Ubiquitin-Mediated Proteolysis. To Be in the Right Place at the Right Moment during Nodule Development

The cell cycle plays a crucial role in plant development. Organogenesis takes place throughout the lifetime and most cells maintain their ability to reenter and to regulate the cell cycle in response to a wide range of endogenous and external signals. In planta, phytohormones, particularly auxin and cytokinin, are essential for cell proliferation. New organs arise from local foci of proliferating cells, called meristems, which either persist or are de novo formed. In the organ primordium (initial stage of an organ before reaching maturity), cell differentiation starts with cell division arrest. These nondividing cells can exit the mitotic cycle with either complete loss or partial activity of the cell cycle. In the latter case, plant cells frequently enter an altered version of the cell cycle, known as endoreduplication cycles or endocycles, where the genome is duplicated without mitosis. Single or multiple rounds of endoreduplication cycles result in the formation of polyploid cells. The physiological role of ploidy is poorly understood.