A SUPPLY OF NITROGEN CAUSES INCREASE IN THE LEVEL OF NADH-DEPENDENT GLUTAMATE SYNTHASE PROTEIN AND IN THE ACTIVITY OF THE ENZYME IN ROOTS OF RICE SEEDLINGS

When rice seedlings, after the growth for 26 days in water alone, were transferred to nutrient medium contained 1 mM NH4Cl, the level of NAD H-dependent glutamate synthase (GOGAT) protein and the activity of the enzyme increased more than 10-fold in root, but not in shoots. Both t he level of the protein and the activity reached a maximum within 24 h . NH4Cl was effective at concentrations as low as 50 mu M. A supply of either 1 mM NaNO3 or 0.5 mM NH4NO3 also caused such increases, but NH 4Cl was most effective. A supply of glutamine or glutamate was less ef fective. The increase was specific to NADH-GOGAT and little change was observed in the levels of ferredoxin-GOGAT and glutamine synthetase i soproteins in roots. These inducible increases in the levels of NADH-G OGAT protein and in its activity were greater in the root-tip region t han at the base of the root. Both 6-methylpurine and cycloheximide com pletely inhibited the effects of NH4Cl. Moreover, the mRNA for NADH-GO GAT in rice roots accumulated markedly within 12 h of the start of a s upply of NH4Cl. A possible role for the rapid response of NADH-GOGAT t o a supply of NH4Cl is discussed.