Vacuolar Ion Channels of Higher Plants

Publisher Summary This chapter discusses vacuolar ion channels of higher plants. The accumulation of ions in the vacuolar lumen is a central attribute in many of the principal functions of vacuoles. Most obviously, as storage compartments, vacuoles are used as a repository for nutrient ions when these are in ample supply. The maximization of cytosolic surface–to–volume ratio by the vacuole is most effectively achieved in energetic terms if the osmotic pressure of the vacuolar lumen—which must balance that of the cytosol—is generated by simple inorganic salts absorbed from the external medium. In respect to inorganic nutrients such as NO 3 – or K + , mobilization occurs in the event that extracellular supply of the nutrient becomes depleted. The ions so released can either be used to bolster cytosolic levels, or can even be exported to rapidly growing tissues where nutritional demand is high. Cellular release of ions, for example during control of cell turgor in halophytes in response to hypotonic conditions, or, in the case of guard cells, stomatal closing stimuli, will inevitably involve ion mobilization from the vacuole if cytosolic volume is to be sustained. The balance between net vacuolar accumulation and release for a given ion will be determined by the relative activities of two classes of transport system. In general, carriers energize transport of ions by coupling the flow of ions to that of protons thermodynamically downhill into the cytosol.