Ureide

Ureide is a class of nitrogen-containing organic compounds derived from purine degradation and is used to describe several related metabolites that serve as nitrogen carriers in plants. The term encompasses compounds such as uric acid, allantoin, and allantoate, with allantoin and allantoate being among the most studied ureides in higher plants.

In plants, ureides function as long-distance transports of fixed nitrogen, particularly in legumes and some woody species. Allantoin and allantoate are synthesized in roots from purine breakdown and are transported in the xylem (and sometimes phloem) to shoots, where they can be metabolized to release ammonium for assimilation. Ureide transport and metabolism are especially important in nitrogen-fixing systems, where nitrogen from nodules is loaded into the ureide pool for distribution throughout the plant. In grasses and many non-ureide-transporting species, ammonium or amino acids may predominate as nitrogen carriers, reflecting a division in nitrogen transport strategies among plant lineages.

Biosynthesis and metabolism of ureides begin with purine catabolism, converting uric acid to allantoin and then to allantoate, with subsequent enzymatic steps releasing ammonium for incorporation into amino acids and other biomolecules. The ureide pathway thus links purine degradation to nitrogen remobilization and utilization within the plant.

Research on ureides informs understanding of plant nitrogen-use efficiency and crop improvement, as environmental conditions and developmental stage influence ureide production, transport, and breakdown.