Regulatory Mechanisms of the Plant G2/M Transition.

The cell cycle is the life of a cell from one mitotic division to the next. In yeast and animals the transition from G2 to mitosis is regulated by the Wee1 kinases and Cdc25 phosphatases. Phosphoregulation of G2/M is also maintained by 14-3-3 proteins, which function in a wide range of additional processes including signal transduction and stress responses. The scope of this thesis was to investigate how the plant G2/M checkpoint functions and how features of the yeast and animal G2/M model apply to the plant model. A better knowledge of the mechanisms that regulate AtCDC25 and AtWEE1 activities was achieved by identifying interaction partners for the two proteins. Both proteins interact with proteins involved in protein biosynthesis, cell division and plant stress responses leading to many hypotheses about the localization, regulation and function of both AtCDC25 and AtWEE1. Moreover, AtWEE1 interacts with proteins involved in ubiquitin-mediated degradation, which might be the mechanism regulating WEE1 protein levels (Chapter 4). Additionally, AtWEE1 interacts with 14-3-3 proteins and its interaction with 14-3-3ω was confirmed in vivo in plant cells (Chapter 5). Furthermore, greater insights into the role of WEE1 in cell cycle regulation and plant development were obtained by investigation of the biochemistry of N. tabacum WEE1 during the cell cycle of synchronized N. tabacum BY-2 cells showing that both WEE1 protein level and kinase activity are sensitive indicators for the timing of mitosis (Chapter 6). Moreover, A. thaliana wee1 T-DNA insertion lines were characterized. Under standard growth conditions the T-DNA insertions in the WEE1 gene only mildly affect the plant root development. However, exposure to hydroxyurea results in a hypersensitivity response leading to a reduced primary root length and decreased rate of lateral root production linking AtWEE1 with both stress responses and plant development (Chapter 7).