Investigating consequences of metal hyperaccumulation by plants for flower-microbe interactions

Bacteria and fungi are ubiquitously associated with all plant organs including flowers and can mediate plant ecological interactions. Those microbes associated with flowers (the floral microbiome) can alter floral traits and change floral attractiveness to pollinators, leading to shifts in plant fitness. Different species of microbes vary in their influence on plant-pollinator interactions. Floral traits that drive distinctions in floral microbiome composition may then foster distinct interactions with pollinators, however currently little is known about the role of flower traits in microbe-mediated plant-pollinator interactions. In my dissertation I address this gap in knowledge by leveraging a natural system of a nickel hyperaccumulating plant, related non-accumulating plants and their floral microbiomes. In my first dissertation chapter I used an amplicon sequencing approach to characterize natural floral microbiomes of two clades of hyper- and non-accumulator plants, and determine the strength of floral metal concentrations, geography, and phylogeny in shaping interspecific patterns of floral microbiome divergence. I show that floral metal and elemental concentrations were the strongest drivers of variation in floral microbiome composition in the clade containing the nickel hyperaccumulator but had little influence on microbiomes of the other plant clade. In my second chapter I showed that bacteria cultured from flowers of the nickel hyperaccumulator exhibited greater tolerance to nickel than bacteria isolated from the non-accumulator. Bacterial taxa which were virtually absent from hyperaccumulator microbiomes in my first thesis chapter were notably susceptible to nickel toxicity in culture. In my third dissertation chapter I assessed bumblebee foraging preferences for nectar containing bacteria and yeast cultured from nickel hyperaccumulator and non-accumulator flowers, and for nickel in nectar. I show that while bumblebees exhibited initial preference against non-accumulator bacteria in the presence of nickel, bees largely ignored variation in nectar microbes while foraging. Together, the three chapters of my dissertation link variation in a flower trait to interspecific divergence in floral microbiome structure and function, and for the first time, demonstrate the limits of divergence microbial consortia to influence plant-pollinator interactions