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) a ctivity on foliar aminoacid levels and on biomass production was examined i n transgenic tobacco. For that, tobacco was transformed via Agrobacterium t umefaciens with a binary vector containing a tobacco GS-2 cDNA downstream o f the leaf-specific soybean ribulose-1,5-bisphosphate carboxylase/oxygenase small subunit gene promotor. Two transgenic tobacco lines with 15- to 18-f old 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 l eaves of the wild type. This discrepancy may reflect a post-transcriptional control of GS-2 protein accumulation. The increased GS-2 activity was corr elated 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 w eight: however, remained unchanged. This result indicates that a process do wnstream 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 cat alysed 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. Incre ased leaf GS-2 activity had, however, no influence on the foliar Fd-GOGAT p rotein abundance. This result suggests that in tobacco leaves, more Fd-GOGA T is present than required to meet the demands of primary ammonium assimila tion and that there is no strong interdependence between GS-2 and Fd-GOGAT protein expression.