THE PECULIAR FEATURES OF PLANT VACUOLES
All eukaryotic cells have inner acidic hydrolytic compartments, which are constituted by vacuoles in plants and yeast, and by lysosomes in animals (Figure 1). The sorting of proteins to these compartments occurs at the Golgi complex and is mediated by specific protein receptors (47, 48). Plant vacuoles are also involved in the maintenance of cell turgor and in the storage of secondary metabolites and proteins (49). The accumulation of unmetabolized molecules in lysosomes, on the other hand, is a pathology, called lysosomal storage disorder, and results from genetic defects in lysosomal proteins (50).
Protein storage in vacuoles occurs mainly in seeds. Indeed, most seed storage proteins are vacuolar, and the accumulation of seed protein in the ER is more a peculiar feature of cereals than a rule (29). Two aspects of the biogenesis and functions of plant vacuoles are relevant for the production of recombinant proteins. First, the high hydrolytic activity of vacuoles of vegetative tissues must be considered, and second, the mechanisms of deposition of vacuolar storage proteins must be understood.