Ammonia Production and Assimilation in Glutamate Synthase Mutants of Arabidopsis thaliana1

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Ammonia production and assimilation1 were examined in photorespiratory mutants of Arabidopsis thaliana L. lacking ferredoxin-dependent glutamate synthase (Fd-GluS) activity. Although photosynthesis was rapidly inhibited in these mutants in normal air, NH4+ continued to accumulate. The accumulation of NH4+ was also seen after an initial lag of 30 minutes in 2% O2, 350 microliters per liter of CO2 and after 90 minutes in 2% O2, 900 microliters per liter of CO2. The accumulation of NH4+ in normal air and low O2 was also associated with an increase in the total pool of amino acid-N and glutamine, and a decrease in the pools of glutamate, aspartate, alanine, and serine. Upon return to dark conditions, or to 21% O2, 1% CO2 in the light, the NH4+ which had accumulated in the leaves was reassimilated into amino acids. The addition of methionine sulfoximine (MSO) resulted in higher accumulations of NH4+ in glutamate synthase mutants and prevented the reassimilation of NH4+ upon return to the dark. The addition of MSO also resulted in the accumulation of NH4+ in glutamate synthase mutants in the light and in 21% O2, 1% CO2. These results indicate that glutamine synthetase is essential for the reassimilation of photorespiratory NH4+ and for primary N assimilation in the leaves and strongly suggest that glutamate dehydrogenase plays only a minimal role in the assimilation of ammonia. Levels of NADH-dependent glutamate synthase (NADH-GluS) appear to be sufficient to account for the assimilation of NH4+ by a GS/NADH-GluS cycle.

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