Calcium transport across plant membranes: mechanisms and functions

Calcium is an essential structural, metabolic and signalling element. The physiological functions of Ca²⁺ are enabled by its orchestrated transport across cell membranes, mediated by Ca2⁺- permeable ion channels, Ca²+-ATPases and Ca²⁺/H⁺ exchangers. Bioinformatics analysis has not determined any Ca²⁺-selective filters in plant ion channels, but electrophysiological tests do reveal Ca²⁺ conductances in plant membranes. The biophysical characteristics of plant Ca²⁺ conductances have been studied in detail and were recently complemented by molecular genetic approaches. Plant Ca²⁺ conductances are mediated by several families of ion channels, including cyclic nucleotide-gated channels (CNGCs), ionotropic glutamate receptors, two-pore channel 1 (TPC1), annexins and several types of mechanosensitive channels. Key Ca²⁺-mediated reactions (e.g. sensing of temperature, gravity, touch and hormones, and cell elongation and guard cell closure) have now been associated with the activities of specific subunits from these families. Structural studies have demonstrated a unique selectivity filter in TPC1, which is passable for hydrated divalent cations. The hypothesis of a ROS-Ca²⁺ hub is discussed, linking Ca²⁺ transport to ROS generation. CNGC inactivation by cytosolic Ca²⁺, leading to the termination of Ca²⁺ signals, is now mechanistically explained. The structure–function relationships of Ca²⁺-ATPases and Ca²⁺/H⁺ exchangers, and their regulation and physiological roles are analysed.