DICTYOSOME POLARITY AND MEMBRANE DIFFERENTIATION IN OUTER CAP CELLS OF THE MAIZE ROOT-TIP

Abstract

Outer rootcap cells of maize produce large numbers of secretory vesicles that ultimately fuse with the plasma membrane to discharge their product from the cell. As a result of the fusion, these vesicles contribute large quantities of membrane to the cell surface. This phenomenon was investigated using sections stained with phosphotungstic acid at low pH (PACP), a procedure in plant cells that specifically stains the plasma membrane. In the maize root tip, the PACP also stains the membranes of the secretory vesicles derived from Golgi apparatus to .apprx. the same density that it stains the plasma membrane. The membranes of the secretory vesicles acquire the staining characteristic while still attached to the Golgi apparatus. The staining progresses across the dictyosome from the forming to the maturing pole, confirming the marked polarity of these dictyosomes. The PACP staining of Golgi apparatus is confined to the membranes of the secretory vesicles. It is largely absent from the central plates or peripheral tubules, and provides an unambiguous example of lateral differentiation of membranes orthogonal to the major polarity axis. In the cytoplasm there were no vesicles other than secretory vesicles bearing polysaccharide that were PACP positive. Even the occasional coated vesicle seen in the vicinity of the Golgi apparatus did not stain. If exocytotic vesicles are present in the maize root cap cell, they are formed in a manner where the PACP-staining constituent is not retained by the internalized membrane. Dictyosome polarity in the maize root cap, and membrane differentiation both across and at right angles to the major polarity axis were confirmed. Endocytotic vesicles, if present, probably exclude the PACP-staining component.