Polyamines and oximes in the tolerance response against ammonium stress in plants
Authors:   , Urra Rodríguez Marina 
Published: November 2023

Several studies have uncovered ammonium-triggered physiological responses in plants, such as changes in cation homeostasis, content of nitrogen-rich molecules with low carbon/nitrogen ratio, redox status and metabolism, gene expression, pH, and phytohormone balance. In this work, the relevance of the urea cycle to polyamine metabolism in higher plants was demonstrated, since these interconnected routes operated as important protective mechanisms limiting ammonium toxicity in Medicago truncatula. Accordingly, high ammonium resulted in the accumulation of ammonium and pathway intermediates, particularly glutamine, arginine, ornithine, and putrescine. A switch in the functioning of the urea cycle in roots, from the arginine decarboxylase to the ornithine decarboxylase-dependent production of putrescine, suggested that the ornithine decarboxylase enzyme may provide greater plasticity to plants to cope with ammonium stress. Moreover, the activity of copper amine oxidase, which releases ammonium from putrescine, was significantly decreased in both shoots and roots, supported by in vitro assays. Finally, transcript levels of urea cycle-related genes were increased and those involved in polyamine catabolism were decreased under high ammonium conditions. Early supplementation of plants with putrescine avoided ammonium stress, preventing the ammonium-induced reduction of shoot and root biomass, primary root growth, and photosynthetic performance index. Exogenous putrescine increased nitrogen isotopic discrimination, indicating not only that putrescine may not impede ammonia entrance, but also the occurrence of an additional isotopic discrimination process as ammonium assimilation. In fact, exogenous putrescine boosted glutamine content and glutamine synthetase1 protein content and transcript levels in roots, supporting that ammonium assimilation was upregulated in the presence of putrescine to avoid the translocation of excess ammonium to shoots. The content of glutamine synthetase2 was decreased in shoots of putrescinetreated plants, which may indicate a lessening of acidic stress within chloroplasts. Furthermore, exogenous putrescine regulated Snitrosoglutathione reductase enzyme and, thus, nitric oxide and Snitrosothiols contents. Considering the results obtained, a new model for the role of putrescine under ammonium stress was proposed, where putrescine may act as a negative regulator of nitric oxide accumulation, but promoting Snitrosoglutathione reductase activity and glutathione disulfide and ammonium contents as plant tolerance response Due to the significance of maintaining nitric oxide homeostasis for effectively dealing with ammonium stress, a potential oxidative source of nitric oxide has been identified. The effect of the auxin precursor, indole-3- acetaldoxime, may be attributed to its capacity to release nitric oxide from the oximic moiety and to induce indole-3-acetic acid accumulation via indole-3- acetaldoxime dehydratase, inhibiting primary root elongation and increasing the number of secondary roots, arising a superroot phenotype. Due to the significance of maintaining nitric oxide homeostasis for effectively dealing with ammonium stress, a potential oxidative source of nitric oxide has been identified. The effect of the auxin precursor, indole-3- acetaldoxime, may be attributed to its capacity to release nitric oxide from the oximic moiety and to induce indole-3-acetic acid accumulation via indole-3- acetaldoxime dehydratase, inhibiting primary root elongation and increasing the number of secondary roots, arising a superroot phenotype.