Mitochondrial complex I dysfunction increases CO₂ efflux and reconfigures metabolic fluxes of day respiration in tobacco leaves

Mutants affected in complex I are useful to understand the role played by mitochondrial electron transport and redox metabolism in cellular homeostasis and signaling. However, their respiratory phenotype is incompletely described and a specific examination of day respiration (R_d) is lacking. Here, we used isotopic methods and metabolomics to investigate the impact of complex I dysfunction on R_d in two respiratory mutants of forest tobacco (Nicotiana sylvestris): cytoplasmic male sterile II (CMSII) and nuclear male sterile 1 (NMS1), previously characterized for complex I disruption. R_d was higher in mutants and the inhibition of leaf respiration by light was lower. Higher R_d values were caused by increased (phosphoenol)pyruvate (PEP) metabolism at the expense of anaplerotic (PEP carboxylase (PEPc) -catalyzed) activity. De novo synthesis of Krebs cycle intermediates in the light was larger in mutants than in the wild-type, although numerically small in all genotypes. Carbon metabolism in mutants involved alternative pathways, such as alanine synthesis, and an increase in amino acid production with the notable exception of aspartate. Our results show that the alteration of NADH re-oxidation activity by complex I does not cause a general inhibition of catabolism, but rather a re-orchestration of fluxes in day respiratory metabolism, leading to an increased CO₂ efflux.