Dark-induced expression of genes for asparagine synthetase and cytosolic glutamine synthetase in radish cotyledons is dependent on the growth stage

In radish, the level of transcripts from the cytosolic glutamine synthetase (GS1) gene (Gln1;1) increased as the cotyledons aged and senesced, After t he transfer to darkness, the level of transcripts from Gln1;1 in senescing (3 weeks after germination) cotyledons which was abundant before transfer, increased, but that in young (1 week after germination) cotyledons which wa s very low before transfer, did not. On the contrary, transcripts from the asparagine synthetase (AS) gene accumulated after the transfer to darkness in young cotyledons but not in senescing cotyledons. The amount of free Gin in the phloem exudates collected from the cotyledons increased during sene scence when the Gln1;1 transcripts accumulated in the cotyledons. The accum ulation of AS transcripts, however, led to the increase in the level of Asn in the cotyledons rather than in the phloem exudates from the cotyledons. These data suggest that the synthesis of Gin is related to the translocatio n of nitrogen from senescing cotyledons, whereas that of Asn is involved in the transient nitrogen storage in the tissue, and also that the synthesis of these amino acids is dependent on the expression of genes for synthetase s of these amino acids.