Alternative Respiratory Pathway Limits Cell Death

Abstract

Three papers explore the function of mitochondrial alternative oxidase (AOX) in tobacco plant cells. Plants can divert electron flow to oxygen from the cytochrome (cyt.) pathway through the AOX pathway. The AOX pathway does not contribute to adenosine triphosphate (ATP) synthesis, but does decrease the production of reactive oxygen species. Robson and Vanlerberghe determined that cells in which AOX was inhibited by antisense (AOXi) were more sensitive to stimuli that produce programmed cell death than were wild-type tobacco cells. Peroxide and salicylic acid treatment led to long-term loss of culture viability, inhibition of mitochondrial and cellular respiration, release of cytochrome c from the mitochondria, and DNA laddering in the AOXi cells. Wild-type cells showed less inhibition of respiration and only transient decrease in culture viability, which was not associated with release of cytochrome c or DNA laddering. Thus, loss of the AOX respiratory pathway causes plant cells to be more likely to undergo programmed cell death. In a second paper, Vanlerberghe et al. compared how wild-type cells and AOXi cells responded to inhibition of the cyt. respiratory pathway. The AOXi cells lost viability in response to inhibition of the cyt. pathway at much lower concentrations of cysteine and had complete loss of respiration compared with wild-type cells, which maintained respiration through increased synthesis of AOX and diversion of electron transport and oxygen consumption through the AOX pathway. The AOXi cells underwent programmed cell death based on the appearance of DNA laddering in response to inhibition of cyt. respiration. The induction of AOX represents a pathway that allows communication between the mitochondria and the nucleus (AOX is encoded by a nuclear gene) to protect cells under conditions of respiratory stress. The final paper (Ordog et al. ) analyzed wild-type and transgenic tobacco plants for AOX (overexpression of AOX or inhibition of AOX) and found that the plants responded normally to tobacco mosiac virus infection, suggesting that AOX induction is not essential for plant viral resistance and programmed cell death associated with infection. C. A. Robson, G. C. Vanlerberghe, Transgenic plant cells lacking mitochondrial alternative oxidase have increased susceptibility to mitochondria-dependent and -independent pathways of programmed cell death. Plant Physiol. 129 , 1908-1920 (2002). [Abstract] [Full Text] G. C. Vanlerberghe, C. A. Tobson, J. Y. H. Yip, Induction of mitochondrial alternative oxidase in response to a cell signal pathway down-regulating the cytochrome pathway prevents programmed cell death. Plant Physiol . 129 , 1829-1842 (2002). [Abstract] [Full Text] S. H. Ordog, V. J. Higgins, G. C. Vanlerberghe, Mitochondrial alternative oxidase is not a critical component of plant viral resistance but may play a role in the hypersensitive response. Plant Physiol . 129 , 1858-1865 (2002). [Abstract] [Full Text]