OVEREXPRESSION OF A SOYBEAN GENE ENCODING CYTOSOLIC GLUTAMINE-SYNTHETASE IN SHOOTS OF TRANSGENIC LOTUS-CORNICULATUS L PLANTS TRIGGERS CHANGES IN AMMONIUM ASSIMILATION AND PLANT DEVELOPMENT

A soybean cytosolic glutamine synthetase gene (GS15) was fused with th e constitutive 35S cauliflower mosaic virus (CaMV) promoter in order t o direct overexpression in Lotus corniculatus L. plants. Following tra nsformation with Agrobacterium rhizogenes, eight independent Lotus tra nsformants were obtained which synthesized additional cytosolic glutam ine synthetase (GS) in the shoots. To eliminate any interference cause d by the T-DNA from the Ri plasmid, three primary transformants were c rossed with untransformed plants and progeny devoid of T-L- and T-R-DN A sequences were chosen for further analyses. These plants had a 50-80 % increase in total leaf GS activity. Plants were grown under differen t nitrogen regimes (4 or 12 mM NH4+) and aspects of carbon and nitroge n metabolism were examined. In roots, an increase in free amino acids and ammonium was accompanied by a decrease in soluble carbohydrates in the transgenic plants cultivated with 12 mM NH4+ in comparison to the wild type grown under the same conditions. Labelling experiments usin g (NH4+)-N-15 were carried out in order to monitor the influx of ammon ium and its subsequent incorporation into amino acids. This experiment showed that both ammonium uptake in the roots and the subsequent tran slocation of amino acids to the shoots was lower in plants overexpress ing GS. It was concluded that the build up of ammonium and the increas e in amino acid concentration in the roots was the result of shoot pro tein degradation. Moreover, following three weeks of hydroponic cultur e early floral development was observed in the transformed plants. As all these properties are characteristic of senescent plants, these fin dings suggest that expression of cytosolic GS in the shoots may accele rate plant development, leading to early senescence and premature flow ering when plants are grown on an ammonium-rich medium.