Light-gated channelrhodopsin sparks proton-induced calcium release in guard cells

Although there has been long-standing recognition that stimuli-induced cytosolic pH alterations coincide with changes in calcium ion (Ca 2+ ) levels, the interdependence between protons (H + ) and Ca 2+ remains poorly understood. We addressed this topic using the light-gated channelrhodopsin Hc KCR2 from the pseudofungus Hyphochytrium catenoides , which operates as a H + conductive, Ca 2+ impermeable ion channel on the plasma membrane of plant cells. Light activation of Hc KCR2 in Arabidopsis guard cells evokes a transient cytoplasmic acidification that sparks Ca 2+ release from the endoplasmic reticulum. A H + -induced cytosolic Ca 2+ signal results in membrane depolarization through the activation of Ca 2+ -dependent SLAC1/SLAH3 anion channels, which enabled us to remotely control stomatal movement. Our study suggests a H + -induced Ca 2+ release mechanism in plant cells and establishes Hc KCR2 as a tool to dissect the molecular basis of plant intracellular pH and Ca 2+ signaling.