One of the distinct organelles of plant cells is the vacuole which is also found in some protist and fungal cells. This organelle is essentially an enclosed compartment that is filled with water that contains both inorganic and organic molecules including enzymes in solution, although in certain cases it can contain solids that have been engulfed by the cell. Some of the major roles that vacuoles play in plant cells include turgor maintenance, accumulation of seed storage proteins, sequestration of deleterious compounds and storage of metabolites. Such functions are dependent directly or indirectly on proteins that are residing in the tonoplast, which is mainly the vacuolar membrane stacked together to form a larger unit, the vacuole.
The tonoplast contains numerous proteins that are responsible in the transport of small solutes and salts; and proteins that are involved in the fusion of membrane and remodeling the allowed delivery of macromolecules by vesicle traffic. In order to fulfill the said function, the proteins in the tonoplast need be synthesized correctly, sorted to the tonoplast and turned over. Since most mature plants normally have a single large central vacuole that occupies more than 30% of the total volume of the cell, and can occupy as much as 80% of certain cells’ volume; the number of tonoplast in a single central volume is therefore dependent on its size, based on the given condition.
The term tonoplast is derived from the word origin Gk tón(os), meaning “tension”, “stretching”, or “tone” and comb, meaning “molded”. The tonoplast is also called as the vacuolar membrane, and it is a cytoplasmic membrane that surrounds a vacuole, thus separating the vacuolar contents from the cytoplasm of the cell. As a membrane, tonoplast is directly involved in the regulation of the movements of ions around the cell and isolating any possible harmful materials that is threatening to the cell.