Microtubular cytoskeletons in nuclear, alveolar, and cellular endosperm of bean (Phaseolus vulgaris) were analyzed immunocytochemically and by electron microscopy to reveal their function during cellularization. Nuclear endosperm showed a fine network of microtubules between the wide-spaced nuclei observed towards the chalazal pole. Near the embryo, where nuclei were densely packed, bundles of microtubules radiated from nuclei. They were formed just before alveolus formation and functioned in spacing nuclei and in forming internuclear, phragmoplast-like structures that gave rise to nonmitosis-related cell plates. During alveolus formation cell plates extended and fused with other newly formed walls, thus forming the walls of alveoli. Growing wall edges of cell plates exhibited arrays of microtubules perpendicular to the plane of the wall, initially. When two growing walls were about to fuse, microtubules of both walls interacted, and because of the interaction of microtubules, the cell walls changed their position. When a growing wall was about to fuse with an already existing wall, such interactions between microtubules were not observed. It is therefore concluded that interactions of microtubules of fusing walls influence shape and position of walls. Thus microtubules control the dynamics of cell wall positioning and initial cell shaping. Key words: cell wall, cellularization, endosperm, microtubule, Phaseolus vulgaris.
