Purine-Derived Ureides Under Drought and Salinity

Purine-derived ureides (allantoin and allantoate) are the dominant nitrogenous (N) products transported from the root nodules to the aerial plant organs by rhizobia-nodulated (Rh+) ureidic legumes. However, ureides are not exclusively synthesized in Rh+ legumes as some evidence suggest a more universal ureide metabolism in C3 and C4 plants to recycle N for growth maintenance. Metabolism of ureides is tightly regulated and highly modulated by abiotic factors, with accumulating evidence describing large similarities among ureidic or amide-producing Rh+, nonrhizobianodulated (Rh-) legumes and nonlegumes, in terms of the critical role played by ureides on plant drought and salinity tolerance. Most reports relate drought or salt sensitivity with increased ureide content in different plant organs, and relate drought or salinity tolerance with the growth trait of high rates of ureide catabolism, regardless of the species and their root nodulating status. However, a considerable number of research articles argue for a hindered rather than active ureide catabolism as the survival trait for plants subjected to periods of mild drought or salinity due to the alternative prime stress signaling function of uric acid and allantoin. Therefore, the selection between the growth or survival trait for breeding programs requires a deep understanding of the purine pathway in Rh+, Rh- legumes and nonlegumes subjected to drought and salinity, based on relevant examples. Finally, this review may benefit both plant scientists and growers in providing detailed insights into ureide metabolism as it relates to abiotic stress responses.