Microtubules and actin filaments direct nuclear movement during the polarisation ofMarchantiaspore cells

ABSTRACT Multicellular organisms typically develop from single cells, the polarity of which establishes the first body axis of the organism. The multicellular haploid stage of land plants develops from a single haploid cell produced by meiosis – the spore. Starting from a non-polar state, these spores develop polarity and divide asymmetrically to establish the first apical-basal axis of the plant body. In the spore of the liverwort, Marchantia polymorpha , we show that the nucleus migrates from the cell centroid to the side of the cell to define the future basal pole. A microtubule organising centre leads this migration by initiating a dense microtubules array towards the cortex at the basal pole. Simultaneously, cortical microtubules disappear from the apical hemisphere but persist near the basal pole. A dense network of fine actin filaments also accumulates between the nucleus and the basal cell cortex. These data demonstrate that microtubules and actin filaments reorganise during the polarisation of the M. polymorpha spore. We speculate that signals orient microtubules and actin filaments during spore polarisation, resulting in the formation of a fine actin filament network between the nucleus and cell cortex that moves the nucleus to the future basal pole. SUMMARY STATEMENT Microtubules and actin filament dynamics are required for the basal movement of the nucleus which establishes cell asymmetry before cell division in the Marchantia spore.