Oxygen is a natural acceptor of electrons in the respiratory pathway of aerobic organisms and in many other biochemical reactions. Aerobic metabolism is always associated with the formation of reactive oxygen species (ROS). ROS may damage biomolecules but are also involved in regulatory functions of photosynthetic organisms. This review presents the main properties of ROS, the formation of ROS in the photosynthetic electron transport chain and in the stroma of chloroplasts, and ROS scavenging systems of thylakoid membrane and stroma. Effects of ROS on the photosynthetic apparatus and their roles in redox signaling are discussed.

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Lipid peroxide (LOOH)-derived aldehydes and ketones (hereafter collectively designated as carbonyls) are recently recognized as cell signals and toxic species in plant stress responses. Carbonyls at low concentrations can induce stress defense genes (Sattler et al. 2006; Weber et al. 2004), while at high concentrations, they can cause damages to cell components (O'Brien et al. 2005). Recent studies have shown that enhanced detoxification of carbonyls in transgenic plants improved their tolerance to various environmental stresses, as follows: Aldehyde reductase from Medicago sativa (alfalfa) overexpressed in Nicotiana tabacum (tobacco) improved tolerance to drought stress (Oberschall et al. 2000), methyl viologen and UV-B (Hideg et al. 2003). Aldehyde dehydrogenase isozymes from Arabidopsis thaliana improved the tolerance of transgenic A. thaliana plants to NaCl, heavy metals, methyl viologen, and H 2 O 2 (Sunkar et al. 2003). Vice versa, the dehydrogenaseknockout mutants were more sensitive to dehydration and salt than wild type (Kotchoni et al. 2006). The observed correlation between the extent of damages and the content of aldehydes, as thiobarbituric acid-reactive substances, in these studies strongly supports that aldehydes are causes of stress-induced damages in vivo. It is known that a great variety of carbonyls are produced from LOOH (Grosch 1987; Kawai et al. 2007) and many of them occur in plant tissues (Schauenstein et al. 1977), but it has been scarcely investigated what carbonyl species are involved in stress-induced damage of plant cells. Among LOOH-derived carbonyls, a,b-unsaturated aldehydes (2-alkenals) have potent cytotoxicity due to their high electrophilicity to form adducts with thiols, amines, and imidazoles in proteins and with nucleotide bases (Esterbauer et al. 1991). To photosynthesis, 2alkenals showed greater toxicity than saturated aldehydes in vitro (Mano et al. 2009). In A. thaliana, we found a novel enzyme, 2-alkenal reductase (AER), to detoxify 2alkenals (Mano et al. 2002). We have made transgenic tobaccos that overexpressed the AER gene At5g16970.

Hydrogen peroxide production in isolated pea thylakoids was studied in the presence of cytochrome c to prevent disproportionation of superoxide radicals outside of the thylakoid membranes. The comparison of cytochrome c reduction with accompanying oxygen uptake revealed that hydrogen peroxide was produced within the thylakoid. The proportion of electrons from water oxidation participating in this hydrogen peroxide production increased with increasing light intensity, and at a light intensity of 630 micromol quanta m(-2) s(-1) it reached 60% of all electrons entering the electron transport chain. Neither the presence of a superoxide dismutase inhibitor, potassium cyanide or sodium azide, in the thylakoid suspension, nor unstacking of the thylakoids appreciably affected the partitioning of electrons to hydrogen peroxide production. Also, osmolarity-induced changes in the thylakoid lumen volume, as well as variation of the lumen pH induced by the presence of Gramicidin D, had negligible effects on such partitioning. The flow of electrons participating in lumen hydrogen peroxide production was found to be near 10% of the total electron flow from water. It is concluded that a considerable amount of hydrogen peroxide is generated inside thylakoid membranes, and a possible mechanism, as well as the significance, of this process are discussed.

Chlorophylls are degraded and flavonoids synthesized during autumn senescence of deciduous trees. In the present study, chlorophyll and flavonol contents of individual leaves were monitored non-destructively throughout the autumn. Loss of chlorophyll and synthesis of flavonols were not gradual. Instead, each leaf maintained steady chlorophyll content until rapid chlorophyll degradation, accompanied by flavonol synthesis, was triggered. In ~1 week, the leaf turns yellow and falls. The pattern was similar in birch (Betula pendula), maple (Acer platanoides) and bird cherry (Prunus padus); in rowan (Sorbus aucuparia), very slow gradual chlorophyll degradation occurred on top of the main pattern.

It was found that the contribution of segments of photosynthetic electron transport chain (PETC) besides Photosystem I (PSI) to oxygen reduction increased with increase in light intensity, and at high intensities achieved 50% at pH 5.0, and was higher than 60% at pH 6.5 and pH 7.8. The data are explained as the result of O2 reduction in plastoquinone (PQ) pool as well as in PSI followed by reduction of superoxide radicals generated in both processes by plastohydroquinone.

The photosynthetic electron transport chain (PETC) is the principal place of appearance of reactive oxygen species (ROS) in plants under illumination. The peculiarities of this process in different segments of the PETC are discussed. Oxygen uptake observed under impaired electron donation to photosystem II is attributed mainly to hydroperoxide formation by reaction of oxygen with organic radicals generated after detachment of electrons by P680(+). Oxygen reduction in the plastoquinone pool is suggested to start with the reaction of O(2) with plastosemiquinone, and to be followed by reduction of superoxide to hydrogen peroxide by plastohydroquinone. The distribution of plastoquinone throughout the thylakoid membrane interior provides for the generation of ROS by this route all along the membrane surface. O(2) reduction at the acceptor side of photosystem I remains poorly understood. The regeneration of antioxidants is stated to be a priority task of photosynthetic electron transport in view of the effectiveness of monodehydroascorbate as electron acceptor. We propose that ROS generation in the plastoquinone pool and the possible formation of hydroperoxides in the vicinity of photosystem II are key processes participating in the primary stages of redox signaling.

Aldehydes and ketones produced from lipid peroxides (oxylipin carbonyls) exhibit a variety of biological activities ranging from the induction of defense genes to the irreversible damage to cells. Short oxylipin carbonyls such as acrolein and (E)-2-pentenal are responsible for tissue injury under environmental stress, but their production mechanism remains unclear. In this study, we elucidated the source fatty acids of short oxylipin carbonyls in leaves. We first established a comprehensive analysis of oxylipin carbonyls for quantitation and structural estimation. Carbonyls were extracted from rosette leaves of Arabidopsis thaliana, derivatized with 2,4-dinitrophenylhydrazine and separated with a reverse-phase HPLC equipped with a photodiode array detector and an Fourier transform ion cyclotron resonance mass spectrometer. Thirty-three distinct carbonyls were detected, in which 19 species were identified on the MS/MS spectrum. Using this analysis system, we compared the carbonyl composition in the leaves between two A. thaliana lines that have different fatty acid composition. The mutant fad7fad8, which lacks the biosynthesis of trienoic fatty acids in the plastid, contained significantly lower amounts of malondialdehyde, acrolein and (E)-2-pentenal and higher amounts of acetone, 3-pentanone, and n-hexanal than the wild type Col-0. This difference in the carbonyl composition agreed with the oxidative degradation of dienoic and trienoic fatty acids in vitro, showing that similar non-enzymatic reactions occur in the thylakoid membrane. Consideration of the formation mechanism of acrolein from trienoic fatty acid suggests that membrane lipids in chloroplasts are constitutively oxidized by singlet oxygen.