Do plastids in Dendrobium cv. Lucky Duan petals function similar to autophagosomes and autolysosomes?

In animal cells a double-membrane-bound structure, the autophagosome, encloses a portion of the cytoplasm. The encapsulated material becomes digested after fusion of the autophagosome with a vesicle containing lytic enzymes. The autophagosome is then termed autolysosome. In intact plants, structures similar to animal autophagosomes/autolysosomes have been found only in a few types of cells. Additionally, some early papers indicated that plastids can function similar to autophagosomes/autolysosomes. Here, we report that plastids in Dendrobium cv. Lucky Duan petals produced an endocytosis-like invagination of the two outer membranes. The opening between the invagination space and the cytoplasm was almost isodiametric, less than 0.2 μm in diameter. The volume of the space formed by the invagination had a maximum of about half of the total plastid volume. Staining of the invagination lumen for acid phosphatase, a marker of organelles showing autophagic activity, was positive. Membranes and numerous ribosomes were observed inside the lumen of the invagination. The structure of the material inside the lumen varied from that of the cytoplasm to uniform electron-translucent, indicating that the enclosed cytoplasmic material became completely digested. No support was found for the idea that the material engulfed by the plastid or the whole plastid became transferred to a vacuole. Taken together, the data suggested the hypothesis that plastids in Dendrobium petal mesophyll cells can function in a way similar to both autophagosomes and autolysosomes in animal cells.     

Formation of autophagosomes in the pinealocytes of the rat and Mongolian gerbil (Meriones unguiculatus): A lysosome wrapping mechanism?

Summary Autophagosome formation in rat and gerbil pinealocytes is described. It starts with the setting up of a tubular acid phosphatase-rich cisterna which gradually wraps around cytoplasmic areas to be catabolized. In light of obtained findings, it seems that the autophagosome formation in pinealocytes is a type of lysosome wrapping mechanism.     

GABARAP-mediated targeting of PI4K2A/PI4KIIα to autophagosomes regulates PtdIns4P-dependent autophagosome-lysosome fusion

For decades, phosphatidylinositol 4-phosphate (PtdIns4P) was considered primarily as a precursor in the synthesis of phosphatidylinositol(4,5)bisphosphate (PtdIns(4,5)P₂). More recently, specific functions for PtdIns4P itself have been identified, particularly in the regulation of intracellular membrane trafficking. PI4K2A/PI4KIIα (phosphatidylinositol 4-kinase type 2 α), one of the 4 enzymes that catalyze PtdIns4P production in mammalian cells, promotes vesicle formation from the trans-Golgi network (TGN) and endosomes. We recently identified a novel function for PI4K2A-derived PtdIns4P, as a facilitator of autophagosome-lysosome (A-L) fusion. We further showed that that this function requires the presence of the autophagic adaptor protein GABARAP (GABA[A] receptor-associated protein), which binds to PI4K2A and recruits it to autophagosomes. The mechanism whereby GABARAP-PI4K2A-PtdIns4P promotes A-L fusion remains to be defined. Based on other examples of phosphoinositide involvement in membrane trafficking, we speculate that it acts by recruiting elements of the membrane docking and fusion machinery.