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Cell-to-cell communication is a key aspect of microbial physiology and population dynamics, and a cornerstone in understanding the evolution of multicellularity. Quorum sensing in bacteria is a canonical example of microbial cell-to-cell signaling, in which bacteria use small molecule signals in order to monitor their population size and modulate their physiology accordingly. We propose that the evolution of plant hormone signaling arose in unicellular green algae, analogously to quorum sensing in bacteria, and that the complexity of these pathways required the recruitment of increasingly specific enzymes to increasingly sophisticated gene networks throughout the course of phytohormone signaling evolution. Using Chlorella sorokiniana UTEX 1230 as a model system, we address the evolution of cell-to-cell signaling from the perspective of phytohormone signaling evolution, particularly the evolution of indole-3-acetic acid (IAA) signaling and abscisic acid (ABA) signaling. We demonstrate that key components of these phytohormone signaling pathways are present in C. sorokiniana, and that these hormones are present and active in the physiology of these organisms. Indeed, the distribution of early auxin signaling related orthologs in the chlorophytes suggests that some auxin signaling machinery was available early in the evolution of plants. Abscisic acid (ABA) is a phytohormone that has been extensively characterized in higher plants for its role in stress response. This dissertation demonstrates that ABA is involved in regulating algal stress responses in Chlorella; additionally, the genome contains orthologs to essential genes in higher plants that control ABA biosynthesis, sensing, and degradation. Transcriptomic studies reveal that treatment with ABA induces dramatic changes in gene expression profiles, including transcripts associated with ABA signaling in higher plants. The physiological effects of phytohormones, together with the presence of phytohormone signaling orthologs, suggest that phytohormone signaling evolved as an intercellular stress response signaling molecule in eukaryotic microalgae prior to the evolution of multicellularity and colonization of land. Advisers: Kenneth W. Nickerson, Wayne R. Riekhof