Renata Szymańska

Plastoquinol is the Main Prenyllipid Synthesized During Acclimation to High Light Conditions in Arabidopsis and is Converted to Plastochromanol by Tocopherol Cyclase

Plants have evolved various strategies to acclimate to high light conditions at different levels of organization. High light stress stimulates synthesis of different antioxidant enzymes and low molecular weight antioxidants, mainly in chloroplasts. In the present studies we showed that plastoquinol, in addition to alpha-tocopherol, is the main lipid-soluble antioxidant synthesized during acclimation of Arabidopsis plants to high light conditions. The level of plastoquinol increased >10-fold and independently of tocopherols, as revealed using tocopherol biosynthetic mutants. The high light-induced increase in plastoquinol level was mainly attributable to the photochemically non-active fraction of this compound localized in plastoglobuli, which are the storage site of prenyllipids for their antioxidant action. Our data also revealed that tocopherol cyclase is required for plastochromanol biosynthesis from plastoquinol in vivo. Plastochromanol accumulated in increasing amounts in leaves during growth and it was also identified in seeds. The obtained data suggest that plastochromanol may, similarly to other prenyllipids, fulfill antioxidant function in leaves and seeds, especially during aging.

γ-Tocopherol dominates in young leaves of runner bean (Phaseolus coccineus) under a variety of growing conditions: The possible functions of γ-tocopherol

It has been shown that young leaves of runner bean (Phaseolus coccineus) plants grown under natural conditions have an unusually high content of gamma-tocopherol, accounting for up to 90% of all tocopherols and 50% of the chlorophyll content. The level of gamma-tocopherol gradually decreased during the first two weeks of leaf development. The high content of gamma-tocopherol in young leaves was not significantly influenced by growth conditions. In contrast to seeds, gamma-tocopherol was also the main tocopherol found in light-grown and etiolated primary leaves of runner bean. The obtained results suggest that gamma-tocopherol decline during leaf development is not only due to conversion of gamma- to alpha-tocopherol but probably also due to degradation of gamma-tocopherol to non-tocochromanol compounds. We have also shown that gamma-tocopherol found in young leaves is mainly localized in thylakoid membranes within chloroplast. In the primary leaves subjected to different abiotic stresses, only during simultaneous drought and light stress, gamma-tocopherolquinone, an oxidation product of gamma-tocopherol, was preferentially accumulated. Since one of the other possible functions of gamma-tocopherol could be its action as a nitric oxide scavenger, young leaves were analyzed for the presence of nitro-gamma-tocopherol. However, despite the use of a sensitive detection method, it was not found. The possible physiological function of the increased level of gamma-tocopherol in the young leaves was discussed.

Singlet oxygen and non-photochemical quenching contribute to oxidation of the plastoquinone-pool under high light stress in Arabidopsis

The redox state of plastoquinone-pool in chloroplasts is crucial for driving many responses to variable environment, from short-term effects to those at the gene expression level. In the present studies, we showed for the first time that the plastoquinone-pool undergoes relatively fast oxidation during high light stress of low light-grown Arabidopsis plants. This oxidation was not caused by photoinhibition of photosystem II, but mainly by singlet oxygen generated in photosystem II and non-photochemical quenching in light harvesting complex antenna of the photosystem, as revealed in experiments with a singlet oxygen scavenger and with Arabidopsis npq4 mutant. The latter mechanism suppresses the influx of electrons to the plastoquinone-pool preventing its excessive reduction. The obtained results are of crucial importance in light of the function of the redox state of the plastoquinone-pool in triggering many high light-stimulated physiological responses of plants.

Novel vitamin E forms in leaves of Kalanchoe daigremontiana and Phaseolus coccineus

In the present study, we isolated novel tocochromanols from green leaves of Kalanchoe daigremontiana and primary leaves of etiolated seedlings of Phaseolus coccineus that were identified as β-, γ-, and δ-tocomonoenols with unsaturation at the terminal isoprene unit of the side chain. The content of γ-tocomonoenol in leaves of etiolated bean increased gradually with the age of seedlings, reaching 50% of the γ-tocopherol level in 40-day-old plants. The content of this compound in leaves was increased by short illumination of etiolated plants and by addition of homogentisic acid, a biosynthetic precursor of tocopherols. These data indicated that γ-tocomonoenol is synthesized de novo from homogentisic acid and tetrahydro-geranylgeraniol diphosphate, a phytol precursor. Based on these results, a biosynthetic pathway of tocomonoenols is proposed.

Tocopherol quinone content of green algae and higher plants revised by a new high-sensitive fluorescence detection method using HPLC -Effects of high light stress and senescence

A rapid, sensitive fluorescence method was applied here for detection of oxidized tocopherol quinones in total plant tissue extracts using HPLC, employing a post-column reduction of these compounds by a Zn column. Using this method, we were able to detect both alpha- and gamma-tocopherol quinones in Chlamydomonas reinhardii with a very high degree of sensitivity. The levels of both compounds increased under high light stress in the presence of pyrazolate in parallel to a decrease in the content of the corresponding tocopherols. The formation of tocopherol quinones from tocopherols was apparently due to their oxidation by singlet oxygen, which is formed in photosystem II under high light stress. alpha-Tocopherol quinone was also detected in a variety of higher plants of different age, and its level was found to increase during senescence in leaves grown under natural conditions. In contrast to alpha-tocopherol quinone, gamma-tocopherol quinone was not found in the higher plant species investigated with the exception of young runner bean leaves, where the levels of both compounds increased dramatically during cold and light stress. Taking advantage of native fluorescence of the reduced alpha-tocopherol quinone (alpha-tocopherol quinol), it can be detected in plant tissue extracts with a high sensitivity. In young runner bean leaves, alpha-tocopherol quinol was found at a level similar to alpha-tocopherol.

Plant-Derived Antioxidants in Disease Prevention

1Department of Medical Physics and Biophysics, Faculty of Physics and Applied Computer Science, AGH University of Science and Technology, Reymonta 19, 30-059 Krakow, Poland 2Department of Biophysics, Faculty of Science, Centre of the Region Haná for Biotechnological and Agricultural Research, Šlechtitelů 27, Palacký University, 78371 Olomouc, Czech Republic 3Department of Plant Physiology and Biochemistry, Faculty of Biochemistry, Biophysics and Biotechnology, Gronostajowa 7, 30-387 Krakow, Poland

Ferredoxin:NADP+ oxidoreductase bound to cytochrome b6f complex is active in plastoquinone reduction: Implications for cyclic electron transport

In this study, we have compared three isolation methods of cytochrome b₆f complex, obtained from spinach (Spinacia oleracea), differing in the preservation of the cytochrome b₆f-associated ferredoxin:NADP+ oxidoreductase (FNR). Although the complexes isolated by all the methods showed the presence of the FNR peptide(s), when incorporated into liposome membranes, the NADPH-PQ (plastoquinone) oxidoreductase activity was not detected for the cytochrome b₆f complex isolated with the original method including a NaBr wash. Some activity was found for the complex isolated with the omission of the wash, but the highest activity was detected for the complex isolated with the use of digitonin. The reaction rate of PQ reduction of the investigated complexes in liposomes was not significantly influenced by the addition of free FNR or ferredoxin. The reaction was inhibited by about 60% in the presence of 2 µM 2-n-nonyl-4-hydroxyquinoline N-oxide, an inhibitor of the cytochrome b₆ f complex at the Q(i) site, while it was not affected by triphenyltin or isobutyl cyanide that interacts with the recently identified heme c(i) . The obtained data indicate that FNR associated with the cytochrome b₆ f complex can participate in the cyclic electron transport as PSI-PQ or NADPH-PQ oxidoreductase. Moreover, we have shown that PQ can be non-enzymatically reduced by ascorbate in liposomes and this reaction might be involved in non-photochemical reduction pathways of the PQ-pool in chloroplasts.

PRENYLLIPIDS AND PIGMENTS CONTENT IN SELECTED ANTARCTIC LICHENS AND MOSSES

The content and relative composition of tocopherols, plastochromanol, plastoquinone and pigments in fifteen Antarctic species (five mosses and ten lichens) were analyzed by HPLC. Total tocopherols in mosses ranged from 90 mg/g (Warnstrofia sarmentosa) to 220 mg/g (Syntrichia magellanica), while in lichens it ranged from 0.89 mg/g in Caloplaca sp. to 45 mg/g in Placopsis contortuplicata. With the exception of Ochrolechia frigida, in all other mosses and lichens species, a-tocopherol accounted for more than 90% of total tocopherols. Plastochromanol was detected in four mosses and two lichen species; the highest level was found in Polytrichastrum alpinum (19.1 mg/g). The highest content of plastoquinone-9 (PQ-9) in mosses was found in Bryum pseudotriquetrum (42.6 mg/g), whereas in lichens it was 24.5 mg/g in Stereocaulon alpinum, and 23.17 mg/g in Umbilicaria antarctica. Pigment composition in mosses was typical for higher plants. Some lichen species lacked chlorophyll b, violaxanthin and b-carotene. Based on these results it is suggested that tocochromanols and carotenoid pigments are involved in the protection of mosses and lichens against the oxidative stress caused by the extreme Antarctic conditions.

Carotenoids Involved in Antioxidant System of Chloroplasts

Carotenoids are tetraterpenoid pigments that are present in the chloroplasts and chromoplasts of the photosynthetic organisms. Carotenoids are divided into two classes: carotenes, composed of carbon and hydrogen and xanthophylls, containing additionally oxygen. In chloroplasts carotenoids play two key roles: they absorb light energy for use in the photosynthesis, and they are also important components of the antioxidant system of chloroplasts protecting photosynthetic apparatus from photodamage. In this chapter properties of carotenoids as effective nonenzymatic plant antioxidants are described. At the beginning the chemical structure of carotenoids in relation to their antioxidant properties is explained. Next, the photoprotective role of two all-trans β-carotene molecules existing in PSII reaction center will be described. In the last part of the chapter the significance of xanthophylls in photoprotection will be discussed. Special attention is given to the role of carotenoids involved in several types of the xanthophyll cycle. Furthermore, the meaning of the xanthophyll cycles as antioxidant systems is discussed. Four essential mechanisms clarifying the protective role of xanthophyll cycles will be presented. Three of these mechanisms show an indirect and one a direct participation of xanthophylls in the photoprotection. The mechanisms based on indirect participation of pigments created as products of the light phase of xanthophyll cycle in quenching of overexcitation include: (I) aggregation-dependent LHCII quenching; (II) light-driven mechanisms in LHCII and (III) charge transfer quenching between Chl a and Zx. Selected results of research on the antioxidant properties of xanthophyll cycle pigments in model systems is also shown. Finally, the role of ascorbate as an antioxidant and as a reductant required to carry out de-epoxidation is also considered.

Vitamin E - Occurrence, Biosynthesis by Plants and Functions in Human Nutrition

OBJECTIVE This review examines various aspects of vitamin E, both in plant metabolism and with regard to its importance for human health. Vitamin E is the collective name of a group of lipidsoluble compounds, chromanols, which are widely distributed in the plant kingdom. Their biosynthetic pathway, intracellular distribution and antioxidant function in plants are well recognized, although their other functions are also considered. CONCLUSION Analytical methods for the determination of vitamin E are discussed in detail. Furthermore, the vitamin E metabolism and its antioxidant action in humans are described. Other nonantioxidant functions of vitamin E are also presented, such as its anti-inflammatory effects, role in the prevention of cardiovascular diseases and cancer, as well as its protective functions against neurodegenerative and other diseases.