Defense against Photooxidative Damage in Plants

Abstract

Plant leaves intercept light, transform it into chemical energy and reductant, i.e., ATP and NADPH, and use these primary products of photosynthesis for assimilatory processes, mainly the reduction of CO₂. If plants are exposed to excess light that cannot be utilized for production of cellular reductant, photosynthetic antennae may transfer excitation energy to ground state oxygen (³O₂) yielding singlet oxygen (¹O₂) (Hideg et al. 1994). Furthermore, light-driven electron transport systems may divert electrons to O₂ instead of NADP⁺, resulting in superoxide radical (O₂^•−) production (Mehler reaction; Mehler 1951). This light-dependent production of reactive oxygen species is generally termed photooxidative stress. Primary oxidants such as O₂^•− give rise to secondary oxidants, namely H₂O₂. The concurrent accumulation of O₂^•− and H₂O₂ is particularly dangerous. In the presence of transition metals, these two oxidants together initiate the production of hydroxyl radicals (•OH) (Fenton reaction). •OH are extremely toxic since they destroy cellular constituents at an almost diffusion-controlled rate (k = 10⁸ to 10¹⁰ M⁻¹S⁻¹ for organic molecules) (Saran et al. 1987). Damage by •OH can only be prevented by controlling the concentrations of their precursors.

Environmental conditions that induce or favor photooxidative stress are part of plants’ everyday life. Thus, generation of reactive intermediates of oxygen metabolism is inevitable in aerobic organisms. Because of their light-harvesting function and production of oxygen, chloroplasts have a particular risk to suffer from oxidative stress. In comparison with animal cells, chloroplasts contain about one to three orders of magnitude higher oxidant concentrations, i.e., 10⁻⁸ to 10⁻⁹ versus...