EFFECTS OF CYCLOHEXIMIDE ON PREPROPHASE BANDS AND PROPHASE SPINDLES IN ONION (ALLIUM-CEPA L) ROOT-TIP CELLS

Effects of cycloheximide (CHM) on preprophase bands (PPBs) of microtub ules (MTs) and on prophase spindle MTs in root Lip cells of onion (All ium cepa L.) were examined. When root tip cells were treated with 36 m u M CHM for 0.5-4 h, the population of cells with a PPB did not decrea se markedly although the population of mitotic cells and that of proph ase cells with a PPB gradually decreased to half of the control root t ips. In prophase cells treated with 11 and 36 mu M CHM for 2 h, the wi dth of the PPB was 1.4 times broader than that in the prophase PPB wit hout CHM. Electron microscopic observation on the cross section of the PPB showed that the number of MTs and the distance between adjacent M Ts in prophase PPBs treated with CHM were similar to those in the earl y developmental stage of PPBs without CHM. The bipolar spindle, that a ppeared in late prophase was not seen in prophase cells treated with 1 1 mu M or higher concentrations of CHM for 2 h. In order to examine di fferences of perinuclear MT arrangement between CHM treated and non-tr eated prophase cells, arrangement of perinuclear MTs was examined by c onfocal laser scanning microscopy. In control cells without CHM, MTs a ppeared on the nuclear surface with several ''branched'' or ''cross ov er'' type MT foci in the cytoplasm when broad PPB formation started. T hese MT foci were replaced by the ''aster'' type MT foci, from which s everal MTs radiated along the nuclear surface. The ''aster'' type MT f oci gradually gathered to form a bipolar spindle. MTs connecting the s pindle pole region and the PPB were seen in late prophase. In CHM-trea ted cells (11-360 mu M for 2 h), ''branched'' and ''cross over'' type MT foci were prominent, even in prophase cells with well condensed chr omosomes. Neither linkages of MTs between the spindle pole region and the PPB nor ''aster'' type MT foci were seen. These observations showe d that CHM prevents the bundling of MTs in the PPB and also inhibits t he formation of ''aster'' type MT foci that is essential for bipolar s pindle development.