In and out of the plant storage vacuole

Abstract

The plant storage vacuole is involved in a wide variety of metabolic functions a great many of which necessitate the transport of substances across the tonoplast. Some solutes, depending on the origin, have to cross the plasma membrane as well. The cell is equipped with a complex web of transport systems, cellular routes, and unique intracellular environments that support their transport and accumulation against a concentration gradient. These are capable of processing a diverse nature of substances of distinct sizes, chemical properties, and origins. In this review we describe the various mechanism involved in solute transport into the vacuole of storage cells with special emphasis placed on solutes arriving through the apoplast. Transport of solutes from the cytosol to the vacuole is carried out by tonoplast-bound ABC transporters, solute/H(+) antiporters, and ion channels whereas transport from the apoplast requires additional plasma membrane-bound solute/H(+) symporters and fluid-phase endocytosis. In addition, and based on new evidence accumulated within the last decade, we re-evaluate the current notion of extracellular solute uptake as partially based on facilitated diffusion, and offer an alternative interpretation that involves membrane bound transporters and fluid-phase endocytosis. Finally, we make several assertions in regards to solute export from the vacuole as predicted by the limited available data suggesting that both membrane-bound carriers and vesicle mediated exocytosis are involved during solute mobilization.