Mercury inhibits the non-photochemical reduction of plastoquinone by exogenous NADPH and NADH: evidence from measurements of the polyphasic chlorophyll a fluorescence rise in spinach chloroplasts |
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Authors: | Haldimann Pierre Tsimilli-Michael Merope |
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Institution: | (1) Laboratory of Bioenergetics, Department of Botany and Plant Biology, University of Geneva, Chemin des Embrouchis 10, 1254 Jussy, Geneva, Switzerland;(2) Present address: Institute of Plant Sciences, University of Bern, Altenbergrain 21, 3013 Bern, Switzerland;(3) Ministry of Education and Culture, Nicosia, 1434, Cyprus |
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Abstract: | Chlorophyll a fluorescence rise kinetics (from 50 μs to 1 s) were used to investigate the non-photochemical reduction of the plastoquinone
(PQ) pool in osmotically broken spinach chloroplasts (Spinacia oleracea L.). Incubation of the chloroplasts in the presence of exogenous NADPH or NADH resulted in significant changes in the shape
of the fluorescence transient reflecting an NAD(P)H-dependent accumulation of reduced PQ in the dark, with an extent depending
on the concentration of NAD(P)H and the availability of oxygen; the dark reduction of the PQ pool was saturated at lower NAD(P)H
concentrations and reached a higher level when the incubation took place under anaerobic conditions than when it occurred
under aerobic conditions. Under both conditions NADPH was more effective than NADH in reducing PQ, however only at sub-saturating
concentrations. Neither antimycin A nor rotenone were found to alter the effect of NAD(P)H. The addition of mercury chloride
to the chloroplast suspension decreased the NAD(P)H-dependent dark reduction of the PQ pool, with the full inhibition requiring
higher mercury concentrations under anaerobic than under aerobic conditions. This is the first time that this inhibitory role
of mercury is reported for higher plants. The results demonstrate that in the dark the redox state of the PQ pool is regulated
by the reduction of PQ via a mercury-sensitive NAD(P)H-PQ oxidoreductase and the reoxidation of reduced PQ by an O2-dependent pathway, thus providing additional evidence for the existence of a chlororespiratory electron transport chain in
higher plant chloroplasts.
This revised version was published online in June 2006 with corrections to the Cover Date. |
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Keywords: | chlorophyll fluorescence chloroplasts chlororespiration mercury plastoquinone Spinacia oleracea |
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