Protein oxidation results from the reaction of amino-acid side chains with reactive oxygen species (ROS) and is mostly irreversible. In non-photosynthetic tissues, mitochondria are a major source of ROS, whereas plastids are the major source in photosynthetic tissues. Oxidized proteins suffer from decreased structural integrity and even loss of function, and their accumulation ultimately leads to cytotoxic aggregates. In mammals, this correlates with aging and is associated with several age-related pathologies. Mammalian proteolytic pathways to clear oxidized proteins are under intensive research, while mechanistic insights into this process in plants are scarce. Ralf and colleagues have identified a protease that is evolutionary conserved in all domains of life. In humans, its malfunction is associated with severe disease, while little is known about its function in plants. Therefore, they generated multiple knockouts in Physcomitrella and obtained Arabidopsis T-DNA mutants. Collectively, their data reveal a function of this enzyme in stress response and in plant aging.