ROOT-SPECIFIC AND SHOOT-SPECIFIC RESPONSES OF INDIVIDUAL GLUTAMINE-SYNTHETASE GENES OF MAIZE TO NITRATE AND AMMONIUM

The responses of the five cytosolic-type glutamine synthetase (GS(1)) genes of maize to treatment of hydroponically grown seedlings with 10 mM KNO3 or 10 mM NH4Cl were analyzed. Non-coding 3' gene-specific hybr idization probes and radioanalytic imaging were used to quantitate ind ividual gene transcript levels in excised roots and shoots before trea tment and at selected times after treatment. Genes GS(1-1) and GS(1-2) exhibited distinct organ-specific responses to treatment with either nitrogen source. The GS(1-1) transcript level increased over three-fol d in roots, but changed little if any in shoots. In contrast, the GS(1 -2) transcript level increased over two-fold in shoots, but decreased in roots after treatment. Increased transcript levels were evident at 4 h after treatment with either nitrogen source, with maximum accumula tions present at 8 h after treatment with ammonium and at 10-12 h afte r treatment with nitrate. The GS(1-3) gene transcript level showed lit tle or no change after treatment with either nitrogen source. The GS(1 -4) gene transcript level remained constant in shoots of treated seedl ings, whereas in roots, it exhibited relatively minor, but complex res ponses to these two nitrogen sources. The GS(1-5) gene transcript is p resent in very small amounts in seedlings, making it difficult to anal yze its response to metabolites in young plants. These results provide support for the possibility that different cytosolic GS genes of maiz e play distinct roles in nitrogen metabolism during plant growth and d ifferentiation.