Hydrogen Peroxide Mediates Premature Senescence Caused by Darkness and Inorganic Nitrogen Starvation in Physcomitrium patens

Leaf senescence accompanied by yellowing and Rubisco degradation occurs prematurely in response to various stresses. However, signaling pathways between stress perception and senescence responses are not understood fully, although previous studies suggest the involvement of reactive oxygen species (ROS). While investigating the physiological functions of autophagy in Physcomitrium patens using wild-type (WT) and autophagy-deficient atg5 strains, we found that Physcomitrium colonies senesce prematurely under dark or nitrogen-deficient conditions, with atg5 senescing earlier than WT. In the present study, we measured cellular H(2)O(2), and examined whether H(2)O(2) mediates premature senescence in Physcomitrium colonies. Methyl viologen, an ROS generator, increased cellular H(2)O(2) levels and caused senescence-like symptoms. H(2)O(2) levels were also elevated to the same plateau levels in WT and atg5 under dark or nitrogen-deficient conditions. The ROS scavenger N-acetylcysteine and the ROS source inhibitor carbonyl cyanide m-chlorophenylhydrazone inhibited the increase in H(2)O(2) levels as well as senescence. Upon transfer to a nitrogen-deficient medium, H(2)O(2) levels increased earlier in atg5 than in WT by ~18 h, whereas atg5 yellowed earlier by >2 days. We conclude that the increased H(2)O(2) levels under dark or nitrogen-deficient conditions mediate premature senescence in Physcomitrium but do not explain the different senescence responses of WT and atg5 cells.