Singlet oxygen oxidation products of carotenoids, fatty acids and phenolic prenyllipids.

Singlet oxygen (1O2) is the major reactive oxygen species ROS causing photooxidative stress in plants which is formed predominantly in the reaction center of photosystem II during photosynthesis. To avoid deleterious effects of 1O2 oxygen on photosynthetic membrane components, plant synthesize a variety of 1O2 quenchers of lipophilic character, such as carotenoids or phenolic prenyllipids (tocopherols, plastochromanol-8, plastoquinol). In the process of chemical quenching of 1O2 by the antioxidants, both short-lived products, such as oxidized carotenoids, or relative long-lived compounds, such as oxidized phenolic prenyllipids are formed. The other target of 1O2 are unsaturated fatty acids of membrane lipids that undergo peroxidation as a result of the reaction. Some of the 1O2 oxidation products, like ß-cyclocitral can be components of 1O2-signallingsignaling pathway leading to acclimatory responses of plants, while some others further fulfill antioxidant functions, like hydroxy-plastochromanol or hydroxy-plastoquinol. As most of the 1O2 oxidation products are specific compounds formed only as a results of 1O2 action, they can be very useful, specific molecular markers of 1O2-dependent oxidative stress in vivo.

Novel and rare prenyllipids - Occurrence and biological activity.

The data presented indicate that there is a variety of unique prenyllipids, often of very limited taxonomic distribution, whose origin, biosynthesis, metabolism and biological function deserves to be elucidated. These compounds include tocoenols, tocochromanol esters, tocochromanol acids, plastoquinones and ubiquinones. Additionally, based on the available data, it can be assumed that there are still unrecognized prenyllipids, like prenylquinols fatty acid esters of the hydroquinone ring, including prenylquinol phosphates, and others, whose biological function might be of great importance. Our knowledge of these compounds is not only important from the scientific point of view, but may also be of practical significance to medicine, pharmacy or cosmetics.

Down-regulation of tomato PHYTOL KINASE strongly impairs tocopherol biosynthesis and affects prenyllipid metabolism in an organ-specific manner.

Tocopherol, a compound with vitamin E (VTE) activity, is a conserved constituent of the plastidial antioxidant network in photosynthetic organisms. The synthesis of tocopherol involves the condensation of an aromatic head group with an isoprenoid prenyl side chain. The latter, phytyl diphosphate, can be derived from chlorophyll phytol tail recycling, which depends on phytol kinase (VTE5) activity. How plants co-ordinate isoprenoid precursor distribution for supplying biosynthesis of tocopherol and other prenyllipids in different organs is poorly understood. Here, Solanum lycopersicum plants impaired in the expression of two VTE5-like genes identified by phylogenetic analyses, named SlVTE5 and SlFOLK, were characterized. Our data show that while SlFOLK does not affect tocopherol content, the production of this metabolite is >80% dependent on SlVTE5 in tomato, in both leaves and fruits. VTE5 deficiency greatly impacted lipid metabolism, including prenylquinones, carotenoids, and fatty acid phytyl esters. However, the prenyllipid profile greatly differed between source and sink organs, revealing organ-specific metabolic adjustments in tomato. Additionally, VTE5-deficient plants displayed starch accumulation and lower CO2 assimilation in leaves associated with mild yield penalty. Taken together, our results provide valuable insights into the distinct regulation of isoprenoid metabolism in leaves and fruits and also expose the interaction between lipid and carbon metabolism, which results in carbohydrate export blockage in the VTE5-deficient plants, affecting tomato fruit quality.

Formation of singlet oxygen and protection against its oxidative damage in Photosystem II under abiotic stress.

Photosystem II (PSII) is exposed to various abiotic stresses associated with adverse environmental conditions such as high light, heat, heavy metals or mechanical injury. Distinctive functional response to adverse environmental conditions is formation of singlet oxygen ((1)O2). In this review, recent progress on mechanistic principles on (1)O2 formation under abiotic stresses is summarized. Under high light, (1)O2 is formed by excitation energy transfer from triplet chlorophylls to molecular oxygen formed by the spin conversion via photosensitization Type II reaction in the PSII antenna complex or by the recombination of (1)[P680(+)Pheo(-)] radical pair in the PSII reaction center. Apart from well-described (1)O2 formation by excitation energy transfer, (1)O2 formation by decomposition of dioxetane and tetroxide is summarized as a potential source of (1)O2 in PSII under heat, heavy metals and mechanical stress. The description of mechanistic principles on (1)O2 formation under abiotic stress allows us to understand how plants respond to adverse environmental conditions in vivo.

Hydroxy-plastochromanol and plastoquinone-C as singlet oxygen products during photo-oxidative stress in Arabidopsis.

In the present study, we have shown that hydroxy-plastochromanol and plastoquinone-C, the hydroxy derivatives of plastochromanol and plastoquinone-9, respectively, are specifically formed from the parent compounds upon action of singlet oxygen and can be regarded as stable, specific, natural products of singlet oxygen action during photo-oxidative stress in vivo. The presented data indicate that plastoquinone-C formation dominates mainly during relatively short periods of high light stress where efficient production of singlet oxygen takes place, whereas hydroxy-plastochromanol is rather formed under conditions of long-term, less pronounced generation of singlet oxygen. An interesting observation was that hydroxy-plastochromanol is formed even at very low light conditions (5-10 µmol photons m(-2) s(-1)), indicating that singlet oxygen is generated not only during high light stress but also its formation by photosystem II is inseparably connected with the functioning of this photosystem even at the lowest light intensities.