Chlororespiration and poising of cyclic electron transport - Plastoquinoneas 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 plastoq uinone would be oxidized in chlororespiration, However, isolated Ndh comple x has low activity with plastoquinone and no terminal oxidase has been foun d in chloroplasts, thus the function of Ndh complex is unknown. Alternative ly, thylakoid hydroquinone peroxidase could oxidize reduced plastoquinone w ith H2O2, By immunoaffinity chromatography, we have purified the plastid Nd h complex of barley (Hordeum vulgare L,) to investigate the electron donor and acceptor specificity. A detergent-containing system was reconstructed w ith thylakoid Ndh complex and peroxidase which oxidized NADH with H2O2 in a plastoquinone-dependent process. This system and the increases of thylakoi d Ndh complex and peroxidase activities under photooxidative stress suggest that the chlororespiratory process consists of the sequence of reactions c atalyzed by Ndh complex, peroxidase (acting on reduced plastoquinone), supe roxide dismutase, and the non-enzymic one-electron transfer from reduced ir on-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, witho ut requirement of light. Chlororespiration consumes reactive species of oxy gen and, eventually, may decrease their production by lowering O-2 concentr ation in chloroplasts, The common plastoquinone pool with photosynthetic el ectron transport suggests that chlororespiratory reactions may poise reduce d and oxidized forms of the intermediates of cyclic electron transport unde r highly fluctuating light intensities.