Control of G2 and Transition to Endocycles in the Plant Cell Cycle

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Plant growth is crucial to agriculture, with important implications for yields of traditional agricultural crops and for production of biomass for biofuels. At the cellular level, plant growth involves two distinct processes, cell division, which produces new cells, and cell expansion, which increases the size of the cells. During division, the cell doubles its DNA content and subsequently the cell divides to form two cells, a process known as the mitotic cell cycle. During the cell expansion phase of growth, many plant cells continue a modified version of the cell cycle, in which the DNA content doubles but the cell does not divide. Examples of economically important cell types that double their DNA without division include cereal endosperm, cotton fibers, and nitrogen-fixing symbiotic nodules in legumes. In addition, this process plays important roles in cell differentiation, and is essential for nitrogen fixation in legume root nodules, but the detailed mechanism of this modified cell cycle remains poorly understood. A protein (known as SIM) has been identified that functions as an inhibitor of cell division, but allows DNA replication. The goal of this project is to understand how SIM regulates the balance between division and DNA replication during plant growth. The project will also play a role in the education of graduate students and undergraduates from diverse backgrounds, will test the effectiveness of the film as a classroom teaching tool, both in learning course content, and on student attitudes towards science, and will train undergraduates in mathematical modeling for the biological sciences.The SIAMESE (SIM) gene encodes a cyclin-dependent kinase (CDK) inhibitor that blocks mitosis without blocking DNA replication. Although SIM is clearly plays a key role in the transition from mitotic growth to endoreplication, the control of the G2/M transition in plants is complex, and many questions remain about how endoreplication is established, and the role of SIM and related proteins in this process. The proposed work will use a combination of molecular genetic manipulations, live cell imaging and mathematical modeling to test specific hypotheses about the role of SIM and the related SMR proteins in the mitosis/endocycle transition. The Specific Objectives of this proposal are to answer the following questions: Can a combination of mathematical modeling, genetic and cell biological experiments be used to create a coherent model of inhibition of G2/M during the transition to endocycles? How is degradation of SIM protein regulated? Do SIM or other SMRs play a role in controlling progression through G2 during the regular mitotic cycle?