Pinocytosis in the root cells of a salt-accumulating halophyte Suaeda altissima and its possible involvement in chloride transport

Ultrastructure of root cells in salt-accumulating halophyte Suaeda altissima (L.) Pall. was examined with transmission electron microscopy. Plants were grown hydroponically on nutrient media containing 3, 50, 250, and 500 mM NaCl. Some plants were exposed to hyperosmotic salt shock by an abrupt increase in NaCl concentration from 50 to 400 mM. Growing S. altissima plants at high NaCl concentrations induced the formation of type I pinocytotic structures in root cells. Type I structures appeared as pinocytotic invaginations of two membranes, the plasmalemma and tonoplast. These invaginations into vacuoles gave rise to freely "floating" multivesicular bodies (MVB) enclosed by a double membrane layer. The pinocytotic invaginations and MVB contained the plasmalemma-derived vesicles and membranes of endosome origin. The hyperosmotic salt shock led to formation of type 2 and type 3 pinocytotic structures. The type 2 structures were formed as pinocytotic invaginations of the tonoplast and gave rise to MVB in vacuoles. Unlike type 1 MVB, the type 2 MVB had only one enclosing membrane, the tonoplast. The type 3 structures appeared as the plasmalemma-derived vesicles located in the periplasmic space. The cytochemical electron-microscopy method was applied to determine the intracellular Cl- localization. This method, based on sedimentation of electron-dense AgCl granules in tissues treated with silver nitrate, showed that the pinocytotic structures of all types contain Cl- ions. The presence of Cl- in pinocytotic structures implies the involvement of these structures in Cl- transport between the apoplast, cytoplasm, and the vacuole.