Induction of mitochondrial alternative oxidase during macronutrient stress decreases growth efficiency of tobacco cells

The impact of plant mitochondrial alternative oxidase (AOX) on the growth efficiency and carbon metabolism of tobacco (Nicotiana tabacum) was investigated. The carbon use efficiency (CUE) of wild type (WT) tobacco cells decreased from 0.50 under complete nutrient conditions to 0.19 and 0.29 under phosphate (P)- and nitrogen (N)-limited conditions, respectively. The drop in CUE in WT cells corresponded with a large increase in AOX respiratory capacity. Conversely, the CUE of transgenic tobacco suspension cells lacking AOX (AS8 line) remained approximately 0.50 under complete, low-P and low-N growth conditions. Under N-limitation, AS8 cells accumulated 2-fold more non-structural carbon (C_ns) and 7-fold more sucrose than WT cells. Under P-limitation, AS8 cells incurred more oxidative damage than WT cells, as indicated by higher lipid peroxidation. In a series of growth chamber experiments, tobacco plants that over-express AOX had less biomass and C_ns than WT plants when grown at high light.