Calcium channels activated by hydrogen peroxide mediate abscisic acid signalling in guard cells

Drought is a major threat to agricultural production. Plants synthesize the hormone abscisic acid (ABA) in response to drought, triggering a signalling cascade in guard cells that results in stomatal closure, thus reducing water loss. ABA triggers an increase in cytosolic calcium in guard cells ([Ca²⁺](cyt)) that has been proposed to include Ca²⁺ influx across the plasma membrane. However, direct recordings of Ca²⁺ currents have been limited and the upstream activation mechanisms of plasma membrane Ca²⁺ channels remain unknown. Here we report activation of Ca²⁺-permeable channels in the plasma membrane of Arabidopsis guard cells by hydrogen peroxide. The H₂O₂-activated Ca²⁺ channels mediate both influx of Ca²⁺ in protoplasts and increases in [Ca²⁺](cyt) in intact guard cells. ABA induces the production of H₂O₂ in guard cells. If H₂O₂ production is blocked, ABA-induced closure of stomata is inhibited. Moreover, activation of Ca²⁺ channels by H₂O₂ and ABA- and H₂O₂-induced stomatal closing are disrupted in the recessive ABA-insensitive mutant gca2. These data indicate that ABA-induced H₂O₂ production and the H₂O₂-activated Ca²⁺ channels are important mechanisms for ABA-induced stomatal closing.