PLANT PHYSIOLOGY , Vol 107, Issue 4 1225-1231, Copyright © 1995 by American Society of Plant Biologists
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BIOCHEMISTRY AND ENZYMOLOGY |
Regulation of Nitrate Reductase during Early Seedling Growth (A Role for Asparagine and Glutamine)
S. Sivasankar and A. Oaks
Department of Botany, University of Guelph, Guelph, Ontario, Canada N1G 2W1
Growth systems that either permit (wet system) or prevent (dry system) the
hydrolysis of endosperm reserves in maize (Zea mays) seedlings were
developed to study the effect of endosperm reserves on the acquisition of
external nitrogen. Three-day-old seedlings treated with 5 mM KNO3 for 24 h
had higher levels of nitrate reductase (NR) activity and protein in shoot
and root tissues in the dry relative to the wet system. This suggests that
the induction of NR is sensitive to products of hydrolysis of endosperm
reserves. Asparagine (1 mM) or glutamine (1 mM), potential products of that
hydrolysis, inhibited the induction of NADH-dependent root NR in the dry
system by about 70%. The inhibition of the induction of NR activity in the
wet system was only about 35%, suggesting that the enzyme in the wet system
was already partially repressed at 3 d. At 5 d, when asparagine and
glutamine levels in the plant tissue had decreased, the induction of root
NR activity was inhibited to a similar extent in the two growth systems by
amide additions. The shoot enzyme was less sensitive to amide additions,
and 10 mM concentrations of either amide was required for a 65% inhibition.
