ureides

Ureides are a class of nitrogen-rich organic compounds derived from the degradation of purines that contain the ureide motif. The best-known ureides in plants are allantoin and allantoate, with uric acid acting as an upstream precursor in many species. Ureides are particularly important in certain plants as forms of organic nitrogen that can be transported over long distances.

Biochemical pathways and transport: In many plants, purine catabolism proceeds from uric acid to allantoin via urate oxidase, then to allantoate, and through subsequent hydrolytic steps to release usable nitrogen. Ureides are produced primarily in root nodules of nitrogen-fixing legumes and are transported through the xylem to shoots, where they may be degraded to provide ammonium and assimilated into amino acids and other nitrogenous compounds. The ureide transport system is regulated in response to nitrogen demand and nodulation activity.

Physiological role and significance: The ureide pathway provides an efficient mechanism for long-distance transport of organic nitrogen in several legumes and related species. This mode of transport contrasts with nitrate or ammonium as the predominant nitrogen forms in many non-ureide plants. Ureide levels can reflect nitrogen status, nodulation activity, and overall nitrogen use efficiency, and they influence downstream processes such as seed protein allocation in some crops.

Distribution and context: Ureide metabolism is well characterized in leguminous crops such as soybeans and common beans, and it occurs in a subset of other woody plants and microorganisms. In addition to nitrogen transport, ureides can participate in nitrogen remobilization and cellular metabolism under various environmental conditions.