Abstract:Nitric oxide is a bioactive molecule involved in many biological events, and has been reported as prooxidant as well as antioxidant in plants. In the present study, the sources of NO production under water stress, the role of NO in water stress-induced H2O2 accumulation and subcellular activities of antioxidant enzymes in leaves of maize (Zea mays L.) plants were investigated. Water stress induced increases in the generation of NO in maize mesphyll cells and the activity of nitric oxide synthase (NOS) in the cytosolic and microsomal fractions of maize leaves. Water stress-induced increases in the production of NO were blocked by the pretreatments with inhibitors of NOS and nitrate reductase (NR), suggesting that NO is produced from NOS and NR in leaves of maize plants exposed to water stress. Water stress also induced increases in the activities of the chloroplastic and cytosolic antioxidant enzymes superoxide dismutase (SOD), ascorbate peroxidase (APX), and glutathione reductase (GR), and the increases in the activities of antioxidant enzymes were reduced by the pretreatments with inhibitors of NOS and NR. Exogenous NO increases the activities of water stress-induced subcellular antioxidant enzymes, which decreases accumulation of hydrogen peroxide (H2O2). Our results suggest that NOS and NR are involved in water stress-induced NO production and NOS is the major source NO. The potential ability of NO to scavenge H2O2 is, at least in part, due to the induction of subcellular antioxidant defense.