Regulation of the Plane of Cell Division in Vacuolated Cells I. The Function of Nuclear Positioning and Phragmosome Formation

Abstract

In small leaf explants from Nautilocalyx lynchii (Hook, f.) Sprague (Gesneriaceae) the vacuolated epidermal cells divide at a more or less predictable time and most often periclinally. A phragmosome (PS) and a band of microtubules (BMT) are formed before mitosis. To deduce the importance of the PS and other premitotic structures for the realization of a new cell wall in a predetermined plane, the cells were studied under normal conditions and after treatments that inhibit PS formation. PS formation was preceded by migration of the nucleus to the centre of the cell and an increase in the number of cytoplasmic strands. Fully developed PSs and BMTs coincided in time and occupied the same position at the cell periphery. The beginning of the formation of a PS always preceded the onset of prophase, but there was no strict correlation between successive stages of prophase and PS formation. Colchicine (COL), oryzalin (OR), and cytochalasin B (CB) inhibited the formation of a PS as well as nuclear positioning and induced abnormally placed cell walls in many of the cells that had been inhibited before mitosis. The abnormally placed cell walls were apparently caused by failing positioning of mitotic spindles resulting in the beginning of cytokinesis at an abnormal site in the cell and insufficient guidance of cell plate plus phragmoplast during cytokinesis. The occurrence of some sharply bent cell walls suggested the existence of a specialized cortical region still present after prolonged COL treatment and a function of such a cortical region to guide the phragmoplast. These results suggest that in vacuolated cells the cytoplasmic strands are important to position the nucleus and that a combination of cytoplasmic strands, PS, and cortical division site is needed to locate a new cell wall at a predetermined place in the cell.