| Summary: | In animals, the impact of ROS production by mitochondria on cell physiology, death, disease and ageing is well recognised. In photosynthetic organisms such as higher plants, however, the chloroplast and peroxisomes are the major sources of ROS during normal metabolism and the importance of mitochondria in oxidative stress and redox signalling is less well established. To address this, the <em>in vivo</em> oxidation state of a mitochondrially-targeted redox-sensitive GFP (mt-roGFP2) was investigated in <em>Arabidopsis</em> leaves. Classical ROS-generating inhibitors of mitochondrial electron transport (rotenone, antimycin A and SHAM) had no effect on mt-ro GFP oxidation when used singly, but combined inhibition of complex III and alternative oxidase by antimycin A and SHAM did cause significant oxidation. Inhibitors of complex IV and aconitase also caused oxidation of mt-ro GFP2. This oxidation was not apparent in the cytosol whereas antimycin A + SHAM also caused oxidation of cytosolic roGOFP2. Menadione had a much greater effect than the inhibitors, causing nearly complete oxidation of roGFP2 in both mitichondria and cytosol. A range of severe abiotic stress treatments (heat, salt, and heavy metal stress) led to oxidation of mt-roGFP2 while hyperosmotic stress had no effect and low temperature caused a slight but significant decrease in oxidation. Similar changes were observed for cytosolic roGFP2. Finally, the recovery of oxidation state of roGFP in mitochondria after oxidation by H₂O₂ treatment was dramatically slower than that of either the cytosol or chlorplast. Together, the results highlight the sensitivity of the mitochondrion to redox perturbation and suggest a potential role in sensing and signalling cellular redox challenge.
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