ascorbic-acid and antheraxanthin

As a response to high light, plants have evolved non-photochemical quenching (NPQ), mechanisms that lead to the dissipation of excess absorbed light energy as heat, thereby minimizing the formation of dangerous oxygen radicals. One component of NPQ is pH dependent and involves the formation of zeaxanthin from violaxanthin. The enzyme responsible for the conversion of violaxanthin to zeaxanthin is violaxanthin de-epoxidase, which is located in the thylakoid lumen, is activated by low pH, and has been shown to use ascorbate (vitamin C) as its reductant in vitro. To investigate the effect of low ascorbate levels on NPQ in vivo, we measured the induction of NPQ in a vitamin C-deficient mutant of Arabidopsis, vtc2-2. During exposure to high light (1,500 micromol photons m(-2) s(-1)), vtc2-2 plants initially grown in low light (150 micromol photons m(-2) s(-1)) showed lower NPQ than the wild type, but the same quantum efficiency of photosystem II. Crosses between vtc2-2 and Arabidopsis ecotype Columbia established that the ascorbate deficiency cosegregated with the NPQ phenotype. The conversion of violaxanthin to zeaxanthin induced by high light was slower in vtc2-2, and this conversion showed saturation below the wild-type level. Both the NPQ and the pigment phenotype of the mutant could be rescued by feeding ascorbate to leaves, establishing a direct link between ascorbate, zeaxanthin, and NPQ. These experiments suggest that ascorbate availability can limit violaxanthin de-epoxidase activity in vivo, leading to a lower NPQ. The results also demonstrate the interconnectedness of NPQ and antioxidants, both important protection mechanisms in plants.

Topics: Arabidopsis; Ascorbic Acid; beta Carotene; Electron Transport; Hydrogen-Ion Concentration; Light; Models, Chemical; Mutation; Oxidoreductases; Photochemistry; Photosynthetic Reaction Center Complex Proteins; Photosystem II Protein Complex; Plant Leaves; Thylakoids; Xanthophylls; Zeaxanthins

Plants of Nicotiana tabacum (O3-tolerant cv Bel-B and O3-sensitive cv Bel-W3) were exposed to 150 ppb of ozone for 5 h; the fumigation produced visual injury in mature leaves, particularly in Bel-W3. After O3-treatment the pigments of the xanthophyll cycle pool decreased in both cvs, with a strong reduction in violaxanthin content, while antheraxanthin and zeaxanthin increased slightly. Under these conditions the content of leaf abscisic acid (ABA) markedly increased, particularly in O3-sensitive cv, indicating that the violaxanthin may have been partially converted into ABA. The control plants of Bel-B showed an ascorbic acid content four times greater than Bel-W3 and the ozone treatment did not produce significant differences in the ascorbic acid content and in the redox state. The two tobacco cvs were found to have similar total glutathione content, however the redox state was lower in O3-sensitive cv and decreased after ozone exposure. Ozone fumigation caused an increase in oxidized glutathione, particularly in Bel-W3, associated with a reduced glutathione reductase (GR) activity and a reduced GR protein content.

Topics: Ascorbic Acid; beta Carotene; Carotenoids; Free Radicals; Glutathione; Glutathione Reductase; Lutein; Nicotiana; Oxidative Stress; Ozone; Pigments, Biological; Plant Leaves; Plants, Toxic; Xanthophylls; Zeaxanthins