ACTIVE CALCIUM-TRANSPORT BY PLANT-CELL MEMBRANES

The cytosolic free calcium concentration of higher plant cells is maintained at about 0.1-mu-M by the action of membrane calcium transporters. These act to remove calcium from the cytosol and expel it to the apoplast or accumulate it in intracellular stores. In this review, the properties and subcellular localizations of these systems are described. The major calcium transporter of the plasma membrane is a calcium pumping ATPase which shows many similarities to its equivalent in mammalian cells. The transporter has been purified from maize coleoptiles and is of M(r) 140 000, binds (and is activated by) calmodulin and shows common antigenicity with the mammalian protein. Higher plant endoplasmic reticulum also contains a calcium pumping ATPase which transports calcium from the cytoplasm and its role and properties, together with those of the tonoplast calcium/proton antiporter are presented. Evidence for calcium accumulation by chloroplasts and mitochondria is considered. The review also deals with the regulation of plant cell membrane calcium transport and its role in providing intracellular pools of calcium for signal transduction.