Calcium transport across plant membranes: mechanisms and functions.

Contents Summary 49 I. Introduction 49 II. Physiological and structural characteristics of plant Ca -permeable ion channels 50 III. Ca extrusion systems 61 IV. Concluding remarks 64 Acknowledgements 64 References 64 SUMMARY: Calcium is an essential structural, metabolic and signalling element. The physiological functions of Ca are enabled by its orchestrated transport across cell membranes, mediated by Ca -permeable ion channels, Ca -ATPases and Ca /H exchangers. Bioinformatics analysis has not determined any Ca -selective filters in plant ion channels, but electrophysiological tests do reveal Ca conductances in plant membranes. The biophysical characteristics of plant Ca conductances have been studied in detail and were recently complemented by molecular genetic approaches. Plant Ca 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 Ca -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-Ca hub is discussed, linking Ca transport to ROS generation. CNGC inactivation by cytosolic Ca , leading to the termination of Ca signals, is now mechanistically explained. The structure-function relationships of Ca -ATPases and Ca /H exchangers, and their regulation and physiological roles are analysed.