PARENT SESSION
Posters P1C Photosynthate transport and transporters. Abstracts (612-613)

Antisense repression reveals a crucial role of the plastidic 2-oxoglutarate/malate translocator DIT1 at the interface between carbon and nitrogen metabolism. Andreas Weber^*,1, Jörg Schneidereit¹, Werner Kaiser², ¹ Department of Plant BiologyS-336 Plant BiologyLabs, East Lansing, MI, USA²

ABSTRACT- In leaves of most angiosperm species, the biosynthesis of 2-oxoglutarate, the carbon substrate for primary ammonia assimilation, is confined to the cytosol and/or the mitochondria. Hence, plastidic ammonia assimilation depends on 2-oxoglutarate import from the cytosol. A plastidic 2-oxoglutarate/malate transporter (DiT1) has been identified and its substrate specificity and kinetic constants have been analyzed in vitro. However, the role of DiT1 in intact plants and its significance for ammonia assimilation remained uncertain. To unravel the in vivo role of DiT1, the expression of the corresponding gene was repressed by antisense technology in transgenic tobacco plants. Antisense repression of DiT1 resulted in a reduced transport capacity for 2-oxoglutarate across the plastid envelope membrane. In consequence, carbon supply for amino acid synthesis was impaired, organic acids accumulated and protein content, photosynthetic capacity, and sugar pools in leaves strongly decreased. The phenotype was consistent with a role of DIT1 in both, primary ammonia assimilation and the re-assimilation of ammonia resulting from the photorespiratory carbon cycle. Unexpectedly, the in situ rate of nitrate reduction was extremely low in ?-DiT1 leaves, although nitrate reductase activity in leaf extracts remained high. We suggest that this reflects a massive oxidation of the cytosolic NADH pool and we propose a novel model for redox export from plastids that is based on the kinetic constants of plastid envelope membrane transporters and on substrate concentrations in the cytosol. These findings and the severe phenotype of the antisense plants establish a crucial role of DiT1 at the interface between carbon and nitrogen metabolism.

KEY WORDS: nitrogen assimilation, chloroplast envelope membrane, dicarboxylate translocator, antisense repression