Ultrastructure of the partially coated reticulum and dictyosomes during endocytosis by soybean protoplasts.

Individual and serial sections were used to obtain detailed information regarding the morphology and distribution of the partially coated reticulum (PCR) and to determine its relationship with dictyosomes in endocytotically active soybean (Glycine max. (L.) Merr.) protoplasts. The results confirm and extend the description of the PCR provided by T.C. Pesacreta and W.J. Lucas (1985, Protoplasma 125, 173-184) from whole cells of selected angiosperms. The PCR of soybean protoplasts consists of a set of interconnected tubular membranes bearing a clathrin-like coat over part of their cytoplasmic surface. A dilation, sometimes containing small vesicles, is frequently seen in this organelle. The PCR often appears associated with dictyosomes but also occurs independent of other cell organelles. Only one example of a direct connection between the PCR and dictyosomes was observed.Following adsorptive endocytosis of cationized ferritin, the label appears in the PCR within 2 min and accumulates with time. It is never observed in the membrane dilations. Serial sectioning established that dictyosomes are labelled with cationized ferritin around the periphery of several cisternae, including those on both sides of the same dictyosome.