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Mitochondria give mixed messages about cell death
Programmed cell death (PCD) occurs in plants and animals as part of particular developmental processes and in response to various stimuli including abiotic stresses. The best characterized form of PCD is apoptosis in animal cells. In one of the signalling pathways that regulate apoptosis, cytochrome c leakage from the mitochondria leads to activation of the apoptotic proteases. The leakage of cytochrome c can occur either non-specifically via the mitochondrial permeability transition pore (opening in response to calcium-induced mitochondrial swelling) or, more specifically, via a channel formed in response to an apoptosis-inducing protein such as BAX. There is disagreement in the literature as to whether any forms of PCD in plants resemble apoptosis. For example, do plant mitochondria behave in a similar way to animal mitochondria? The paper by Virolainen et al. (Helsinki University, pp. 509-516) addresses this question. In plants that cannot adapt to anoxia, exposure to low oxygen tension may lead to PCD. This group have therefore studied the effects of anoxia on mitochondria from wheat roots, and have revealed a complex situation. Anoxia initially causes a loss of membrane potential, shrinkage of the matrix and efflux of Ca2+. However, if high concentrations of Ca2+ are present externally, the mitochondria exhibit some re-uptake of Ca2+ (the extent of which is limited by the lack of oxygen) followed by swelling and efflux of cytochrome c. This is consistent with observations that cytochrome c release occurs in some forms of PCD in other plants. However, before concluding that cyotchrome c from mitochondria is part of a signalling pathway leading to PCD in anoxic wheat roots, we need information on the calcium status of the root cells themselves; we also need to know what happens 'downstream' of the cytochrome c efflux. Clearly there is interesting and valuable research still to be done.
Professor J. A. BryantUniversity of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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