Chlororespiration and poising of cyclic electron transport -: Plastoquinone as electron transporter between thylakoid NADH dehydrogenase and peroxidase

Polypeptides encoded by plastid ndh genes form a complex (Ndh) which could reduce plastoquinone with NADH, Through a terminal oxidase, reduced plastoquinone would be oxidized in chlororespiration, However, isolated Ndh complex has low activity with plastoquinone and no terminal oxidase has been found in chloroplasts, thus the function of Ndh complex is unknown. Alternatively, thylakoid hydroquinone peroxidase could oxidize reduced plastoquinone with H2O2, By immunoaffinity chromatography, we have purified the plastid Ndh complex of barley (Hordeum vulgare L,) to investigate the electron donor and acceptor specificity. A detergent-containing system was reconstructed with thylakoid Ndh complex and peroxidase which oxidized NADH with H2O2 in a plastoquinone-dependent process. This system and the increases of thylakoid Ndh complex and peroxidase activities under photooxidative stress suggest that the chlororespiratory process consists of the sequence of reactions catalyzed by Ndh complex, peroxidase (acting on reduced plastoquinone), superoxide dismutase, and the non-enzymic one-electron transfer from reduced iron-sulfur protein (FeSP) to O-2, When FeSP is a component of cytochrome b(6).f complex or of the same Ndh complex, O-2 may be reduced with NADH, without requirement of light. Chlororespiration consumes reactive species of oxygen and, eventually, may decrease their production by lowering O-2 concentration in chloroplasts, The common plastoquinone pool with photosynthetic electron transport suggests that chlororespiratory reactions may poise reduced and oxidized forms of the intermediates of cyclic electron transport under highly fluctuating light intensities.