The production of reactive oxygen species (ROS), such as O2- and H2O2, is an unavoidable consequence of aerobic metabolism. In plant cells the mitochondrial electron transport chain (ETC) is a major site of ROS production. In addition to complexes I-IV, the plant mitochondrial ETC contains a non-proton-pumping alternative oxidase as well as two rotenone-insensitive, non-proton-pumping NAD(P)H dehydrogenases on each side of the inner membrane: NDex on the outer surface and NDin on the inner surface. Because of their dependence on Ca2+, the two NDex may be active only when the plant cell is stressed. Complex I is the main enzyme oxidizing NADH under normal conditions and is also a major site of ROS production, together with complex III. The alternative oxidase and possibly NDin(NADH) function to limit mitochondrial ROS production by keeping the ETC relatively oxidized. Several enzymes are found in the matrix that, together with small antioxidants such as glutathione, help remove ROS. The antioxidants are kept in a reduced state by matrix NADPH produced by NADP-isocitrate dehydrogenase and non-proton-pumping transhydrogenase activities. When these defenses are overwhelmed, as occurs during both biotic and abiotic stress, the mitochondria are damaged by oxidative stress.

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are produced in many places in living cells and at an increased rate during biotic or abiotic stress. ROS and RNS participate in signal transduction, but also modify cellular components and cause damage. We first look at the most common ROS and their properties. We then consider the ways in which the cell can regulate their production and removal. We critically assess current knowledge about modifications of polyunsaturated fatty acids (PUFAs), DNA, carbohydrates, and proteins and illustrate this knowledge with case stories wherever possible. Some oxidative breakdown products, e.g., from PUFA, can cause secondary damage. Other oxidation products are secondary signaling molecules. We consider the fate of the modified components, the energetic costs to the cell of replacing such components, as well as strategies to minimize transfer of oxidatively damaged components to the next generation.

The metabolism of aerobic organisms continuously produces reactive oxygen species. Although potentially toxic, these compounds also function in signaling. One important feature of signaling compounds is their ability to move between different compartments, e.g. to cross membranes. Here we present evidence that aquaporins can channel hydrogen peroxide (H 2 O 2 ). Twenty-four aquaporins from plants and mammals were screened in five yeast strains differing in sensitivity toward oxidative stress. Expression of human AQP8 and plant Arabidopsis TIP1;1 and TIP1;2 in yeast decreased growth and survival in the presence of H 2 O 2 . Further evidence for aquaporin-mediated H 2 O 2 diffusion was obtained by a fluorescence assay with intact yeast cells using an intracellular reactive oxygen species-sensitive fluorescent dye. Application of silver ions (Ag ؉ ), which block aquaporin-mediated water diffusion in a fast kinetics swelling assay, also reversed both the aquaporindependent growth repression and the H 2 O 2 -induced fluorescence. Our results present the first molecular genetic evidence for the diffusion of H 2 O 2 through specific members of the aquaporin family.Hydrogen peroxide (H 2 O 2 ) 2 belongs to the group of reactive oxygen species (ROS). ROS are generated in a number of key metabolic processes in cells like the electron transport chain in the inner mitochondrial membrane (1) and, specific for plants, the chloroplast thylakoid membrane (2).Because ROS can potentially damage proteins, lipids, and nucleic acids, cells have a number of ROS-scavenging systems that are able to remove these molecules and to maintain a relatively low and constant ROS concentration (3). However, ROS are also intermediates in various signal transduction pathways and have been shown to initiate responses to various stresses and disorders (for recent reviews, see Refs. 4 and 5). Arabidopsis mutants lacking an NADPH oxidase were not able to respond adequately to potassium deficiency (6) and were impaired in stomatal closure (7), providing genetic evidence for a role of NADPH oxidase in signaling.ROS are interconvertible molecules including singlet oxygen, superoxide, hydroxyl radical, and H 2 O 2 . H 2 O 2 has a distinctive set of features compared with other ROS. (i) It is not charged, (ii) it is not a radical, (iii) it possesses an intermediate oxidation number, (iv) it is relatively stable under physiological conditions, and (v) catalase can disproportionate it into water and molecular oxygen without the expense of reduction equivalents.Although substantial progress has been made regarding the formation and scavenging of ROS, little is known about their transport from the site of origin to the place of action or detoxification. Recently three studies from mammalian systems have provided evidence that H 2 O 2 , in addition to the well studied role in intracellular signaling, is also used as an intercellular signal molecule (8 -10). This implies that a necessary step within these signal transduction pathways is the transport of H 2 O 2 ...

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