Leaf-specific overexpression of plastidic glutamine synthetase stimulates the growth of transgenic tobacco seedlings.
The impact of increased plastidic glutamine synthetase (GS-2; EC 6.1. 3.2) activity on foliar amino-acid levels and on biomass production was examined in transgenic tobacco. For that, tobacco was transformed via Agrobacterium tumefaciens with a binary vector containing a tobacco GS-2 cDNA downstream of the leaf-specific soybean ribulose-1,5-bisphosphate carboxylase/oxygenase small subunit gene promotor. Two transgenic tobacco lines with 15- to 18-fold higher foliar GS-2 transcript levels than the wild type were obtained. The GS-2 protein pools and the specific GS-2 activities were, however, only 2- to 2.3-fold higher in the leaves of the transgenic plants than in the leaves of the wild type. This discrepancy may reflect a post-transcriptional control of GS-2 protein accumulation. The increased GS-2 activity was correlated with a decrease in the leaf ammonium pool (3.7-fold) and an increase in the levels of some free amino acids, including glutamate (2. 5-fold) and glutamine (2.3-fold). The accumulation of soluble protein per unit fresh weight, however, remained unchanged. This result indicates that a process downstream of the synthesis of the primary organic products of N-assimilation is limiting leaf protein accumulation. Nevertheless, the overexpression of GS-2 stimulated the growth rate of the transgenic tobacco seedlings which, consequently, were larger (20-30% on a fresh-weight basis) than wild-type seedlings grown under identical conditions. This result suggests that GS-2 is the rate-limiting enzyme during biomass production in tobacco seedlings. The requirement for glutamate as the ammonium acceptor in the reaction catalysed by GS-2 may imply that there is co-regulation of GS-2 and ferredoxin dependent glutamate synthase (Fd-GOGAT; EC 1.4.7.1) gene expression. Increased leaf GS-2 activity had, however, no influence on the foliar Fd-GOGAT protein abundance. This result suggests that in tobacco leaves, more Fd-GOGAT is present than required to meet the demands of primary ammonium assimilation and that there is no strong interdependence between GS-2 and Fd-GOGAT protein expression.
Migge
A
A
Lehrstuhl für Genetik, Fakultät für Biologie, Universität Bielefeld, Postfach 10 01 31, D-33501 Bielefeld, Germany.
Carrayol
E
E
Hirel
B
B
Becker
T W
TW
D000594:Amino Acid Oxidoreductases / Q000378:metabolism; D000596:Amino Acids / Q000378:metabolism; D002352:Carrier Proteins / Q000378:metabolism; D005974:Glutamate-Ammonia Ligase / Q000235:genetics* / Q000378:metabolism; D007527:Isoenzymes / Q000378:metabolism; D045342:Light-Harvesting Protein Complexes; D050901:Nitrate Reductase; D009565:Nitrate Reductases / Q000378:metabolism; D045322:Photosynthetic Reaction Center Complex Proteins / Q000378:metabolism; D045332:Photosystem II Protein Complex; D063245:Plant Development; D018515:Plant Leaves / Q000201:enzymology* / Q000235:genetics; D010940:Plant Proteins / Q000378:metabolism; D010944:Plants / Q000201:enzymology / Q000235:genetics*; D030821:Plants, Genetically Modified; D010947:Plants, Toxic; D018087:Plastids / Q000201:enzymology*; D011401:Promoter Regions, Genetic / Q000235:genetics; D000644:Quaternary Ammonium Compounds / Q000378:metabolism; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000235:genetics; D012273:Ribulose-Bisphosphate Carboxylase / Q000235:genetics; D014026:Nicotiana / Q000201:enzymology / Q000235:genetics* / Q000254:growth & development; D014158:Transcription, Genetic
D000596:Amino Acids; D002352:Carrier Proteins; D007527:Isoenzymes; D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; D010940:Plant Proteins; D000644:Quaternary Ammonium Compounds; D012333:RNA, Messenger; D011993:Recombinant Fusion Proteins; D000594:Amino Acid Oxidoreductases; C042233:glutamate synthase (ferredoxin); D009565:Nitrate Reductases; D050901:Nitrate Reductase; D012273:Ribulose-Bisphosphate Carboxylase; D005974:Glutamate-Ammonia Ligase