Mitosis, or cell division, involves a duplication of the chromosomes of the mitotic cell, which are subsequently distributed between two daughter cells. Contrastingly, endoreplication involves chromosomal DNA replication without intervening mitosis or cytokinesis, leading to an increase in the ploidy level. Mitosis and endoreplication are thus two different paths a cell can follow during the cell cycle. The process of endoreplication is widespread among eukaryotes, although most prevailing in plants. However, despite its common nature, the physiological role of endoreplication is poorly understood. Often a correlation between the DNA content and cell size is observed, but this relationship is not universal. Other hypotheses link endoreplication with metabolic activity, maintenance of the optimal ratio between nuclear and organellar DNA, or protection against stress. Additionally, the endoreplication process probably plays an important role in the differentiation of post-mitotic cells, as endocycle onset often characterizes the switch between cell proliferation and differentiation. As such, comprehending how cells start to endoreplicate might help to understand how cell cycle exit is initiated.
Mesophyl labelled GFP nuclei
To gain a full understanding of the physiological significance of the endoreplication cycle, we use the root tip and the first developing leaf pair of Arabidopsis thaliana as a model system. In both model systems, cells gradually exit the mitotic cell cycle and engage into an endocycle. Our molecular analyses indicate that the onset of endoreplication is controlled through a robust network, including the involvement of positive and negative feedback mechanisms. Currently, we use key regulators of this network to perturb endocycle onset in a tissue- and cell-specific manner, followed by a comprehensive analysis of the effects on plant growth under control and stress conditions. Central to this work is the development of a three-dimensional endoploidy cartogram of the complete root and leaf. The results of this project may eventually result in the development of new strategies to cope with yield losses induced by environmental stresses.
Tissue-specific modification of endoploidy affecting leaf morphogenesis
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Heyman J., Van den Daele H., De Wit K., Boudolf V., Berckmans B., Verkest A., Kamei C.L.A., De Jaeger, G, Koncz C. and De Veylder L. (2011). The Arabidopsis thaliana ULTRAVIOLET-B-INSENSITIVE 4 protein maintains cell division activity by temporal inhibition of the anaphase-promoting complex/cyclosome. Plant Cell 23, 4394-4410.
Radziejwoski A., Vlieghe K., Lammens T., Berckmans B., Maes S., Jansen M., Knappe C., Albert A., Seidlitz H.K., Bahnweg G., Inzé D., Cools, and De Veylder L. (2011). Atypical E2F activity coordinates PHR1 photolyase gene transcription with endoreduplication onset. EMBO J. 30, 355-363.
Gaamouche T., Manes C.L., Kwiatkowska D., Berckmans B., Koumproglou R., Maes S., Beeckman T., Vernoux T., Doonan J. T.raas J., Inzé D. and De Veylder L. (2010). Cyclin-dependent kinase activity retains the shoot apical meristem cells in an undifferentiated state. Plant J. 64, 26-37.
Lammens T., Boudolf V., Kheibarshekan L., Zalmas L.P., Gaamouche T., Maes S., Vanstraelen M., Kondorosi E., La Thangue N.B., Govaerts W., Inzé D. and De Veylder L. (2008). Atypical E2F activity restrains APC/CCCS52A2 function obligatory for endocycle onset. Proc. Natl. Acad. Sci. USA 105, 14721-14726.
Boudolf V., Lammens T., Boruc J., Van Leene J., Van Den Daele H., Maes S., Van Isterdael G., Russinova E., Kondorosi E., Witters E., De Jaeger G., Inzé D. and De Veylder L. (2009). CDKB1;1 forms a functional complex with CYCA2;3 to suppress endocycle onset. Plant Physiol. 150, 1482-1493.