Mitochondrial complex I impairment and differential carbon monoxide sensitivity of cytochrome c oxidase in wild type and CMS II mutants of Nicotiana sylvestris

Plant mitochondria unlike their animal counterpart have some unique features with highly branched respiratory chain. The present work was undertaken in order to investigate the effect of loss/dysfunction of plant mitochondrial complex I on the relative flux of electrons through alternative oxidase (AOX) and cytochrome oxidase. Loss of a major subunit of mitochondrial complex I in cytoplasmic male sterile II (CMS II) mutant of Nicotiana sylvestris caused respiratory redox perturbations, as evident from the differential CO sensitivity of cytochrome oxidase. The leaf segments of CMS II mutant when exposed to CO under dark aerobic condition were insensitive to the inhibition of cytochrome oxidase, as against the wild type (WT). The differential CO response of WT and CMS II mutants appeared to be due to differences in the redox state of cytochrome a3 (cyt a3), the terminal electron acceptor during in situ respiration. Cyt a3 appeared to be more in its oxidized form in CMS II and hence unable to form cyt a3-CO complex. Pre-treatment of CMS II leaves with 2,4-dinitrophenol, an uncoupler of oxidative phosphorylation increased the CO response. The slight increase in rotenone-insensitive respiration of CMS II could be attributed partly to enhanced flux of electrons through cytochrome pathway to compensate for the loss of phosphorylation site and partly through AOX, which was induced by nitrate.