A Key Role of Xanthophylls That Are Not Embedded in Proteins in Regulation of the Photosynthetic Antenna Function in Plants, Revealed by Monomolecular Layer Studies

Mechanisms shaping the synergism of zeaxanthin and PsbS in photoprotective energy dissipation in the photosynthetic apparatus of plants

Safe operation of photosynthesis is vital to plants and is ensured by the activity of processes protecting chloroplasts against photo-damage. The harmless dissipation of excess excitation energy is considered to be the primary photoprotective mechanism and is most effective in the combined presence of PsbS protein and zeaxanthin, a xanthophyll accumulated in strong light as a result of the xanthophyll cycle. Here we address the problem of specific molecular mechanisms underlying the synergistic effect of zeaxanthin and PsbS. The experiments were conducted with Arabidopsis thaliana, using wild-type plants, mutants lacking PsbS (npq4), and mutants affected in the xanthophyll cycle (npq1), with the application of molecular spectroscopy and imaging techniques. The results lead to the conclusion that PsbS interferes with the formation of densely packed aggregates of thylakoid membrane proteins, thus allowing easy exchange and incorporation of xanthophyll cycle pigments into such structures. It was found that xanthophylls trapped within supramolecular structures, most likely in the interfacial protein region, determine their photophysical properties. The structures formed in the presence of violaxanthin are characterized by minimized dissipation of excitation energy. In contrast, the structures formed in the presence of zeaxanthin show enhanced excitation quenching, thus protecting the system against photo-damage.

Combined dynamics of the 500-600 nm leaf absorption and chlorophyll fluorescence changes in vivo: Evidence for the multifunctional energy quenching role of xanthophylls

Carotenoids (Cars) regulate the energy flow towards the reaction centres in a versatile way whereby the switch between energy harvesting and dissipation is strongly modulated by the operation of the xanthophyll cycles. However, the cascade of molecular mechanisms during the change from light harvesting to energy dissipation remains spectrally poorly understood. By characterizing the in vivo absorbance changes (ΔA) of leaves from four species in the 500-600 nm range through a Gaussian decomposition, while measuring passively simultaneous Chla fluorescence (F) changes, we present a direct observation of the quick antenna adjustments during a 3-min dark-to-high-light induction. Underlying spectral behaviours of the 500-600 nm ΔA feature can be characterized by a minimum set of three Gaussians distinguishing very quick dynamics during the first minute. Our results show the parallel trend of two Gaussian components and the prompt Chla F quenching. Further, we observe similar quick kinetics between the relative behaviour of these components and the in vivo formations of antheraxanthin (Ant) and zeaxanthin (Zea), in parallel with the dynamic quenching of singlet excited chlorophyll a (¹Chla*) states. After these simultaneous quick kinetical behaviours of ΔA and F during the first minute, the 500-600 nm feature continues to increase, indicating a further enhanced absorption driven by the centrally located Gaussian until 3 min after sudden light exposure. Observing these precise underlying kinetic trends of the spectral behaviour in the 500-600 nm region shows the large potential of in vivo leaf spectroscopy to bring new insights on the quick redistribution and relaxation of excitation energy, indicating a key role for both Ant and Zea.

Determination of water-soluble vitamins and carotenoids in Brazilian tropical fruits by High Performance Liquid Chromatography

Vitamins are organic compounds essential for normal physiological functioning and they need to be provided in adequate amounts by the diet. They are nutrients mainly associated to fruit consumption, playing an important role in the cellular function, growth and development of individuals. The present study aimed to analyze levels of vitamins B, C and carotenoids of fruits from the agrobiodiversity of Northeastern Brazil, among them cajuí (Anacardium spp), murici (Byrsonima crassifolia (L.) Kunth), pequi (Caryocar coriaceum Wittm.), jenipapo (Genipa americana L.), mangaba (Hancornia speciosa Gomes), bacuri (Platonia insignis Mart.), cajá (Spondias mombin L.), umbu-cajá (Spondias bahiensis P. Carvalho, Van den Berg & M. Machado), umbu (Spondias tuberosa Arruda), pitanga (Eugenia uniflora L.), araçá (Psidium sobralianum Landrum & Proença). The vitamins were quantified using the analytical method High Performance Liquid Chromatography (HPLC). Vitamin B complex levels varied from 0.003 ± 0.01 mg/100 g to 6.107 ± 0.06 mg/100 g. Vitamin C ranged from 0.36 ± 0.06 mg/100 g to 253.92 ± 9.02 mg/100 g. Carotenoid values ranged from 0.12 ± 0.02 μg/100 g to 395.63 ± 113.69 μg/100 g. Thus, the profile of water-soluble vitamins and carotenoids of the fruits analyzed was quantified. Therefore, these fruits can provide varied amounts of vitamins important to human health. However, it is interesting for the individual to consume fruits in a diversified manner, avoiding monotony and thus guaranteeing the daily intake of more nutrients.

Lipid Dependence of Xanthophyll Cycling in Higher Plants and Algae

The xanthophyll cycles of higher plants and algae represent an important photoprotection mechanism. Two main xanthophyll cycles are known, the violaxanthin cycle of higher plants, green and brown algae and the diadinoxanthin cycle of Bacillariophyceae, Xanthophyceae, Haptophyceae, and Dinophyceae. The forward reaction of the xanthophyll cycles consists of the enzymatic de-epoxidation of violaxanthin to antheraxanthin and zeaxanthin or diadinoxanthin to diatoxanthin during periods of high light illumination. It is catalyzed by the enzymes violaxanthin or diadinoxanthin de-epoxidase. During low light or darkness the back reaction of the cycle, which is catalyzed by the enzymes zeaxanthin or diatoxanthin epoxidase, restores the epoxidized xanthophylls by a re-introduction of the epoxy groups. The de-epoxidation reaction takes place in the lipid phase of the thylakoid membrane and thus, depends on the nature, three dimensional structure and function of the thylakoid lipids. As the xanthophyll cycle pigments are usually associated with the photosynthetic light-harvesting proteins, structural re-arrangements of the proteins and changes in the protein-lipid interactions play an additional role for the operation of the xanthophyll cycles. In the present review we give an introduction to the lipid and fatty acid composition of thylakoid membranes of higher plants and algae. We introduce the readers to the reaction sequences, enzymes and function of the different xanthophyll cycles. The main focus of the review lies on the lipid dependence of xanthophyll cycling. We summarize the current knowledge about the role of lipids in the solubilization of xanthophyll cycle pigments. We address the importance of the three-dimensional lipid structures for the enzymatic xanthophyll conversion, with a special focus on non-bilayer lipid phases which are formed by the main thylakoid membrane lipid monogalactosyldiacylglycerol. We additionally describe how lipids and light-harvesting complexes interact in the thylakoid membrane and how these interactions can affect the structure of the thylakoids. In a dedicated chapter we offer a short overview of current membrane models, including the concept of membrane domains. We then use these concepts to present a model of the operative xanthophyll cycle as a transient thylakoid membrane domain which is formed during high light illumination of plants or algal cells.