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 the transfer to dark ness, 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 was 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 Gln in the phloem exudates collected from the cotyledons increased during senescence when the Gln1;1 transcripts accumulated in the cotyledons. The accumulation 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 Gln is related to the translocation 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 de pendent on the expression of genes for synthetases of these amino acids.