1. Division of Biology, Cell and Developmental Biology, and Center for Molecular Genetics, University of California at San Diego, La Jolla, California 92093-0116, USA
2. Technische Universität München, Lehrstuhl für Botanik, D-85350 Freising–Weihenstephan , Germany
3. These authors contributed equally to this work

Correspondence to: Julian I. Schroeder¹ Correspondence and requests for materials should be addressed to J.I.S. (e-mail: Email: julian@biomail.ucsd.edu).

Abstract

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 loss¹. ABA triggers an increase in cytosolic calcium in guard cells ([Ca²⁺]_cyt)^{2, 3, 4, 5, 6} that has been proposed to include Ca²⁺ influx across the plasma membrane^{3, 5, 7, 8, 9}. However, direct recordings of Ca ²⁺ currents have been limited³ 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 [Ca ²⁺]_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 H₂O ₂-activated Ca²⁺ channels are important mechanisms for ABA-induced stomatal closing.