Diurnal changes in ammonia assimilation in transformed tobacco plants expressing ferredoxin-dependent glutamate synthase mRNA in the antisense orientation

The diurnal changes in carbon and nitrogen metabolite concentrations were studied in transgenic tobacco plants (Nicotiana tabacum L. cv. Xanthi) expressing an antisense ferredoxin-dependent glutamate synthase (Fd-GOGAT; EC 1.4.7.1) mRNA. In parallel, enzyme activities, and abundance of the proteins and transcripts involved in ammonia assimilation were monitored during the day/night cycle in both transgenic and untransformed control plants. When the transgenic plants were transferred from non-photorespiratory (high CO2) to photorespiratory conditions (air), photorespiratory NH4+ accumulated in the leaves during the second half of the light period; it decreased progressively through the night until the beginning of the next light period. Glutamine and 2-oxoglutarate (2-OG) also accumulated during the second part of the light period; this was followed by a decrease at the end of the night. A concomitant increase in asparagine was observed during the night, suggesting that this amino acid serves as a temporary storage compound for the partial elimination of excess photorespiratory ammonia. We also observed that the direction of the glutamate reaction varied during the day/night cycle such that a higher ratio of aminating/deaminating activity occurred in the first half of the light period. This was correlated with the decline in NH4+ and 2-OG concentrations. Both of these observations suggest that alternative metabolic pathways may be transiently activated during a day/night cycle in plants with low Fd-GOGAT activity. (C) 2002 Elsevier Science Ireland Ltd. All rights reserved.