How plant cells make a mitotic spindle, without involvement of centrosomes?

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Centrosomes play a role on formation of a mitotic spindle. It has been a mystery how plant cells form a mitotic spindle in the absence of the centrosomes. We proposed that mitotic spindles in plants are formed by branching of existing microtubules. To evaluate the hypothesis, we examined mechanism of spindle formation by analyzing localization of markers of centrosomes and microtubules during cell division of tobacco suspension culture cells. Gamma-tubulin, which localizes in the centrosome and nucleate microtubules there, dispersed in the spindle and moved in the spindle. In phragmoplast formation, new microtubules were elongated from the existing microtubules.