INCREASING CORN AND TOBACCO SHOOT MASS, ROOT MASS AND SEED YIELD BY TRANSGENE MANIPULATION OF NITROGEN ASSIMILATION.

¹ The Dept. of Plant and Soil Science, Molecular Science Program, Southern Illinois Univ., Carbondale, IL 62091-4415 USA

² 1 Pioneer Hi-Bred International Inc., Trait and Technology Development. 7300 NW 62nd Ave., PO Box 1004, Johnson Iowa 50131-1004 USA

³ Garst Inc., ICI Seeds, Highway 210 West, PO Box 500, Slater IA 50244 USA

In all microorganisms and plants the flow of nitrogen into organic compounds occurs via ammonium. In plants ammonium assimilation occurs largely through the activity of glutamine synthetase, a high affinity low capacity enzyme. Whereas in many microorganisms glutamate dehydrogenase (GDH), a low affinity high capacity enzyme, is responsible for ammonium assimilation. GDH uses less energy and therefore carbon per NH3 assimilated. We have transformed plants with a bacterial gene that encodes NADP specific GDH, an enzyme that can assimilate ammonium. The transformed plants express GDH specific activities many fold higher than the GS activity in a cell free assays with saturating substrate concentrations and flux of ammonium to aminoacids is increased. The GDH activity gives the transgenic plants tolerance to herbicides that inhibit GS activity and to toxic concentrations of ammonium. Resistance to phosphinothricin is linearly correlated with NADPH GDH activity. With tobacco the biomass of plants is increased about 10% in the field and greenhouse. The field yield increase reflects an increase in canopy area that was consistent over 4 field trials (1995-1997). Nitrogen response is improved in suboptimal and super optimal fertilization regimes. The relative field yield benefit is stronger under water stress (40%) so NADP GDH activity may increase both yield and yield stability. In the greenhouse shoot biomass was increased about 10 % and root biomass about 30 % among none stressed plants. In the transgenic corn hybrids (H99xB73; H99xBD68) and inbreds kernel yield per plant, plant height and plant biomass are increased by about 10%. Protein contents are raised marginally and aminoacid quality is altered. Therefore, we conclude that GDH activity alters plant growth by improving the metabolic efficiency of assimilation of ammonium by plant cells and by increasing stress tolerance.The effect of the transgene expression on genes regulating C and N partitioning will be examined.