IRON-DEPENDENT OXYGEN-FREE RADICAL GENERATION IN PLANTS SUBJECTED TO ENVIRONMENTAL-STRESS - TOXICITY AND ANTIOXIDANT PROTECTION

Iron has a pivotal and dual role in free radical chemistry in all orga nisms. On the one hand, free Fe can participate in Fenton reactions an d catalyze ('catalytic Fe') the generation of hydroxyl radical and oth er toxic oxygen species. On the other hand, Fe is a constituent of the antioxidant enzymes catalase, ascorbate peroxidase, guaiacol peroxida se, and ferro-superoxide dismutase. Protein Fe is Fenton inactive but can be released from proteins upon attack by activated oxygen. Healthy , unstressed plants avoid the interaction of catalytic Fe and peroxide s by disposing of Fe in vacuoles and apoplast, by sequestering Fe in f erritin, and by having high levels of antioxidant enzymes and metaboli tes in most subcellular compartments. However, when plants are exposed to a variety of adverse conditions, including chilling, high light, d rought and paraquat, oxidative stress ensues due primarily to the decr ease in antioxidant defenses but also to the increase in free radical production mediated by catalytic Fe. The latter accumulates in many st ressed plant tissues. Oxidative stress may lead to metabolic dysfuncti on and ultimately to plant cell death, so it needs to be estimated con veniently by quantifying the oxidation products of lipids (malondialde hyde and other cytotoxic aldehydes), proteins (total carbonyls, methio nine sulfoxide, 2-oxohistidine), and DNA (8-hydroxyguanine, 5-hydroxyc ytosine). Protein oxidation appears to be a more sensitive and precoci ous marker than is lipid peroxidation, and DNA damage may also prove t o be a useful marker for stress studies in plants.