Purine metabolism is a fundamental pathway of plant primary metabolism and is also known to be used in tropical legumes like soybean and common bean to produce ureides for long-distance nitrogen transport. Some enzymes and transporters involved in ureide biosynthesis are yet unknown. This work describes the identification of a novel xanthosine monophosphate-specific phosphatase, XMPP. By LC-MS-based metabolite analysis of XMPP-deficient nodules generated using CRISPR-Cas9 mutagenesis, it was demonstrated that this phosphatase is involved in ureide biosynthesis. XMPP is conserved in vascular plants so that another aim of this thesis was its characterization in Arabidopsis thaliana. Metabolite analysis of XMPP mutants in context of other mutants of the purine catabolism showed that XMP dephosphorylation represents an entry point into purine catabolism and that it is operative in seeds, seedlings, vegetative and reproductive rosettes and that it is of special importance in extended darkness and likely also under biotic stress. It was found that the expression of XMPP protein is under tight control and that it is strongly induced in the extended night and under plant defense-related conditions like methyl jasmonate treatment and infiltration with Pseudomonas syringae. In the context of the extended night a new model is proposed which suggests that nucleotides, in particular adenylates, may serve as transient alternative energy source at the beginning of the extended night when the starch reservoir is depleted and amino acids are not yet available as alternative respiratory substrates. In the context of plant defense, it is suggested that XMPP and purine catabolism could be part of the innate immune response to contain oxidative bursts to the infection sites and thereby protect the surrounding plant tissue.