Engineering the lutein epoxide cycle into Arabidopsis thaliana

Although sunlight provides the energy necessary for plants to survive and grow, light can also damage reaction centers of photosystem II (PSII) and reduce photochemical efficiency. To prevent damage, plants possess photoprotective mechanisms that dissipate excess excitation. A subset of these mechanisms is collectively referred to as NPQ, or nonphotochemical quenching of chlorophyll a fluorescence. The regulation of NPQ is intrinsically linked to the cycling of xanthophylls that affects the kinetics and extent of the photoprotective response. The violaxanthin cycle (VAZ cycle) and the lutein epoxide cycle (LxL cycle) are two xanthophyll cycles found in vascular plants. The VAZ cycle has been studied extensively, owing in large part to its presence in model plant species where mutants are available to aid in its characterization. In contrast, the LxL cycle is not found in model plants, and its role in photosynthetic processes has been more difficult to define. To address this challenge, we introduced the LxL cycle into Arabidopsis thaliana and functionally isolated it from the VAZ cycle. Using these plant lines, we showed an increase in dark-acclimated PSII efficiency associated with Lx accumulation and demonstrated that violaxanthin deepoxidase is responsible for the light-driven deepoxidation of Lx. Conversion of Lx to L was reversible during periods of low light and occurred considerably faster than rates previously described in nonmodel species. Finally, we present clear evidence of the LxL cycle’s role in modulating a rapid component of NPQ that is necessary to prevent photoinhibition in excess light.

Antioxidative Responses of Tobacco Expressing a Bacterial Glutathione Reductase

Reports on stress response of tobacco expressing a bacterial glutathione reductase (GR) do not agree. To clarify this situation we investigated several parameters using the tobacco BelW3 line and its transformant BelW3gor expressing an E. coli GR. This alteration in the activity of GR led to an ambiguous modification of the antioxidative system. In contrast to the wild type, the transgenic tobacco suffered lipid peroxidation under moderate light intensities, while it was found to be more resistant towards oxidative stress induced by paraquat or hydrogen peroxide. Transcript levels for violaxanthin deepoxidase and cytosolic Cu-Zn-superoxide dismutase were strongly reduced in BelW3gor plants as compared to BelW3.

Interconversion of Carotenoids and Quinones after Onset of Photosynthesis in Chloroplasts of Higher Plants

The interconversion of carotenoids and quinones was investigated in beech and spinach leaves as well as isolated intact spinach chloroplasts following a dark-light transition. It is shown that isolated intact chloroplasts which are preincubated for 2 h at pH 7.6 in the dark and re­illuminated with strong white light are capable not only of deepoxidizing violaxanthin into antheraxanthin and zeaxanthin but simultaneously change the redox state of the plastoquinone- pool in their thylakoid membrane. At the same time as violaxanthin is deepoxidized plastohydroquinone-9 is oxidized to plastoquinone-9. If the light is turned off zeaxanthin is epoxidized into antheraxanthin and violaxanthin but no significant change in the redox state of the plastoquinone-pool occurred. It is concluded that the deepoxidation of violaxanthin is connected to the photosynthetic electron transport in that way that an acidification of the intrathylakoidal compartment by the vectorial release of protons from the water photooxidizing enzyme system and the plastoquinone- pool is required for the activation of the violaxanthin deepoxidase. This may be taken as further evidence that violaxanthin deepoxidase is located at the inner side of the thylakoid membrane. Additional evidence for this location site is given by the observation that neither deepoxidation of violaxanthin nor photooxidation of plastohydroquinone-9 occurred after onset of photosyn­thesis if non cyclic electron transport was inhibited by DCMU.