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 the constitutive 35S cauliflower mosaic virus (CaMV) promoter in order to direct overexpression in Lotus corniculatus L. plants. Following transformation with Agrobacterium rhizogenes, eight independent Lotus transformants were obtained which synthesized additional cytosolic glutamine synthetase (GS) in the shoots. To eliminate any interference caused by the T-DNA from the Ri plasmid, three primary transformants were crossed with untransformed plants and progeny devoid of T-L- and T-R-DNA sequences were chosen for further analyses. These plants had a 50-80% increase in total leaf GS activity. Plants were grown under different nitrogen regimes (4 or 12 mM NH4+) and aspects of carbon and nitrogen 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 using (NH4+)-N-15 were carried out in order to monitor the influx of ammonium and its subsequent incorporation into amino acids. This experiment showed that both ammonium uptake in the roots and the subsequent translocation of amino acids to the shoots was lower in plants overexpressing GS. It was concluded that the build up of ammonium and the increase in amino acid concentration in the roots was the result of shoot protein degradation. Moreover, following three weeks of hydroponic culture early floral development was observed in the transformed plants. As all these properties are characteristic of senescent plants, these findings suggest that expression of cytosolic GS in the shoots may accelerate plant development, leading to early senescence and premature flowering when plants are grown on an ammonium-rich medium.