Abstract
We have been investigating the physiological roles of autophagy using cultured tobacco cells, Arabidopsis roots, and the protonemata of Physcomitrella. When tobacco cells cultured in a nutrient-sufficient medium are transferred to sucrose-deprived medium, the net degradation of cellular proteins and rRNA occurs, and these components decreased to approximately half the initial values in 2 days. The degradation of protein and rRNA are inhibited by the autophagy inhibitor 3-methyladenine, suggesting that cells degrade their own constituents by autophagy for recycling and obtaining energy under nutrient starvation conditions. Root tips excised from Arabidopsis seedlings grow in a culture medium. Their growth depends on sucrose in the culture medium. In atg5 mutants, in which the macroautophagic pathway is impaired, the growth of root tips is retarded. This suggests that autophagy contributes to cell growth. When the protonemata of Physcomitrella are placed in the dark, senescence accompanying the degradation of chlorophyll and Rubisco is induced. It proceeds faster in atg5 mutants than in the wild type. This suggests that senescence is negatively regulated by autophagy.
