Urease and nitrogen recycling and the role of nickel as an essencial micronutrient

The fact that Ni is part of the active site of urease in plants, and this enzyme is responsible for the elimination of toxic levels of urea in plants has been the justification for classifying this element as essential. In contrast, urease activity has been almost strictly related to the urea produced in the conversion of arginine to ornithine and especially in seeds. In vegetative tissues, leaves, for example, the levels of urease activity is very low. Thus, there are few studies that actually show that Ni is essential in plants, being restricted mainly to soybean, and even fewer studies that highlight the role of urease in plant metabolism. Recent studies have shown that urease, and consequently Ni, may play a role in the recycling of N between metabolic pools in plants under stress. Two experiments were performed using Arabidopsis thalinana plants receiving treatments with nitrogen sources (urea and arginine) and nickel. A third experiment used same species plants under drought stress. Analysis for gene expressions, aminoacids concentration and polyamines concentrations were part of all three experiments. Those data indicated that Ni treatment caused changes in a variety of metabolic pathways beyond arginine and urea¿s reactions. Levels of arginine, ornithine, urea, glutamate, glutamine, proline, GABA, citruline, agmatine e polyamines were altered by the treatments. Genes for the enzymes of these substances¿s metabolic pathways had their expression changed as well. Those changes seem to be connected with the nitrogen source applied to the plants on the experiments. The third experiment indicates that osmoprotectors, as proline and polyamines, may have their accumulation affected by metabolic pathways that may be prone to interferences caused by Ni. Urease and Ni may be important factors not only to intracellular urea detoxification but also for the N metabolism as a whole (AU)