ABSTRACT
Vacuoles are ubiquitous, multifaceted, indispensable, and acidic organelles. The vacuoles occupy a large part of the plant cells. Typical plant vacuoles are 50 to 100 µm in diameter and accumulate many soluble substances, such as inorganic ions, organic acids, enzymes, and secondary metabolites. The acidic environment in the vacuole is essential not only for the hydrolytic enzymes but also for the active transport and accumulation of substances. Plant vacuoles have two distinct proton-transporting enzymes, namely H+- translocating inorganic pyrophosphatase (V-PPase) and H+-ATPase (V-ATPase, vacuolar-type H+-ATPase). V-ATPase is known as the third ion-translocating ATPase in addition to the F-type and P-type ATPases (see Chapter 1). V-PPase is a unique proton pump different from these three types of ATPases. V-PPase uses inorganic pyrophosphate (PPi, diphosphate) as an energy source comparable to ATP.
