Modulation of Antioxidant Defense System and NADPH Oxidase in Pluchea Indica Leaves by Water Deficit Stress

We investigated changes in antioxidant content and antioxidant enzyme activity in polyethylene glycol (PEG)-induced water deficit stressed Pluchea indica leaves. We also used diphenylene iodonium (DPI), a plasma membrane NADPH oxidase suicide inhibitor, to examine the role of reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase in the induction of antioxidant defense systems. PEG decreased water content and the reduction ability of 2,3,5-triphenyltetrazolium chloride (TTC), but increased malondialdehyde (MDA), total peroxide, O2 - and H2O2 contents, indicating water deficit-dependent oxidative stress. Total ascorbate (AsA), AsA contents, and AsA/oxidized AsA ratios were increased by moderate water deficit (-0.6 MPa) conditions, while total glutathione (GSH), GSH contents and GSH/GSSG ratios increased as water potential decreased. Superoxide dismutase (SOD) activity was not affected after 24 h of PEG treatment but decreased after 48 h. Catalase (CAT) activity increased as water potential decreased while peroxidase (POX) activity increased only at -1.2 MPa. Ascorbate peroxidase (APX) activity increased under moderate water deficit and glutathione reductase (GR) activity increased as water potential decreased. DPI depressed the induction of antioxidant accumulation and antioxidant enzyme activity by water deficit, indicating that reactive oxygen species (ROS) signals are involved in activating defense systems in P. indica in response to oxidative stress. We conclude that moderate water deficit stress induces both NADPH oxidase-mediated non-enzymatic and enzymatic oxidative defense mechanisms in ROS scavenging in P. indica leaves.