K+ and pH homeostasis in plant cells is controlled by a synchronized K+/H+ antiport at the plasma and vacuolar membrane

Stomatal movement involves ion transport across the plasma membrane (PM) and vacuolar membrane (VM) of guard cells. However, the coupling mechanisms of ion transporters in both membranes and their interplay with Ca2+ and pH changes are largely unclear. Here, we investigated transporter networks in tobacco guard cells and mesophyll cells using multiparametric live-cell ion imaging and computational simulations. K+ and anion fluxes at both, PM and VM, affected H+ and Ca2+, as changes in extracellular KCl or KNO3 concentrations were accompanied by cytosolic and vacuolar pH shifts and changes in [Ca2+](cyt) and the membrane potential. At both membranes, the K+ transporter networks mediated an antiport of K+ and H+. By contrast, net transport of anions was accompanied by parallel H+ transport, with differences in transport capacity for chloride and nitrate. Guard and mesophyll cells exhibited similarities in K+/H+ transport but cell type-specific differences in [H+](cyt) and pH-dependent [Ca2+](cyt) signals. Computational cell biology models explained mechanistically the properties of transporter networks and the coupling of transport across the PM and VM. Our integrated approach indicates fundamental principles of coupled ion transport at membrane sandwiches to control H+/K+ homeostasis and points to transceptor-like Ca2+/H+-based ion signaling in plant cells.