Leaf-specific overexpression of plastidic glutamine synthetase stimulates the growth of transgenic tobacco seedlings

被引:112
作者
Migge, A
Carrayol, E
Hirel, B
Becker, TW
机构
[1] Univ Bielefeld, Fak Biol, Lehrstuhl Genet, D-33501 Bielefeld, Germany
[2] INRA, Lab Metab & Nutr Plantes, F-78026 Versailles, France
关键词
ammonium assimilation; glutamate synthase; glutamine synthetase; Nicotiana (NH4+ assimilation); overexpression (glutamine synthetase); transgenic tobacco;
D O I
10.1007/PL00008132
中图分类号
Q94 [植物学];
学科分类号
071001 ;
摘要
The impact of increased plastidic glutamine synthetase (GS-2; EC 6.1.3.2) activity on foliar aminoacid 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.
引用
收藏
页码:252 / 260
页数:9
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