Cellular quiescence, defined as reversible cell cycle arrest, is a fundamental mechanism that organisms use to maintain tissue homeostasis or to overcome adverse conditions. In fact, most cells spend the majority of their life span in quiescence. Despite the importance, remarkably little is known about the regulation of quiescence. For photosynthetic organisms, it requires additional effort to maintain the state of quiescence, that is, to halt the photosynthetic machinery in a way that it can restart momentarily when conditions improve. Thus far, how plant cells exit quiescence, and regain competence to divide is entirely unknown. The unicellular green alga Chlamydomonas reinhardtii was chosen as a reference model to study quiescence in photosynthetic eukaryotes for several reasons: First, quiescence and cell division can be discretely defined and controlled by manipulating the nutrient availability. Second, microalgae tend to accumulate triacylglycerols only under the growth-limiting, quiescent state, which has long hampered efforts toward the efficient generation of biofuel feedstocks from microalgae. Mechanistic insights into the cellular quiescence hold great potential to uncouple this inverse relationship.In this dissertation, I present the discovery of the protein Compromised Hydrolysis of Triacylglycerols 7 (CHT7), a repressor of cellular quiescence in Chlamydomonas that ensures the reversibility of quiescence. Moreover, I conducted comparative transcriptomics using the cht7 mutant, which is unable to orderly progress from quiescence, to uncover the CHT7 regulon and transcriptional programs unique to the exit of quiescence. In addition, I found that lipid droplets coordinate lipid and protein dynamics during quiescence and are important for the timeliness of quiescence exit. Overall, my findings make substantial progress towards the comprehensive understanding of cellular quiescence in photosynthetic cells, and promises to provide important insights into the regulation of cellular behavior in multicellular organisms as well.
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