Developmental genetics of the soybean urease isozymes

The soybean (Glycine max [L.] Merr.) contains two urease isozymes whose expression is regulated in a tissue‐specific and temporal manner. The ubiquitous urease is expressed in all tissues examined (leaf, embryo, seed coat, cell culture); the embryo‐specific urease is synthesized exclusively in the developing embryo. The embryo‐specific urease accumulates during seed development while the ubiquitous urease is found in highest levels during early development of both leaves and seeds. We have isolated mutants which fall in three phenotypic classes lacking one or both urease isozyme activities. Genetic analysis has thus far identified three unlinked loci which control the expression of urease(s). Genomic and cDNA clones of urease structural genes have also been recovered and we are working to assign these to genetic loci by sequence and RFLP analyses. That the ubiquitous urease isozyme is expressed in cell culture makes it possible to include cell culture in physiological and developmental studies. Additionally, we have developed direct selections for urease‐negative mutants, and their revertants, in cell culture. These selections will facilitate the study of the expression of cloned urease genes in genetically transformed tissue.