In land plants, the cell plate partitions the daughter cells at cytokinesis. The cell plate initiallyforms between daughter nuclei and expands centrifugally until reaching the plasma membrane.The centrifugal development of the cell plate is driven by the centrifugal expansion ofthe phragmoplast microtubule array, but the molecular mechanism underlying this expansionis unknown. Here, we show that the phragmoplast array comprises stable microtubulebundles and dynamic microtubules. We find that the dynamic microtubules are nucleated byg-tubulin on stable bundles. The dynamic microtubules elongate at the plus ends and formnew bundles preferentially at the leading edge of the phragmoplast. At the same time, theyare moved away from the cell plate, maintaining a restricted distribution of minus ends. Wepropose that cycles of attachment of gamma-tubulin complexes onto the microtubule bundles,microtubule nucleation and bundling, accompanied by minus-end-directed motility, drive thecentrifugal development of the phragmoplast.
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