Cross-talk between calcium and reactive oxygen species originated from NADPH oxidase in abscisic acid-induced antioxidant defence in leaves of maize seedlings

The signal interactions between calcium (Ca²⁺) and reactive oxygen species (ROS) originated from plasma membrane NADPH oxidase in abscisic acid (ABA)-induced antioxidant defence were investigated in leaves of maize (Zea mays L.) seedlings. Treatment with ABA led to significant increases in the activity of plasma membrane NADPH oxidase, the production of leaf O₂^-, and the activities of several antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and glutathione reductase (GR). However, such increases were blocked by the pretreatment with Ca²⁺ chelator EGTA or Ca²⁺ channel blockers La³⁺ and verapamil, and NADPH oxidase inhibitors such as diphenylene iodonium (DPI), imidazole and pyridine. Treatment with Ca²⁺ also significantly induced the increases in NADPH oxidase activity, O₂^- production and the activities of antioxidant enzymes, and the increases were arrested by pretreatment with the NADPH oxidase inhibitors. Treatment with oxidative stress induced by paraquat, which generates O₂^-, led to the induction of antioxidant defence enzymes, and the upregulation was suppressed by the pretreatment of Ca²⁺ chelator and Ca²⁺ channel blockers. Our data suggest that a cross-talk between Ca²⁺ and ROS originated from plasma membrane-bound NADPH oxidase is involved in the ABA signal transduction pathway leading to the induction of antioxidant enzyme activity, and Ca²⁺ functions upstream as well as downstream of ROS production in the signal transduction event in plants.