Molecular Genetics of Superoxide Dismutases in Plants

Abstract

Aerobic organisms gain a significant energetic advantage by using molecular oxygen as the terminal oxidant in respiration. However, although dioxygen itself is relatively unreactive and harmless, it has the potential to be partially reduced to form toxic intermediate species such as singlet oxygen (¹O₂), the superoxide (O₂^•−) and hydroxyl (• OH) radicals, and hydrogen peroxide (H₂O₂). These molecules are formed during normal cellular functions and as a consequence of various environmental insults to which organisms are exposed. Consequently, their formation and destruction constitute a highly regulated cellular phenomenon. Many diverse cellular reactions and most intracellular compartments have been implicated in the generation of such reactive oxygen species (ROS). In plants, chloroplasts readily generate the highly reactive singlet oxygen and superoxide via direct donation of excitation energy or electrons to oxygen from the photosynthetic electron transport chain. An activation energy of approximately 22 kcal/mole is required to raise molecular O₂ from its ground state to its first singlet state. In higher plants, this energy is readily obtained from light quanta via such transfer molecules as chlorophyll (Chl) (Foote 1976).

All ROS are extremely reactive and cytotoxic in all organisms. ROS can react with unsaturated fatty acids to cause peroxidation of essential membrane lipids in the plasmalemma or intracellular organelles. Peroxidation of the plasmalemma leads to leakage of cellular contents, rapid desiccation, and cell death. Intracellular membrane damage can affect respiratory activity in mitochondria, cause pigment breakdown, and cause loss of carbon-fixing ability in chloroplasts. In the presence of metal ions such...