Ultrastructural Analyses of Senescence Associated Dismantling of Chloroplasts Revisited

During leaf senescence, chloroplasts are transformed into gerontoplasts involving typical structural changes that have been revealed by electron microscopy for more than 30 years. The structural changes involved in chloroplast-to-gerontoplast transition affect the organization of the thylakoid membrane system which is progressively degraded. In parallel, the number and size of plastoglobules was observed to increase. The internal changes in the structure of chloroplasts occurring during leaf senescence are accompanied by a change from an ellipsoid to a round shape and by a reduction in volume. Recent results on Rubisco degradation involving modern cell biology approaches suggest that plastids during senescence release material including Rubisco and other stromal proteins for degradation outside the organelle. In order to get further insight into the structural changes associated with chloroplast dismantling, we have revisited the pertinent literature and furthermore analyzed the ultrastructure of chloroplasts at different stages of barley leaf senescence and under different conditions leading to yellowing of the leaves. Specific changes at the periphery of chloroplasts at certain stages during aging might be related to an exchange of material between chloroplasts and the endoplasmic reticulum. Electron microscopy cannot, however, discriminate between anterograde and retrograde vesicle movements. Electron lucent areas in the matrix of chloroplasts indicate that protein degradation occurs not only outside but also inside the organelle. In many studies it has been observed that the number of plastids per cell declines at late stages of senescence. Our ultrastructural analyses of leaves senescing under field conditions showed that chloroplasts as well as gerontoplasts are surrounded by membranous structures before they are engulfed by the vacuole. Thus, the autophagy pathway appears to be involved in senescence. Many results of electron microscopical analyses of leaf senescence indicate that there exist several mechanisms of chloroplast dismantling. However, further studies by live-cell imaging, immunolabeling and cryo-electron microscopical methods on defined material of plants grown under strictly controlled and comparable conditions will be required for elucidating the mechanisms involved.