Deletion of the single UreG urease activation gene in soybean NIL lines: characterization and pleiotropic effects

The soybean eu3-a mutant (formerly, eu3-e1) lacks all ureolytic activity. Eu3 encodes urease accessory (Ni insertion) protein, UreG. Eu3 (Glycine max v1.1 Glyma08g08970.1) is the only UreG-encoding gene in the soybean genome. Here we show that the eu3-a lesion is a 2.4 kb deletion, beginning 50 bp upstream of the transcription start, and covering 94 % of the deduced amino acid sequence of UreG, explaining the urease activity-null phenotype of eu3-a. We established near-isogenic lines (NILs), eu3-a versus Eu3, and confirmed a striking accumulation of endogenous urea in eu3-a and none in Eu3, and the greater resistance of eu3-a leaf discs to external urea. However, incubation of discs in light alleviated urea sensitivity in Eu3, apparently by providing C skeletons for urea-released NH3. Hence, urea, endogenous and exogenous, is apparently less toxic than urea-derived NH3. eu3-a and Eu3 plants, however, did not differ in seed Ni content, nor in pool sizes of metabolites that generate urea via arginine. The NILs differed in patterns of nodulation by Bradyrhizobium japonicum and by Bradyrhizobium elkanii. Ureases have been reported to be fungitoxic. However, eu3-a and Eu3 NIL seedlings were equally sensitive to the pathogenic Basidiomycete fungus, Rhizoctonia solani. Since eu3-a accumulates at least some apo-urease, we examined a specific anti-fungal role of the seed urease in seedlings of a second NIL pair, differing in the presence (Eu1) and absence (eu1-a) of the seed urease. Again, this second NIL pair exhibited equal sensitivity to R. solani. To our knowledge, we report only the second test of fungal resistance influenced by urease levels in intact plants.