Poster: Membrane Transport

Abs # 1222: Antisense repression of the plastidic 2-oxoglutarate/malate translocator (DiT1) has massive impact on plant nitrogen metabolism

Presenter:	Schneidereit, Joerg , joerg_schneidereit@web.de
Authors	Schneidereit, Joerg  (A)   Fluegge, Ulf I (B)   Kaiser, Werner M (C)   Weber, Andreas  (A)
Affiliations:	(A): Michigan State University (B): Universtiy of Cologne (C): University of Würzburg

The assimilation of ammonia resulting from nitrate reduction and the photorespiratory carbon cycle is catalyzed by glutamine synthetase (GS₂) and glutamate synthase (GOGAT) that are both located in the plastid stroma. The actual precursor molecule for the assimilation of ammonia (2-oxoglutarate) is imported into the plastid from the cytosol. This import occurs in strict counter-exchange with malate and is mediated by the plastidic 2-oxoglutarate/malate translocator DiT1. We have generated transgenic tobacco plants showing antisense repression of the 2-oxoglutarate/malate translocator. These plants display a severe phenotype characterized by a strong decrease in biomass production, deformed leave shape, bleached regions around the leaf vasculature and delayed onset of flowering. Ammonium accumulates in transgenics during the day and the levels of major amino acids are altered. Cultivation of the transgenics on different nitrogen regimens revealed that the transgenics do not mobilize nitrate from stores after transfer from nitrate to ammonia nutrient solutions. Nitrate reductase activity and transcript levels, however, were increased in the antisense plants. Consequently, the transgenic plants display a discrepancy between nitrate reductase activity and the in vivo rate of nitrate reduction, indicating that nitrate reduction in the 2-oxoglutarate/malate translocator antisense plants is not restricted by availability of nitrate or nitrate reductase activity but by an other, not yet identified factor. We will present a detailed characterization of the transcript levels, enzymatic activities, and total as well as sub-cellular metabolite levels in the transgenic plants and in leaves in response to feeding of 2-oxoglutarate and amino acids.