ATG5-knockout mutants of Physcomitrella provide a platform for analyzing the involvement of autophagy in senescence processes in plant cells

To identify the physiological functions of autophagy in plants, we disrupted ATG5 , an autophagy-related gene, in Physcomitrella , and confirmed that atg5 mutants are deficient in the process of autophagy. On carbon or nitrogen starvation medium, atg5 colonies turned yellow earlier than the wild-type (WT) colonies, showing that Physcomitrella atg5 mutants, like yeast and Arabidopsis , are sensitive to nutrient starvation.

In the dark, even under nutrient-sufficient conditions, colonies turned yellow and the net degradation of chlorophyll and Rubisco protein occurred together with the upregulation of several senescence-associated genes. Yellowing reactions were inhibited by the protein synthesis inhibitor cycloheximide, suggesting that protonemal colonies undergo dark-induced senescence like the green leaves of higher plants. Such senescence responses in the dark occurred earlier in atg5 colonies than WT colonies.