The Endosomal System of Plants: Charting New and Familiar Territories

Abstract

Almost 30 years ago, it was suggested that turgor pressure may prevent membrane invagination at the PM of a plant cell with a wall (Cram, 1980). However, endocytosis was demonstrated in protoplasts a few years later with the help of electron dense tracers (Hillmer et al., 1986; Tanchak and Fowke, 1987). Accordingly, it was then calculated that turgor pressure varies and fluctuates enormously throughout the plant and that turgor values below 5 bar do not impede endocytosis (Saxton and Breidenbach, 1988; Gradmann and Robinson, 1989). Even in guard cells, where the highest turgor values have been recorded, there are clathrin-coated pits at the PM (Doohan and Palevitz, 1980) and the internalization of fluorescently labeled PM proteins has been measured (Meckel et al., 2004). Despite such arguments, doubts about plant cells performing endocytosis continued to be fueled by erroneous observations on fluid phase endocytosis using the sulfonated dye Lucifer Yellow (Oparka and Hawes, 1992; Baluska et al., 2004; Aniento and Robinson, 2005). Only with the introduction of amphiphilic styryl dyes, especially FM4-64, which inserts into the outer lipid bilayer of the PM and then moves in a temperature-dependent manner to the tonoplast (Bolte et al., 2004; Meckel et al., 2004), can one say that the dissenting voices on plant endocytosis have finally been silenced.