Date of Award:

12-2023

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Biology

Committee Chair(s)

Robert Schaeffer

Committee

Robert Schaeffer

Committee

Monica Borghi

Committee

Karen Beard

Abstract

Fire blight, a devastating disease of pome fruit trees caused by the bacterium Erwinia amylovora, can cause millions of dollars in losses for producers each year around the globe. Management approaches that involve use of antibiotics, such as streptomycin, can be effective; although concerns exist over pollinator and crop health when using them regularly. Recently, there have been developments that allow for biological agents such as microbes to curtail fire blight infection. These agents work by competing with Erwinia for resources or space, producing antibacterial compounds, or even killing Erwinia cells on contact. Unfortunately, these agents do not yet work as well as traditional antibiotic strategies thus prompting the need for further research and development. Erwinia infects plants through the flower, so understanding how to suppress Erwinia growth within the flower is essential to reducing the damage caused by fire blight. Floral microbial communities are generally dominated by one species with the first species to arrive conferring a great advantage by being able to utilize the few resources available as well as strongly affecting the nectar environment (i.e. making it more acidic). The consequences faced by later arriving species are called priority effects, and make establishment of a late arriver very difficult.

In my first chapter I evaluate the strength of priority effects exerted by prospective microbes on the bacterial pathogen Erwinia amylovora. This framework breaks priority effects into three axes: overlap in amino acid use, impact to the nectar environment (pH and sugar use), and resource requirement. I then test which of these mechanisms drive priority effects exerted by each microbe against Erwinia. To do this, I performed a laboratory study that looked at suppression of pathogen growth by each microbe in artificial pear nectar. I then assessed whether the microbe suppressed growth by utilizing similar resources, acidifying the nectar environment, or other mechanisms. In my laboratory study, I found that microbe impacts on nectar acidity and sugar content and concentration were the strongest predictors of priority effects.

In my second chapter I consider these floral microbial interactions more broadly in a review article that examines how community assembly processes are subject to change in the context of a shifting climate. I discuss floral microbiome assembly and function in the context of increasing stresses to both host and microbe. These stresses may include hotter temperatures, drought, and more frequent bouts of heavy precipitation. I first highlight mechanisms by which microbial dispersal may be affected by altered environmental conditions. I secondly discuss how upon reaching the flower, microbes are likely to face harsher conditions within floral nectar, which can mostly be attributed to plant responses to these same stresses. I then discuss how shifts in microbe and plant metabolism are likely to affect microbe-microbe and microbe-plant interactions. Finally, I propose ideas for what consequences may arise for both plant and microbe due to compounding environmental stresses.

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