The Urea Cycle in Connection to Polyamine Metabolism in Higher Plants: New Perspectives on a Central Pathway

The ornithine-urea cycle is a biochemical pathway primarily found in animals, where it plays a crucial role in the re-assimilation of ammonium and the removal of excess nitrogen in the form of urea. In lower photosynthetic eukaryotes, it contributes to metabolic responses during episodes of high nitrogen availability. In higher plants, although historically overlooked, compelling evidence indicates the pivotal role of the urea cycle in different aspects of plant physiology and metabolism. In particular, it is associated with the metabolism of polyamines during stress. Unlike in animals and lower photosynthetic eukaryotes, in higher plants, the urea cycle is not complete due to the lack of the carbamoyl phosphate synthase-I enzyme that incorporates ammonium into the cycle. Higher plants only possess a type-II carbamoyl phosphate synthase-II that introduces glutamine into the cycle, which is also metabolically linked to arginine and polyamine metabolism. Putrescine accumulation is a metabolic hallmark of different types of abiotic stresses, such as drought, salinity, ammonium stress, iron and phosphorus deficiency, and low temperatures. Notably, the exogenous application of polyamines, such as putrescine or spermine, enhances tolerance to abiotic stress, a process in which the free radical nitric oxide appears to play a role. Overall, this review article attempts to bring together the current knowledge on the functionality of the constituent enzymes and metabolites of the urea cycle and discuss the importance of this pathway in relation to the metabolism of polyamine in higher plants.