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PLANT PHYSIOLOGY , Vol 109, Issue 1 169-175, Copyright © 1995 by American Society of Plant Biologists
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BIOCHEMISTRY AND ENZYMOLOGY |
Urease Is Not Essential for Ureide Degradation in Soybean
N. E. Stebbins and J. C. Polacco
Department of Biochemistry and Interdisciplinary Plant Group, University of Missouri-Columbia, Columbia, Missouri 65211
The hypothesis that soybean (Glycine max L. [Merrill]) catabolizes ureides
to urea to a physiologically significant extent was tested and rejected.
Urease-negative (eu3-e1/eu3-e1) plants were supported by fixed N2 or by 2
mM NH4NO3, 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 [14C-ureido]allantoate to [14C]urea, whereas 40 to
70% of [14C-guanido]arginine was recovered as [14C]urea.
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