Iodide accumulation provides kelp with an inorganic antioxidant impacting atmospheric chemistry.

Brown algae of the Laminariales (kelps) are the strongest accumulators of iodine among living organisms. They represent a major pump in the global biogeochemical cycle of iodine and, in particular, the major source of iodocarbons in the coastal atmosphere. Nevertheless, the chemical state and biological significance of accumulated iodine have remained unknown to this date. Using x-ray absorption spectroscopy, we show that the accumulated form is iodide, which readily scavenges a variety of reactive oxygen species (ROS). We propose here that its biological role is that of an inorganic antioxidant, the first to be described in a living system. Upon oxidative stress, iodide is effluxed. On the thallus surface and in the apoplast, iodide detoxifies both aqueous oxidants and ozone, the latter resulting in the release of high levels of molecular iodine and the consequent formation of hygroscopic iodine oxides leading to particles, which are precursors to cloud condensation nuclei. In a complementary set of experiments using a heterologous system, iodide was found to effectively scavenge ROS in human blood cells.

Impact and interaction of lipophilic antioxidants in mutants and transgenic plants.

Carotenoids and tocopherols are lipophilic antioxidants with important functions in plants and humans. Due to their nutritional value and putative health benefits, they have become a focus of intensive research. The identification of all genes of the carotenoid and tocopherol biosynthesis has enabled the manipulation of their biosynthetic pathways, aiming for quantitative and qualitative improvement. In plants, carotenoids and tocopherols are of crucial importance because of their protective abilities, which help to keep them alive even under light stress conditions. A wealth of information has accumulated concerning the responses of plants to various environmental stress factors. Here, we summarize some of the recent data concentrating on the impact and possible interaction of lipophilic antioxidants in mutants and transgenic plants with altered status of lipophilic antioxidants.

Coordinate up-regulation of carotenoid biosynthesis as a response to light stress in Synechococcus PCC7942.

Carotenoid biosynthesis is up-regulated by strong light in the cyanobacterium Synechococcus. By blocking off the pathway at the level of phytoene conversion, light-dependent accumulation of phytoene was observed. Real-time PCR studies demonstrated that four genes of the carotenogenic pathway are under transcriptional control. These were the genes encoding phytoene synthase, phytoene desaturase, zeta-carotene desaturase and beta-carotene hydroxylase. The transcript of the first three follow a similar kinetics, whereas the transcript of beta-carotene hydroxylase increased much faster upon transfer to high light. Promoter activities were determined with transcriptional fusions to chloramphenicol acyltransferase as reporter enzyme. The activity of the promoter of the phytoene desaturase/synthase operon was higher under strong light.

Expression of xanthophyll biosynthetic genes during light-dependent chloroplast differentiation.

In higher plants, etioplast to chloroplast differentiation is characterized by dramatic ultrastructural changes of the plastid and a concomitant increase in chlorophylls and carotenoids. Whereas the formation and function of carotenes and their oxygenated derivatives, the xanthophylls, have been well studied, little is known about the regulation of the genes involved in xanthophyll biosynthesis. Here, we analyze the expression of three xanthophyll biosynthetic genes (i.e. beta-carotene hydroxylase [bhy], zeaxanthin epoxidase [zep], and violaxanthin de-epoxidase [vde]) during de-etiolation of seedlings of tobacco (Nicotiana tabacum L. cv Samsun) under different light conditions. White-light illumination caused an increase in the amount of all corresponding mRNAs. The expression profiles of bhy and zep not only resembled each other but were also similar to the pattern of a gene encoding a major light-harvesting protein of photosystem II. This finding indicates a coordinated synthesis during formation of the antenna complex. In contrast, the expression pattern of vde was clearly different. Furthermore, the gene expression of bhy was shown to be modulated after illumination with different white-light intensities. The expression of all xanthophyll biosynthetic genes under examination was up-regulated upon exposure to red, blue, and white light. Gene expression of bhy and vde but not of zep was more pronounced under red-light illumination, pointing at an involvement of the phytochrome system. Expression analysis in the presence of the photosynthetic electron transport inhibitors 3-(3,4-dichlorophenyl)-1,1-dimethyl-urea and 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone indicated a redox control of transcription of two of the xanthophyll biosynthetic genes (bhy and zep).