Increased respiratory restriction during phosphate-limited growth in transgenic tobacco cells lacking alternative oxidase

We found that mitochondrial alternative oxidase (AOX) protein and the capac ity for CN-resistant respiration are dramatically increased in wild-type to bacco (Nicotiana tabacum) suspension-cultured cells in response to growth u nder P limitation, and antisense (AS8) tobacco cells unable to induce AOX u nder these conditions have altered growth and metabolism. Specifically, we round that the respiration of AS8 cells was restricted during P-limited gro wth, when the potential for severe adenylate control of respiration (at the level of C supply to the mitochondrion and/or at the level of oxidative ph osphorylation) is high due to the low cellular levels of ADP and/or inorgan ic P. As a result of this respiratory restriction, AS8 cells had altered gr owth, morphology, cellular composition, and patterns of respiratory C flow to amino acid synthesis compared with wild-type cells with abundant AOX pro tein. Also, AS8 cells under P limitation displayed high in vivo rates of ge neration of active oxygen species compared with wild-type cells. This diffe rence could be abolished by an uncoupler of mitochondrial oxidative phospho rylation. Our results suggest that induction of non-phosphorylating AOX res piration (like induction of adenylate and inorganic P-independent pathways in glycolysis) is an important plant metabolic adaptation to P limitation. By preventing severe respiratory restriction, AOX acts to prevent both redi rections in C metabolism and the excessive generation of harmful active oxy gen species in the mitochondrion.