Cell Cycle Regulation, Protein Ubiquitination and Plant Development

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Cell division is one of the most fundamental processes underlying plant growth and development. Further, most cells produced from cell divisions by meristematic cells will eventually differentiate into other cell types. The long-term objectives of my research are: to gain fundamental knowledge on the key mechanisms regulating plant cell proliferation and differentiation, to understand how the cell cycle is linked to other biological processes in plants, and to use the new knowledge for practical applications. The cell division cycle in eukaryotes is regulated by cyclin-dependent kinases (CDKs), which can be regulated by several different mechanisms. Previously, we identified a family of plant-specific CDK inhibitors called ICKs (interactor/inhibitor of CDK) or KRPs (Kip-related proteins). For the last two decades our research has provided important understanding on these plant cell cycle regulators. The proposed research will determine how the levels of ICK proteins are regulated, and how ICKs may respond to various conditions serving to integrate environmental signals.Many proteins in eukaryotic species are regulated by protein ubiquitination, which involves a cascade of three biochemical reactions, catalyzed by three enzymes named as E1, E2 and E3 respectively. In the process, different polyubiquitin chains can be linked through different lysine (K) residues of ubiquitin. Previously, we identified the first plant E2 enzymes responsible for catalyzing K63-linked and K11-linked protein ubiquitination. The proposed research will further determine the molecular properties and regulation of UBC22 for K11-linked ubiquitination.ICKs and UBC22 have important roles in plant sexual development. In most plants, the female gametophyte development starts when one subepidermal cell in a young ovule differentiates into a megaspore mother cell (MMC). The MMC then produces four megaspores, one of which survives and becomes the functional megaspore (FM). Our recent results indicate that multiple ICK/KRPs function redundantly in normal plants to ensure the development of one embryo sac and thus one embryo per ovule. Further, when UBC22 is inactivated, the FM could not form normally in most ovules. The proposed work will investigate further how ICKs and UBC22 function in sexual plant reproduction. The proposed research is significant for a number of reasons. (a) It will provide new insight on a family of important cell cycle regulators, the newly identified UBC22 and Lys11-linked protein ubiquitination in plants. (b) The two lines of fundamental research have converged in plant sexual reproductive development. The knowledge generated could be useful for enhancing seed production and control of embryo development. (c) The two fundamental areas of plant biology along with diverse approaches and techniques to be used will provide excellent opportunities to train highly qualified personnel.