**3. Repercussion of urease activity elimination in plants**

In higher plants it appears that urea can be assimilated only by urease action. Urease-negative plants and cultures, induced genetically [30] with urease inhibitor [31, 32] or by nickel deprivation [33–36], have been observed either to accumulate urea or to be blocked in the ability to employ urea as a nitrogen source. All plant [37] and bacterial [3] ureases are probably nickel metalloenzymes. Seed ureases from jackbean [7, 38] and soybean [39] have been shown to contain nickel. Duckweed plants [40] and callus of soybean, rice, and tobacco [34, 37] are dependent on nickel for maximal growth with urea as sole nitrogen source. Urease appears to be the only nickel-requiring enzyme in plants since, as indicated below, nickel-deprived soybean plants have the same phenotype as those genetically blocked in urease

synthesis [41]. Thus, higher plants appear to lack the ATP-dependent urea amidohydrolase reported in algae and yeast [42–45]. This biotin-containing carboxylase/ hydrolase appears not to be a nickel metalloenzyme and has a urea-assimilatory function (e.g., [46] in these urease-negative lower eukaryotes. In an interesting example of the potential of urease to provide nitrogen for the plant, [47] developed transgenic tobacco plants engineered for resistance to cyanamide. The resistance gene, from soil fungus *Myrothecium verrucaria,* codes a cyanamide hydratase that converts cyanamide to urea. Although urea levels are raised in such plants, the endogenous urease apparently hydrolyzes much of the liberated urea.
