Establishment of division plane and mitosis in monoplastidic guard mother cells of Selaginella

Stomatogenesis, the determinate developmental pathway leading to formation of a pair of specialized guard cells, was studied in monoplastidic cells ofSelaginella. Observations of living cells followed by immunofluorescence microscopy of the same cells made it possible to correlate changes in cytoskeletal organization with developmental events. The guard mother cell divides in a plane perpendicular to previous divisions and this shift in polarity is marked by morphogenetic plastid migration, as well as by extensive reorganization of cytoskeletal arrays. The single plastid divides and daughter plastids move to a position opposite each other (incipient spindle poles). The axis defined by the opposing plastids rotates in the cell before becoming fixed in position with polar plastids adjacent to the lateral anticlinal walls. Plastid polarity predicts spindle orientation and the plane of division. Once division polarity is defined by plastid position, which will remain unchanged throughout mitosis and cytokinesis, cortical microtubules become reorganized from radial to longitudinal (relative to the long axis of the leaf). The initially random cortical F‐actin also becomes aligned longitudinally. A wide preprophase band of microtubules and F‐actin is formed at right angles to the spindle axis. Plastid‐based microtubules establish the preprophase spindle and also connect to the preprophase band. The mitotic spindle remains anchored at the polar plastids. After mitosis, a phragmoplast that forms among microtubules emanating from plastids and nuclei develops in the plane marked previously by the preprophase band. Mitosis is completed in 1 h 15 min ± 3 min (mean ± S.E.).