Nitrogen is one of the major plant nutrients limiting crop production worldwide. In many parts of the world the availability of N fertilizers is limited, whereas in other parts of the world too much N fertilizer is applied, leading to serious negative environmental consequences. The use of N fertilizers accordingly needs to be optimized in order to make agriculture more sustainable. One pathway to achieve such optimization is to improve plant N use efficiency (NUE) by developing new crop genotypes with improved yield per unit of N fertilizer applied.

For this purpose, more and better knowledge about bottlenecks in plant N assimilation is needed. Based on a reverse genetics strategy embracing characterization of knockout mutants in the model plant species Arabidopsis, the results obtained in this PhD study have provided new information about the specific roles of two genes Gln1;1 and Gln1;2 which encode different isoforms of the key N-assimilatory enzyme cytosolic glutamine synthetase (GS1).

In the single knockout mutant gln1;2 and in the double knockout mutant gln1;1:gln1;2, seed germination and seedling establishment were distinctly impaired. The negative effect of Gln1;2 deficiency was associated with reduced N remobilization from the cotyledons. During reproductive growth, both the single and double Gln1;2-knockout mutants showed decreased seed yield due to fewer siliques, less seeds per silique and lower dry weight per seed. The gln1;1 single mutant had normal seed yield structure but primary root development during seed germination was reduced in the presence of external N. Gln1;2 promoter-GFP construct showed that Gln1;2 expression was localized to the vascular cells of roots, petals, and stamens. A novel compensatory interaction between Gln1;1 and Gln1;2 was shown since shoot Gln1;2 was up-regulated to compensate for the absence of Gln1;1 in N assimilation and ammonium detoxification. It is concluded that Gln1;2 plays an important role in N remobilization for both seedling establishment and seed yield structure in Arabidopsis.