Formation of Reactive Oxygen Species

The bleaching effects of many heavy metals in light have been known for a long time and are connected with the formation of reactive oxygen species (ROS; Asada 1999). The main important ROS are singlet oxygen ('O2) and the hydroxy radical (HO*), because both are highly reactive, carrying out oxidation reactions with many organic molecules at their sites of formation during their short lives. The generation of ROS is a general phenomenon; higher plants developed a highly sophisticated antioxidant system during the course of evolution. This consists of several enzymes (superoxide dismutases, catalases, ascorbate oxidases, glutathione peroxidases and glutathione reductases) and antioxidant substrates (ascorbate, glutathione and a-tocopherol). The main sources of ROS under control conditions (an absence of toxic concentrations of heavy metals) are photosynthetic and respiratory electron transport processes. Only when the capacity of cells to suppress the concentrations of ROS is exceeded do these species then damage cells over a long period. Heavy metals play many roles in this respect (Sharma and Agrawal 2005):

• They directly disturb electron transport, causing electrons to be transferred to oxygen instead of the natural electron acceptors in chloroplasts and mitochondria

• Disturbances to metabolic reactions feed back to electron transport, as just described

• Redox-active metals in different oxidation states under physiological conditions can participate in the Fenton and Haber-Weiss reaction (c.f. Shaw et al. 2004), producing hydroxyl radicals

• Inactivation and downregulation of enzymes of the antioxidant defence system

• Depletion of antioxidant substrates.

It has been shown on several occasions that lipid peroxidation (Bertrand and Poirier 2005) is just a consequence of oxidative stress, such as that caused by glutathione depletion (Schutzendubel and Polle 2002).

As long as the stress is not too high, plants often respond by inducing antioxidant enzymes together with rather unspecific stress proteins, such as heat-shock proteins (Clemens 2006).

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