Using yeast to trace a history of expanding auxin signals

Abstract

Auxin is a hormone that plays essential roles in almost all growth and developmental processes across allplants. The way cells interpret responses to auxin is regulated by the auxin signal transduction pathway - a tangled network of proteins with overlapping functions that turn auxin signals into transcriptional responses. I set out to learn more about this signaling network: how did it come to be so expanded in flowering plants? What different roles do components play within a given plant species, or across distantly related flowers? Where do the fundamental transcriptional mechanisms that control the functions of these pathways originate from? Previously, the Nemhauser Lab has developed a synthetic auxin response circuit in yeast to facilitate studying the molecular functions of signaling components. I used this circuit to explore these questions. Firstly, I hypothesized that auxin receptors in Arabidopsis thaliana were diverging in their functions. I used the Visualizing Variation (ViVa) tool to process 1001 information on natural variation in these receptors across accessions to learn more about the role of these proteins across different populations. I was able to use this to create hypotheses about which proteins were most functionally important. Secondly, I hypothesized that the fundamental mechanisms of auxin signaling were conserved across distantly related flowering plants. I recapitulated the maize auxin response signaling network mixing and matching its components with those of Arabidopsis to see if I could produce functional signals and explore differences in their sensitivities. I found auxin signaling networks to be highly conserved in their functions, and that this circuit can be used to rapidly characterize and compare the functions of auxin signaling components across distantly related plant species. Alex Leydon previously characterized the LisH domain of TPL as being sufficient to induce a robust and derepressible repressive function in the auxin response circuit. In my last chapter, I hypothesized that the LisH domain’s repressive function is conserved across LisH-containing proteins across eukaryotes. I was able to use the auxin response circuit and high-throughput domain libraries to test this function across different natural sequences. I found that this LisH repressive function was widely conserved, and functional in both yeast and tobacco. Understanding the roles auxin plays in plant growth is essential to our understanding of the diversity of form and function in the plant world, and to our ability to engineer plant development to better suit our needs.