Host Specificity, Negative Feedbacks, and Pathogen Defense in the Plant Phyllosphere Microbiome

Abstract

My dissertation research spans several topics in plant microbial ecology. In two research projects, I have explored whether host specificity influences fungal endophyte community structure in plant leaves across several ecotypes of switchgrass (Panicum virgatum) and across 19 plant species within the Asteraceae family. In these works, I found contrasting results for the importance of host specificity, where fungal endophytes did not preferentially colonize specific host ecotypes within a single species, but did preferentially colonize specific host species within a single family. Additionally, I also found that more phylogenetically related host species within the Asteraceae family shared more similar fungal endophyte communities than more phylogenetically distant hosts. In another portion of my research, I applied a novel extension of the plant-soil feedback framework to microbiota associated with aboveground tissues, termed “plant-phyllosphere feedback”. In this work, I found that all four species tested experienced strong negative plant-phyllosphere feedback suggesting that phyllosphere, like rhizosphere (belowground), microbiota can potentially mediate plant species coexistence via negative feedbacks. In a final work, I tested whether traits displayed by bacterial endophytes in vitro can be used to reliably predict disease reduction outcomes in planta across variable climatic conditions using wheat plants and the fungal pathogen Fusarium graminearum. I did not ultimately find that in vitro trait assessments were good predictors of disease reduction outcomes in planta. However, my analyses did reveal differences among bacterial endophytes in their resilience to variable climatic conditions and degree of pathogen antagonism, emphasizing the importance of considering the abiotic environment for studies of putatively beneficial plant microbiota. Through my dissertation research, I have provided evidence for the extent and limitations of host specificity in the aboveground plant microbiome, the potential role of aboveground plant microbes in mediating species coexistence, and their role in reducing pathogenic outcomes in an agriculturally relevant host.