Functional analysis of mob-like genes in Arabidopsis thaliana

The MOBs (Mps1-One Binder) family is a group of cell cycle-associated, non-catalytic proteins highly conserved in eukaryotes, whose founding members are implicated in mitotic exit and coordination of cell polarity with cell cycle progression. Mob1 proteins have been demonstrated to be important for both mitosis completion and cell plate formation in yeast. In particular, in Saccharomyces cerevisiae Mob1p is an essential regulator of the localization and activity of Dbf2 protein kinase, a component of the mitotic exit network (MEN). MEN components were also found in the higher eukaryotes, indeed Mob1 and Dbf2-related proteins (NDR – nuclear Dbf2 related) have been found in animal cell suggesting that their role might be conserved through evolution. Members of the NDR family are essential components of pathways that control important cellular processes, such as mitotic exit, cytokinesis, cell proliferation and morphogenesis, and apoptosis. In plants, MOB1-like proteins were studied only in Medicago sativa, where they are expressed in a cell cycle-dependent manner and are localized in the cell division midplane during cytokinesis. In this plant Mob1-like transcripts and proteins seem also to be associated with the onset of programmed cell death. The aim of this thesis was to better understand the role of MOB-like proteins in plants. Using Arabidopsis thaliana as a model organism, we have pursued two main aims: the development and characterization of transgenic lines with altered expression of Mob-like genes and the identification of plant Dbf2-related protein/s. In conclusion we have demonstrated the importance of the MOB-A protein in Arabidopsis thaliana growth and development. Specifically we showed that the reduction of Mob-A level affects cell morphology, cell size and cell proliferation in root tip, the regular progression of megasporogenesis and the formation of functional embryo sacs in reproductive organs. The whole results seem to indicate that Mob-A has more than one role in plant growth and development. Given the complexity of the interactions it is possible that MOB-A belongs to specific networks depending on the interactor and/or that the activation of different pathways is organism, tissue and/or cellular context dependent. It is also likely that different isoforms of MOB-like proteins belonging to different pathways can substitute each other, making mutant analysis more complex. However the future identification of their binding partners may shed light in determining the functions of these proteins.