Oxidative reactions in axenically seedling roots of olive (Olea europaea, L.)

Abstract

The production of reactive oxygen species (ROS) plays important roles in the life cycle and in the stress response and defence mechanisms of plants. Various enzyme systems are involved in the formation of ROS in the apoplast, including plasmalemma NADPH oxidase and apoplastic peroxidases. The production of O 2 ·− and apoplastic peroxidase and exogenous NADH oxidation activities are all strongly dependent on the age of roots—the younger the root, the greater the activity. Apoplastic production of ROS is shown in the root by using specific histochemical probes, this ROS production is growing zone dependent. In the present study, using olive seedlings, differences were also observed between cultivars, especially in O 2 ·− production by the Verdial cultivar which was well above that of other cultivars studied. In all the cultivars, treatment of roots with methyl jasmonate (MeJA) or methyl salicylate (MeSA) increased O 2 ·− production. Similar results were observed for peroxidase activity, but not for the oxidation of exogenous NADH which was either unaffected (MeJA) or even partially inhibited (MeSA). A conclusion was that MeJA or MeSA induced apoplastic production of ROS does not use exogenous NADH. Treatment with diphenylene iodonium (DPI) reduced the formation of O 2 ·− , but affected neither peroxidase nor NADH oxidation activities. Cyanide inhibited O 2 ·− production and peroxidase and NADH oxidation activities. Treatment with MnCl2 had a strong stimulatory effect on peroxidase and NADH oxidation activities, but much less on O 2 ·− production. Finally, azide greatly reduced all activities, but especially O 2 ·− production. Together, these results indicate a relationship between oxidative activities and the processes of root growth, and that those activities are also dependent on the cultivar, as well as an involvement of peroxidases and plasmalemma NADPH oxidase in apoplast ROS production which is sensitive to DPI, azide, and cyanide but relatively insensitive to MnCl2, while exogenous NADH oxidation is linked to peroxidase activity.