**4. Physiology of leguminous soybean ureases and their functions**

Carbon followed by nitrogen is the major limiting element for the performance of plant [84], and a regular demand for utilization of nitrogen targeting the development of mechanism that are efficient for the uptake of nitrogen and metabolic pathways for remobilization if nitrogen in plants [85, 86]. A pressure like this eventually led to nitrogen content reduction of proteins from plants [87]. Since urea is known to be a major source of nitrogen in plants, arginase' activity is found to be the only known pathway involved in the *in vivo* generation of urea; it may be generated also due to ureides and purines degradation [88], even though this pathway happens to attract so much debate. The only way urea can be assimilated is after it has been hydrolyzed by urease into carbon dioxide and ammonia [89], this happens to be the major physiological function attributed to plant ureases [85]. Re-assimilation of ammonia will then be performed by an enzyme called glutamine synthetase utilizing glutamate as substrate [64]. The activity of urease is found in almost all

#### *Function of Urease in Plants with Reference to Legumes: A Review DOI: http://dx.doi.org/10.5772/intechopen.102646*

species of plant and is found to be ubiquitous in all tissues of plants [62, 68, 90], this shows the great advantage of its physiological function to the entire plant. For ages, the importance of urease-mediated metabolism of urea in plant was not regarded because, there was an assumption that plants have difficulty in taking up urea, otherwise they are broken down by soil microorganisms leading to the absorption of nitrate and ammonia. Presently it has been established that plant can absorb urea from the soil actively, via the activity of a committed transporters urea [91] and can also process urea imported from the soil efficiently, even though the concentrations is high. These discoveries focuses at urease as an area for research on improving plant nitrogen metabolism urea that is urea based, the widely utilized fertilizer globally [92]. *G. max* happens to be an exciting model for research based on the physiological function of urease in import plants, because it has so far become the only plant that the genome have been sequenced which shows above one isoform of the enzyme. Soy bean urease ub-SBU has been identified to be the isoform accountable for recycling every urea that is derived metabolically [93–95]. This has been shown because mutants with noes-SBU never assemble urea in any tissue and do not have any deterioration on urea utilization as the major source of nitrogen, although ub-SBU is available at levels only 0.1–0.3% that of es-SBU [61, 93]. *G. max* mutants that have no activity of ub-SBU show a distinguishing feature phenotype consisting of necrosis of the tips of leaf, because of urea "burn", and accumulation of urea in many tissues [41, 96]. Urease is the only Nickel dependent enzyme discovered in plants and the same phenotype as mentioned earlier, has been studied for plants grown under the deprivation of Nickel [35].

It is interesting to note that there is no physiological function, either assimilatory or that of any other characteristics, could possibly be seen due to bountiful es-SBU. In reality, types of cultured cotyledons that are wild were not able to grow due to urea presence, because of a sudden increase in pH as a result released ammonia that was not controlled. A similar effect was not seen on mutants having ub-SBU only [41]. It was concluded that es-SBU could possibly be associated in protection against predators in plant. For the case of microbes or attack of insect, protection of chemical was postulated. Using a model like this, immature embryo that is infected would bring about the release of arginase release from mitochondria that is ruptured which will generate urea in abundance from the pool of arginine and urease from the cytoplasm would transform urea to ammonia rapidly [96]. This hypothesis still waiting for demonstration to be carried out, meanwhile a report has been published stating that mutants that lack the activity of urease were highly susceptible to be infected by microbes [97].
