Mechanisms of cytosolic calcium elevation in plants: the role of ion channels, calcium extrusion systems and NADPH oxidase-mediated ‘ROS-Ca2+ Hub'

Elevation in the cytosolic free calcium is crucial for plant growth, development and adaptation. Calcium influx into plant cells is mediated by Ca²⁺ depolarisation-activated, hyperpolarisation-activated and voltage-independent Ca²⁺-permeable channels (DACCs, HACCs and VICCs respectively). These channels are encoded by the following gene families: (1) cyclic nucleotide-gated channels (CNGCs), (2) ionotropic glutamate receptors (GLRs), (3) annexins, (4) ‘mechanosensitive channels of small (MscS) conductance’-like channels (MSLs), (5) ‘mid1-complementing activity’ channels (MCAs), Piezo channels, and hyperosmolality-induced [Ca²⁺]_cyt. channel 1 (OSCA1). Also, a ‘tandem-pore channel1’ (TPC1) catalyses Ca²⁺ efflux from the vacuole in response to the plasma membrane-mediated Ca²⁺ elevation. Recent experimental data demonstrated that Arabidopsis thaliana (L.) Heynh. CNGCs 2, 5–10, 14, 16 and 18, GLRs 1.2, 3.3, 3.4, 3.6 and 3.7, TPC1, ANNEXIN1, MSL9 and MSL10,MCA1 and MCA2, OSCA1, and some their homologues counterparts in other species, are responsible for Ca²⁺ currents and/or cytosolic Ca²⁺ elevation. Extrusion of Ca²⁺ from the cytosol is mediated by Ca²⁺-ATPases and Ca²⁺/H⁺ exchangers which were recently examined at the level of high resolution crystal structure. Calcium-activated NADPH oxidases and reactive oxygen species (ROS)-activated Ca²⁺ conductances form a self-amplifying ‘ROS-Ca²⁺hub’, enhancing and transducing Ca²⁺ and redox signals. The ROS-Ca²⁺ hub contributes to physiological reactions controlled by ROS and Ca²⁺, demonstrating synergism and unity of Ca²⁺ and ROS signalling mechanisms.