Ultrastructure of vacuolar inclusions in root tips

Summary Vacuoles in plant cells often contain inclusions which at early stages of development are bounded by a single membrane. The inclusion bodies (IBs) comprise a diversity of forms and various stages of differentiation are recognizable. IBs are divided into two categories: those which have a matrix without internal membranes, and those which contain cytoplasmic organelles and other membranous material. The internal membranes may be tightly coiled or in the form of vesicles. IBs develop from invaginations of the tonoplast which become detached into the vacuole. They are initiated mainly during active cell growth but may remain within the vacuole in differentiated cells. Various components contribute to the contents of IBs: endoplasmic reticulum, nuclear envelope, Golgi vesicles, extruded portions of mitochondria and plastids, ribosomes and groundplasm. In most IBs the limiting membrane and contents eventually disappear within the vacuole. Some IBs prior to their breakdown within the vacuole also function as sites for the formation of material not found elsewhere in the cell. The disappearance of IBs from vacuoles suggests that such vacuoles behave as lysosomes.