**3.1 Effect urease on protein deposition and embryo development**

Given that urease is the plant's only means of assimilating urea, the next question is the metabolic version of "If you are so smart how come you are not rich?" When applied to urease it would read, "If you're so important how come the plant survives without you?" Indeed, completely urease negative mutant soybean plants develop to maturity and produce a relatively good yield of seeds that germinate at normal frequency to propagate another generation. However, if the role of urease is to recycle urea nitrogen generated from arginine (and possibly ureide) degradation, then a protein-rich plant such as soybean may provide a "suppressing" background for a urease defect. Soybean has indeed been intensively bred for large and protein-rich seeds. However, even in soybean we question the dispensability of urease. Ureasenegative mutant plants [41] and nickel-deprived wild type [35, 36] exhibit necrotic leaf tips, apparently due tourea "burn." Similar observations were made in nickeldeprived tomato [48, 49]. More important, perhaps, than the deleterious effects of leaf burn is the loss of significant quantities of nitrogen in a urea dead-end, lost nitrogen that could have a significant negative impact on seed protein deposition during pod fill. The assessment of the agronomic impact of a urease-negative phenotype on soybean performance requires extensive field testing of isogenic or nearly isogenic paired urease-positive and urease-negative lines, preferably in multiple environments. To obtain these lines we are currently engaged in the long process of backcrossing, generation advance, selection for uniformity in maturity group and plant architecture, etc., and amplification of seed stocks [50]. Our prediction that total seed protein of plant will decrease as suggested by [36], who reported a correlation between seed yield and nickel content per seed. However, there was too much variation in their material to obtain a statistically significant difference. It was observed that at the time of flowering completely urease-negative soybean mutants accumulated approximately 100 dry wt of total leaf (green plus necrotic tip) [41]. Assuming that 10% of the leaf dry weight is protein (16% nitrogen), much of which is destined to provide amino acids during pod fill, accumulated urea (47% nitrogen) represents 18% of the nitrogen in leaf protein. The developing soybean embryo does not generate urea [41]. Thus, other than recycling maternally derived urea, urease appears to play no direct role in embryo metabolism. However, it is possible that in monocots a urease role is more critical in embryo development. It has been reported that nickel is essential for development of viable barley embryos [51]. Post dormancy grains could not be rescued by nickel, whereas developing grains could be rescued by the feeding of nickel to the maternal plant. The role of nickel in barley embryo development is not known and may be unrelated to urease. (Urease is the only nickel metalloenzyme yet identified in plants). However, it is possible that a loss of urease activity under nickel-deprivation conditions leads either to urea poisoning or to nitrogen starvation of the embyro. It would be informative to study barley embryo development *in vitro,* where both nickel availability and nitrogen source can be manipulated.
