Dissection of autophagy in tobacco BY-2 cells under sucrose starvation conditions using the vacuolar H+-ATPase inhibitor concanamycin A and the autophagy-related protein Atg8

Tobacco BY-2 cells undergo autophagy in sucrose-free culture medium, which is the process mostly responsible for intracellular protein degradation under these conditions. Autophagy was inhibited by the vacuolar H⁺-ATPase inhibitors concanamycin A and bafilomycin A₁, which caused the accumulation of autophagic bodies in the central vacuoles. Such accumulation did not occur in the presence of the autophagy inhibitor 3-methyladenine, and concanamycin in turn inhibited the accumulation of autolysosomes in the presence of the cysteine protease inhibitor E-64c. Electron microscopy revealed not only that the autophagic bodies were accumulated in the central vacuole, but also that autophagosome-like structures were more frequently observed in the cytoplasm in treatments with concanamycin, suggesting that concanamycin affects the morphology of autophagosomes in addition to raising the pH of the central vacuole. Using BY-2 cells that constitutively express a fusion protein of autophagosome marker protein Atg8 and green fluorescent protein (GFP), we observed the appearance of autophagosomes by fluorescence microscopy, which is a reliable morphological marker of autophagy, and the processing of the fusion protein to GFP, which is a biochemical marker of autophagy. Together, these results suggest the involvement of vacuole type H⁺-ATPase in the maturation step of autophagosomes to autolysosomes in the autophagic process of BY-2 cells. The accumulation of autophagic bodies in the central vacuole by concanamycin is a marker of the occurrence of autophagy; however, it does not necessarily mean that the central vacuole is the site of cytoplasm degradation.