Stress and Redox Metabolism

Plants are submitted to a wide variety of environmental stresses, like high light, drought, nutrient deficiency, and temperature that can induce oxidative stress. In addition, intermediates in electron transfer reactions may react with other cell constituents and produce radicals and reactive oxygen species. Reactive oxygen species are responsible for major damages to membrane lipids and proteins. Consequently, an efficient antioxidant network is essential for protecting cell functions, particularly in chloroplasts. A whole set of metabolites such as glutathione and ascorbate, carotenoids, tocopherols, and enzymes such as superoxide dismutase (SOD), ascorbate peroxidase (APX), and glutathione peroxidase are known elements of antioxidant metabolism (for review, see Mullineaux and Karpinski 2002). The presence of a-tocopherol and xanthophylls in envelope membranes (see above)

is a first element for this system to play a role in oxidative stress responses. Furthermore, several proteins that could be involved in oxidative stress responses were identified in Arabidospis envelope membranes (Table 4): namely a phospholipid hydroperoxide glutathione peroxidase (GPX2, At2g25080) and an ascorbate peroxydase (APX3, At4g35000). Two members of the peroxiredoxins (Prx, for review see Dietz 2007) class of enzymatic antioxidants, PrxA (At3g11630) and PrxQ (At3g26060), were also identified (Table 4). However, PrxQ was also assigned to be bound to thylakoids (Lamkemeyer et al. 2006). In fact, most of these proteins do not have any transmembrane domains and are probably bound to, and active at, the stromal surface of the envelope membranes.

The stromal Cu/Zn SOD contains Cu, which is toxic at high concentration, due to generation of oxygen and hydroxyl free radicals and the oxidation of dithiols to disulfide in proteins (see for instance Shingles et al. 2004) . Indeed, Seigneurin-Berny et al. (2006) demonstrated that Arabidopsis plants with a mutation in the gene encoding the envelope protein HMA1, a member of the metal-transporting P1B-type ATPases family (see above), have a photosensitivity phenotype under high light. This demonstrates that Cu homeostasis in chloroplast, and therefore Cu transport across the envelope, is essential for an efficient photoprotection, especially under high light.

One original way to prevent photodamage in chloroplasts when plants are exposed to high light levels could be chloroplast avoidance movement, in which chloroplasts move from the cell surface to the periphery of cells under high light conditions. Kasahara et al. -2002) have shown that Arabidopsis chupl mutants (devoid of CHUP1 envelope protein, see above) are defective in chloroplast avoidance movement and are therefore more susceptible to damage in high light than wild-type plants.

Finally, several envelope (or envelope-bound) proteins are induced upon stress. For example, GLX1 (At1g11840) and ERD4 (At1g30360) are induced upon drought (Seki et al. 2001), and the damages caused to membrane proteins submitted to an oxidative stress require the presence of active repair mechanisms, such as chaperones and proteases (see above).

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