XVI International Botanical Congess

Among all aerobic organisms, the cellular dioxygen concentration is the highest in photosynthetic tissues of plants, and thus even under normal metabolic conditions, the intracellular production of activated oxygen species (AOS) in chloroplasts and other cellular compartments is inevitable. Various environmental perturbations, including exposure to drought, temperature extremes, high light intensity, UV-B irradiation, and tropospheris ozone, exacerbate this condition. AOS are also generated in response to invading pathogens and serve a dual purpose, directly through the oxidative burst" and indirectly as signaling molecules in eliciting the defense response. To cope with oxidative stress, all cellular compartments contain various antioxidants. These include metabolites that are effective scavengers of AOS, such as glutathione, carotenoids and vitamin C and E, numerous enzymes that rid the cell of various AOS molecules, and other enzymes that maintain a pool of antioxidants in their reduced state. Together these antioxidants act to minimize oxidative damage by controlling AOS accumulation and regulate gene expression through the production of appropriate signals.