The regulation of xanthophyll cycle activity and of non-photochemical fluorescence quenching by two alternative electron flows in the diatoms Phaeodactylum tricornutum and Cyclotella meneghiniana |
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Authors: | Irina Grouneva |
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Affiliation: | Institute of Biology I, University of Leipzig, Johannisallee 21-23, D-04103 Leipzig, Germany |
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Abstract: | Intact cells of diatoms are characterized by a rapid diatoxanthin epoxidation during low light periods following high light illumination while epoxidation is severely restricted in phases of complete darkness. The present study shows that rapid diatoxanthin epoxidation is dependent on the availability of the cofactor of diatoxanthin epoxidase, NADPH, which cannot be generated in darkness due to the inactivity of PSI. In the diatom Phaeodactylum tricornutum, NADPH production during low light is dependent on PSII activity, and addition of DCMU consequently abolishes diatoxanthin epoxidation. In contrast to P. tricornutum, DCMU does not affect diatoxanthin epoxidation in Cyclotella meneghiniana, which shows the same rapid epoxidation in low light both in the absence or presence of DCMU. Measurements of the reduction state of the PQ pool and PSI activity indicate that, in the presence of DCMU, NADPH production in C. meneghiniana occurs via alternative electron transport, which includes electron donation from the chloroplast stroma to the PQ pool and, in a second step, from PQ to PSI. Similar electron flow to PQ is also observed during high light illumination of DCMU-treated P. tricornutum cells. In contrast to C. meneghiniana, the electrons are not directed to PSI, but most likely to a plastoquinone oxidase. This chlororespiratory electron transport leads to the establishment of an uncoupler-sensitive proton gradient in the presence of DCMU, which induces diadinoxanthin de-epoxidation and NPQ. In C. meneghiniana, electron flow to the plastoquinone oxidase is restricted, and consequently, diadinoxanthin de-epoxidation and NPQ is not observed after addition of DCMU. |
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Keywords: | Chl a, chlorophyll a DCMU, 3-(3&prime ,4&prime -dichlorophenyl)-1,1-dimethylurea DDE, diadinoxanthin de-epoxidase Ddx, diadinoxanthin DES, de-epoxidation state of the Ddx cycle pigment pool Dtx, diatoxanthin Fo, Fm, fluorescence intensity measured with all reaction centers open (Fo) or closed (Fm) by a saturation pulse FNR, ferredoxin-NADP+-oxidoreductase HL, high light LL, low light NADH, nicotinamide adenine dinucleotide, reduced form NADPH, nicotinamide adenine dinucleotide phosphate, reduced form Ndh, NAD(P)H dehydrogenase NPQ, non-photochemical chlorophyll fluorescence quenching OJIP curve, fast rise in fluorescence defined by its transient states P700, reaction centre pigments of PS I (primary electron donor of PS I) PAR, photosynthetically active radiation PS, photosystem PQ, plastoquinone Vx, violaxanthin Zx, zeaxanthin |
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