Urease Is Not Essential for Ureide Degradation in Soybean

Abstract

The hypothesis that soybean (Clycine max 1. [Merrill]) catabolizes ureides to urea to a physiologically significant extent was tested and rejected. Urease-negative (eu3-el/eu3-el) plants were supported by fixed N, or by 2 mM NH,NO,, so that xylem-borne nitrogen contained predominantly ureides (allantoin and allantoic acid) or amide amino acids, respectively. Seed nitrogen yield was equal on either nitrogen regime, although 35-d-old fixing plants accumulated about 6 times more leaf urea. In callus, lack of an active urease reduced growth on either arginine or allantoin as the sole nitrogen source, but the reduction was greater on arginine (73%) than on allantoin (39%). Furthermore, urease-negative cells accumulated 17 times more urea than urease-positive cells on arginine; for allantoin the ratio was 1.8. Urease-negative callus accumulated urea at 3 % the rate of seedlings. To test whether urea accumulating in urease-negative seedlings was derived from ureides, seeds were first allowed to imbibe in 1 mM allopurinol, an inhibitor of ureide formation. Seedling ureides were decreased by 90%, but urea levels were unchanged. Thus, ureides are poor precursors of urea, which was confirmed in seedlings that converted no more than 5% of seed-absorbed [' 4C-ureido]

Arginase Is Inoperative in Developing Soybean Embryos1

“…Arg degradation by arginase during germination is consistent with the sharp increase in urea in urease-deficient mutant seedlings (Fig. 6;Stebbins and Polacco, 1995). Urease has been proposed to function coordinately with arginase in the utilization of seed protein reserves during germination (Thompson, 1980).…”

“…The homozygous eu3-e1/eu3-e1 mutant lacks the activities of all soybean ureases. Eu3/eu3-e1 heterozygous plants (Stebbins et al, 1991;Stebbins and Polacco, 1995) have urease levels similar to wild-type Eu3/Eu3 plants. All urease-negative eu3-e1/eu3-e1 seeds used in this study were derived from selfed Eu3/ eu3-e1 plants.…”

The Ureide-Degrading Reactions of Purine Ring Catabolism Employ Three Amidohydrolases and One Aminohydrolase in Arabidopsis, Soybean, and Rice

“…Consistently, it was reported that in soybean, half of the nitrogen from allantoate was released as urea, presumably during ureidoglycolate degradation (Todd and Polacco, 2004). However, earlier studies could not find any evidence for a urea intermediate in the course of ureide catabolism in soybean (Winkler et al, 1985(Winkler et al, , 1987Stebbins and Polacco, 1995).…”

Nickel Deficiency Disrupts Metabolism of Ureides, Amino Acids, and Organic Acids of Young Pecan Foliage

“…UGL or UGAH), with one predominating over the other. For example, in soybean, ureidoglycolate catabolism was approximately 95% via UGAH and approximately 5% via UGL; thus, urease did not appear to be essential for ureide catabolism in this species (Stebbins and Polacco, 1995), whereas in chickpea (Cicer arietinum) UGL appears to be the predominant path (Munoz et al, 2001). The accumulation of ureidoglycolate and quantitative reduction in urea from Ni-D foliage suggests that the UGL activity is being impaired.…”