The role of urea in soybean nitrogen metabolism will be examined by comparing normal plants and mutant plants lacking one or both urease isozymes, namely the abundant embryo- specific urease and the ubiquitous urease. The former isozyme is synthesized exclusively in the developing embryo while the latter is found in all soybean tissues examined. While urease-negative plants are viable they exhibit leaf-tip necrosis associated with urea buildup. This project examines the sources of the urea and the effects of blocking urea assimilation on soybean yield. Radiolabeled urea accumulation from guanido-labelled arginine and from allantoin will be measured. The effects of the urease blocks on assimilation of ureides (allantoin and allantoic acid) and of arginine in cultured cells and in hydroponic plants will be studied. Field trials of urease-negative plants will assess protein and seed yield. In addition to the urease mutants already in hand, new mutants will be screened for lacking arginase and allantoinase. Upon determining the tissue specificity, it will be determined if these mutational blocks prevent urea accumulation in the affected tissues in urease-negative backgrounds and whether they affect arginine and ureide assimilation, respectively.%%% These studies will add to our knowledge of the metabolism of ureides and arginine in soybean. The former are an important transport form of fixed nitrogen while the latter likely has a nitrogen storage role in addition to its role as a polyamine precursor.***//
