Complex I Impairment, Respiratory Compensations, and Photosynthetic Decrease in Nuclear and Mitochondrial Male Sterile Mutants of Nicotiana sylvestris

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Abstract
We have previously shown that in Nicotiana sylvestris cytoplasmic male-sterile (CMS) mutants where the mtDNA lacks the nad7 gene coding for a subunit of respiratory Complex I (NADH:ubiquinone oxidoreductase, EC 1.6.5.3), glycine (Gly) oxidation was lower than in the wild type and insensitive to rotenone, suggesting Complex I dysfunction. In contrast, the oxidation rate of exogenous NADH and the capacity of the cyanide-resistant respiration (AOX) were enhanced. Here we report that, in contrast to Gly, the rate of malate oxidation was not affected, but proceeded totally in a rotenone-insensitive pathway, strongly suggesting that survival of CMS plants depends on the activation of internal and external alternative NAD(P) H dehydrogenases and that Gly decarboxylase activity depends on Complex I functioning. A similar defect in Complex I activity and Gly oxidation was found in the NMS1 nuclear mutant, defective in the processing of the nad4transcript, but alternative NAD(P) H dehydrogenases were less activated. In CMS and NMS1, the fraction of the AOX pathway was increased, as compared to wild type, associated with higher amounts ofaox transcripts, AOX protein, and plant resistance to cyanide. Non-phosphorylating respiratory enzymes maintained normal in vivo respiration levels in both mutants, but photosynthesis was decreased, in correlation with lower leaf conductance, emphasizing mitochondrial control on photosynthesis.

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