THE PLANT VACUOLE.

Plant cells are unique in containing large acidic vacuoles which occupy most of the cell volume. The vacuolar H+-ATPase (V-ATPase) is the enzyme responsible for acidifying the central vacuole, although it is also present on Golgi and coated vesicles. Many secondary transport processes are driven by the proton-motive force generated by the V-ATPase, including reactions required for osmoregulation, homeostasis, storage, plant defense and many other functions. However, a second proton pump, the V-PPase, serves as a potential back-up system and may, in addition, pump potassium. The plant V-ATPase is structurally similar to other eukaryotic V-ATPases and its subunits appear to be encoded by small multigene families. These multigene families may play important roles in the regulation of gene expression and in the sorting of V-ATPase isoforms to different organelles.