title: Supplementary ultraviolet-B radiation induces a rapid reversal of the diadinoxanthin cycle in the strong light-exposed diatom Phaeodactylum tricornutum.
abstract: A treatment of the diatom Phaeodactylum tricornutum with high light (HL) in the visible range led to the conversion of diadinoxanthin (Dd) to diatoxanthin (Dt). In a following treatment with HL plus supplementary ultraviolet (UV)-B, the Dt was rapidly epoxidized to Dd. Photosynthesis of the cells was inhibited under HL + UV-B. This is accounted for by direct damage by UV-B and damage because of the UV-B-induced reversal of the Dd cycle and the associated loss of photoprotection. The reversal of the Dd cycle by UV-B was faster in the presence of dithiothreitol, an inhibitor of the Dd de-epoxidase. Our results imply that the reversal of the Dd cycle by HL + UV-B was caused by an increase in the rate of gross Dt epoxidation, whereas the de-epoxidation of Dd was unaffected by UV-B. This is further supported by our finding that the in vitro de-epoxidation activity and the affinity toward the cosubstrate ascorbic acid of the Dd de-epoxidase were both unaffected by UV-B pretreatment of intact cells. We provide evidence that Dt epoxidation is normally down-regulated by a high pH gradient under HL. It is proposed that supplementary UV-B affected the pH gradient across the thylakoid membrane, which disrupted the down-regulation of Dt epoxidation and led to the observed increase in the rate of Dt epoxidation.
authors.affiliation: Institut für Allgemeine Botanik, Johannes Gutenberg Universität, Saarstrasse 21, 55099 Mainz, Germany.
authors.forename: Heiko
authors.initials: H
authors.lastname: Mewes
authors.forename: Michael
authors.initials: M
authors.lastname: Richter
mesh_terms: D000222:Adaptation, Physiological / Q000502:physiology* / Q000528:radiation effects; D003624:Darkness; D017377:Diatoms / Q000378:metabolism* / Q000528:radiation effects; D004229:Dithiothreitol / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D008027:Light; D010088:Oxidoreductases / Q000187:drug effects / Q000378:metabolism / Q000528:radiation effects; D015398:Signal Transduction / Q000502:physiology / Q000528:radiation effects; D020524:Thylakoids / Q000378:metabolism; D014466:Ultraviolet Rays; D024341:Xanthophylls / Q000378:metabolism* / Q000528:radiation effects
chemical_list: D024341:Xanthophylls; C033808:diadinoxanthin; C103105:diatoxanthin; D010088:Oxidoreductases; C023111:epoxidase; D004229:Dithiothreitol