The vacuole is, in terms of volume, the dominant organelle of the vast majority of mature plant cells. Many of its functions have long been recognised (Dainty, 1968; Boiler & Wiemken, 1986; Matile, 1987; Raven, 1987). Thus, the vacuole provides a storage compartment for nutrients and metabolites, exhibits lysosomal characteristics as a major lytic compartment, and (by virtue of its size and relative metabolic inertness) enables the cell to achieve a large volume without compromising the high cytoplasmic surface-to-volume ratio that is necessary for efficient gas and nutrient exchange with the environment.

Increasingly, however, additional functions for the vacuole are being discovered. The presence of ion translocating phosphohydrolases at the vacuolar membrane (the tohoplast) implies that the vacuole plays an important role in cellular energetics (Rea & Sanders, 1987). Indeed the H+-pumping pyrophosphatase at this membrane appears (in photosynthetic tissue at least) to constitute the only means of hydrolysis of cytosolic pyrophosphate (PPi), which suggests that the organelle must be primarily responsible for the removal of PPi produced during biosynthetic reactions (Weiner et al, 1987). Furthermore, in accord with its role as a storage organelle, the vacuole seems likely to constitute a major source of second messengers which can be released into the cytoplasm during signal transduction (Boudet & Ranjeva, 1989).