Role of Phragmoplast Filaments in Cell-Plate Formation

Abstract

With the differential interference microscope (Nomarski system), it is possible to observe the interaction of filaments of the phragmoplast with micron-size refractile particles and vesicles, very close to the resolution limit of the microscope, during formation of the cell plate. Filaments occupying the birefringent zone of the phragmoplast (i.e. remnants of the continuous and interzonal fibres of the mitotic spindle) shorten and thicken in their mid-region to become spindle-shaped. The thickenings later fuse laterally to form the cell plate. During the process of thickening, vesicles can be seen to move toward the plate in close association with the filaments on both sides of the phragmoplast. These observations, taken together with information from electron micrographs of thin sections of Haemanthus endosperm and other dividing cells, confirm the notion that the fibrillar component of the phragmoplast has a transport function. The sizes of particles entering the phragmoplast region by saltation suggest that they correspond to Golgi bodies, which have been suspected of producing vesicles that contribute to cell-plate formation. Neither light nor electron micrographs have yet provided any insight into the mechanism of transport of these vesicles.