Accelerated Cell Death in Podospora Autophagy Mutants

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Abstract
Although autophagy is characteristic of type II programmed cell death (PCD), its role in cell death is currently debated. Both cell death-promoting and prosurvival roles of autophagy have been reported depending on the organism and the cell type. In filamentous fungi, a cell death reaction known as an incompatibility reaction occurs when cells of unlike genotype fuse. Cell death by incompatibility is characterized by a dramatic vacuolar enlargement and cell lysis. In Podospora anserina , autophagy is induced early during this cell death reaction. Cell death by incompatibility in Podospora is a model of type II PCD used here to assess the role of autophagy in this type of cell death. We have inactivated Pa ATG1 , the Podospora ortholog of the Saccharomyces cerevisiae ATG1 gene involved in the early steps of autophagy in yeast. The ΔPa ATG1 mutant displays developmental defects characteristic of abrogated autophagy in Podospora . Using the green fluorescent protein-PaATG8 autophagosome marker, we show that autophagy is abolished in this mutant. Neither cell death by incompatibility nor vacuolization are suppressed in ΔPa ATG1 and ΔPa ATG8 autophagy mutants, indicating that a vacuolar cell death reaction without autophagy occurs in Podospora . Our results thus provide a novel example of a type II PCD reaction in which autophagy is not the cause of cell death. In addition, we found that cell death is accelerated in ΔPa ATG null mutants, suggesting that autophagy has a protective role in this type II PCD reaction.