The remobilization of metabolites during stress and senescence plays an important role in optimal plant adaptation to the environment. The plant molybdenum co-factor (MoCo) and flavin-containing enzyme xanthine dehydrogenase (XDH; EC 18.104.22.168) are pivotal for purine remobilization, and catalyze the conversion of the purine catabolic products hypoxanthine and xanthine to uric acid, which is subsequently degraded to the ureides allantoin and allantoate. We observed that in wild-type plants conditions of extended darkness or increasing leaf age caused induction of transcripts related to purine catabolism, resulting in marked accumulation of the purine catabolic products allantoin and allantoate. In contrast, Arabidopsis mutants of XDH, Atxdh1, accumulated xanthine and showed premature senescence symptoms, as exemplified by enhanced chlorophyll degradation, extensive cell death and upregulation of senescence-related transcripts. When dark-treated mutant lines were re-exposed to light, they showed elevated levels of reactive oxygen species (ROS) and a higher mortality rate compared with wild-type plants. Interestingly, the level of ROS and mortality could be attenuated by the addition of allantoin and allantoate, suggesting that these metabolites can act as scavengers of ROS. The results highlight a crucial need for the controlled maintenance of ureide levels mediated by AtXDH1 activity during dark stress and ageing, and point to the dual functionality of ureides as efficient stores of nitrogen and as cellular protectants. Thus, the regulation of ureide levels by Atxdh1 has general implications for optimal plant survival during nutrient remobilization, such as occurs during normal growth, dark stress and senescence.