Non-Photochemical Reduction of Intersystem Electron Carriers in Chloroplasts of Higher Plants and Algae

Abstract

Besides reactions involved in the « Z » scheme of photosynthesis, additional electron transport pathways, such as cyclic electron flow around PSI or chlororespiration, have been reported in algal and in higher plant chloroplasts (1-4). These pathways generally involve a reduction of the intersystem electron transport chain (plastoquinones and/or the cyt b ₆ f complex) by stromal components such as NAD(P)H or reduced ferredoxin. Different electron carriers have been proposed to mediate the non-photochemical reduction of inter-system electron transport chain. The discovery of chloroplast genes showing homologies with genes encoding subunits of the mitochondrial complex I (ndh genes), was taken as an evidence for the existence of a NADH dehydrogenase complex I in chloroplasts. Since, the existence of such a complex has been reported from non-denaturing gel electrophoresis (5) and recently, the NDH complex has been purified and partially characterized from pea chloroplasts (6). Inactivation of chloroplast ndh genes by plastid transformation has been undertaken in order to assess the role of the NDH complex during photosynthesis. However, if the disruption of ndh genes allowed to show that the NDH complex is functional in vivo and is involved in the dark re-reduction of the PQ pool after a period of illumination (7) a participation of the NDH complex during photosynthesis was not shown. Surprisingly, no clear phenotype was observed in inactivated transformants (7), suggesting that the NDH complex has a marginal role during photosynthesis. Other mediators have been proposed to be involved in the non-photochemical reduction of inter-system electron carriers: these include the putative ferredoxinquinone-oxidoreductase activity (FOR), FNR (4) and more recently a NAD(P)H-PQ oxidoreductase activity characterized in potato chloroplasts that would be different from the NDH complex, from FNR, and from FQR (8).