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

Abstract

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 CO 2) to photorespiratory conditions (air), photorespiratory NH 4 + 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 NH 4 + 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.