The different fates of mitochondria and chloroplasts during dark-induced senescence in Arabidopsis leaves

Senescence is an active process allowing the reallocationof valuable nutrients from the senescing organ towardsstorage and/or growing tissues.Using Arabidopsis thalianaleaves from both whole darkened plants (DPs) and individuallydarkened leaves (IDLs), we investigated the fate ofmitochondria and chloroplasts during dark-induced leafsenescence. Combining in vivo visualization of fates of thetwo organelles by three-dimensional reconstructions ofabaxial parts of leaves with functional measurements ofphotosynthesis and respiration, we showed that the twoexperimental systems displayed major differences during6 d of dark treatment. In whole DPs, organelles werelargely retained in both epidermal and mesophyll cells.However, while the photosynthetic capacity was maintained,the capacity of mitochondrial respiration decreased.In contrast, IDLs showed a rapid decline in photosyntheticcapacity while maintaining a high capacity for mitochondrialrespiration throughout the treatment. In addition, wenoticed an unequal degradation of organelles in the differentcell types of the senescing leaf. From these data, wesuggest that metabolism in leaves of the whole DPs enters a‘stand-by mode’ to preserve the photosynthetic machineryfor as long as possible. However, in IDLs, mitochondriaactively provide energy and carbon skeletons for the degradationof cell constituents, facilitating the retrieval ofnutrients. Finally, the heterogeneity of the degradation processesinvolved during senescence is discussed with regardto the fate of mitochondria and chloroplasts in the differentcell types.