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 (Ca²⁺) levels, the interdependence between protons (H⁺) and Ca²⁺ remains poorly understood. We addressed this topic using the light-gated channelrhodopsin HcKCR2 from the pseudofungus Hyphochytrium catenoides, which operates as a H⁺ conductive, Ca²⁺ impermeable ion channel on the plasma membrane of plant cells. Light activation of HcKCR2 in Arabidopsis guard cells evokes a transient cytoplasmic acidification that sparks Ca²⁺ release from the endoplasmic reticulum. A H⁺-induced cytosolic Ca²⁺ signal results in membrane depolarization through the activation of Ca²⁺-dependent SLAC1/SLAH3 anion channels, which enabled us to remotely control stomatal movement. Our study suggests a H⁺-induced Ca²⁺ release mechanism in plant cells and establishes HcKCR2 as a tool to dissect the molecular basis of plant intracellular pH and Ca²⁺ signaling.