Expression analysis of a cytosolic glutamine synthetase gene in cotyledonsof Scots pine seedlings: developmental, light regulation and spatial distribution of specific transcripts

The expression of a cytosolic glutamine synthetase (GS1; EC 6.3.1.2) gene w as examined in cotyledons of Scots pine seedlings. Light strongly stimulate d GS1 mRNA accumulation during development. Similarly, steady-state levels of GS1 transcripts increased in dark-grown seedlings transferred to light a nd decreased in dark-adapted seedlings. Light/dark adaptation affected rbcS and lhcb2 mRNA levels and chlorophyll contents in the same manner. Light-g rown seedlings in the presence of the herbicide norflurazon showed a drasti c decrease in mRNA for GS and photosynthetic proteins, whereas the effect o f the herbicide on mitochondrial beta-ATP synthase mRNA was limited. These results indicate that factors associated with developing chloroplasts could be required for maximal GS1 gene expression during seedling development. T he level of GS polypeptide, determined by immunoblot, was up-regulated duri ng seedling development in the light or dark. However, the levels of the po lypeptide detected were unaltered by the light/dark adaptation treatments. The analysis of GS1 mRNA association with polysomes indicated that the disc repancies between GS protein and mRNA levels are not a result of a differen tial translational rate of the transcript in darkness relative to light. Tw o GS isoproteins with different isoelectric point were resolved by two-dime nsional PAGE in light- and dark-germinated plants. The relative abundance o f one of them was markedly affected by light and correlated with the observ ed changes in GS mRNA, suggesting that the other form is not a product deri ved from the detected transcript. In situ hybridization of cotyledon sectio ns showed the presence of GS1 mRNAs in mesophyll and phloem cells confirmin g gene expression in photosynthetic tissues. High levels of transcript were detected also in meristematic cells of apical primordia. These data sugges t a dual role for the GS1 gene associated with chloroplast development/acti vity and glutamine biosynthesis for nitrogen mobilization during early grow th of Scots pine.