The development of the preprophase band (PPB) of microtubules (MT) in meristematic plant cells was studied by using antibodies to pig brain tubulin and indirect immunofluorescence microscopy. The PPB is first visible as a wide band of MT that are arranged only slightly more densely than flanking MT of the cortical interphase array. MT progressively become more tightly packed together, and other cortical MT are no longer seen as the PPB matures. The surface of the nuclear envelope (NE) displays no tubulin fluorescence during interphase but begins to fluoresce in the early stages of PPB development, and its intensity progressively increases thereafter. The pattern at the NE is usually diffuse at first, suggesting the presence of nonpolymerized tubulin, but fibers along the NE can be resolved at later stages. MT, arranged either radially or as a meshwork, can occur between the nucleus and cell cortex, and sometimes appear to connect the PPB and NE directly. Isolated preprophase nuclei from cells ruptured during processing often retain the PPB in its normal orientation, indicating stable linkages between the nucleus and PPB. Fluorescent cross-bars perpendicular to the axis of the MT were resolved in some PPB, suggesting lateral linkages. This suggestion is reinforced by the presence of PPB that hold together as a ribbon of MT in certain preparations, allowing PPB to be isolated from the rest of the cytoplasm and the nucleus.

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