NAD(P)H dehydrogenase-dependent, antimycin A-sensitive electron donation to plastoquinone in tobacco chloroplasts

Cyclic electron transport around PSI through the NAD(P)H dehydrogenase complex (NDH) in tobacco leaf disks, measured as an increase in the dark level of Chi fluorescence after the onset of darkness, was inhibited by antimycin A, an inhibitor of ferredoxin quinone reductase (FQR), suggesting that antimycin A inhibits not only the FQR-mediated cyclic flow but also the NDH-dependent how. This electron how was inhibited also by amytal, an inhibitor of mitochondrial NDH and by nigericin. The reduction of plastoquinone was detected when NADPH and ferredoxin were added to the suspension of the osmotically ruptured chloroplasts of the wild type and NDH-defective mutant. Because the addition of NADPH alone did not induce the reduction, membrane-bound ferredoxin NADP(+) reductase (FNR) was supposed to reduce ferredoxin, which may be a more direct electron donor for the plastoquinone reduction. The presence of two types of reducing enzymes was suggested from the bi-phasic inhibition of plastoquinone reduction by antimycin A in the wild type. It is proposed that the reducing activity inhibited by antimycin A at a low concentration is attributed to FQR and the less sensitive activity to NDH.