Chloroplasts are partially mobilized to the vacuole by autophagy

Excluding the central vacuole, chloroplasts constitute the largest compartment within the leaf cells of plants and contain approximately 80 percent of the total leaf nitrogen, mainly as proteins. Much of this nitrogen is allocated to the carbon-fixing enzyme in photosynthesis, Rubisco. During senescence, plants can mobilize nitrogen from chloroplasts in older leaves to other organs, such as developing seeds. Whereas bulk degradation of the cytosol and organelles in plants occurs by autophagy, the role of autophagy in the degradation of chloroplast proteins is still unclear. We have recently demonstrated that stroma-targeted green fluorescent protein (GFP), DsRed, and GFP-labeled Rubisco can be mobilized to the vacuole of living cells via Rubisco-containing bodies, in an ATG gene-dependent manner. Our results indicate the presence of a specific autophagic pathway for chloroplast stromal proteins, which does not cause chloroplast lysis. Here, we also discuss the involvement of stroma-filled tubules, stromules, which are important for the structural flexibility of the organelle, on the autophagic transfer of stromal proteins to the vacuole.