Calcium-calmodulin is required for abscisic acid-induced antioxidant defense and functions both upstream and downstream of H₂O₂ production in leaves of maize (Zea mays) plants

• Using pharmacological and biochemical approaches, the role of calmodulin (CaM) and the relationship between CaM and hydrogen peroxide (H ₂O₂) in abscisic acid (ABA)-induced antioxidant defense in leaves of maize (Zea mays) plants were investigated. • Treatment with ABA or H₂O₂ led to significant increases in the concentration of cytosolic Ca²⁺ in the protoplasts of mesophyll cells and in the expression of the calmodulin 1 (CaM1) gene and the content of CaM in leaves of maize plants, and enhanced the expression of the antioxidant genes superoxide dismutase 4 (SOD4), cytosolic ascorbate peroxidase (cAPX), and glutathione reductase 1 (GR1) and the activities of the chloroplastic and cytosolic antioxidant enzymes. The up-regulation of the antioxidant enzymes was almost completely blocked by pretreatments with two CaM antagonists. • Pretreatments with CaM antagonists almost completely inhibited ABA-induced H₂O₂ production throughout ABA treatment, but pretreatment with an inhibitor or scavenger of reactive oxygen species (ROS) did not affect the initial increase in the contents of CaM induced by ABA. • Our results suggest that Ca²⁺-CaM is involved in ABA-induced antioxidant defense, and that cross-talk between Ca²⁺-CaM and H₂O₂ plays a pivotal role in ABA signaling.