Quantitative analysis of changes in spatial distribution and plus-end geometry of microtubules involved in plant-cell cytokinesis.

The cell plate of higher plants is formed within a ribosome-excluding cell plate assembly matrix. Phragmoplast microtubules facilitate cell-plate formation by forming a scaffold that directs Golgi-derived vesicles to the forming cell plate. Here, we analyse the effects of the cell-plate assembly matrix on phragmoplast microtubule plus-end geometry by electron tomography of cryogenically fixed Arabidopsis meristem cells. Five distinct microtubules plus-end geometries are seen--blunt, extended, horned, flared and hybrid extended/horned. We have quantified and mapped these types of plus-end morphology during the different stages of cell-plate formation and analysed the effects of cell-plate assembly matrix association on microtubule plus-end morphologies. Our results show that somatic-type phragmoplast microtubules do not interdigitate at the cell plate mid-line. The cell-plate assembly matrix is shown to stabilize microtubule plus ends, as evidenced by the fact that of these microtubules that do not terminate in such a matrix, 40-80% are horn-shaped (shrinking), whereas of those that end in such a matrix, 50-70% are blunt (metastable). Also, a third of the blunt-ended microtubules within the cell-plate assembly matrix end at a distance of approximately 30 nm from the cell plate.