An assessment of the pathways of dark reduction and oxidation of the plastoquinone pool in thylakoid membranes of higher plants and green algae

Abstract

Several possible `chlororespiratory¿ pathways have been proposed for both reduction and oxidation of the plastoquinone pool in the dark in cells of both higher plants and green algae. However, bioinformatic analysis of the recently published genome of Arabidopsis thaliana sets limits to the types of respiratory-related components that could be present in higher plant thylakoids: for example, homologues of cytochrome c oxidase, succinate dehydrogenase and alternative NDH-2 enzymes in thylakoids can be ruled out and there are also no thylakoid-targetted homologues of the NADH-reactive fragment of mitochondrial complex I (the 75kDa, 51 kDa and 30 kDa subunits) that might provide an NADH dehydrogenase fragment for the other complex I homologues that are encoded in the chloroplast genome. In parallel to these considerations, direct assay of pathways of dark reduction and oxidation of plastoquinone has been made in intact leaves and in cells of Chlamydomonas reinhardtii by monitoring P700+ reduction, the slow phase of the carotenoid bandshift and turnover kinetics of the cytochrome bf complex. Combination of these measurements with effects of a wide range of specific inhibitors of various quinone-reactive sites has allowed further definition of the types of pathways and their extents.