**2.1 The precursor of urea**

Arginine, by the action of arginase, is the immediate precursor of urea in the mammalian arginine (urea) cycle. Although an active functional arginine cycle in plants has been debated [18], it is clear that arginase (EC 3.5.3.1) is widely spread in the plant world. Plant arginases resemble the animal enzyme in their high pH optima (approximately 9.7) and Mn2+ requirement [18–22]. As arginase is widely spread, so is its substrate abundant: arginine is a main nitrogen transport and plants' storage compound. It is the nitrogen main transport compound of deciduous [23] and coniferous [24] trees and a major component of underground storage organs (bulbs, roots, tubers [25]). It was shown to be among the amino acids in the seeds of 379 angiosperms that is predominant [26]. It was recalculated (in mol %) the reported average seed amino acid composition of these 379 species. Arginine accounted for 7.7% of seed amino acids and its "N-weighted" contribution was 21.1% of total amino acid nitrogen, the highest contribution of any amino acid, with glutamine a close second (18.6% based on the assumption that half of the glutamate in protein hydrolyses came from glutamine) [26]. In Glycine max, arginine contains 18% of seed protein-bound nitrogen [27]. At least 50% of free amino acid N in developing seeds of pea [28] and soybean [27] is in the arginine pool. In addition, many legume seeds are exceedingly rich in free canavanine [29], an arginine analog. Half of arginine nitrogen (and two-thirds of canavanine nitrogen), in the guanidino moiety, is convertible to urea by arginase kanavinase action.
