SYMMETRICAL REORGANIZATIONS OF RADIATING MICROTUBULES AROUND PREMITOTIC AND POSTMITOTIC NUCLEI OF DIVIDING CELLS ORGANIZED WITHIN INTACT ROOT-MERISTEMS

Using indirect immunofluorescence, the system of radiating endoplasmic microtubules (REMTs) within intact cells of meristematic root tissues of maize has been examined throughout the cell cycle, paying special attention to its relationship with the pre- and post-mitotic nuclei wi th which it is associated. At early interphase, REMTs are not uniforml y disposed around the nucleus but grow out from faint, though easily r ecognizable, perinuclear foci. During S and G2 phases, REMTs increase in number and have a close association with the assembly of the prepro phase band (PPB) MT array. Later, when the cortical part of the PPB di sintegrates, the REMTs align along the nuclear surface, predicting the long axis of the future mitotic spindle. In contrast to naturally wal l-less cells, or to cells with perturbed cell walls, these pre-mitotic , as well as the subsequently formed post-mitotic cells display symmet rical rearrangements of their REMTs around the nuclear surface. Mitoti c cells sectioned in the median plane show a symmetrical quadripolar M T organization which is obvious at all stages of mitosis. The symmetri cal redistributions of the REMTs which occur during the cell cycle are perturbed, or even prevented, by treatments with chemical or with phy sical anti-MT agents. Nuclei of cells so treated accumulate REMTs, but fail to redistribute them symmetrically. As a result, the pre- and po st-mitotic nuclei of root cells treated with anti-MT agents resemble, with respect to their REMTs, the corresponding nuclei of wall-less pla nt cells, or of cells which have perturbed cell walls. The dynamic REM Ts which connect the pre- and post-mitotic nuclei with the cell periph ery are suggested as being involved in sensing the position of dividin g cells within the intact plant organ. This property of REMTs enables them to spatially control the sequential alignment of cell division pl anes of immobile walled plant cells which underlies the morphogenesis of higher plant organs.