In animals, cyclin-dependent kinase inhibitors (CKIs) are important regulators of cell cycle progression. Recently, putative CKIs were also identified in plants, and in previous studies, Arabidopsis thaliana plants misexpressing CKIs were found to have reduced endoreplication levels and decreased numbers of cells consistent with a function of CKIs in blocking the G1-S cell cycle transition. Here, we demonstrate that at least one inhibitor from Arabidopsis, ICK1/KRP1, can also block entry into mitosis but allows S-phase progression causing endoreplication. Our data suggest that plant CKIs act in a concentrationdependent manner and have an important function in cell proliferation as well as in cell cycle exit and in turning from a mitotic to an endoreplicating cell cycle mode. Endoreplication is usually associated with terminal differentiation; we observed, however, that cell fate specification proceeded independently from ICK1/KRP1-induced endoreplication. Strikingly, we found that endoreplicated cells were able to reenter mitosis, emphasizing the high degree of flexibility of plant cells during development. Moreover, we show that in contrast with animal CDK inhibitors, ICK1/KRP1 can move between cells. On the one hand, this challenges plant cell cycle control with keeping CKIs locally controlled, and on the other hand this provides a possibility of linking cell cycle control in single cells with the supracellular organization of a tissue or an organ
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