Cytology of budding and cleavage in tobacco mesophyll protoplasts cultivated in saline medium

Tobacco (Nicotiana tabacum L., cv. Maryland) mesophyll protoplasts cultivated in saline medium divide by bud formation, migration of one nucleus into the bud, and subsequent furrowing. This process was investigated light and electron microscopically. The cytoplasm of the growing bud is richer in dictyosomes, rough endoplasmic reticulum profiles, mitochondria, and small vacuoles than is the cytoplasm of the mother cell, but in early stages lacks plastids. Only patches of wall material are found; most of the cell surface appears naked. Oriented sections of the cleavage furrow do not reveal a contractile ring of microfilaments under the fixation conditions used. The furrow is flanked by numerous microtubules, and is rich in coated vesicles. Nuclear division appears normal, but the phragmoplast vesicles appear empty, and the phragmoplast seems to disintegrate again later. The nucleus migrating into the bud does not show any signs of associated contractile structures. The results demonstrate that, in principle, higher plant cells are capable of a mode of division usually said to be "yeast-like". The events of karyokinesis and cell plate formation are not therefore obligatorily linked processes.