of foliar inoculants in conservation contexts. Together, this dissertation provides the first characterization of the sagebrush leaf microbiome, a highly detailed study of succession of the leaf microbiome over time, a direct observation of environmental context modifying the interaction between a host plant and its leaf microbiome, and a look to the future with the hope that we can include the underappreciated leaf microbiome in conservation strategies. Because interactions (between humans) are also essential to education, I have included a reflection on my educational activities during the course of my Ph.D.