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Soybean (Glycine max (L.) Merr.) protoplasts have been surface-labelled with cationized ferritin, and the fate of the label has been followed ultrastructurally. Endocytosis of the label occurs via the coated-membrane system. The pathway followed by the label, once it has been taken into the interior of the protoplast, appears to be similar to that found during receptor-mediated endocytosis in animal cells. Cationized ferritin is first seen in coated vesicles but rapidly appears in smooth vesicles. Labelled, partially coated vesicles are occasionally observed, indicating that the smooth vesicles may have arisen by the uncoating of coated vesicles. Structures which eventually become labelled with cationized ferritin include multivesicular bodies, dictyosomes, large smooth vesicles, and a system of partially coated reticula.

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.

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