Autophagy accelerates cell death after desiccation and hydration stress in Physcomitrium

Autophagy is the process responsible for degrading cytoplasmic components in lysosomes and vacuoles. Autophagy-deficient mutants are generally more sensitive to environmental stress than their wild-type (WT) plants. In this study, however, we found that autophagy-deficient (atg) mutants of Physcomitrium, in which an autophagy-related gene, either ATG5 or ATG7, is disrupted, are more tolerant to extreme desiccation stress than WT plants. The colonies were treated with the phytohormone abscisic acid, desiccated using silica gel, and hydrated on a nutrient-sufficient medium. atg mutant colonies survived this stress program and produced green colonies whereas the WT colonies did not. Evans Blue staining revealed obvious cell death in both colonies, but the rate was higher in WT than in atg5 colonies. atg cells had a higher protein content, accumulated more sucrose, and had greater water-holding capacity than WT cells. These factors likely reduce the rate of cell death with less damage during the stress program. On the other hand, inhibition of autophagy during the hydration step with the inhibitor 3-methyladenine increased survival, suggesting that autophagy occurring during the hydration step is involved in cell death. We conclude that Physcomitrium autophagy-deficient cells are more tolerant to extreme desiccation stress than WT cells due to higher water-holding capacity and the promotion of cell death by autophagy through an unknown mechanism. These findings challenge the conventional understanding of the role of autophagy in cell survival against stress and highlight the importance of further research in this area.