Cell polarity, asymmetric division & cytokinesis

We are interested in the mechanisms that control cell polarity and asymmetric division and how this is integrated with cell fate determination in male gametophyte development. With the use of transgenic cell-specific markers we showed that vegetative cell fate is a default pathway independent of cell division and that division asymmetry is essential for differentiation of the germline [Eady et al 1995; Twell et al 1998]. This inspired genetic screens to identify mutants defective in cell polarity and cell fate specification and led to the characterisation of several novel classes of mutants [Howden et al 1998; Park et al 1998; Lalanne et al 2002; Durbarry et al 2005] and genes that are required for asymmetric division, cytokinesis and pollen maturation [Twell et al 2002; Oh et al 2005].

Cell polarity & asymmetric division

gemini pollen (gem) mutants disturb cell polarity, asymmetric division and cytokinesis during gametophyte development [Park et al 1998, 2004; Park and Twell 2001]. Map-based cloning led to the identification GEM1 as the major XMAP215-family microtubule associated protein (identical to MOR1) that has an essential role in cortical microtubule organisation [Twell et al, 2002]. MOR1/GEM1 binds to both cortical and mitotic microtubule arrays including the spindle and phragmoplast [Twell et al 2002]. These data highlight the requirement for MOR1/GEM1 in microtubule re-organisation that occurs during polarity establishment and cytokinesis at pollen mitosis I. We have developed tools to monitor microtubule dynamics during male gametophyte development and in mutants defective in polarity establishment, cytokinesis and cell cycle progression. Our results provide direct evidence for the function of MAP215/Dis1 family protein TMBP200 in the organisation of microtubule arrays critical for male germline formation in plants [Oh et al 2010a, 2010b].

Cytokinesis

tionew1.jpgCytokinesis is the final step in cell division after mitosis leading to cell separation. To avoid disastrous mis-separation cytokinesis must occur at the right time and place. This is particularly important in plants, where cells are encased within rigid cell walls and cellular pattern and plant architecture are strongly influenced by the division plane. Unlike animal cells that divide by forming cleavage furrows, plant cells have adopted unique strategies by building a cell plate centrifugally. This involves a highly orchestrated interplay between the nucleus, membranes and cytoskeletal arrays.

tionew2.jpgtwo-in-one (tio) mutants produce binucleate pollen, which results from failure of cytokinesis at pollen mitosis I. TIO is required for conventional and non-conventional modes of cytokinesis and is a key player in the plant cytokinesis machinery. We identified TIO as an ancient protein kinase closely related to Fused (Fu) kinases which have divergent roles in slime moulds, flies and humans [Oh et al 2005]. We discovered that TIO is required for expansion of the phragmoplast and forms a signalling module with the phragmoplast-associated Kinesin-12-family [Oh et al 2012]. We are investigating the localisation, interactions and signalling role of TIO in gametophytic and sporophytic cytokinesis together with our partners in South Korea (Dr S-A Oh & Prof. SK Park, Kyungpook National University).

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