Ammonia Production and Assimilation in Glutamate Synthase Mutants of Arabidopsis thaliana

Ammonia production and assimilation(1) 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, NH(4) (+) continued to accumulate. The accumulation of NH(4) (+) was also seen after an initial lag of 30 minutes in 2% O(2), 350 microliters per liter of CO(2) and after 90 minutes in 2% O(2), 900 microliters per liter of CO(2). The accumulation of NH(4) (+) in normal air and low O(2) 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% O(2), 1% CO(2) in the light, the NH(4) (+) which had accumulated in the leaves was reassimilated into amino acids. The addition of methionine sulfoximine (MSO) resulted in higher accumulations of NH(4) (+) in glutamate synthase mutants and prevented the reassimilation of NH(4) (+) upon return to the dark. The addition of MSO also resulted in the accumulation of NH(4) (+) in glutamate synthase mutants in the light and in 21% O(2), 1% CO(2). These results indicate that glutamine synthetase is essential for the reassimilation of photorespiratory NH(4) (+) 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 NH(4) (+) by a GS/NADH-GluS cycle.