The nucleolar structure and nucleolar proteins as indicators of cell proliferation events in plants

Cell proliferation is a crucial cellular process which influences development. In plants, meristems are formed by actively proliferating cells, in which the main expression of proliferation is the existence of a cell division cycle. Many cell activities are influenced by the cell proliferation status and cell cycle progression, among them ribosome biogenesis, which is morphologically expressed as the nucleolus. The connection is established through nucleolar proteins, which regulate the synthesis and processing of preribosomal precursors and, at the same time, are targets of various cell cycle regulators, such as certain kinases. Nucleolin is one of these nucleolar proteins, whose level increases with cell proliferation and depends on the cell cycle stages. Not only the levels, but also other important features of the protein, such as its distribution in situ in the nucleolus, its phosphorylation and its physiological degradation, depend on these parameters. Furthermore, since the nucleolar structure is highly sensitive to functional variations, distinct nucleolar structures, regarding the nucleolar size and the distribution of nucleolar subcomponents, have been defined for each period of the cell cycle, using synchronized cells. In addition to increasing our knowledge of cellular physiology, these relationships can be used to mark the proliferative state of the cell and the periods of cell cycle.