Isoprenoid quinones are one of the most important groups of compounds occurring in membranes of living organisms. These compounds are composed of a hydrophilic head group and an apolar isoprenoid side chain, giving the molecules a lipid-soluble character. Isoprenoid quinones function mainly as electron and proton carriers in photosynthetic and respiratory electron transport chains and these compounds show also additional functions, such as antioxidant function. Most of naturally occurring isoprenoid quinones belong to naphthoquinones or evolutionary younger benzoquinones. Among benzoquinones, the most widespread and important are ubiquinones and plastoquinones. Menaquinones, belonging to naphthoquinones, function in respiratory and photosynthetic electron transport chains of bacteria. Phylloquinone K(1), a phytyl naphthoquinone, functions in the photosynthetic electron transport in photosystem I. Ubiquinones participate in respiratory chains of eukaryotic mitochondria and some bacteria. Plastoquinones are components of photosynthetic electron transport chains of cyanobacteria and plant chloroplasts. Biosynthetic pathway of isoprenoid quinones has been described, as well as their additional, recently recognized, diverse functions in bacterial, plant and animal metabolism.

Although light is essential for photosynthesis, excess light can damage the photosynthetic apparatus and deregulate other cellular processes. Thus, protective integrated regulatory responses that can dissipate excess of absorbed light energy and simultaneously optimize photosynthesis and other cellular processes under variable light conditions can prove highly adaptive. Here, we show that the local and systemic responses to an excess light episode are associated with photoelectrophysiological signaling (PEPS) as well as with changes in nonphotochemical quenching and reactive oxygen species levels. During an excess light incident, PEPS is induced by quantum redox changes in photosystem II and in its proximity and/or by changes in glutathione metabolism in chloroplasts. PEPS is transduced, at least in part, by bundle sheath cells and is light wavelength specific. PEPS systemic propagation speed and action potential are dependent on ASCORBATE PEROXIDASE2 function. Excess light episodes are physiologically memorized in leaves, and the cellular light memory effect is specific for an excess of blue (450 nm) and red (650 nm) light of similar energy. It is concluded that plants possess a complex and dynamic light training and memory system that involves quantum redox, reactive oxygen species, hormonal, and PEPS signaling and is used to optimize light acclimation and immune defenses.

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We have described a direct, high-performance liquid chromatography-based method of estimation of the total level of plastoquinone (PQ) in leaves, the redox state of total (photoactive and non-photoactive) PQ, as well as the redox state of the PQ-pool that is applicable to any illumination conditions. This method was applied to Arabidopsis thaliana leaves but it can be applied to any other plant species. The obtained results show that the level of total PQ was 25+/-3 molecules/1000 chlorophyll (Chl) molecules in relation to foliar total Chl content. The level of the photoactive PQ, i.e., the PQ-pool, was about 31% of the total PQ present in Arabidopsis leaves that corresponds to about 8 PQ molecules/1000 Chl molecules. The reduction level of the non-photoactive PQ fraction, present outside thylakoids in chloroplasts, was estimated to account for about 49%. The measurements of the redox state of the PQ-pool showed that the pool was reduced during the dark period in about 24%, and during the light period (150 micromol/m(2).s) the reduction of the PQ-pool increased to nearly 100%. The obtained results were discussed in terms of the activity of chlororespiration pathways in Arabidopsis and the regulatory role of the redox state of PQ-pool in various physiological and molecular processes in plants.

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.

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