Tubular Connections between Dictyosomes and Forming Secretory Vesicles in Plant Golgi Apparatus

The Golgi apparatus of most plant and animal cells occurs as a system of dictyosomes (4), often dispersed throughout a large portion of the cytoplasm. Each dictyosome consists of a stack of cisternae (usually 5 or 6) and associated vesicles. The central region of a typical cisterna is sacor platelike and about a/~ /z in diameter. Tubular proliferations extend from this central structure, unit ing at intervals to form an anastomosing network of tubules (reference 1 and Figs. 1 and 2). In plant ceils, two vesicle types consistently form part of the tubular network (reference 1 and Figs. 1 and 2). These vesicles represent regions of specialization different from the usual secretion granules or vesicles reported for maize root tip (5), slimesecreting cells of insectivores (8), and root hairs (9). Secretion vesicles are generally lost during dictyosome isolation (6), but their relationship to the cisternal tubules can be visualized from sectioned material. Due to the usual cup-shape of the dictyosomes, tangential sections through an entire cisterna are rare. However, partial tangential sections are numerous and were used to clarify the mode of secretion vesicle at tachment to the cisternal tubules. Tangential ly sectioned dictyosomes from glutaraldehyde-fixed, OsO4-postfixed maize root tip have the same tubular cisternal form as isolated dictyosomes. The secretion product is contained in forming vesicles attached to the cisternae by one or more tubules (Figs. 3 and 4). To clarify the tubular nature of plant dictyosomes and to preclude artifacts solely due to glutaraldehyde, we include results with tissues fixed in KMnO4 (Figs. 5 and 6), confirming the above observation. The oft published (5, 10) crosssectional view of a KMnO4-fixed dictyosome is shown on the right in Fig. 6. Only in tangential section is the tubular nature of the structure clearly revealed (at the left of Fig. 6). Thus, the tubular character of the plant dictyosome is not restricted to cold-stored onion (1) and storage tissues (radish of Fig. 1 and cauliflower of Fig. 2) in which the dictyosomes might be expected to be relatively inactive. I t is also present in dictyosomes of actively secreting cells. Preliminary observations on other tissues suggest that the tubular nature of dictyosome cisternae is widespread and not necessarily restricted to plant cells. Tha t cisternal tubules are preserved by KMnO4 fixation demonstrates that they are not artifacts attr ibutable to glutaraldehyde stabilization. A tubular attachment for 2 forms of secretion vesicles of cap cells of maize root tip has been demonstrated (Figs. 3 to 6). During formation of secretion vesicles, the cisternal tubules provide a channel for transfer of materials and possibly serve to segregate the activities of the cisternal lumen from those of the forming vesicles.